Only about 2% of known marine species inhabit the pelagic environment. This means that they live in the water column as opposed to the benthic organisms that live in or on the sea floor. Deep-sea organisms generally inhabit bathypelagic (1000–4000m deep) and abyssopelagic (4000–6000m deep) zones. However, characteristics of deep-sea organisms, such as bioluminescence can be seen in the mesopelagic
(200–1000m deep) zone as well. The mesopelagic zone is the disphotic
zone, meaning light there is minimal but still measurable. The oxygen
minimum layer exists somewhere between a depth of 700m and 1000m deep
depending on the place in the ocean. This area is also where nutrients
are most abundant. The bathypelagic and abyssopelagic zones are aphotic, meaning that no light penetrates this area of the ocean. These zones make up about 75% of the inhabitable ocean space.
The epipelagic zone (0–200m) is the area where light penetrates
the water and photosynthesis occurs. This is also known as the photic
zone. Because this typically extends only a few hundred meters below the
water, the deep sea, about 90% of the ocean volume, is in darkness. The
deep sea is also an extremely hostile environment, with temperatures
that rarely exceed 3 °C (37.4 °F) and fall as low as −1.8 °C (28.76 °F)
(with the exception of hydrothermal vent ecosystems that can exceed
350 °C, or 662 °F), low oxygen levels, and pressures between 20 and
1,000 atmospheres (between 2 and 100 megapascals).
Environment
Scale diagram of the layers of the pelagic zone
In the deep ocean, the waters extend far below the epipelagic zone,
and support very different types of pelagic fishes adapted to living in
these deeper zones. In deep water, marine snow is a continuous shower of mostly organic detritus falling from the upper layers of the water column. Its origin lies in activities within the productive photic zone. Marine snow includes dead or dying plankton, protists (diatoms),
fecal matter, sand, soot and other inorganic dust. The "snowflakes"
grow over time and may reach several centimetres in diameter, travelling
for weeks before reaching the ocean floor. However, most organic
components of marine snow are consumed by microbes, zooplankton
and other filter-feeding animals within the first 1,000 metres of their
journey, that is, within the epipelagic zone. In this way marine snow
may be considered the foundation of deep-sea mesopelagic and benthicecosystems:
As sunlight cannot reach them, deep-sea organisms rely heavily on
marine snow as an energy source. Since there is no light in the deep sea
(aphotic), there is a lack of primary producers. Therefore, most
organisms in the bathypelagic rely on the marine snow from regions
higher in the vertical column.
Some deep-sea pelagic groups, such as the lanternfish, ridgehead, marine hatchetfish, and lightfish families are sometimes termed pseudoceanic
because, rather than having an even distribution in open water, they
occur in significantly higher abundances around structural oases,
notably seamounts and over continental slopes. The phenomenon is explained by the likewise abundance of prey species which are also attracted to the structures.
Hydrostatic pressure increases by 1 atmosphere for every 10m in depth.
Deep-sea organisms have the same pressure within their bodies as is
exerted on them from the outside, so they are not crushed by the extreme
pressure. Their high internal pressure, however, results in the reduced
fluidity of their membranes because molecules are squeezed together.
Fluidity in cell membranes increases efficiency of biological functions,
most importantly the production of proteins, so organisms have adapted
to this circumstance by increasing the proportion of unsaturated fatty
acids in the lipids of the cell membranes.
In addition to differences in internal pressure, these organisms have
developed a different balance between their metabolic reactions from
those organisms that live in the epipelagic zone. David Wharton, author
of Life at the Limits: Organisms in Extreme Environments, notes
"Biochemical reactions are accompanied by changes in volume. If a
reaction results in an increase in volume, it will be inhibited by
pressure, whereas, if it is associated with a decrease in volume, it
will be enhanced". This means that their metabolic processes must ultimately decrease the volume of the organism to some degree.
Humans seldom encounter frilled sharks alive, so they pose little danger (though scientists have accidentally cut themselves examining their teeth).
Most fish that have evolved in this harsh environment are not capable
of surviving in laboratory conditions, and attempts to keep them in
captivity have led to their deaths. Deep-sea organisms contain
gas-filled spaces (vacuoles). Gas is compressed under high pressure and
expands under low pressure. Because of this, these organisms have been
known to blow up if they come to the surface.
Characteristics
An annotated diagram of the basic external features of an abyssal grenadier and standard length measurements.
Bigeye tuna cruise the epipelagic zone at night and the mesopelagic zone during the day
The fish of the deep-sea have evolved various adaptations to survive
in this region. Since many of these fish live in regions where there is
no natural illumination, they cannot rely solely on their eyesight for locating prey and mates and avoiding predators; deep-sea fish have evolved
appropriately to the extreme sub-photic region in which they live. Many
of these organisms are blind and rely on their other senses, such as
sensitivities to changes in local pressure and smell, to catch their
food and avoid being caught. Those that aren't blind have large and
sensitive eyes that can use bioluminescent light. These eyes can be as much as 100 times more sensitive to light than human eyes. Rhodopsin
(Rh1) is a protein found in the eye’s rod cells that helps animals see
in dim light. While most vertebrates usually have one Rh1 opsin gene,
some deep sea fish have several Rh1 genes, and one species, the silver spinyfin (Diretmus argenteus), has 38.
This proliferation of Rh1 genes may help deep sea fish to see in the
depths of the ocean. Also, to avoid predation, many species are dark to
blend in with their environment.
Many deep-sea fish are bioluminescent,
with extremely large eyes adapted to the dark. Bioluminescent organisms
are capable of producing light biologically through the agitation of
molecules of luciferin, which then produce light. This process must be
done in the presence of oxygen. These organisms are common in the
mesopelagic region and below (200m and below). More than 50% of deep-sea
fish, as well as some species of shrimp and squid, are capable of
bioluminescence. About 80% of these organisms have photophores – light
producing glandular cells that contain luminous bacteria bordered by
dark colourings. Some of these photophores contain lenses, much like
those in the eyes of humans, which can intensify or lessen the emanation
of light. The ability to produce light only requires 1% of the
organism's energy and has many purposes: It is used to search for food
and attract prey, like the anglerfish; claim territory through patrol;
communicate and find a mate, and distract or temporarily blind predators
to escape. Also, in the mesopelagic where some light still penetrates,
some organisms camouflage themselves from predators below them by
illuminating their bellies to match the colour and intensity of light
from above so that no shadow is cast. This tactic is known as
counter-illumination.
The lifecycle of deep-sea fish can be exclusively deep water
although some species are born in shallower water and sink upon
maturation. Regardless of the depth where eggs and larvae reside, they
are typically pelagic. This planktonic — drifting — lifestyle requires
neutral buoyancy. In order to maintain this, the eggs and larvae often
contain oil droplets in their plasma.
When these organisms are in their fully matured state they need other
adaptations to maintain their positions in the water column. In general,
water's density causes upthrust — the aspect of buoyancy that makes
organisms float. To counteract this, the density of an organism must be
greater than that of the surrounding water. Most animal tissues are
denser than water, so they must find an equilibrium to make them float. Many organisms develop swim bladders
(gas cavities) to stay afloat, but because of the high pressure of
their environment, deep-sea fishes usually do not have this organ.
Instead they exhibit structures similar to hydrofoils in order to
provide hydrodynamic lift. It has also been found that the deeper a fish
lives, the more jelly-like its flesh and the more minimal its bone
structure. They reduce their tissue density through high fat content,
reduction of skeletal weight — accomplished through reductions of size,
thickness and mineral content — and water accumulation makes them slower and less agile than surface fish.
Due to the poor level of photosynthetic light reaching deep-sea environments, most fish need to rely on organic matter sinking from higher levels, or, in rare cases, hydrothermal vents for nutrients. This makes the deep-sea much poorer in productivity
than shallower regions. Also, animals in the pelagic environment are
sparse and food doesn't come along frequently. Because of this,
organisms need adaptations that allow them to survive. Some have long
feelers to help them locate prey or attract mates in the pitch black of
the deep ocean. The deep-sea angler fish in particular has a long
fishing-rod-like adaptation protruding from its face, on the end of
which is a bioluminescent piece of skin that wriggles like a worm to
lure its prey. Some must consume other fish that are the same size or
larger than them and they need adaptations to help digest them
efficiently. Great sharp teeth, hinged jaws, disproportionately large
mouths, and expandable bodies are a few of the characteristics that
deep-sea fishes have for this purpose. The gulper eel is one example of an organism that displays these characteristics.
Fish in the different pelagic and deep water benthic zones are
physically structured, and behave in ways, that differ markedly from
each other. Groups of coexisting species within each zone all seem to
operate in similar ways, such as the small mesopelagic vertically migrating plankton-feeders, the bathypelagic anglerfishes, and the deep water benthic rattails.
Ray finned
species, with spiny fins, are rare among deep sea fishes, which
suggests that deep sea fish are ancient and so well adapted to their
environment that invasions by more modern fishes have been unsuccessful. The few ray fins that do exist are mainly in the Beryciformes and Lampriformes,
which are also ancient forms. Most deep sea pelagic fishes belong to
their own orders, suggesting a long evolution in deep sea environments.
In contrast, deep water benthic species, are in orders that include many
related shallow water fishes.
Mesopelagic fish
Mesopelagic fish
Most
mesopelagic fishes are small filter feeders which ascend at night to
feed in the nutrient rich waters of the epipelagic zone. During the day,
they return to the dark, cold, oxygen deficient waters of the
mesopelagic where they are relatively safe from predators. Lanternfish account for as much as 65% of all deep sea fish biomass and are largely responsible for the deep scattering layer of the world's oceans.
Most of the rest of the mesopelagic fishes are ambush predators, like this sabertooth fish
which uses its telescopic, upward-pointing eyes to pick out prey
silhouetted against the gloom above. Their recurved teeth prevent a
captured fish from backing out.
The Antarctic toothfish have large, upward looking eyes, adapted to detecting the silhouettes of prey fish.
The barreleye has barrel-shaped, tubular eyes which are generally directed upwards but can be swivelled forward
The telescopefish has large, forward-pointing telescoping eyes with large lenses
Below the epipelagic zone, conditions change rapidly. Between
200 metres and about 1000 metres, light continues to fade until there is
almost none. Temperatures fall through a thermocline to temperatures between 3.9 °C (39 °F) and 7.8 °C (46 °F). This is the twilight or mesopelagic
zone. Pressure continues to increase, at the rate of one atmosphere
every 10 metres, while nutrient concentrations fall, along with
dissolved oxygen and the rate at which the water circulates.
Sonar operators, using the newly developed sonar technology
during World War II, were puzzled by what appeared to be a false sea
floor 300–500 metres deep at day, and less deep at night. This turned
out to be due to millions of marine organisms, most particularly small
mesopelagic fish, with swim bladders that reflected the sonar. These
organisms migrate up into shallower water at dusk to feed on plankton.
The layer is deeper when the moon is out, and can become shallower when
clouds pass over the moon. This phenomenon has come to be known as the deep scattering layer.
Most mesopelagic fish make daily vertical migrations,
moving at night into the epipelagic zone, often following similar
migrations of zooplankton, and returning to the depths for safety during
the day. These vertical migrations often occur over large vertical distances, and are undertaken with the assistance of a swim bladder.
The swim bladder is inflated when the fish wants to move up, and, given
the high pressures in the messoplegic zone, this requires significant
energy. As the fish ascends, the pressure in the swim bladder must
adjust to prevent it from bursting. When the fish wants to return to the
depths, the swim bladder is deflated. Some mesopelagic fishes make daily migrations through the thermocline,
where the temperature changes between 50 °F (10 °C) and 69 °F (20 °C),
thus displaying considerable tolerances for temperature change.
These fish have muscular bodies, ossified bones, scales, well
developed gills and central nervous systems, and large hearts and
kidneys. Mesopelagic plankton feeders have small mouths with fine gill rakers, while the piscivores have larger mouths and coarser gill rakers.
Mesopelagic fish are adapted for an active life under low light
conditions. Most of them are visual predators with large eyes. Some of
the deeper water fish have tubular eyes with big lenses and only rod cells that look upwards. These give binocular vision and great sensitivity to small light signals. This adaptation gives improved terminal vision at the expense of lateral vision, and allows the predator to pick out squid, cuttlefish, and smaller fish that are silhouetted against the gloom above them.
Mesopelagic fish usually lack defensive spines, and use colour to camouflage themselves from other fish. Ambush predators
are dark, black or red. Since the longer, red, wavelengths of light do
not reach the deep sea, red effectively functions the same as black.
Migratory forms use countershaded silvery colours. On their bellies, they often display photophores producing low grade light. For a predator from below, looking upwards, this bioluminescence
camouflages the silhouette of the fish. However, some of these
predators have yellow lenses that filter the (red deficient) ambient
light, leaving the bioluminescence visible.
The brownsnout spookfish, a species of barreleye, is the only vertebrate known to employ a mirror, as opposed to a lens, to focus an image in its eyes.
Sampling by deep trawling indicates that lanternfish account for as much as 65% of all deep-sea fish biomass. Indeed, lanternfish are among the most widely distributed, populous, and diverse of all vertebrates, playing an important ecological role as prey for larger organisms. The estimated global biomass of lanternfish is 550–660 million tonnes, several times the entire world fisheries catch. Lanternfish also account for much of the biomass responsible for the deep scattering layer of the world's oceans.
Bigeye tuna
are an epipelagic/mesopelagic species that eats other fish. Satellite
tagging has shown that bigeye tuna often spend prolonged periods
cruising deep below the surface during the daytime, sometimes making
dives as deep as 500 metres. These movements are thought to be in
response to the vertical migrations of prey organisms in the deep scattering layer.
The stoplight loosejaw has a lower jaw
one-quarter as long as its body. The jaw has no floor and is attached
only by a hinge and a modified tongue bone. Large fang-like teeth in the
front are followed by many small barbed teeth.
The stoplight loosejaw is also one of the few fishes that produce red bioluminescence. As most of their prey cannot perceive red light, this allows it to hunt with an essentially invisible beam of light.
Long-snouted lancetfish.
Lancetfish are ambush predators which spend all their time in the
mesopelagic zone. They are among the largest mesopelagic fishes (up to
2 metres).
The daggertooth paralyses other mesopelagic fish when it bites them with its dagger-like teeth
Bathypelagic fish
Bathypelagic fish
The humpback anglerfish
is a bathypelagic ambush predator, which attracts prey with a
bioluminescent lure. It can ingest prey larger than itself, which it
swallows with an inrush of water when it opens its mouth.
Many bristlemouth species, such as the "spark anglemouth" above,
are also bathypelagic ambush predators which can swallow prey larger
than themselves. They are among the most abundant of all vertebrate
families.
Young, red flabby whalefish make nightly vertical migrations into the lower mesopelagic zone to feed on copepods.
When males make the transition to adults, they develop a massive liver,
and then their jaws fuse shut. They no longer eat, but continue to
metabolise the energy stored in their liver.
The Sloane's viperfish can make nightly migrations from bathypelagic depths to near surface waters.
The widespread fangtooth has the largest teeth of any fish, proportionate to body size.
Despite their ferocious appearance, bathypelagic fish are usually
weakly muscled and too small to represent any threat to humans.
Below the mesopelagic zone it is pitch dark. This is the midnight (or bathypelagic zone), extending from 1000 metres to the bottom deep water benthic zone. If the water is exceptionally deep, the pelagic zone below 4000 metres is sometimes called the lower midnight (or abyssopelagic zone). Temperatures in this zone range from 1 to 4 degrees celsius and is completely aphotic.
Conditions are somewhat uniform throughout these zones; the
darkness is complete, the pressure is crushing, and temperatures,
nutrients and dissolved oxygen levels are all low.
Bathypelagic fish have special adaptations to cope with these conditions – they have slow metabolisms
and unspecialized diets, being willing to eat anything that comes
along. They prefer to sit and wait for food rather than waste energy
searching for it. The behaviour of bathypelagic fish can be contrasted
with the behaviour of mesopelagic fish. Mesopelagic fish are often
highly mobile, whereas bathypelagic fish are almost all lie-in-wait
predators, normally expending little energy in movement.
The dominant bathypelagic fishes are small bristlemouth and anglerfish; fangtooth, viperfish, daggertooth and barracudina
are also common. These fishes are small, many about 10 centimetres
long, and not many longer than 25 cm. They spend most of their time
waiting patiently in the water column for prey to appear or to be lured
by their phosphors. What little energy is available in the bathypelagic
zone filters from above in the form of detritus, faecal material, and
the occasional invertebrate or mesopelagic fish. About 20 percent of the food that has its origins in the epipelagic zone falls down to the mesopelagic zone, but only about 5 percent filters down to the bathypelagic zone.
Bathypelagic fish are sedentary, adapted to outputting minimum
energy in a habitat with very little food or available energy, not even
sunlight, only bioluminescence. Their bodies are elongated with weak, watery muscles and skeletal
structures. Since so much of the fish is water, they are not compressed
by the great pressures at these depths. They often have extensible,
hinged jaws with recurved teeth. They are slimy, without scales.
The central nervous system is confined to the lateral line and
olfactory systems, the eyes are small and may not function, and gills, kidneys and hearts, and swim bladders are small or missing.
These are the same features found in fish larvae, which suggests that during their evolution, bathypelagic fish have acquired these features through neoteny. As with larvae, these features allow the fish to remain suspended in the water with little expenditure of energy.
Despite their ferocious appearance, these beasts of the deep are
mostly miniature fish with weak muscles, and are too small to represent
any threat to humans.
The swim bladders of deep sea fish are either absent or scarcely
operational, and bathypelagic fish do not normally undertake vertical
migrations. Filling bladders at such great pressures incurs huge energy
costs. Some deep sea fishes have swim bladders which function while they
are young and inhabit the upper epipelagic zone, but they wither or
fill with fat when the fish move down to their adult habitat.
The most important sensory systems are usually the inner ear, which responds to sound, and the lateral line, which responds to changes in water pressure. The olfactory system can also be important for males who find females by smell.
Bathypelagic fish are black, or sometimes red, with few photophores.
When photophores are used, it is usually to entice prey or attract a
mate. Because food is so scarce, bathypelagic predators are not
selective in their feeding habits, but grab whatever comes close enough.
They accomplish this by having a large mouth with sharp teeth for
grabbing large prey and overlapping gill rakers which prevent small prey that have been swallowed from escaping.
It is not easy finding a mate in this zone. Some species depend on bioluminescence, where bioluminescent patterns are unique to specific species. Others are hermaphrodites, which doubles their chances of producing both eggs and sperm when an encounter occurs. The female anglerfish releases pheromones to attract tiny males. When a male finds her, he bites on to her and never lets go. When a male of the anglerfish species Haplophryne mollis bites into the skin of a female, he releases an enzyme
that digests the skin of his mouth and her body, fusing the pair to the
point where the two circulatory systems join up. The male then
atrophies into nothing more than a pair of gonads. This extreme sexual dimorphism ensures that, when the female is ready to spawn, she has a mate immediately available.
Many forms other than fish live in the bathypelagic zone, such as squid, large whales, octopuses, sponges, brachiopods, sea stars, and echinoids, but this zone is difficult for fish to live in.
The pelican eel
uses its mouth like a net by opening its large mouth and swimming at
its prey. It has a luminescent organ at the tip of its tail to attract
prey.
The black swallower, with its distensible stomach, is notable for its ability to swallow, whole, bony fishes ten times its mass.
Female Haplophryne mollis anglerfish trailing attached males which have atrophied into a pair of gonads, for use when the female is ready to spawn.
Adaptation to high pressure
As a fish moves deeper into the sea, the weight of the water overhead exerts increasing hydrostatic pressure on the fish. This increased pressure amounts to about one standard atmosphere
for every 10 meters in depth (one atmosphere is the pressure exerted at
the surface of the sea by the atmosphere above). For a fish at the
bottom of the bathypelagic zone, this pressure amounts to about 400 atmospheres (nearly 6000 pounds per square inch).
Deep sea organisms possess adaptations at cellular and
physiological levels that allow them to survive in environments of great
pressure. Not having these adaptations limits the depths at which
shallow-water species can operate. High levels of external pressure
affects how metabolic processes and biochemical reactions proceed. The
equilibrium of many chemical reactions is disturbed by pressure, and
pressure can inhibit processes which result in an increase in volume.
Water, a key component in many biological processes, is very susceptible
to volume changes, mainly because constituents of cellular fluid have
an effect on water structure. Thus, enzymatic reactions that induce
changes in water organization effectively change the system's volume. Proteins responsible for catalyzing reactions are typically held together by weak bonds and the reactions usually involve volume increases.
To adapt to this change, the protein structure and reaction
criteria of deep sea fish have been adapted to withstand pressure in
order to perform reactions in these conditions. In high pressure
environments, bilayer cellular membranes experience a loss of fluidity. Deep-sea cellular membranes favor phospholipid bilayers with a higher proportion of unsaturated fatty acids, which induce a higher fluidity than their sea-level counterparts.
The rattail Coryphaenoides armatus (abyssal grenadier) on the Davidson Seamount at 2253 meters depth.
Deep sea species exhibit lower changes of entropy and enthalpy
compared to surface level organisms, since a high pressure and low
temperature environment favors negative enthalpy changes and reduced
dependence on entropy-driven reactions. From a structural standpoint, globular proteins of deep sea fish due to the tertiary structure of G-actin are relatively rigid compared to those of surface level fish.
The fact that proteins in deep sea fish are structurally different from
surface fish is apparent from the observation that actin from the
muscle fibers of deep sea fish are extremely heat resistant; an
observation similar to what is found in lizards. These proteins are
structurally strengthened by modification of the bonds in the tertiary
structure of the protein which also happens to induce high levels of
thermal stability. Proteins are structurally strengthened to resist pressure by modification of bonds in the tertiary structure.
Therefore, high levels of hydrostatic pressure, similar to high body
temperatures of thermophilic desert reptiles, favor rigid protein
structures.
Na+/K+ -ATPase is a lipoprotein enzyme that plays a prominent role in osmoregulation
and is heavily influenced by hydrostatic pressure. The inhibition of
Na+/K+ -ATPase is due to increased compression due to pressure. The
rate-limiting step of the Na+/K+ -ATPase reaction induces an expansion
in the bilayer surrounding the protein, and therefore an increase in
volume. An increase in volume makes Na+/K+ -ATPase reactivity
susceptible to higher pressures. Even though the Na+/K+ -ATPase activity
per gram of gill tissue
is lower for deep sea fishes, the Na+/K+ -ATPases of deep sea fishes
exhibit a much higher tolerance of hydrostatic pressure compared to
their shallow-water counterparts. This is exemplified between the
species C. acrolepis (around 2000m deep) and its hadalpelagic counterpart C. armatus (around 4000m deep), where the Na+/K+ -ATPases of C. armatus are much less sensitive to pressure. This resistance to pressure can be explained by adaptations in the protein and lipid moieties of Na+/K+ -ATPase.
Sampling via deep trawling indicates that lanternfish account for as much as 65% of all deep-sea fish biomass. Indeed, lanternfish are among the most widely distributed, populous, and diverse of all vertebrates, playing an important ecological role as prey for larger organisms. With an estimated global biomass of 550–660 million metric tons, several times the entire world fisheries catch, lanternfish also account for much of the biomass responsible for the deep scattering layer of the world's oceans. In the Southern Ocean, Myctophids provide an alternative food resource to krill for predators such as squid and the king penguin.
Although these fish are plentiful and prolific, currently only a few
commercial lanternfish fisheries exist: these include limited operations
off South Africa, in the sub-Antarctic, and in the Gulf of Oman.
Endangered species
A 2006 study by Canadian scientists has found five species of deep-sea fish – blue hake, spiny eel – to be on the verge of extinction due to the shift of commercial fishing from continental shelves to the slopes of the continental shelves, down to depths of 1600 meters. The slow reproduction of these fish – they reach sexual maturity at about the same age as human beings – is one of the main reasons that they cannot recover from the excessive fishing.
Intertidal ecology is the study of intertidalecosystems, where organisms live between the low and high tide lines. At low tide, the intertidal is exposed whereas at high tide, the intertidal is underwater. Intertidal ecologists therefore study the interactions between intertidal organisms and their environment, as well as between different species
of intertidal organisms within a particular intertidal community. The
most important environmental and species interactions may vary based on
the type of intertidal community being studied, the broadest of
classifications being based on substrates—rocky shore and soft bottom communities.
Organisms living in this zone have a highly variable and often hostile environment, and have evolved various adaptations to cope with and even exploit these conditions. One easily visible feature of intertidal communities is vertical zonation,
where the community is divided into distinct vertical bands of specific
species going up the shore. Species ability to cope with abiotic factors associated with emersion stress, such as desiccation determines their upper limits, while biotic interactions e.g.competition with other species sets their lower limits.
Intertidal habitats can be characterized as having either hard or soft bottoms substrates. Rocky intertidal communities occur on rocky shores, such as headlands, cobble beaches, or human-made jetties. Their degree of exposure may be calculated using the Ballantine Scale. Soft-sediment habitats include sandy beaches, and intertidal wetlands (e.g., mudflats and salt marshes). These habitats differ in levels of abiotic,
or non-living, environmental factors. Rocky shores tend to have higher
wave action, requiring adaptations allowing the inhabitants to cling
tightly to the rocks. Soft-bottom habitats are generally protected from
large waves but tend to have more variable salinity levels. They also offer a third habitable dimension: depth. Thus, many soft-sediment inhabitants are adapted for burrowing.
Environment
A rock, seen at low tide, exhibiting typical intertidal zonation.
A specimen of the shell Pinna nobilis exposed by low tide
Because intertidal organisms endure regular periods of immersion and
emersion, they essentially live both underwater and on land and must be
adapted to a large range of climatic conditions. The intensity of
climate stressors varies with relative tide height because organisms
living in areas with higher tide heights are emersed for longer periods
than those living in areas with lower tide heights. This gradient of
climate with tide height leads to patterns of intertidal zonation,
with high intertidal species being more adapted to emersion stresses
than low intertidal species. These adaptations may be behavioral (i.e.
movements or actions), morphological (i.e. characteristics of external
body structure), or physiological (i.e. internal functions of cells and organs).
In addition, such adaptations generally cost the organism in terms of
energy (e.g. to move or to grow certain structures), leading to
trade-offs (i.e. spending more energy on deterring predators leaves less
energy for other functions like reproduction).
Intertidal organisms, especially those in the high intertidal, must cope with a large range of temperatures.
While they are underwater, temperatures may only vary by a few degrees
over the year. However, at low tide, temperatures may dip to below
freezing or may become scaldingly hot, leading to a temperature range
that may approach 30 °C (86 °F) during a period of a few hours. Many
mobile organisms, such as snails and crabs, avoid temperature
fluctuations by crawling around and searching for food at high tide and
hiding in cool, moist refuges (crevices or burrows) at low tide.
Besides simply living at lower tide heights, non-motile organisms may
be more dependent on coping mechanisms. For example, high intertidal
organisms have a stronger stress response, a physiological response of
making proteins that help recovery from temperature stress just as the
immune response aids in the recovery from infection.
Intertidal organisms are also especially prone to desiccation
during periods of emersion. Again, mobile organisms avoid desiccation
in the same way as they avoid extreme temperatures: by hunkering down in
mild and moist refuges. Many intertidal organisms, including Littorina
snails, prevent water loss by having waterproof outer surfaces, pulling
completely into their shells, and sealing shut their shell opening. Limpets (Patella)
do not use such a sealing plate but occupy a home-scar to which they
seal the lower edge of their flattened conical shell using a grinding
action. They return to this home-scar after each grazing excursion,
typically just before emersion. On soft rocks, these scars are quite
obvious. Still other organisms, such as the algae Ulva and Porphyra, are able to rehydrate and recover after periods of severe desiccation.
The level of salinity
can also be quite variable. Low salinities can be caused by rainwater
or river inputs of freshwater. Estuarine species must be especially euryhaline,
or able to tolerate a wide range of salinities. High salinities occur
in locations with high evaporation rates, such as in salt marshes and
high intertidal pools. Shading by plants, especially in the salt marsh,
can slow evaporation and thus ameliorate salinity stress. In addition,
salt marsh plants tolerate high salinities by several physiological
mechanisms, including excreting salt through salt glands and preventing
salt uptake into the roots.
In addition to these exposure stresses (temperature, desiccation,
and salinity), intertidal organisms experience strong mechanical
stresses, especially in locations of high wave action. There are myriad ways in which the organisms prevent dislodgement due to waves.
Morphologically, many mollusks (such as limpets and chitons) have
low-profile, hydrodynamic shells. Types of substrate attachments include
mussels' tethering byssal threads and glues, sea stars'
thousands of suctioning tube feet, and isopods' hook-like appendages
that help them hold on to intertidal kelps. Higher profile organisms,
such as kelps, must also avoid breaking in high flow locations, and they
do so with their strength and flexibility. Finally, organisms can also
avoid high flow environments, such as by seeking out low flow
microhabitats. Additional forms of mechanical stresses include ice and
sand scour, as well as dislodgment by water-borne rocks, logs, etc.
For each of these climate stresses, species exist that are
adapted to and thrive in the most stressful of locations. For example,
the tiny crustacean copepod Tigriopus thrives in very salty, high intertidal tidepools, and many filter feeders
find more to eat in wavier and higher flow locations. Adapting to such
challenging environments gives these species competitive edges in such
locations.
During tidal immersion, the food supply to intertidal organisms is subsidized by materials carried in seawater, including photosynthesizingphytoplankton and consumer zooplankton. These plankton are eaten by numerous forms of filter feeders—mussels, clams, barnacles, sea squirts, and polychaete worms—which filter seawater in their search for planktonic food sources. The adjacent ocean is also a primary source of nutrients for autotrophs, photosynthesizing producers ranging in size from microscopic algae (e.g. benthic diatoms) to huge kelps and other seaweeds. These intertidal producers are eaten by herbivorous grazers, such as limpets that scrape rocks clean of their diatom layer and kelp crabs that creep along blades of the feather boa kelp Egregia eating the tiny leaf-shaped bladelets. Crabs are eaten by goliath grouper, which are then eaten by sharks. Higher up the food web, predatory consumers—especially voracious starfish—eat other grazers (e.g. snails) and filter feeders (e.g. mussels). Finally, scavengers, including crabs and sand fleas, eat dead organic material, including dead producers and consumers.
In addition to being shaped by aspects of climate, intertidal
habitats—especially intertidal zonation patterns—are strongly influenced
by species interactions, such as predation, competition, facilitation,
and indirect interactions. Ultimately, these interactions feed into the
food web structure, described above. Intertidal habitats have been a
model system for many classic ecological studies, including those
introduced below, because the resident communities are particularly
amenable to experimentation.
One dogma of intertidal ecology—supported by such classic
studies—is that species' lower tide height limits are set by species
interactions whereas their upper limits are set by climate variables.
Classic studies by Robert Paine
established that when sea star predators are removed, mussel beds
extend to lower tide heights, smothering resident seaweeds. Thus,
mussels' lower limits are set by sea star predation. Conversely, in the
presence of sea stars, mussels' lower limits occur at a tide height at
which sea stars are unable to tolerate climate conditions.
Competition, especially for space, is another dominant
interaction structuring intertidal communities. Space competition is
especially fierce in rocky intertidal habitats, where habitable space is
limited compared to soft-sediment habitats in which three-dimensional
space is available. As seen with the previous sea star example, mussels
are competitively dominant when they are not kept in check by sea star
predation. Joseph Connell's research on two types of high intertidal barnacles, Balanus balanoides, now Semibalanus balanoides, and a Chthamalus stellatus, showed that zonation patterns could also be set by competition between closely related organisms. In this example, Balanus outcompetes Chthamalus at lower tide heights but is unable to survive at higher tide heights. Thus, Balanus
conforms to the intertidal ecology dogma introduced above: its lower
tide height limit is set by a predatory snail and its higher tide height
limit is set by climate. Similarly, Chthamalus, which occurs in a
refuge from competition (similar to the temperature refuges discussed
above), has a lower tide height limit set by competition with Balanus and a higher tide height limit is set by climate.
Although intertidal ecology has traditionally focused on these
negative interactions (predation and competition), there is emerging
evidence that positive interactions are also important. Facilitation refers to one organism helping another without harming itself. For example, salt marsh plant species of Juncus and Iva
are unable to tolerate the high soil salinities when evaporation rates
are high, thus they depend on neighboring plants to shade the sediment,
slow evaporation, and help maintain tolerable salinity levels.
In similar examples, many intertidal organisms provide physical
structures that are used as refuges by other organisms. Mussels,
although they are tough competitors with certain species, are also good
facilitators as mussel beds provide a three-dimensional habitat to
species of snails, worms, and crustaceans.
All of the examples given so far are of direct interactions:
Species A eat Species B or Species B eats Species C. Also important are
indirect interactions
where, using the previous example, Species A eats so much of Species B
that predation on Species C decreases and Species C increases in number.
Thus, Species A indirectly benefits Species C. Pathways of indirect
interactions can include all other forms of species interactions. To
follow the sea star-mussel relationship, sea stars have an indirect
negative effect on the diverse community that lives in the mussel bed
because, by preying on mussels and decreasing mussel bed structure,
those species that are facilitated by mussels are left homeless.
Additional important species interactions include mutualism, which is seen in symbioses between sea anemones and their internal symbiotic algae, and parasitism, which is prevalent but is only beginning to be appreciated for its effects on community structure.
Current topics
Humans are highly dependent on intertidal habitats for food and raw materials,
and over 50% of humans live within 100 km of the coast. Therefore,
intertidal habitats are greatly influenced by human impacts to both
ocean and land habitats. Some of the conservation issues associated with
intertidal habitats and at the head of the agendas of managers and
intertidal ecologists are:
1. Climate change: Intertidal species are challenged by several of the effects of global climate change, including increased temperatures, sea level rise,
and increased storminess. Ultimately, it has been predicted that the
distributions and numbers of species will shift depending on their
abilities to adapt (quickly!) to these new environmental conditions.
Due to the global scale of this issue, scientists are mainly working to
understand and predict possible changes to intertidal habitats.
2. Invasive species:
Invasive species are especially prevalent in intertidal areas with high
volumes of shipping traffic, such as large estuaries, because of the
transport of non-native species in ballast water. San Francisco Bay, in which an invasive Spartina cordgrass from the east coast is currently transforming mudflat communities into Spartina
meadows, is among the most invaded estuaries in the world. Conservation
efforts are focused on trying to eradicate some species (like Spartina)
in their non-native habitats as well as preventing further species
introductions (e.g. by controlling methods of ballast water uptake and
release).
3. Marine protected areas:
Many intertidal areas are lightly to heavily exploited by humans for
food gathering (e.g. clam digging in soft-sediment habitats and snail,
mussel, and algal collecting in rocky intertidal habitats). In some
locations, marine protected areas have been established where no
collecting is permitted. The benefits of protected areas may spill over
to positively impact adjacent unprotected areas. For example, a greater
number of larger egg capsules of the edible snail Concholepus in
protected vs. non-protected areas in Chile indicates that these
protected areas may help replenish snail stocks in areas open to
harvesting. The degree to which collecting is regulated by law differs with the species and habitat.
Simplified schematic of only the lunar portion of Earth's tides, showing (exaggerated) high tides at the sublunar point and its antipode for the hypothetical case of an ocean of constant depth without land. Solar tides not shown.
In Maine
(U.S.), low tide occurs roughly at moonrise and high tide with a high
Moon, corresponding to the simple gravity model of two tidal bulges; at
most places however, the Moon and tides have a phase shift.
Tide tables can be used for any given locale to find the predicted times and amplitude (or "tidal range"). The predictions are influenced by many factors including the alignment of the Sun and Moon, the phase and amplitude of the tide (pattern of tides in the deep ocean), the amphidromic systems of the oceans, and the shape of the coastline and near-shore bathymetry (see Timing).
They are however only predictions, the actual time and height of the
tide is affected by wind and atmospheric pressure. Many shorelines
experience semi-diurnal tides—two nearly equal high and low tides each day. Other locations have a diurnal tide—one high and low tide each day. A "mixed tide"—two uneven magnitude tides a day—is a third regular category.
Tides vary on timescales ranging from hours to years due to a number of factors, which determine the lunitidal interval. To make accurate records, tide gauges
at fixed stations measure water level over time. Gauges ignore
variations caused by waves with periods shorter than minutes. These data
are compared to the reference (or datum) level usually called mean sea level.
While tides are usually the largest source of short-term
sea-level fluctuations, sea levels are also subject to forces such as
wind and barometric pressure changes, resulting in storm surges, especially in shallow seas and near coasts.
Tidal phenomena are not limited to the oceans, but can occur in
other systems whenever a gravitational field that varies in time and
space is present. For example, the shape of the solid part of the Earth
is affected slightly by Earth tide, though this is not as easily seen as the water tidal movements.
Characteristics
Types of tides (See Timing (below) for coastal map)
Tide changes proceed via the two main stages:
The water stops falling, reaching a local minimum called low tide.
The water stops rising, reaching a local maximum called high tide.
In some regions, there are additional two possible stages:
Sea level rises over several hours, covering the intertidal zone; flood tide.
Sea level falls over several hours, revealing the intertidal zone; ebb tide.
Oscillating currents produced by tides are known as tidal streams or tidal currents. The moment that the tidal current ceases is called slack water or slack tide.
The tide then reverses direction and is said to be turning. Slack water
usually occurs near high water and low water, but there are locations
where the moments of slack tide differ significantly from those of high
and low water.
Tides are commonly semi-diurnal (two high waters and two low waters each day), or diurnal
(one tidal cycle per day). The two high waters on a given day are
typically not the same height (the daily inequality); these are the higher high water and the lower high water in tide tables. Similarly, the two low waters each day are the higher low water and the lower low water. The daily inequality is not consistent and is generally small when the Moon is over the Equator.
The following reference tide levels can be defined, from the highest level to the lowest:
Highest astronomical tide (HAT) – The highest tide which can be predicted to occur. Note that meteorological conditions may add extra height to the HAT.
Mean high water springs (MHWS) – The average of the two high tides on the days of spring tides.
Mean high water neaps (MHWN) – The average of the two high tides on the days of neap tides.
Mean sea level (MSL) – This is the average sea level. The MSL is constant for any location over a long period.
Mean low water neaps (MLWN) – The average of the two low tides on the days of neap tides.
Mean low water springs (MLWS) – The average of the two low tides on the days of spring tides.
Tidal constituents are the net result of multiple influences
impacting tidal changes over certain periods of time. Primary
constituents include the Earth's rotation, the position of the Moon and
Sun relative to the Earth, the Moon's altitude (elevation) above the
Earth's Equator, and bathymetry. Variations with periods of less than half a day are called harmonic constituents. Conversely, cycles of days, months, or years are referred to as long period constituents.
Tidal forces affect the entire earth,
but the movement of solid Earth occurs by mere centimeters. In
contrast, the atmosphere is much more fluid and compressible so its
surface moves by kilometers, in the sense of the contour level of a
particular low pressure in the outer atmosphere.
In most locations, the largest constituent is the principal lunar semi-diurnal, also known as the M2 tidal constituent or M2 tidal constituent. Its period is about 12 hours and 25.2 minutes, exactly half a tidal lunar day, which is the average time separating one lunar zenith from the next, and thus is the time required for the Earth to rotate once relative to the Moon. Simple tide clocks
track this constituent. The lunar day is longer than the Earth day
because the Moon orbits in the same direction the Earth spins. This is
analogous to the minute hand on a watch crossing the hour hand at 12:00
and then again at about 1:05½ (not at 1:00).
The Moon orbits the Earth in the same direction as the Earth
rotates on its axis, so it takes slightly more than a day—about 24 hours
and 50 minutes—for the Moon to return to the same location in the sky.
During this time, it has passed overhead (culmination) once and underfoot once (at an hour angle
of 00:00 and 12:00 respectively), so in many places the period of
strongest tidal forcing is the above-mentioned, about 12 hours and 25
minutes. The moment of highest tide is not necessarily when the Moon is
nearest to zenith or nadir, but the period of the forcing still determines the time between high tides.
Because the gravitational field created by the Moon weakens with
distance from the Moon, it exerts a slightly stronger than average force
on the side of the Earth facing the Moon, and a slightly weaker force
on the opposite side. The Moon thus tends to "stretch" the Earth
slightly along the line connecting the two bodies. The solid Earth
deforms a bit, but ocean water, being fluid, is free to move much more
in response to the tidal force, particularly horizontally (see equilibrium tide).
As the Earth rotates, the magnitude and direction of the tidal
force at any particular point on the Earth's surface change constantly;
although the ocean never reaches equilibrium—there is never time for the
fluid to "catch up" to the state it would eventually reach if the tidal
force were constant—the changing tidal force nonetheless causes
rhythmic changes in sea surface height.
When there are two high tides each day with different heights
(and two low tides also of different heights), the pattern is called a mixed semi-diurnal tide.
The
semi-diurnal range (the difference in height between high and low
waters over about half a day) varies in a two-week cycle. Approximately
twice a month, around new moon and full moon when the Sun, Moon, and Earth form a line (a configuration known as a syzygy), the tidal force due to the Sun reinforces that due to the Moon. The tide's range is then at its maximum; this is called the spring tide. It is not named after the season, but, like that word, derives from the meaning "jump, burst forth, rise", as in a natural spring.
Spring tides are sometimes referred to as syzygy tides.
When the Moon is at first quarter
or third quarter, the Sun and Moon are separated by 90° when viewed
from the Earth, and the solar tidal force partially cancels the Moon's
tidal force. At these points in the lunar cycle, the tide's range is at
its minimum; this is called the neap tide, or neaps. "Neap" is an Anglo-Saxon word meaning "without the power", as in forðganges nip (forth-going without-the-power).
Neap tides are sometimes referred to as quadrature tides.
Spring tides result in high waters that are higher than average, low waters that are lower than average, "slack water"
time that is shorter than average, and stronger tidal currents than
average. Neaps result in less extreme tidal conditions. There is about a
seven-day interval between springs and neaps.
Spring tide: Sun and Moon on the same side (0°)
Neap tide: Sun and Moon at 90°
Spring tide: Sun and Moon at opposite sides (180°)
Neap tide: Sun and Moon at 270°
Spring tide: Sun and Moon at the same side (cycle restarts)
The changing distance separating the Moon and Earth also affects tide heights. When the Moon is closest, at perigee, the range increases, and when it is at apogee, the range shrinks. Six or eight times a year perigee coincides with either a new or full moon causing perigean spring tides with the largest tidal range.
The difference between the height of a tide at perigean spring tide and
the spring tide when the moon is at apogee depends on location but can
be large as a foot higher.
Other constituents
These include solar gravitational effects, the obliquity (tilt) of
the Earth's Equator and rotational axis, the inclination of the plane of
the lunar orbit and the elliptical shape of the Earth's orbit of the
Sun.
A compound tide (or overtide) results from the shallow-water interaction of its two parent waves.
Phase and amplitude
M2
tidal constituent. Red is most extreme (highest highs, lowest lows),
with blues being least extreme. White cotidal lines converge in blue
areas indicating little or no tide. The curved arcs around these
convergent areas are amphidromic points.
They show the direction of the tides, each indicating a synchronized
6-hour period. Tidal ranges generally increase with increasing distance
from amphidromic points. Tide waves move around these points, generally
counterclockwise in the N. Hemisphere and clockwise in the S. Hemisphere
Because the M2 tidal constituent dominates in most locations, the stage or phase
of a tide, denoted by the time in hours after high water, is a useful
concept. Tidal stage is also measured in degrees, with 360° per tidal
cycle. Lines of constant tidal phase are called cotidal lines, which are analogous to contour lines of constant altitude on topographical maps, and when plotted form a cotidal map or cotidal chart.
High water is reached simultaneously along the cotidal lines extending
from the coast out into the ocean, and cotidal lines (and hence tidal
phases) advance along the coast. Semi-diurnal and long phase
constituents are measured from high water, diurnal from maximum flood
tide. This and the discussion that follows is precisely true only for a
single tidal constituent.
For an ocean in the shape of a circular basin enclosed by a
coastline, the cotidal lines point radially inward and must eventually
meet at a common point, the amphidromic point. The amphidromic point is at once cotidal with high and low waters, which is satisfied by zero tidal motion. (The rare exception occurs when the tide encircles an island, as it does around New Zealand, Iceland and Madagascar.)
Tidal motion generally lessens moving away from continental coasts, so
that crossing the cotidal lines are contours of constant amplitude
(half the distance between high and low water) which decrease to zero
at the amphidromic point. For a semi-diurnal tide the amphidromic point
can be thought of roughly like the center of a clock face, with the hour
hand pointing in the direction of the high water cotidal line, which is
directly opposite the low water cotidal line. High water rotates about
the amphidromic point once every 12 hours in the direction of rising
cotidal lines, and away from ebbing cotidal lines. This rotation, caused
by the Coriolis effect,
is generally clockwise in the southern hemisphere and counterclockwise
in the northern hemisphere. The difference of cotidal phase from the
phase of a reference tide is the epoch. The reference tide is the
hypothetical constituent "equilibrium tide" on a landless Earth
measured at 0° longitude, the Greenwich meridian.
In the North Atlantic, because the cotidal lines circulate
counterclockwise around the amphidromic point, the high tide passes New
York Harbor approximately an hour ahead of Norfolk Harbor. South of Cape
Hatteras the tidal forces are more complex, and cannot be predicted
reliably based on the North Atlantic cotidal lines.
Investigation into tidal physics was important in the early development of celestial mechanics,
with the existence of two daily tides being explained by the Moon's
gravity. Later the daily tides were explained more precisely by the
interaction of the Moon's and the Sun's gravity.
Seleucus of Seleucia theorized around 150 BC that tides were caused by the Moon. The influence of the Moon on bodies of water was also mentioned in Ptolemy's Tetrabiblos.
In De temporum ratione (The Reckoning of Time) of 725 Bede
linked semidurnal tides and the phenomenon of varying tidal heights to
the Moon and its phases. Bede starts by noting that the tides rise and
fall 4/5 of an hour later each day, just as the Moon rises and sets 4/5
of an hour later. He goes on to emphasise that in two lunar months (59 days) the Moon circles the Earth 57 times and there are 114 tides. Bede then observes that the height of tides varies over the month. Increasing tides are called malinae and decreasing tides ledones and that the month is divided into four parts of seven or eight days with alternating malinae and ledones. In the same passage he also notes the effect of winds to hold back tides. Bede also records that the time of tides varies from place to place. To the north of Bede's location (Monkwearmouth) the tides are earlier, to the south later.
He explains that the tide "deserts these shores in order to be able
all the more to be able to flood other [shores] when it arrives there"
noting that "the Moon which signals the rise of tide here, signals its
retreat in other regions far from this quarter of the heavens".
Medieval understanding of the tides was primarily based on works of Muslim astronomers, which became available through Latin translation starting from the 12th century. Abu Ma'shar (d. circa 886), in his Introductorium in astronomiam, taught that ebb and flood tides were caused by the Moon. Abu Ma'shar discussed the effects of wind and Moon's phases relative to the Sun on the tides. In the 12th century, al-Bitruji (d. circa 1204) contributed the notion that the tides were caused by the general circulation of the heavens.
Simon Stevin, in his 1608 De spiegheling der Ebbenvloet (The theory of ebb and flood),
dismissed a large number of misconceptions that still existed about
ebb and flood. Stevin pleaded for the idea that the attraction of the
Moon was responsible for the tides and spoke in clear terms about ebb,
flood, spring tide and neap tide, stressing that further research needed to be made.
In 1609 Johannes Kepler also correctly suggested that the gravitation of the Moon caused the tides, which he based upon ancient observations and correlations.
Galileo Galilei in his 1632 Dialogue Concerning the Two Chief World Systems, whose working title was Dialogue on the Tides,
gave an explanation of the tides. The resulting theory, however, was
incorrect as he attributed the tides to the sloshing of water caused by
the Earth's movement around the Sun. He hoped to provide mechanical
proof of the Earth's movement. The value of his tidal theory is
disputed. Galileo rejected Kepler's explanation of the tides.
Isaac Newton
(1642–1727) was the first person to explain tides as the product of the
gravitational attraction of astronomical masses. His explanation of the
tides (and many other phenomena) was published in the Principia (1687) and used his theory of universal gravitation to explain the lunar and solar attractions as the origin of the tide-generating forces.
Newton and others before Pierre-Simon Laplace
worked the problem from the perspective of a static system (equilibrium
theory), that provided an approximation that described the tides that
would occur in a non-inertial ocean evenly covering the whole Earth. The tide-generating force (or its corresponding potential)
is still relevant to tidal theory, but as an intermediate quantity
(forcing function) rather than as a final result; theory must also
consider the Earth's accumulated dynamic tidal response to the applied
forces, which response is influenced by ocean depth, the Earth's
rotation, and other factors.
Maclaurin used Newton's theory to show that a smooth sphere
covered by a sufficiently deep ocean under the tidal force of a single
deforming body is a prolate
spheroid (essentially a three-dimensional oval) with major axis
directed toward the deforming body. Maclaurin was the first to write
about the Earth's rotational effects on motion. Euler realized that the tidal force's horizontal component (more than the vertical) drives the tide. In 1744 Jean le Rond d'Alembert studied tidal equations for the atmosphere which did not include rotation.
In 1770 James Cook's barqueHMS Endeavour grounded on the Great Barrier Reef.
Attempts were made to refloat her on the following tide which failed,
but the tide after that lifted her clear with ease. Whilst she was
being repaired in the mouth of the Endeavour River
Cook observed the tides over a period of seven weeks. At neap tides
both tides in a day were similar, but at springs the tides rose 7 feet
(2.1 m) in the morning but 9 feet (2.7 m) in the evening.
Others including Kelvin and Henri Poincaré further developed Laplace's theory. Based on these developments and the lunar theory of E W Brown describing the motions of the Moon, Arthur Thomas Doodson developed and published in 1921 the first modern development of the tide-generating potential in harmonic form: Doodson distinguished 388 tidal frequencies. Some of his methods remain in use.
Brouscon's Almanach of 1546: Tidal diagrams "according to the age of the moon".
From ancient times, tidal observation and discussion has increased in
sophistication, first marking the daily recurrence, then tides'
relationship to the Sun and moon. Pytheas travelled to the British Isles about 325 BC and seems to be the first to have related spring tides to the phase of the moon.
In the 2nd century BC, the Hellenistic astronomerSeleucus of Seleucia correctly described the phenomenon of tides in order to support his heliocentric theory. He correctly theorized that tides were caused by the moon, although he believed that the interaction was mediated by the pneuma. He noted that tides varied in time and strength in different parts of the world. According to Strabo
(1.1.9), Seleucus was the first to link tides to the lunar attraction,
and that the height of the tides depends on the moon's position relative
to the Sun.
The Naturalis Historia of Pliny the Elder
collates many tidal observations, e.g., the spring tides are a few days
after (or before) new and full moon and are highest around the
equinoxes, though Pliny noted many relationships now regarded as
fanciful. In his Geography, Strabo described tides in the Persian Gulf
having their greatest range when the moon was furthest from the plane
of the Equator. All this despite the relatively small amplitude of Mediterranean basin tides. (The strong currents through the Euripus Strait and the Strait of Messina puzzled Aristotle.) Philostratus discussed tides in Book Five of The Life of Apollonius of Tyana. Philostratus mentions the moon, but attributes tides to "spirits". In Europe around 730 AD, the Venerable Bede
described how the rising tide on one coast of the British Isles
coincided with the fall on the other and described the time progression
of high water along the Northumbrian coast.
The first tide table in China was recorded in 1056 AD primarily for visitors wishing to see the famous tidal bore in the Qiantang River.
The first known British tide table is thought to be that of John
Wallingford, who died Abbot of St. Albans in 1213, based on high water
occurring 48 minutes later each day, and three hours earlier at the Thames mouth than upriver at London.
In 1614 Claude d'Abbeville published the work “Histoire de la mission de pères capucins en l’Isle de Maragnan et terres circonvoisines”, where he exposed that the Tupinambá people already had an understanding of the relation between the Moon and the tides before Europe.
William Thomson (Lord Kelvin) led the first systematic harmonic analysis of tidal records starting in 1867. The main result was the building of a tide-predicting machine
using a system of pulleys to add together six harmonic time functions.
It was "programmed" by resetting gears and chains to adjust phasing and
amplitudes. Similar machines were used until the 1960s.
The first known sea-level record of an entire spring–neap cycle was made in 1831 on the Navy Dock in the Thames Estuary. Many large ports had automatic tide gauge stations by 1850.
John Lubbock was one of the first to map co-tidal lines, for Great Britain, Ireland and adjacent coasts, in 1840. William Whewell expanded this work ending with a nearly global chart in 1836.
In order to make these maps consistent, he hypothesized the existence
of a region with no tidal rise or fall where co-tidal lines meet in the
mid-ocean. The existence of such an amphidromic point, as they are now known, was confirmed in 1840 by Captain William Hewett, RN, from careful soundings in the North Sea.
The tidal force produced by a massive object (Moon, hereafter) on a
small particle located on or in an extensive body (Earth, hereafter) is
the vector difference between the gravitational force exerted by the
Moon on the particle, and the gravitational force that would be exerted
on the particle if it were located at the Earth's center of mass.
Whereas the gravitational force
subjected by a celestial body on Earth varies inversely as the square
of its distance to the Earth, the maximal tidal force varies inversely
as, approximately, the cube of this distance. If the tidal force caused by each body were instead equal to its full gravitational force (which is not the case due to the free fall
of the whole Earth, not only the oceans, towards these bodies) a
different pattern of tidal forces would be observed, e.g. with a much
stronger influence from the Sun than from the Moon: The solar
gravitational force on the Earth is on average 179 times stronger than
the lunar, but because the Sun is on average 389 times farther from the
Earth, its field gradient is weaker. The tidal force is proportional to
where M is the mass of the heavenly body, d is its distance, ρ is its average density, and r is its radius. The ratio r/d
is related to the angle subtended by the object in the sky. Since the
sun and the moon have practically the same diameter in the sky, the
tidal force of the sun is less than that of the moon because its average
density is much less, and it is only 46% as large as the lunar. More precisely, the lunar tidal acceleration (along the Moon–Earth axis, at the Earth's surface) is about 1.1 × 10−7g, while the solar tidal acceleration (along the Sun–Earth axis, at the Earth's surface) is about 0.52 × 10−7g, where g is the gravitational acceleration at the Earth's surface.
The effects of the other planets vary as their distances from Earth
vary. When Venus is closest to Earth, its effect is 0.000113 times the
solar effect. At other times, Jupiter or Mars may have the most effect.
The lunar gravity differential field at the Earth's surface is known as the tide-generating force.
This is the primary mechanism that drives tidal action and explains two
equipotential tidal bulges, accounting for two daily high waters.
The ocean's surface is approximated by a surface referred to as the geoid, which takes into consideration the gravitational force exerted by the earth as well as centrifugal force
due to rotation. Now consider the effect of massive external bodies
such as the Moon and Sun. These bodies have strong gravitational fields
that diminish with distance and cause the ocean's surface to deviate
from the geoid. They establish a new equilibrium ocean surface which
bulges toward the moon on one side and away from the moon on the other
side. The earth's rotation relative to this shape causes the daily tidal
cycle. The ocean surface tends toward this equilibrium shape, which is
constantly changing, and never quite attains it. When the ocean surface
is not aligned with it, it's as though the surface is sloping, and water
accelerates in the down-slope direction.
Equilibrium
The equilibrium tide is the idealized tide assuming a landless Earth.
It would produce a tidal bulge in the ocean, with the shape of an ellipsoid elongated towards the attracting body (Moon or Sun).
It is not caused by the vertical pull nearest or farthest from
the body, which is very weak; rather, it is caused by the tangent or
"tractive" tidal force, which is strongest at about 45 degrees from the
body, resulting in a horizontal tidal current.
Laplace's tidal equations
Ocean depths are much smaller than their horizontal extent. Thus, the response to tidal forcing can be modelled using the Laplace tidal equations which incorporate the following features:
The vertical (or radial) velocity is negligible, and there is no vertical shear—this is a sheet flow.
The Coriolis effect appears as an inertial force (fictitious) acting laterally to the direction of flow and proportional to velocity.
The surface height's rate of change is proportional to the negative
divergence of velocity multiplied by the depth. As the horizontal
velocity stretches or compresses the ocean as a sheet, the volume thins
or thickens, respectively.
The boundary conditions dictate no flow across the coastline and free slip at the bottom.
The Coriolis effect (inertial force) steers flows moving towards
the Equator to the west and flows moving away from the Equator toward
the east, allowing coastally trapped waves. Finally, a dissipation term
can be added which is an analog to viscosity.
Amplitude and cycle time
The theoretical amplitude
of oceanic tides caused by the Moon is about 54 centimetres (21 in) at
the highest point, which corresponds to the amplitude that would be
reached if the ocean possessed a uniform depth, there were no
landmasses, and the Earth were rotating in step with the Moon's orbit.
The Sun similarly causes tides, of which the theoretical amplitude is
about 25 centimetres (9.8 in) (46% of that of the Moon) with a cycle
time of 12 hours. At spring tide the two effects add to each other to a
theoretical level of 79 centimetres (31 in), while at neap tide the
theoretical level is reduced to 29 centimetres (11 in). Since the orbits
of the Earth about the Sun, and the Moon about the Earth, are
elliptical, tidal amplitudes change somewhat as a result of the varying
Earth–Sun and Earth–Moon distances. This causes a variation in the tidal
force and theoretical amplitude of about ±18% for the Moon and ±5% for
the Sun. If both the Sun and Moon were at their closest positions and
aligned at new moon, the theoretical amplitude would reach 93
centimetres (37 in).
Real amplitudes differ considerably, not only because of depth
variations and continental obstacles, but also because wave propagation
across the ocean has a natural period of the same order of magnitude as
the rotation period: if there were no land masses, it would take about
30 hours for a long wavelength surface wave to propagate along the
Equator halfway around the Earth (by comparison, the Earth's lithosphere has a natural period of about 57 minutes). Earth tides,
which raise and lower the bottom of the ocean, and the tide's own
gravitational self attraction are both significant and further
complicate the ocean's response to tidal forces.
Earth's tidal oscillations introduce dissipation at an average rate of about 3.75 terawatts.
About 98% of this dissipation is by marine tidal movement.
Dissipation arises as basin-scale tidal flows drive smaller-scale flows
which experience turbulent dissipation. This tidal drag creates torque
on the moon that gradually transfers angular momentum to its orbit, and a
gradual increase in Earth–moon separation. The equal and opposite
torque on the Earth correspondingly decreases its rotational velocity.
Thus, over geologic time, the moon recedes from the Earth, at about 3.8
centimetres (1.5 in)/year, lengthening the terrestrial day.
Day length has increased
by about 2 hours in the last 600 million years. Assuming (as a crude
approximation) that the deceleration rate has been constant, this would
imply that 70 million years ago, day length was on the order of 1%
shorter with about 4 more days per year.
The shape of the shoreline and the ocean floor changes the way that
tides propagate, so there is no simple, general rule that predicts the
time of high water from the Moon's position in the sky. Coastal
characteristics such as underwater bathymetry
and coastline shape mean that individual location characteristics
affect tide forecasting; actual high water time and height may differ
from model predictions due to the coastal morphology's effects on tidal
flow. However, for a given location the relationship between lunar altitude and the time of high or low tide (the lunitidal interval)
is relatively constant and predictable, as is the time of high or low
tide relative to other points on the same coast. For example, the high
tide at Norfolk, Virginia, U.S., predictably occurs approximately two and a half hours before the Moon passes directly overhead.
Land masses and ocean basins act as barriers against water moving
freely around the globe, and their varied shapes and sizes affect the
size of tidal frequencies. As a result, tidal patterns vary. For
example, in the U.S., the East coast has predominantly semi-diurnal
tides, as do Europe's Atlantic coasts, while the West coast
predominantly has mixed tides. Human changes to the landscape can also significantly alter local tides.
Observation and prediction
Timing
The
same tidal forcing has different results depending on many factors,
including coast orientation, continental shelf margin, water body
dimensions.
The tidal forces due to the Moon and Sun generate very long waves
which travel all around the ocean following the paths shown in co-tidal charts.
The time when the crest of the wave reaches a port then gives the time
of high water at the port. The time taken for the wave to travel around
the ocean also means that there is a delay between the phases of the
Moon and their effect on the tide. Springs and neaps in the North Sea, for example, are two days behind the new/full moon and first/third quarter moon. This is called the tide's age.
The ocean bathymetry greatly influences the tide's exact time and height at a particular coastal point. There are some extreme cases; the Bay of Fundy,
on the east coast of Canada, is often stated to have the world's
highest tides because of its shape, bathymetry, and its distance from
the continental shelf edge.
Measurements made in November 1998 at Burntcoat Head in the Bay of
Fundy recorded a maximum range of 16.3 metres (53 ft) and a highest
predicted extreme of 17 metres (56 ft).
Similar measurements made in March 2002 at Leaf Basin, Ungava Bay in northern Quebec
gave similar values (allowing for measurement errors), a maximum range
of 16.2 metres (53 ft) and a highest predicted extreme of 16.8 metres
(55 ft). Ungava Bay and the Bay of Fundy lie similar distances from the continental shelf edge, but Ungava Bay is free of pack ice for about four months every year while the Bay of Fundy rarely freezes.
Southampton in the United Kingdom has a double high water caused by the interaction between the M2 and M4 tidal constituents (Shallow water overtides of principal lunar). Portland has double low waters for the same reason. The M4 tide is found all along the south coast of the United Kingdom, but its effect is most noticeable between the Isle of Wight and Portland because the M2 tide is lowest in this region.
Because the oscillation modes of the Mediterranean Sea and the Baltic Sea
do not coincide with any significant astronomical forcing period, the
largest tides are close to their narrow connections with the Atlantic
Ocean. Extremely small tides also occur for the same reason in the Gulf of Mexico and Sea of Japan. Elsewhere, as along the southern coast of Australia, low tides can be due to the presence of a nearby amphidrome.
Analysis
A regular water level chart
Isaac Newton's
theory of gravitation first enabled an explanation of why there were
generally two tides a day, not one, and offered hope for a detailed
understanding of tidal forces and behavior. Although it may seem that
tides could be predicted via a sufficiently detailed knowledge of
instantaneous astronomical forcings, the actual tide at a given location
is determined by astronomical forces accumulated by the body of water
over many days. In addition, accurate results would require detailed
knowledge of the shape of all the ocean basins—their bathymetry, and coastline shape.
Current procedure for analysing tides follows the method of harmonic analysis introduced in the 1860s by William Thomson.
It is based on the principle that the astronomical theories of the
motions of Sun and Moon determine a large number of component
frequencies, and at each frequency there is a component of force tending
to produce tidal motion, but that at each place of interest on the
Earth, the tides respond at each frequency with an amplitude and phase
peculiar to that locality. At each place of interest, the tide heights
are therefore measured for a period of time sufficiently long (usually
more than a year in the case of a new port not previously studied) to
enable the response at each significant tide-generating frequency to be
distinguished by analysis, and to extract the tidal constants for a
sufficient number of the strongest known components of the astronomical
tidal forces to enable practical tide prediction. The tide heights are
expected to follow the tidal force, with a constant amplitude and phase
delay for each component. Because astronomical frequencies and phases
can be calculated with certainty, the tide height at other times can
then be predicted once the response to the harmonic components of the
astronomical tide-generating forces has been found.
The main patterns in the tides are
the twice-daily variation
the difference between the first and second tide of a day
the spring–neap cycle
the annual variation
The Highest Astronomical Tide is the perigean spring tide when both the Sun and Moon are closest to the Earth.
When confronted by a periodically varying function, the standard approach is to employ Fourier series, a form of analysis that uses sinusoidal functions as a basis
set, having frequencies that are zero, one, two, three, etc. times the
frequency of a particular fundamental cycle. These multiples are called harmonics of the fundamental frequency, and the process is termed harmonic analysis.
If the basis set of sinusoidal functions suit the behaviour being
modelled, relatively few harmonic terms need to be added. Orbital paths
are very nearly circular, so sinusoidal variations are suitable for
tides.
For the analysis of tide heights, the Fourier series approach has
in practice to be made more elaborate than the use of a single
frequency and its harmonics. The tidal patterns are decomposed into
many sinusoids having many fundamental frequencies, corresponding (as in
the lunar theory)
to many different combinations of the motions of the Earth, the Moon,
and the angles that define the shape and location of their orbits.
For tides, then, harmonic analysis is not limited to harmonics of a single frequency.
In other words, the harmonies are multiples of many fundamental
frequencies, not just of the fundamental frequency of the simpler
Fourier series approach. Their representation as a Fourier series
having only one fundamental frequency and its (integer) multiples would
require many terms, and would be severely limited in the time-range for
which it would be valid.
The study of tide height by harmonic analysis was begun by Laplace, William Thomson (Lord Kelvin), and George Darwin. A.T. Doodson extended their work, introducing the Doodson Number
notation to organise the hundreds of resulting terms. This approach has
been the international standard ever since, and the complications arise
as follows: the tide-raising force is notionally given by sums of
several terms. Each term is of the form
where A is the amplitude, ω is the angular frequency usually given in degrees per hour corresponding to t measured in hours, and p is the phase offset with regard to the astronomical state at time t = 0 . There is one term for the Moon and a second term for the Sun. The phase p of the first harmonic for the Moon term is called the lunitidal interval
or high water interval. The next step is to accommodate the harmonic
terms due to the elliptical shape of the orbits. Accordingly, the value
of A is not a constant but also varying with time, slightly, about some average figure. Replace it then by A(t) where A is another sinusoid, similar to the cycles and epicycles of Ptolemaic theory. Accordingly,
which is to say an average value A with a sinusoidal variation about it of magnitude Aa, with frequency ωa and phase pa. Thus the simple term is now the product of two cosine factors:
Given that for any x and y
it is clear that a compound term involving the product of two cosine terms each with their own frequency is the same as three
simple cosine terms that are to be added at the original frequency and
also at frequencies which are the sum and difference of the two
frequencies of the product term. (Three, not two terms, since the whole
expression is .)
Consider further that the tidal force on a location depends also on
whether the Moon (or the Sun) is above or below the plane of the
Equator, and that these attributes have their own periods also
incommensurable with a day and a month, and it is clear that many
combinations result. With a careful choice of the basic astronomical
frequencies, the Doodson Number annotates the particular additions and
differences to form the frequency of each simple cosine term.
Tidal prediction summing constituent parts. The tidal coefficients are defined on the page theory of tides.
Remember that astronomical tides do not include weather
effects. Also, changes to local conditions (sandbank movement, dredging
harbour mouths, etc.) away from those prevailing at the measurement time
affect the tide's actual timing and magnitude. Organisations quoting a
"highest astronomical tide" for some location may exaggerate the figure
as a safety factor against analytical uncertainties, distance from the
nearest measurement point, changes since the last observation time,
ground subsidence, etc., to avert liability should an engineering work
be overtopped. Special care is needed when assessing the size of a
"weather surge" by subtracting the astronomical tide from the observed
tide.
Careful Fourier data analysis over a nineteen-year period (the National Tidal Datum Epoch in the U.S.) uses frequencies called the tidal harmonic constituents. Nineteen years is preferred because the Earth, Moon and Sun's relative positions repeat almost exactly in the Metonic cycle of 19 years, which is long enough to include the 18.613 year lunar nodaltidal constituent. This analysis can be done using only the knowledge of the forcing period,
but without detailed understanding of the mathematical derivation,
which means that useful tidal tables have been constructed for
centuries.
The resulting amplitudes and phases can then be used to predict the
expected tides. These are usually dominated by the constituents near
12 hours (the semi-diurnal constituents), but there are major constituents near 24 hours (diurnal) as well. Longer term constituents are 14 day or fortnightly, monthly, and semiannual. Semi-diurnal tides dominated coastline, but some areas such as the South China Sea and the Gulf of Mexico are primarily diurnal. In the semi-diurnal areas, the primary constituents M2 (lunar) and S2 (solar)
periods differ slightly, so that the relative phases, and thus the
amplitude of the combined tide, change fortnightly (14 day period).
In the M2 plot above, each cotidal line differs
by one hour from its neighbors, and the thicker lines show tides in
phase with equilibrium at Greenwich. The lines rotate around the amphidromic points counterclockwise in the northern hemisphere so that from Baja California Peninsula to Alaska and from France to Ireland the M2 tide propagates northward. In the southern hemisphere this direction is clockwise. On the other hand, M2
tide propagates counterclockwise around New Zealand, but this is
because the islands act as a dam and permit the tides to have different
heights on the islands' opposite sides. (The tides do propagate
northward on the east side and southward on the west coast, as predicted
by theory.)
The exception is at Cook Strait
where the tidal currents periodically link high to low water. This is
because cotidal lines 180° around the amphidromes are in opposite phase,
for example high water across from low water at each end of Cook
Strait. Each tidal constituent has a different pattern of amplitudes,
phases, and amphidromic points, so the M2 patterns cannot be used for other tide components.
Tides at Bridgeport, Connecticut, U.S. during a 30-day period.
Tides at Bridgeport, Connecticut, U.S. during a 400-day period.
Tidal
patterns in Cook Strait. The south part (Nelson) has two spring tides
per month, versus only one on the north side (Wellington and Napier).
Because the Moon is moving in its orbit around the Earth and in the
same sense as the Earth's rotation, a point on the Earth must rotate
slightly further to catch up so that the time between semi-diurnal tides
is not twelve but 12.4206 hours—a bit over twenty-five minutes extra.
The two peaks are not equal. The two high tides a day alternate in
maximum heights: lower high (just under three feet), higher high (just
over three feet), and again lower high. Likewise for the low tides.
When the Earth, Moon, and Sun are in line (Sun–Earth–Moon, or
Sun–Moon–Earth) the two main influences combine to produce spring tides;
when the two forces are opposing each other as when the angle
Moon–Earth–Sun is close to ninety degrees, neap tides result. As the
Moon moves around its orbit it changes from north of the Equator to
south of the Equator. The alternation in high tide heights becomes
smaller, until they are the same (at the lunar equinox, the Moon is
above the Equator), then redevelop but with the other polarity, waxing
to a maximum difference and then waning again.
Current
The tides' influence on current or flow is much more difficult to analyze, and data is much more difficult to collect. A tidal height is a scalar quantity and varies smoothly over a wide region. A flow is a vector quantity,
with magnitude and direction, both of which can vary substantially with
depth and over short distances due to local bathymetry. Also, although a
water channel's center is the most useful measuring site, mariners
object when current-measuring equipment obstructs waterways. A flow
proceeding up a curved channel may have similar magnitude, even though
its direction varies continuously along the channel. Surprisingly, flood
and ebb flows are often not in opposite directions. Flow direction is
determined by the upstream channel's shape, not the downstream channel's
shape. Likewise, eddies may form in only one flow direction.
Nevertheless, tidal current analysis is similar to tidal heights
analysis: in the simple case, at a given location the flood flow is in
mostly one direction, and the ebb flow in another direction. Flood
velocities are given positive sign, and ebb velocities negative sign.
Analysis proceeds as though these are tide heights.
In more complex situations, the main ebb and flood flows do not
dominate. Instead, the flow direction and magnitude trace an ellipse
over a tidal cycle (on a polar plot) instead of along the ebb and flood
lines. In this case, analysis might proceed along pairs of directions,
with the primary and secondary directions at right angles. An
alternative is to treat the tidal flows as complex numbers, as each
value has both a magnitude and a direction.
Tide flow information is most commonly seen on nautical charts,
presented as a table of flow speeds and bearings at hourly intervals,
with separate tables for spring and neap tides. The timing is relative
to high water at some harbour where the tidal behaviour is similar in
pattern, though it may be far away.
As with tide height predictions, tide flow predictions based only
on astronomical factors do not incorporate weather conditions, which
can completely change the outcome.
The tidal flow through Cook Strait
between the two main islands of New Zealand is particularly
interesting, as the tides on each side of the strait are almost exactly
out of phase, so that one side's high water is simultaneous with the
other's low water. Strong currents result, with almost zero tidal height
change in the strait's center. Yet, although the tidal surge normally
flows in one direction for six hours and in the reverse direction for
six hours, a particular surge might last eight or ten hours with the
reverse surge enfeebled. In especially boisterous weather conditions,
the reverse surge might be entirely overcome so that the flow continues
in the same direction through three or more surge periods.
A further complication for Cook Strait's flow pattern is that the tide at the south side (e.g. at Nelson)
follows the common bi-weekly spring–neap tide cycle (as found along the
west side of the country), but the north side's tidal pattern has only one cycle per month, as on the east side: Wellington, and Napier.
The graph of Cook Strait's tides shows separately the high water
and low water height and time, through November 2007; these are not
measured values but instead are calculated from tidal parameters
derived from years-old measurements. Cook Strait's nautical chart offers
tidal current information. For instance the January 1979 edition for
41°13·9’S 174°29·6’E (north west of Cape Terawhiti) refers timings to Westport
while the January 2004 issue refers to Wellington. Near Cape Terawhiti
in the middle of Cook Strait the tidal height variation is almost nil
while the tidal current reaches its maximum, especially near the
notorious Karori Rip. Aside from weather effects, the actual currents
through Cook Strait are influenced by the tidal height differences
between the two ends of the strait and as can be seen, only one of the
two spring tides at the north west end of the strait near Nelson has a
counterpart spring tide at the south east end (Wellington), so the
resulting behaviour follows neither reference harbour.
Tidal energy can be extracted by two means: inserting a water turbine
into a tidal current, or building ponds that release/admit water
through a turbine. In the first case, the energy amount is entirely
determined by the timing and tidal current magnitude. However, the best
currents may be unavailable because the turbines would obstruct ships.
In the second, the impoundment dams are expensive to construct, natural
water cycles are completely disrupted, ship navigation is disrupted.
However, with multiple ponds, power can be generated at chosen times. So
far, there are few installed systems for tidal power generation (most
famously, La Rance at Saint Malo,
France) which face many difficulties. Aside from environmental issues,
simply withstanding corrosion and biological fouling pose engineering
challenges.
Tidal power proponents point out that, unlike wind power systems,
generation levels can be reliably predicted, save for weather effects.
While some generation is possible for most of the tidal cycle, in
practice turbines lose efficiency at lower operating rates. Since the
power available from a flow is proportional to the cube of the flow
speed, the times during which high power generation is possible are
brief.
Navigation
US civil and maritime uses of tidal data
Tidal flows are important for navigation, and significant errors in
position occur if they are not accommodated. Tidal heights are also
important; for example many rivers and harbours have a shallow "bar" at
the entrance which prevents boats with significant draft from entering at low tide.
Until the advent of automated navigation, competence in
calculating tidal effects was important to naval officers. The
certificate of examination for lieutenants in the Royal Navy once declared that the prospective officer was able to "shift his tides".
Tidal flow timings and velocities appear in tide charts or a tidal stream atlas.
Tide charts come in sets. Each chart covers a single hour between one
high water and another (they ignore the leftover 24 minutes) and show
the average tidal flow for that hour. An arrow on the tidal chart
indicates the direction and the average flow speed (usually in knots) for spring and neap tides. If a tide chart is not available, most nautical charts have "tidal diamonds" which relate specific points on the chart to a table giving tidal flow direction and speed.
The standard procedure to counteract tidal effects on navigation is to (1) calculate a "dead reckoning"
position (or DR) from travel distance and direction, (2) mark the chart
(with a vertical cross like a plus sign) and (3) draw a line from the
DR in the tide's direction. The distance the tide moves the boat along
this line is computed by the tidal speed, and this gives an "estimated
position" or EP (traditionally marked with a dot in a triangle).
Tidal Indicator, Delaware River, Delaware c. 1897. At the time shown in the figure, the tide is 1+1⁄4
feet above mean low water and is still falling, as indicated by
pointing of the arrow. Indicator is powered by system of pulleys, cables
and a float. (Report Of The Superintendent Of The Coast & Geodetic
Survey Showing The Progress Of The Work During The Fiscal Year Ending
With June 1897 (p. 483))
Nautical charts display the water's "charted depth" at specific locations with "soundings" and the use of bathymetric contour lines to depict the submerged surface's shape. These depths are relative to a "chart datum",
which is typically the water level at the lowest possible astronomical
tide (although other datums are commonly used, especially historically,
and tides may be lower or higher for meteorological reasons) and are
therefore the minimum possible water depth during the tidal cycle.
"Drying heights" may also be shown on the chart, which are the heights
of the exposed seabed at the lowest astronomical tide.
Tide tables list each day's high and low water heights and times.
To calculate the actual water depth, add the charted depth to the
published tide height. Depth for other times can be derived from tidal
curves published for major ports. The rule of twelfths
can suffice if an accurate curve is not available. This approximation
presumes that the increase in depth in the six hours between low and
high water is: first hour — 1/12, second — 2/12, third — 3/12, fourth —
3/12, fifth — 2/12, sixth — 1/12.
Biological aspects
Intertidal ecology
A rock, seen at low water, exhibiting typical intertidal zonation.
Intertidal ecology is the study of ecosystems between the low- and high-water lines along a shore. At low water, the intertidal zone is exposed (or emersed), whereas at high water, it is underwater (or immersed). Intertidal ecologists therefore study the interactions between intertidal organisms and their environment, as well as among the different species.
The most important interactions may vary according to the type of
intertidal community. The broadest classifications are based on
substrates — rocky shore or soft bottom.
Intertidal organisms experience a highly variable and often hostile environment, and have adapted to cope with and even exploit these conditions. One easily visible feature is vertical zonation,
in which the community divides into distinct horizontal bands of
specific species at each elevation above low water. A species' ability
to cope with desiccation determines its upper limit, while competition with other species sets its lower limit.
The approximately fortnightly tidal cycle has large effects on intertidal and marine organisms. Hence their biological rhythms tend to occur in rough multiples of this period. Many other animals such as the vertebrates, display similar rhythms. Examples include gestation and egg hatching. In humans, the menstrual cycle lasts roughly a lunar month, an even multiple of the tidal period. Such parallels at least hint at the common descent of all animals from a marine ancestor.
Other tides
When oscillating tidal currents in the stratified ocean flow over uneven bottom topography, they generate internal waves with tidal frequencies. Such waves are called internal tides.
Shallow areas in otherwise open water can experience rotary tidal
currents, flowing in directions that continually change and thus the
flow direction (not the flow) completes a full rotation in 12+1⁄2 hours (for example, the Nantucket Shoals).
In addition to oceanic tides, large lakes can experience small tides and even planets can experience atmospheric tides and Earth tides. These are continuum mechanical phenomena. The first two take place in fluids. The third affects the Earth's thin solid crust surrounding its semi-liquid interior (with various modifications).
Lake tides
Large lakes such as Superior and Erie can experience tides of 1 to 4 cm (0.39 to 1.6 in), but these can be masked by meteorologically induced phenomena such as seiche. The tide in Lake Michigan is described as 1.3 to 3.8 cm (0.5 to 1.5 in) or 4.4 cm (1+3⁄4 in).
This is so small that other larger effects completely mask any tide, and as such these lakes are considered non-tidal.
Atmospheric tides are negligible at ground level and aviation altitudes, masked by weather's
much more important effects. Atmospheric tides are both gravitational
and thermal in origin and are the dominant dynamics from about 80 to 120
kilometres (50 to 75 mi), above which the molecular density becomes too
low to support fluid behavior.
Earth tides or terrestrial tides affect the entire Earth's mass, which acts similarly to a liquid gyroscope
with a very thin crust. The Earth's crust shifts (in/out, east/west,
north/south) in response to lunar and solar gravitation, ocean tides,
and atmospheric loading. While negligible for most human activities,
terrestrial tides' semi-diurnal amplitude can reach about 55 centimetres
(22 in) at the Equator—15 centimetres (5.9 in) due to the Sun—which is
important in GPS calibration and VLBI measurements. Precise astronomical angular measurements require knowledge of the Earth's rotation rate and polar motion, both of which are influenced by Earth tides. The semi-diurnal M2 Earth tides are nearly in phase with the Moon with a lag of about two hours.
Galactic tides
Galactic tides are the tidal forces exerted by galaxies on stars within them and satellite galaxies orbiting them. The galactic tide's effects on the Solar System's Oort cloud are believed to cause 90 percent of long-period comets.
Misnomers
Tsunamis, the large waves that occur after earthquakes, are sometimes called tidal waves, but this name is given by their resemblance to the tide, rather than any causal link to the tide. Other phenomena unrelated to tides but using the word tide are rip tide, storm tide, hurricane tide, and black or red tides. Many of these usages are historic and refer to the earlier meaning of tide as "a portion of time, a season".
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