Limnology

Limnology(/lɪmˈnɒlədʒi/lim-NOL-ə-jee;fromAncient Greekλίμνη(límnē)'lake', and-λογία(-logía)'study of') is the study of inlandaquatic ecosystems.^[1] The study of limnology includes aspects of thebiological,chemical,physical,andgeologicalcharacteristics offreshandsaline,natural and man-madebodies of water.This includes the study oflakes,reservoirs,ponds,rivers,springs,streams,wetlands,andgroundwater.^[2]Water systems are often categorized as either running (lotic) or standing (lentic).^[3]

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 theseecosystemsusing 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 systemcreated 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 ecologyandhydrobiology,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 byFrançois-Alphonse Forel(1841–1912) who established the field with his studies ofLake Geneva.Interest in the discipline rapidly expanded, and in 1922August Thienemann(a German zoologist) andEinar Naumann(a Swedish botanist) co-founded theInternational Society of Limnology(SIL, fromSocietas Internationalis Limnologiae). Forel's original definition of limnology, "theoceanographyof lakes ", was expanded to encompass the study of all inland waters,^[2]and influencedBenedykt Dybowski's work onLake Baikal.

Prominent early American limnologists includedG. Evelyn HutchinsonandEd Deevey.^[10]At theUniversity of Wisconsin-Madison,Edward A. Birge,Chancey Juday,Charles R. Goldman,andArthur D. Haslercontributed to the development of theCenter for Limnology.^[11]^[12]

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.^[13]Themorphometryof 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]Riverandstreamsystem 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 areaandlake 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]Wetlandsvary 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 interactions

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 euphoticzone. 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]

Thermal stratification

Similar to light zonation, thermalstratificationor 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 lessturbidthe 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. Theepilimnionis 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. Thethermoclineis 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^oC (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 resistanceis the energy needed to mix these strata of different temperatures.^[19]

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 $\displaystyle \int$ A_z(θ_sz-θ_wz)^[19]

Chemical properties

The chemical composition of water in aquatic ecosystems is influenced by natural characteristics and processes includingprecipitation,underlyingsoilandbedrockin thedrainage basin,erosion,evaporation,andsedimentation.^[13]All bodies of water have a certain composition of bothorganicandinorganicelements 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]

Allochthonoussources 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]

Oxygen and carbon dioxide

Dissolved oxygenand dissolvedcarbon dioxideare often discussed together due their coupled role inrespirationandphotosynthesis.Dissolved oxygen concentrations can be altered by physical, chemical, and biological processes and reaction. Physical processes including wind mi xing 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 asphytoplanktonand aquaticalgae,increases dissolved oxygen concentrations while simultaneously reducing carbon dioxide concentrations, since carbon dioxide is taken up during photosynthesis.^[22]Allaerobic organismsin 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 thedaylighthours, while only respiration occurs duringdarkhours or in dark portions of an ecosystem. The balance between dissolved oxygen production and consumption is calculated as theaquatic metabolism rate.^[23]

Vertical changes in the concentrations of dissolved oxygen are affected by both wind mi xing 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 mi xing 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 and phosphorus

Nitrogenandphosphorusare ecologically significant nutrients in aquatic systems. Nitrogen is generally present as agasin aquatic ecosystems however most water quality studies tend to focus onnitrate,nitriteandammonialevels.^[13]Most of these dissolved nitrogen compounds follow a seasonal pattern with greater concentrations in thefallandwintermonths compared to thespringandsummer.^[13]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]

Biological properties

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.^[24]

Lake trophic classification

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 productionand low levels ofnutrients.A eutrophic lake has high levels of primary productivity due to very high nutrient levels.Eutrophicationof a lake can lead toalgal blooms.Dystrophiclakes have high levels ofhumic matterand 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

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.^[25]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.^[25]Moreover, thebiodiversityof 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.^[25]

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 theAssociation for the Sciences of Limnology and Oceanography,theAsociación Ibérica de Limnología,theInternational Society of Limnology,thePolish Limnological Society,the Society of Canadian Limnologists, and theFreshwater Biological Association.^{[citation needed]}

References

^Kumar, Arvind (2005).Fundamentals of Limnology.APH Publishing.ISBN 9788176489195.
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^Marsh, G. Alex; Fairbridge, Rhodes W. (1999). "Lentic and lotic ecosystems".Environmental Geology.Dordrecht: Springer Netherlands. pp. 381–388.doi:10.1007/1-4020-4494-1_204.ISBN 978-1-4020-4494-6.Retrieved2022-04-21.
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