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, termedlandscape limnology, studies, manages, and seeks to conserve theseecosystems using a landscape perspective, by explicitly examining connections between an aquatic ecosystem and itsdrainage basin. Recently, the need to understand global inland waters as part of theEarth system created a sub-discipline called global limnology.[4] This approach considers processes in inland waters on a global scale, like the role of inland aquatic ecosystems in globalbiogeochemical cycles.[5][6][7][8][9]
Limnology is closely related toaquatic ecology andhydrobiology, 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.
Physical properties of aquatic ecosystems are determined by a combination of heat, currents, waves and other seasonal distributions of environmental conditions.[13] Themorphometry 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.[14][15]River andstream system morphometry is driven by underlying geology of the area as well as the general velocity of the water.[13] 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 landscapedrainage density,lake surface area andlake shape.[15]
Other types of aquatic systems which fall within the study of limnology areestuaries. Estuaries are bodies of water classified by the interaction of a river and the ocean or sea.[13]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.[13] The volume and quality of water in underground aquifers rely on the vegetation cover, which fosters recharge and aids in maintaining water quality.[16]
Light zonation is the concept of how the amount of sunlight penetration into water influences the structure of a body of water.[13] 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 thephotic 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 theaphotic zone.[13] 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.[16]
Similar to light zonation, thermalstratification 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 lessturbid the water, the more light is able to penetrate, and thus heat is conveyed deeper in the water.[17] 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. Theepilimnion 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. Thethermocline is an area within the water column where water temperatures rapidly decrease.[17] The bottom layer is thehypolimnion, which tends to have the coldest water because its depth restricts sunlight from reaching it.[17] 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°C (the temperature of maximum density) many lakes can experience an inverse thermal stratification in winter.[18] These lakes are oftendimictic, with a brief spring overturn in addition to longer fall overturn. Therelative thermal resistance is the energy needed to mix these strata of different temperatures.[19]
An annual heat budget, also shown as θa, 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 (Az) multiplied by the difference between the summer (θsz) and winter (θwz) temperatures orAz(θsz-θwz)[19]
The chemical composition of water in aquatic ecosystems is influenced by natural characteristics and processes includingprecipitation, underlyingsoil andbedrock in thedrainage basin,erosion,evaporation, andsedimentation.[13] All bodies of water have a certain composition of bothorganic andinorganic 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.[17]
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 particulateorganic carbon, areautochthonous. In aquatic food webs, the portion of biomass derived from allochthonous material is then named "allochthony".[20] In streams and small lakes, allochthonous sources of carbon are dominant while in large lakes and the ocean, autochthonous sources dominate.[21]
Dissolved oxygen and dissolvedcarbon dioxide are often discussed together due their coupled role inrespiration andphotosynthesis. 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.[22] Biologically, both photosynthesis and aerobic respiration affect dissolved oxygen concentrations.[17] Photosynthesis byautotrophic organisms, such asphytoplankton and aquaticalgae, increases dissolved oxygen concentrations while simultaneously reducing carbon dioxide concentrations, since carbon dioxide is taken up during photosynthesis.[22] Allaerobic 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 thedaylight hours, while only respiration occurs duringdark hours or in dark portions of an ecosystem. The balance between dissolved oxygen production and consumption is calculated as theaquatic metabolism rate.[23]
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 oforganic 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.[17] 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 consideredhypoxic.[22] When dissolved oxygen concentrations are approximately 0 milligrams per liter, conditions areanoxic. Both hypoxic and anoxic waters reduce available habitat for organisms that respire oxygen, and contribute to changes in other chemical reactions in the water.[22]
Nitrogen is a nutrient central for the function of aquatic ecosystems. Nitrogen is generally present as adissolved gas (N2) in aquatic ecosystems, however due to the high energy requirement of utilising N2 most organisms tend not to use it.[24] Therefore, most water quality studies tend to focus onnitrate,nitrite andammonia levels.[13][24] Most of these dissolved nitrogen compounds follow a seasonal pattern with greater concentrations in thefall andwinter months compared to thespring andsummer.[13]
Another important nutrient in aquatic systems isphosphorus. 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.[13] Dissolved phosphorus is also crucial to all living things, is often very limiting to primary productivity in freshwater, and has its own distinctive ecosystemcycling.[17]
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.[25]
One way to classify lakes (or other bodies of water) is with thetrophic state index.[2] An oligotrophic lake is characterized by relatively low levels ofprimary production and low levels ofnutrients. A eutrophic lake has high levels of primary productivity due to very high nutrient levels.Eutrophication of a lake can lead toalgal blooms.Dystrophic lakes have high levels ofhumic matter and typically have yellow-brown, tea-coloured waters.[2] 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.[citation needed]
Tropical limnology is a unique and important subfield of limnology that focuses on the distinct physical, chemical, biological, and cultural aspects of freshwater systems intropical regions.[26] The physical and chemical properties of tropical aquatic environments are different from those intemperate regions, with warmer and more stable temperatures, higher nutrient levels, and more complex ecological interactions.[26] Moreover, thebiodiversity 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.[26]
^Cole, J. J.; Prairie, Y. T.; Caraco, N. F.; McDowell, W. H.; Tranvik, L. J.; Striegl, R. G.; Duarte, C. M.; Kortelainen, P.; Downing, J. A.; Middelburg, J. J.; Melack, J. (23 May 2007). "Plumbing the Global Carbon Cycle: Integrating Inland Waters into the Terrestrial Carbon Budget".Ecosystems.10 (1):172–185.Bibcode:2007Ecosy..10..172C.CiteSeerX10.1.1.177.3527.doi:10.1007/s10021-006-9013-8.S2CID1728636.
^Tranvik, Lars J.; Downing, John A.; Cotner, James B.; Loiselle, Steven A.; Striegl, Robert G.; Ballatore, Thomas J.; Dillon, Peter; Finlay, Kerri; Fortino, Kenneth; Knoll, Lesley B.; Kortelainen, Pirkko L.; Kutser, Tiit; Larsen, Soren; Laurion, Isabelle; Leech, Dina M.; McCallister, S. Leigh; McKnight, Diane M.; Melack, John M.; Overholt, Erin; Porter, Jason A.; Prairie, Yves; Renwick, William H.; Roland, Fabio; Sherman, Bradford S.; Schindler, David W.; Sobek, Sebastian; Tremblay, Alain; Vanni, Michael J.; Verschoor, Antonie M.; von Wachenfeldt, Eddie; Weyhenmeyer, Gesa A. (November 2009)."Lakes and reservoirs as regulators of carbon cycling and climate".Limnology and Oceanography.54 (6part2):2298–2314.Bibcode:2009LimOc..54.2298T.doi:10.4319/lo.2009.54.6_part_2.2298.hdl:10852/11601.
^Welch, P.S. (1935).Limnology (Zoological Science Publications). United States of America: McGraw-Hill.ISBN978-0-07-069179-7.{{cite book}}:ISBN / Date incompatibility (help)[page needed]
^Eby, G.N., 2004, Principles of Environmental Geochemistry: Thomson Brooks/Cole, Pacific Grove, CA., 514 pp.
^abcdDodds, Walter K. (2010).Freshwater ecology: concepts and environmental applications of limnology. Whiles, Matt R. (2nd ed.). Burlington, MA: Academic Press.ISBN978-0-12-374724-2.OCLC784140625.[page needed]
^Cole, Jonathan J.; Caraco, Nina F. (2001). "Carbon in catchments: connecting terrestrial carbon losses with aquatic metabolism".Marine and Freshwater Research.52 (1): 101.Bibcode:2001MFRes..52..101C.doi:10.1071/mf00084.S2CID11143190.
