In temperate latitudes, lakes undergo an annual mixing cycle. The mixing cycle is a result of the fact that the density of water varies with temperature.

[T] temperature density relationship (Mihelcic, 1999, Fig. 5-32)

Of particular importance, is the fact that the maximum density of water occurs at 4 °C rather than at 0 °C, i.e. the T vs.r relationship is not linear. The mixing cycle occurs as lakes warm through the spring and summer and cool through fall and winter. Depending on their relative size and depths (usually >10m), lakes thermally stratify in summer and winter and experience periods of complete mixing in fall and spring. In a stratified lake an upper layer of less dense water (warmer in summer, cooler in winter) floats on a lower layer of more dense water. The layers have special names: epilimnion, metalimnion, and hypolimnion.

[T] thermal stratification pattern (Mihelcic, 1999, Fig. 5-33)

The plane which defines the steepest temperature gradient in the metalimnion is termed the thermocline.

[T] temperature profile in a stratified lake (Mihelcic, 1999, Fig. 5-33)

This phenomena is nicely illustrated by summer and winter temperature profiles from Lake Ekoln in Sweden.

[T] summer stratification (Welch, 1982, Fig 3.4)

[T] winter stratification (Welch, 1982, Fig 3.6)

Seasonality in temperature conditions and mixing status follows a spring (mixed), summer (stratified), fall (mixed), winter (stratified) pattern. The term turnover refers to the completely mixed periods in spring and fall. A lake which mixes twice per year is termed dimictic.

[T] temperature cycles in deep lakes (Mihelcic, 1999, Fig. 5-34)

The yearly distribution of temperature in lakes is often well described by isotherms, shown here again for Lawrence Lake, Michigan and Clear Lake, California.

[T] Lawrence Lake temperature isotherms (Wetzel, 1983)

[T] Clear Lake temperature isotherms (Horne and Goldman, 1994)

Observations from Lake Superior further illustrate seasonality in temperature.

[T] temperature in Lake Superior - temporal development at 4 stations

[T] temperature in Lake Superior - spatial development over the season

Another feature of the temperature cycle is upwelling. These wind-driven, episodic events are known to introduce nutrients from deep ocean waters, stimulating productivity at the surface. Their importance in freshwaters is less well known.

[T] upwellings (Horne and Goldman, 1994)

[T] upwellings in Lake Superior (courtesy of Dr. Judy Budd)

Of the more than 100 chemical elements, only 20-30 are constituents of living things.

[T] chemical composition of living things (Mihelcic, 1999, Table 5-4)

Of these, 5 are of particular importance in mediating water quality conditions in lakes. The basic chemical elements or nutrients that make up the substance of living systems are continuously moved through lakes in what are termed biogeochemical cycles.

Carbon cycle - key features of the carbon cycle in lakes are the processes of photosynthesis

and respiration

The photosynthetic fixation of atmospheric CO₂ by green plants is the source of most of the biomass in the world and serves to fix the sun's energy in chemical bonds. The simple carbohydrates so formed are used to manufacture a myriad of other organic compounds, including more complex carbohydrates, fats, and proteins. Plants and animals use that the chemical energy stored in organic matter for maintenance and growth, transforming that organic matter back to CO₂ through respiration.

[T] the carbon cycle (Wiersma, 1986, Fig. 2.12)

Oxygen cycle - oxygen and carbon are closely linked. The most important source of oxygen is photosynthesis by green plants and the most important oxygen sink is respiration. Remember that, in lakes, photosynthesis is carried out by algae and aquatic plants and that respiration is carried out by all organisms: plants, animals, and microbes.

[T] the oxygen cycle (Wiersma, 1986, Fig. 2.17)

Nitrogen cycle - nitrogen, oxygen, and carbon are closely linked as well. Nitrogen in the form of ammonia or nitrate is taken up directly from the water by plants. Atmospheric nitrogen can also be converted to ammonia by select plant species through a process termed nitrogen fixation. Once within the plants, ammonia and nitrate are incorporated into a variety of organic nitrogen compounds, i.e. amino acids (the building blocks of proteins) and nucleic acids, which then travel through food webs. When organic nitrogen compounds are released to the environment, either as excreta or through death and decomposition, the nitrogen recycling process begins. The transformation of organic nitrogen to nitrate is a complex process involving a variety of microorganisms and, notably, consuming oxygen.

The step where organic-N goes to ammonia is termed ammonification and the conversion of ammonia to nitrite and nitrate is called nitrification. Under anaerobic conditions, nitrate can be converted to nitrogen gas or nitrous oxide through a process termed denitrification.

The resultant nitrogen gas returns to the atmosphere. The two processes illustrated here are of great and fundamental importance to both surface water quality management (nutrients, toxicity) and wastewater treatment.

[T] nitrogen cycle (Mihelcic, 1999, Fig. 5-28)

Phosphorus cycle - phosphorus is most commonly the nutrient limiting plant growth in lakes. Excessive additions of phosphorus create nuisance conditions of algal growth in lakes and a variety of attendant water quality problems. Phosphorus in lakes is basically present in two forms: soluble P, available for uptake by plants, and particulate P either already in plants or associated with abiotic particulate matter. Sources of soluble/particulate P and subsequent transformations are important to lake behavior as they influence the potential of P inputs to foster algal growth. Phosphorus reaching the bottom of lakes can be recycled to the water column, creating water quality problems long after external inputs have been eliminated.

[T] the phosphorus cycle (Wiersma, 1986, Fig. 2.10)

Sulfur cycle - sulfur is linked to the carbon cycle, reaching lakes in the form of sulfates (SO₄⁼) and as organic materials such as amino acids (proteins). Hydrogen sulfide (H₂S), a material which is toxic to aquatic life at very low concentrations, is an important part of the sulfur cycle in lakes. Hydrogen sulfide is formed through the decomposition of sulfur-containing amino acids and through bacterial reduction of sulfate. Oxidation of H₂S is an oxygen-consuming process.

[T] the sulfur cycle (Mihelcic, 1999, Fig. 5-30)

These and other chemical reactions, which drive the functioning of lake and river ecosystems are summarized in the table below.

[T] redox reactions (Mihelcic, 1999, p. 239)

[T] Onondaga Lake isopleths: T, O₂, pH, NO₃, NH₃, Fe, P, S, CH₄

Ultimately energy and material flow drives ecosystem activity.

[T] energy and material flow (Mihelcic, 1999, Fig 5-2)

It starts out with the sun. Energy flow in lakes and aquatic ecosystems begins when the sun's energy is captured photosynthetically, a process termed primary production. The energy, now present in the form of organic matter, is transferred up the food chain from producers to primary consumers (herbivores), and to secondary consumers (carnivore). Multilevel consumers (eating both plants and animals) are termed omnivores. Losses occur at each level due to respiration and decomposition. Because of the inefficiency of energy transfer at each level, relatively little biomass (standing crop) and energy remain at the top of the pyramid.

[T] Lake Superior food chain (modified from Mihelcic, 1999, Fig 5-15)

[T] energy transfer (Mihelcic, 1999, Fig 5-18)

[T] energy pyramid (Mihelcic, 1999, Fig 5-19)

The biology of lakes can also be examined from the perspective of its biotic composition. First, some definitions. The fundamental unit is the species - a group of individuals which can successfully breed with each other. Populations are members of a single species that live together in a particular locality. All the plant and animal populations interacting in a given environment is termed a community. That system of living organisms plus the nonliving or abiotic components make up the ecosystem.

[T] population, community, and ecosystem (Mihelcic, 1999, Fig 5-2)

Thus a population of walleye makes up part of the fish community of a lake ecosystem.

Classification of the biota in aquatic systems may be based on the habitat and function (niche) of the organism. In lakes and rivers, the primary producers are algae and macrophytes. Algae are (generally) microscopic plants which may live free-floating in the water column (phytoplankton) or attached to surfaces such as rocks and pilings (periphyton). Macrophytes are true higher plants, larger than algae, which grow submerged, floating or emergent. Algae are generally most important in the open waters of lakes, while macrophytes often dominate in rivers and in the shallow waters of lakes.

Primary consumers in lakes include microscopic free-floating animals such as protozoans, rotifers, cladocerans, and copepods (zooplankton) and herbivorous fish, e.g. the lake whitefish. Secondary consumers include some zooplankton and piscivorous fish, e.g. the lake trout. Also of importance are detritivores, insect larvae, worms, crustaceans and some fish which utilize the sediments as a food source. Collectively, the organisms which inhabit the sediments are termed benthos.

[T] organism groups (Milhelcic, 1999, Fig. 5-3)