Limnology ( lim-NOL-?-jee; from Greek ?????, limne, "lake" and ?????, logos, "knowledge"), is the study of inland waters. It is often regarded as a division of ecology or environmental science. It covers the biological, chemical, physical, geological, and other attributes of all inland waters (running and standing waters, both fresh and saline, natural or man-made). This includes the study of lakes and ponds, rivers, springs, streams and wetlands. A more recent sub-discipline of limnology, termed landscape limnology, studies, manages, and conserves these aquatic ecosystems using a landscape perspective.
Limnology is closely related to aquatic ecology and hydrobiology, which study aquatic organisms in particular regard to their hydrological environment. Although limnology is sometimes equated with freshwater science, this is erroneous since limnology also comprises the study of inland salt lakes.
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.
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. Another type of aquatic system which falls within the study of limnology is 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.
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 portion of the electromagnetic spectrum which is reflected when sunlight hits the surface of the water is known as albedo.
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 how each layer has different temperature variations. The less turbid the water, the more light is able to penetrate, and thus heating a thicker depth of 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 which define thermal stratification in a lake. The first is the epilimnion which is closest to the surface and experiences primarily wind circulation although the water is generally uniformally warm because of the close proximity to the surface. The layer below is often called the thermocline and is an area within the water column which tends to experience a rapid decrease in temperature. Finally, the layer which is the bottom-most within the body of water is the hypolimnion which has uniformally cold water because of its depth which restricts sunlight from reaching it. In temperate lakes, fall-season cooling of surface water to 4 °C (the highest density of water) results in turnover of the water column.
The chemical composition of water in a natural environment is influenced mainly by precipitation, type of soil and bedrock in the watershed, erosion, evaporation and sedimentation. All bodies of water have a certain composition of both organic and inorganic elements and compounds.
There are hundreds of variables which are considered to play a role in water quality however a few have been determined to be of greater interest regarding the role they play in aquatic ecosystem health. While certain biological activities have an impact on dissolved gas concentrations, nutrients, etc. human activity is one of the strongest influences on water quality.
Dissolved oxygen is an element which is necessary for a number of biological and chemical reactions which are critical to the proper functioning of the ecosystem. Some of the biological processes which alter the concentrations of dissolved oxygen include photosynthesis and aquatic organism respiration. Due to the role that photosynthesis plays in dissolved oxygen concentrations in a body of water. Oxygen profiles are affected by photosynthesis, wind mixing of surface waters, and respiration or organic matter, such that oxygen declines similar to the temperature profile. These profiles are based on similar principles as thermal stratification and light penetration. Since dissolved oxygen concentrations are driven primarily by photosynthesis, the amount of sunlight is a limiting factor in terms of how much photosynthesis can occur within the different levels of the water column where light is readily available. This means that dissolved oxygen levels are generally lower as you move deeper into the body of water because of the lower availability of light in those parts of the water.
Dissolve oxygen and dissolved carbon dioxide are often discussed together due the role they both play in aquatic organism respiration. These organisms absorb dissolved oxygen from the water to use in respiration and expel carbon dioxide as a byproduct of this process. Carbon dioxide tends to have an inverse diurnal relationship with oxygen.
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.
Limnology classifies lakes (or other bodies of water) according to the trophic state index. An oligotrophic lake is characterised 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 has 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.