Freshwater biology

From Wikipedia, the free encyclopedia

 

Oxsjön, a lake in Sweden. Freshwater biology focuses on environments like lakes.

A pond in the Oconee River Floodplain in Georgia, whose surface is covered in duckweed but still contains fish.

Freshwater biology is the scientific biological study of freshwater ecosystems and is a branch of limnology. This field seeks to understand the relationships between living organisms in their physical environment. These physical environments may include rivers, lakes, streams, ponds, lakes, reservoirs, or wetlands. Knowledge from this discipline is also widely used in industrial processes to make use of biological processes involved with sewage treatment and water purification. Water presence and flow is an essential aspect to species distribution and influences when and where species interact in freshwater environments.

In the UK, the Freshwater Biological Association based near Windermere in Cumbria was one of the early institutions to research the biology of freshwater and promote the concepts of trophism in lakes and demonstrated the process of migration from oligotrophic water through mesotrophic to marsh.

Freshwater biology is also used to study the effects of climate change and increased human impact on both aquatic systems and wider ecosystems. Freshwater organisms, vertebrates especially, appear to be at a higher extinction risk from climate change than terrestrial or marine organisms. 

Freshwater habitats

Freshwater habitats support a wide variety of organisms with habitats including rivers, streams, lakes, ponds, and wetlands.

Rivers and streams

Running water is a type of freshwater habitat that mainly consists of rivers and streams. Running, fast-moving waters have a higher oxygen content, allowing different species to thrive and making pollution easier to combat. Running water is an open system, meaning it is not isolate and exchanges matter and energy with other systems. Being an open system, a lot of organic matter found in running water is from land runoff or further sediment upstream and this matter is an important source of food for many species. Flowing bodies of water begin at the headwaters, which include springs, lakes, and snowmelt,  and travel to their mouths, typically another moving water channel or the ocean. The characteristics of the streams and rivers change throughout their journey from the source to the mouth. For example, the water at the source is clearer, has a higher oxygen content, lower temperatures, and heterotrophs common species. In the middle, the width usually expands and the species diversity increases due to temperature and oxygen content changes, including aquatic green plants and algae. The water at the mouth has a lower oxygen concentration and is murkier due to the sediment that has been collected and traveled along the length of the river or stream. This increased sediment decreases the amount of light that is able to penetrate the water and there is less diversity of flora and the lower oxygen lowers the diversity of fauna.

The riparian zone is the area along a riverbank or streambank that is home to vital, high moisture plants. These plants create a buffer between the land and the running water system, protecting it from pollution and flooding. Additionally, these plants provide a large habitat for many wetland species, a large number of which are endangered or threatened. Lastly, riparian plants shade the water from sunlight, reducing the heat stress on the water and aquatic life, while also providing nutrients in the form of organic matter.

Lakes and ponds

Standing water is a type of freshwater habitat that mainly consists of lakes and ponds. This habitat has limited species diversity because they are isolated from one another and other water systems, unlike running water. Standing water experiences the process of stratification, which is when water is layered due to the oxygen content. Stratification does not occur in running water because of the fast moving water that mixes water with varying oxygen content together. The topmost layer has the most oxygen and as depth increases, the oxygen decreases. Stratification can be physically felt in the temperature of the water, as the uppermost layer of water is warmer than deeper water because it has been heated by the sun.

Standing water can be divided into three zones based on depth and distance from shore. The littoral zone is the uppermost layer and the warmest water found in lakes and ponds, as the sun directly heats is. This zone hosts the most biodiversity in standing water, with a wide variety of organisms found here, vital to the health of the ecosystem and an important aspect of the diet of organisms in the habitat, like algae, aquatic plants, clams, insects, fish, crustaceans, and amphibians. The limnetic zone is found below the littoral zone. This zone has lower temperatures, is fairly well-lit, and is occupied by a smaller variety of organisms, including phytoplankton, zooplankton, and fish. The plankton found in this zone play a crucial role in the food web of the habitat and support the diet of many important organisms. The deepest zone is the profundal zone, with very little light, colder temperatures, and higher density than the previous layers. When plankton die they fall into this layer and provide nutrients to the fauna that live in this layer. These faunas are called heterotrophs, meaning they eat dead organisms and use oxygen for cellular respiration, resulting in lower oxygen content in the profundal zone.

The thermocline is the transitionary zone between the warm, surface water and the deeper water at a cooler temperature. The limited mixing and movement of water that occurs in standing water occurs at the thermocline. The mixing of the layers of water in standing water mostly comes from seasonal overturn. During the fall and spring, there is a mixing of layers usually due to wind that circulates oxygen and creates a more uniform temperature throughout the water system. The shore zone is the transitional zone between the water systems and land, similar to the riparian zone seen in running water systems. This area functions in much the same way as the riparian zone, the plants protecting the water from pollution, flooding, and heat stress, while also providing nutrients and habitats for aquatic and wetland species.

Wetlands

Wetlands are a specific type of standing water habitats that include marshes, swamps, and bogs. Due to the waterlogged and submerged nature of the land, the anaerobic conditions of wetlands are unique and support the highest species diversity of all ecosystems. Wetlands slow the decomposition of organic matter, creating layers of rich organic material that provides important nutrients for species in the system. The fauna that reside in wetlands are called hydrophytes, meaning they are adapted to very moist and humid conditions. Wetlands are the home to a large number of bird, amphibian, insect, reptile, grass, and tree species that cannot inhabit any other system, making them at risk to endangerment, as wetlands are being destroyed for urban development and agriculture. Wetlands help combat pollution and climate change, as they filter pollutants and store a large amount of carbon from the biosphere in their moist soil and still water, despite the small amount of land they occupy. Additionally, wetlands provide flood and storm protection, as the system can absorb large amounts of excess water. Wetland's ability to absorb water also assists groundwater recharge, which is very important for human water use, as usable freshwater sources are dwindling. Wetlands are not only freshwater habitats and systems, as there are salt marshes and bogs that support different species.

Freshwater organisms

Freshwater organisms are generally divided into the categories of benthic and pelagic organisms, as these are the two zones of life found in the freshwater biome. Freshwater organism can include invertebrates, insects, fish, amphibians, mammals, birds, aquatic plants, and planktons.

Threats

The most common cause of water pollution is stormwater runoff from developed areas, like pavement and rooftops. Stormwater runoff is moving rain and snowmelt that has not been absorbed. The impervious surfaces used in domestic and urban construction replace soil that used to absorb stormwater, increasing the amount of runoff traveling farther distances. This excess runoff can collect pollutants as it eventually makes its way into streams, rivers, lakes, wetlands, and even aquifers, polluting important freshwater ecosystems and usable water. Additionally, increased flooding and erosion can be caused by the increased stormwater runoff.

Pollution of flowing water

Rivers and streams drain water that falls on upland areas, and this moving water dissolves pollutants at a faster rate than standing water. However, due to the high production and placement of pollutants in these moving waters, the waters become polluted faster than the pollutant dilution rate, leading to over polluted rivers and streams. All three of the major contributors to pollution – industry, agriculture, and cities – are commonly found along moving waters, adding to the over-pollution of rivers and streams. Just the knowledge that fast moving waters can dilute pollutants has encouraged even more pollution, further adding to the pollution issue. Another issue contributing to the destruction of rivers and streams is the physical alteration of these moving waters, mainly in the form of dams, diversion of water, channel alteration, and land development. These alterations affect water temperature, water flow patterns, and increase sediment, destroying important habitat conditions for many aquatic organisms and reducing water quality.

An area of contention regarding the pollution of streams and rivers is the concept that the pollution upstream affects the people downstream. A factory’s waste upstream may contaminate someone’s drinking water downstream. This especially becomes an issue with bodies of moving water that border multiple countries or states, as what one country or state does upstream can drastically affect what the downstream country or state is able to do. 

Pollution of standing water

Lakes and ponds experience much of the same pollution as rivers and streams, but are polluted at a quicker rate due to slower moving waters, no water flow outlets, and amount of water. Standing water circulates much less than moving waters, with the deeper water layers only moving during seasonal changes twice a year. Lakes and ponds are basins into which running water usually flows and accumulates, meaning that the pollutants also accumulate with no outlet. Lakes and ponds contain less water than most rivers and streams, meaning smaller lakes and ponds are polluted at faster rates.

Eutrophication is the process of abundant plant growth, a dominating threat to standing waters. If chemical nutrients for aquatic plant growth that were previously limited become available, plant populations will increase rapidly. This excessive plant population growth decreases the oxygen content of the water, and other aquatic life suffocates. Human waste often contains these chemical nutrients, like phosphorus in fertilizers, and in combination with the poor water circulation in standing waters, causes pollution and organism depletion. Much of the pollution issues that affect ponds and lakes also affect wetlands, as the water circulation of wetlands is also slow.

Groundwater pollution and depletion

Surface water is where groundwater is being expressed, with wetlands being the largest examples of the water table being near or at the surface. The water found in freshwater habitats are the combination of surface flow, precipitation, and groundwater expression. This relationship between groundwater and surface water means that groundwater pollution affects surface freshwater pollution as well.

According to an Environmental Protection Agency survey, about a quarter of the United States’ usable groundwater is contaminated. Groundwater is the only source of drinking water for about half of the United States. As human populations increase and industrialize, the demand for groundwater is increasing, but the pollution of groundwater is also increasing. The pollution of groundwater is easy to achieve due to the slow circulation of water, even slower than that of lakes and ponds. The water must navigate through small holes in the aquifer rock, moving on average only a couple of inches each day. The rate of groundwater recharge is the time it takes for groundwater to replenish itself and extremely slow, leading to water shortages, as humans remove water from aquifers faster than the rate of recharge. Due to such slow circulation of water, groundwater can remain polluted for decades, as the natural purification processes are so slow.

 

Lake Hawea, New Zealand

Limnology (/lɪmˈnɒlədʒi/ lim-NOL-ə-jee; from Ancient Greek λίμνη (límnē) 'lake', and -λογία (-logía) 'study of') is the study of inland aquatic ecosystems. The study of limnology includes aspects of the biological, chemical, physical, and geological characteristics of fresh and saline, natural and man-made bodies of water. This includes the study of lakes, reservoirs, ponds, rivers, springs, streams, wetlands, and groundwater. Water systems are often categorized as either running (lotic) or standing (lentic).

Limnology includes the study of the drainage basin, movement of water through the basin and biogeochemical changes that occur en route. A more recent sub-discipline of limnology, termed landscape limnology, studies, manages, and seeks to conserve these ecosystems using a landscape perspective, by explicitly examining connections between an aquatic ecosystem and its drainage basin. Recently, the need to understand global inland waters as part of the Earth system created a sub-discipline called global limnology. This approach considers processes in inland waters on a global scale, like the role of inland aquatic ecosystems in global biogeochemical cycles.

Limnology is closely related to aquatic ecology and hydrobiology, which study aquatic organisms and their interactions with the abiotic (non-living) environment. While limnology has substantial overlap with freshwater-focused disciplines (e.g., freshwater biology), it also includes the study of inland salt lakes.

History

The term limnology was coined by François-Alphonse Forel (1841–1912) who established the field with his studies of Lake Geneva. Interest in the discipline rapidly expanded, and in 1922 August Thienemann (a German zoologist) and Einar Naumann (a Swedish botanist) co-founded the International Society of Limnology (SIL, from Societas Internationalis Limnologiae). Forel's original definition of limnology, "the oceanography of lakes", was expanded to encompass the study of all inland waters, and influenced Benedykt Dybowski's work on Lake Baikal.

Prominent early American limnologists included G. Evelyn Hutchinson and Ed Deevey. At the University of Wisconsin-Madison, Edward A. Birge, Chancey Juday, Charles R. Goldman, and Arthur D. Hasler contributed to the development of the Center for Limnology.

General limnology

Physical properties

Physical properties of aquatic ecosystems are determined by a combination of heat, currents, waves and other seasonal distributions of environmental conditions. The morphometry of a body of water depends on the type of feature (such as a lake, river, stream, wetland, estuary etc.) and the structure of the earth surrounding the body of water. Lakes, for instance, are classified by their formation, and zones of lakes are defined by water depth. River and stream system morphometry is driven by underlying geology of the area as well as the general velocity of the water. Stream morphometry is also influenced by topography (especially slope) as well as precipitation patterns and other factors such as vegetation and land development. Connectivity between streams and lakes relates to the landscape drainage density, lake surface area and lake shape.

Other types of aquatic systems which fall within the study of limnology are estuaries. Estuaries are bodies of water classified by the interaction of a river and the ocean or sea. Wetlands vary in size, shape, and pattern however the most common types, marshes, bogs and swamps, often fluctuate between containing shallow, freshwater and being dry depending on the time of year. The volume and quality of water in underground aquifers rely on the vegetation cover, which fosters recharge and aids in maintaining water quality.

Light interactions

Light zonation is the concept of how the amount of sunlight penetration into water influences the structure of a body of water. These zones define various levels of productivity within an aquatic ecosystems such as a lake. For instance, the depth of the water column which sunlight is able to penetrate and where most plant life is able to grow is known as the photic or euphotic zone. The rest of the water column which is deeper and does not receive sufficient amounts of sunlight for plant growth is known as the aphotic zone. The amount of solar energy present underwater and the spectral quality of the light that are present at various depths have a significant impact on the behavior of many aquatic organisms. For example, zooplankton's vertical migration is influenced by solar energy levels.

Thermal stratification

Similar to light zonation, thermal stratification or thermal zonation is a way of grouping parts of the water body within an aquatic system based on the temperature of different lake layers. The less turbid the water, the more light is able to penetrate, and thus heat is conveyed deeper in the water. Heating declines exponentially with depth in the water column, so the water will be warmest near the surface but progressively cooler as moving downwards. There are three main sections that define thermal stratification in a lake. The epilimnion is closest to the water surface and absorbs long- and shortwave radiation to warm the water surface. During cooler months, wind shear can contribute to cooling of the water surface. The thermocline is an area within the water column where water temperatures rapidly decrease. The bottom layer is the hypolimnion, which tends to have the coldest water because its depth restricts sunlight from reaching it. In temperate lakes, fall-season cooling of surface water results in turnover of the water column, where the thermocline is disrupted, and the lake temperature profile becomes more uniform. In cold climates, when water cools below 4^oC (the temperature of maximum density) many lakes can experience an inverse thermal stratification in winter. These lakes are often dimictic, with a brief spring overturn in addition to longer fall overturn. The relative thermal resistance is the energy needed to mix these strata of different temperatures.

Lake Heat Budget

An annual heat budget, also shown as θ_a, is the total amount of heat needed to raise the water from its minimum winter temperature to its maximum summer temperature. This can be calculated by integrating the area of the lake at each depth interval (A_z) multiplied by the difference between the summer (θ_sz) and winter (θ_wz) temperatures or ${\displaystyle {\displaystyle \int }}$A_z(θ_sz-θ_wz)

Chemical properties

The chemical composition of water in aquatic ecosystems is influenced by natural characteristics and processes including precipitation, underlying soil and bedrock in the drainage basin, erosion, evaporation, and sedimentation. All bodies of water have a certain composition of both organic and inorganic elements and compounds. Biological reactions also affect the chemical properties of water. In addition to natural processes, human activities strongly influence the chemical composition of aquatic systems and their water quality.

Allochthonous sources of carbon or nutrients come from outside the aquatic system (such as plant and soil material). Carbon sources from within the system, such as algae and the microbial breakdown of aquatic particulate organic carbon, are autochthonous. In aquatic food webs, the portion of biomass derived from allochthonous material is then named "allochthony". In streams and small lakes, allochthonous sources of carbon are dominant while in large lakes and the ocean, autochthonous sources dominate.

Oxygen and carbon dioxide

Dissolved oxygen and dissolved carbon dioxide are often discussed together due their coupled role in respiration and photosynthesis. Dissolved oxygen concentrations can be altered by physical, chemical, and biological processes and reaction. Physical processes including wind mixing can increase dissolved oxygen concentrations, particularly in surface waters of aquatic ecosystems. Because dissolved oxygen solubility is linked to water temperatures, changes in temperature affect dissolved oxygen concentrations as warmer water has a lower capacity to "hold" oxygen as colder water. Biologically, both photosynthesis and aerobic respiration affect dissolved oxygen concentrations. Photosynthesis by autotrophic organisms, such as phytoplankton and aquatic algae, increases dissolved oxygen concentrations while simultaneously reducing carbon dioxide concentrations, since carbon dioxide is taken up during photosynthesis. All aerobic organisms in the aquatic environment take up dissolved oxygen during aerobic respiration, while carbon dioxide is released as a byproduct of this reaction. Because photosynthesis is light-limited, both photosynthesis and respiration occur during the daylight hours, while only respiration occurs during dark hours or in dark portions of an ecosystem. The balance between dissolved oxygen production and consumption is calculated as the aquatic metabolism rate.

Lake cross-sectional diagram of the factors influencing lake metabolic rates and concentration of dissolved gases within lakes. Processes in gold text consume oxygen and produce carbon dioxide while processes in green text produce oxygen and consume carbon dioxide.

Vertical changes in the concentrations of dissolved oxygen are affected by both wind mixing of surface waters and the balance between photosynthesis and respiration of organic matter. These vertical changes, known as profiles, are based on similar principles as thermal stratification and light penetration. As light availability decreases deeper in the water column, photosynthesis rates also decrease, and less dissolved oxygen is produced. This means that dissolved oxygen concentrations generally decrease as you move deeper into the body of water because of photosynthesis is not replenishing dissolved oxygen that is being taken up through respiration. During periods of thermal stratification, water density gradients prevent oxygen-rich surface waters from mixing with deeper waters. Prolonged periods of stratification can result in the depletion of bottom-water dissolved oxygen; when dissolved oxygen concentrations are below 2 milligrams per liter, waters are considered hypoxic. When dissolved oxygen concentrations are approximately 0 milligrams per liter, conditions are anoxic. Both hypoxic and anoxic waters reduce available habitat for organisms that respire oxygen, and contribute to changes in other chemical reactions in the water.

Nitrogen and phosphorus

Nitrogen and phosphorus are ecologically significant nutrients in aquatic systems. Nitrogen is generally present as a gas in aquatic ecosystems however most water quality studies tend to focus on nitrate, nitrite and ammonia levels. Most of these dissolved nitrogen compounds follow a seasonal pattern with greater concentrations in the fall and winter months compared to the spring and summer. Phosphorus has a different role in aquatic ecosystems as it is a limiting factor in the growth of phytoplankton because of generally low concentrations in the water. Dissolved phosphorus is also crucial to all living things, is often very limiting to primary productivity in freshwater, and has its own distinctive ecosystem cycling.

Biological properties

Lake George, New York, United States, an oligotrophic lake

Role in ecology

Lakes "are relatively easy to sample, because they have clear-cut boundaries (compared to terrestrial ecosystems) and because field experiments are relatively easy to perform.", which make then especially useful for ecologists who try to understand ecological dynamics.

Lake trophic classification

One way to classify lakes (or other bodies of water) is with the trophic state index. An oligotrophic lake is characterized by relatively low levels of primary production and low levels of nutrients. A eutrophic lake has high levels of primary productivity due to very high nutrient levels. Eutrophication of a lake can lead to algal blooms. Dystrophic lakes have high levels of humic matter and typically have yellow-brown, tea-coloured waters. These categories do not have rigid specifications; the classification system can be seen as more of a spectrum encompassing the various levels of aquatic productivity.

Tropical limnology

Tropical limnology is a unique and important subfield of limnology that focuses on the distinct physical, chemical, biological, and cultural aspects of freshwater systems in tropical regions. The physical and chemical properties of tropical aquatic environments are different from those in temperate regions, with warmer and more stable temperatures, higher nutrient levels, and more complex ecological interactions. Moreover, the biodiversity of tropical freshwater systems is typically higher, human impacts are often more severe, and there are important cultural and socioeconomic factors that influence the use and management of these systems.

Professional organizations

People who study limnology are called limnologists. These scientists largely study the characteristics of inland fresh-water systems such as lakes, rivers, streams, ponds and wetlands. They may also study non-oceanic bodies of salt water, such as the Great Salt Lake. There are many professional organizations related to limnology and other aspects of the aquatic science, including the Association for the Sciences of Limnology and Oceanography, the Asociación Ibérica de Limnología, the International Society of Limnology, the Polish Limnological Society, the Society of Canadian Limnologists, and the Freshwater Biological Association.