Limnology

thumb|upright=1.3|[[Lake Hāwea, New Zealand]]

Limnology is the study of inland aquatic ecosystems. Pronounced (), the name comes . It 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, 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 the underlying geology of the area as well as the general velocity of the water.

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 the cooling of the water surface. The thermocline is an area within the water column where water temperatures rapidly decrease. These lakes are often dimictic, with a brief spring overturn in addition to a 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 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 or <math>\displaystyle \int</math>Az(θsz-θwz) 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.

thumb|upright=2.2|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. Therefore, most water quality studies tend to focus on nitrate, nitrite and ammonia levels.

Lake trophic classification

One way to classify lakes (or other bodies of water) is with the trophic state index. 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., the Society for Freshwater Science, and the Freshwater Biological Association.