Seasonal distribution of dissolved oxygen in lakes
Diffusion of oxygen from the atmosphere into and within water is a relatively slow process. Turbulent mixing of water is required for dissolved oxygen to be distributed in equilibrium with that of the atmosphere. Subsequent distribution of oxygen in the water of thermally stratified lakes is controlled by a number of solubility conditions, hydrodynamics, inputs from photosynthesis, and losses to chemical and metabolic oxidations (processes with compounds that gain oxygen).
Orthograde oxygen profile (summer)
In oligotrophic (unproductive) lakes that undergo thermal stratification in the summer, the oxygen content of the epilimnion decreases as the water temperature increases. The oxygen content of the hypolimnion is higher than that of the epilimnion because the saturated colder water from spring turnover experiences limited oxygen consumption. This oxygen distribution is known as an orthograde oxygen profile.
Clinograde oxygen profile (summer)
In eutrophic (productive) lakes, the loading of organic matter to the hypolimnion and sediments increases the consumption of dissolved oxygen. As a result, the oxygen content of the hypolimnion of thermally stratified lakes is reduced progressively during the period of summer stratification - usually most rapidly at the deepest portion of the basin where a lower volume of water is exposed to the intensive oxygen consuming processes of decomposition at the surface of the lake sediments. This oxygen distribution is known as a clinograde oxygen profile. Extreme clinograde oxygen distributions occur in meromictic lakes where the monimolimnion may have permanent oxygen deficits.
Redistribution of oxygen (fall, winter, spring)
Oxygen saturation at existing water temperatures returns throughout the water column during fall overturn. The freezing of lakes during the winter prevents much of the exchange of oxygen with the atmosphere. The oxygen content of lower depths in productive lakes are reduced, but not to the extent observed in the summer because of colder water temperatures throughout the water column, resulting in greater oxygen solubility and reduced respiration by aquatic organisms. In the spring, the water is mixed and oxygen becomes saturated throughout the water column.
Metalimnetic Oxygen Maximum
The metalimnetic oxygen maximum distribution occurs when the oxygen content in the metalimnion is supersaturated in relation to levels in the epilimnion and hypolimnion. The resulting positive heterograde oxygen curve is usually caused by extensive photosynthetic activity by algae in the metalimnion.
Metalimnetic Oxygen Minimum
The metalimnetic oxygen minimum distribution occurs less frequently than metalimnetic oxygen maxima and results in a negative heterograde oxygen curve. This oxygen distribution results most often when oxygen consumption from decomposition exceeds oxygen inputs as the rate of organic matter sinking from the epilimnion is slowed upon entering the colder, denser metalimnetic water, allowing greater time for decomposition to occur in this layer of water.
Variations in concentration
Epilimnetic oxygen concentrations vary on a daily basis in productive lakes. Rapid fluctuations between super-saturation and under-saturation of oxygen can result when daily photosynthetic contributions and night respiratory oxygen consumption exceed turbulent exchange with the atmosphere.
Extensive and rapid decay of littoral plants or phytoplankton can result in large reductions in the oxygen content of small, shallow lakes leading to the death of large numbers of aquatic animals. This is known as summerkill.
Winterkills can occur where oxygen is depleted under the ice and large numbers of fish may die. Since most fish would not overwinter in lakes without oxygen, the variable period of autumn mixing prior to ice cover may be a deciding factor in the winterkill. The eventual total mixing or overturn is of great importance because only then can nutrients and oxygen be uniformly distributed. The problem may be aggravated when snow cover prevents light penetration and slows oxygen production through photosynthesis during the winter. Winterkill is common in shallow, productive temperate lakes.
Nitrogen: Lake Trophic Status and the Transformation of Nitrogen
Denitrification in Eutrophic (Nutrient Rich) Lakes
Denitrification only occurs at low oxygen levels, and hence is typically restricted to sediments, although it also occurs in the deoxygenated hypolimnia of some lakes. In eutrophic lakes that are stratified, concentrations of N2 may decline in the epilimnion because of reduced solubility as temperatures rise and increase in the hypolimnion from denitrification of nitrate (NO3) to nitrite (NO2) to inorganic nitrogen (N2). Nitrite (NO2) rarely accumulates except in the metalimnion and hypolimnion of eutrophic lakes. Concentrations of nitrite in lakes are usually very low unless organic pollution is high.
Lake Trophic Status and Nitrification in the Hypolimnion
In eutrophic lakes, anoxia results in increased levels of ammonia (due to lack of Nitrosomonas metabolism) and nitrite (due to lack of Nitrobacter metabolism) with increasing depth in the hypolimnion. In oligotrophic lakes, high oxygen concentrations permit metabolism of ammonia to nitrate, resulting in low levels of nitrite and ammonia and high levels of nitrate in the hypolimnion. When the hypolimnion of a eutrophic lake becomes anaerobic, bacterial nitrification of ammonia ceases and the NH4+ concentrations increase.
Phosphorus: The Involvement of Phytoplankton in Phosphorus Cycling
Although it is only needed in small amounts, phosphorus is one of the more common growth-limiting elements for phytoplankton (various kinds of minute, floating aquatic plants). These shortages arise because of the geochemical shortage of phosphorus in many drainage basins together with the lack of any biological pathway enabling phosphate fixation similar to that enabling nitrogen fixation. Phytoplankton are only able to use phosphorus in the phosphate (PO4) form for growth.
In deep stratified lakes there is a limited replenishment of phosphate in surface waters and the quantity of "available" phosphorus in late winter may determine the level of phytoplankton growth that can develop in the summer. Intensive algal growth in spring usually depletes phosphate to levels in the surface waters. Hence, phytoplankton growth during the summer usually occurs using phosphate excreted by animals feeding on phytoplankton. Direct sediment resupply is important in the summer in shallow areas.
Rooted aquatic plants often obtain large quantities of phosphorus from the sediments and can release large amounts into the water. When phosphate levels are low in surface waters, phytoplankton excrete extracellular enzymes called alkaline phosphatases, which have the ability to free phosphate bound to organic molecules. Since phosphate (in contrast to nitrate) is readily adsorbed to soil particles and does not move easily with groundwater, high inflows of total phosphorus are due to erosion of particles from steep slopes with easily erodible soils. Intensive algal growth in spring usually depletes lake phosphate to low levels - Intensive algal growth in spring usually depletes lake phosphate to low levels. Agricultural, domestic, and industrial wastes are major sources of soluble phosphate and frequently contribute to lake eutrophication and algal blooms.
Organic Carbon: Variation in Lake Colour Resulting from Humic Compounds
Rain which falls on the soil has a chance to leach humics, while rain falling directly on a lake does not. The greater the area of the drainage basin relative to the surface area of the lake, the higher the input of humic acids from the soil versus dilution from rain onto the lake. Humic acids undergo slow degradation once they reach a lake due to sunlight and biodegradation. As the retention time of water in a lake increases, the lake becomes less coloured because the humic acids are exposed to sunlight and living organisms for a longer interval. When the slope of a watershed is steep, rainfall moves rapidly to the lake allowing little time for contact with the soil during which humic acids are picked up. Steep watersheds also tend to have less organic content in their soils and therefore contribute to less humics. Relatively level watersheds allow rainwater to sit in the soil for a longer time. They also have larger accumulations of organic material in the soil and more areas of bog and wetland which contribute colour to runoff. Calcium is also known to precipitate humic acids. Hence, the water in lakes on sedimentary rocks that are largely composed of calcium carbonate is less coloured than those in hard igneous rock settings because of the precipitation of humic acids.