Fresh water ecology

The study of fresh water habitats is called limnology , coming from the Greek word limnos, meaning "pool, lake, or swamp." Fresh water habitats are normally divided into two groups: the study of standing bodies of water such as lakes and ponds (called lentic ecosystems) and the study of rivers, streams, and other moving sources of water (called lotic ecosystems). Another important area that should be included is fresh water wetlands .

The historical roots of limnology go back to F. A. Forel, who studied Lake Geneva, Switzerland, in the late 1800s and E. A. Birge and C. Juday, who studied lakes in Wisconsin in the early 1900s. More recently, the modern "father" of limnology can arguably be attributed to G. Evelyn Hutchinson, who died in 1991 after teaching at Yale University for more than 40 years. Among his prolific writings are four treatises on limnology which offer the most detailed descriptions of lakes that have been published.

Fresh water ecology is an intriguing field because of the great diversity of aquatic habitats. For example, lakes can be formed in different ways: volcanic origin such as Crater Lake, Oregon; tectonic (earth movement) origin like Lake Tahoe in California/Nevada and Lake Baikal in Siberia; glacially-derived lakes like the Great Lakes or smaller kettle hole or cirque lakes; oxbow lakes which form as rivers change their meandering paths; and human-created reservoirs. Aquatic habitats are strongly influenced by the surrounding watershed , and lakes in the same geographic area tend to be of the same origin and have similar water quality .

Lakes are characteristically non-homogeneous. Physical, chemical, and biological factors contribute to both horizontal and vertical zonations. For example, light penetration creates an upper photic (lighted) zone and a deeper aphotic (unlit) zone. Phytoplankton (microscopic algae such as diatoms, desmids, and filamentous algae) inhabit the photic zone and produce oxygen through photosynthesis . This creates an upper productive area called the trophogenic (productive) zone. The deeper area where respiration prevails is called the tropholytic (unproductive) zone. Zooplankton (microscopic invertebrates such as cladocerans, copepods, and rotifers) and nekton (free-swimming animals such as fish) inhabit both of these zones. The boundary area where oxygen produced from photosynthesis equals that consumed through respiration is called the compensation depth. Near-shore areas where light penetrates to the bottom and aquatic macrophytes such as cattails and bulrushes grow, are called the littoral zone . This is typically the most productive area, and it is more pronounced in ponds than in lakes. Open water areas are called the limnetic zone, where most plankton inhabit. Some species of zooplankton are found more concentrated in deeper waters during the day and in greater numbers in the upper waters during the night. One explanation for this vertical migration is that these zooplankton, often large and sometimes pigmented, are avoiding visuallyfeeding planktivorous fish. These zooplankton are thus able to feed on phytoplankton in the trophogenic zone during periods of darkness, and then swim to deeper waters during daylight hours. Phytoplankton are also adapted for existence in the limnetic zone. Some species are quite small in size (less than 20 microns in diameter and called nannoplankton), allowing them to be competitive at nutrient uptake due to their high surface-to-volume ratio. Other groups form large colonies, often with spines, lessening the negative impacts of herbivory and sinking. Blue-green algae produce oils that help them float on or near the water's surface. Some are able to fix atmospheric nitrogen (called nitrogen fixation ), giving them a competitive advantage in low-nitrogen conditions. Other species of blue-greens produce toxic chemicals , making them inedible to herbivores. There have even been reports of cattle deaths following ingestion of water with dense growths of these algae.

Lakes can be isothermal (uniform temperature from top to bottom) during some times of the year, but during the summer months they are typically thermally stratified with an upper, warm layer called the epilimnion (upper lake), and a colder, deeper layer called the hypolimnion (lower lake). These zones are separated by the metalimnion, which is determined by the depths with a temperature change of more than 1°C per meter depth, called the thermocline . The summer temperature stratification creates a density gradient that effectively prevents mixing between zones.

Wind is another physical factor that influences aquatic habitats, particularly in lakes with broad surface areas exposed to the main direction of the wind, called fetch. Strong winds can produce internal standing waves called seiches that create a rocking motion in the water once the wind dies down. Other types of wind-generated water movements include Ekman spirals and Langmuir cells. Deep or chemically-stratified lakes that never completely mix are called meromictic. Lakes in tropical regions that mix several times a year are called polymictic. In regions with severe winters resulting in ice covering the surface of the lake, mixing normally occurs only during the spring and fall when the water is isothermal. These lakes are called dimictic, and the mixing process is called overturn. People living downwind of these lakes often notice a rotten egg smell caused by the release of hydrogen sulfide. This gas is a product of benthic (bottom-dwelling) bacteria that inhabit the anaerobic muds of productive lakes (discussed below in more detail).

Lakes that receive a low-nutrient input remain fairly unproductive, and are called oligotrophic (low nourished). These lakes typically have low concentrations of phytoplankton, with diatoms being the main representative. Moderately productive lakes are called mesotrophic. Eutrophic (well nourished) lakes receive more nutrient input and are therefore more productive. They are typically shallower than oligotrophic lakes and have more accumulated bottom sediments that often experience summer anoxia. These lakes have an abundance of algae, particularly blue-greens (Cynaobacteria) which are often considered nuisance algae because they float on the surface and out-compete the other algae for nutrients and light.

Most lakes naturally age and become more productive over time; however, large, deep lakes may remain oligotrophic. The maturing process is called eutrophication and it is regulated by the input of nutrients which are needed by the algae for growth. Definitive limnological studies done in the 1970s concluded that phosphorus is the key limiting nutrient in most lakes. Thus, the accelerated input of this chemical into streams, rivers, and eventually lakes by excess fertilization, sewage input (both human and animal), and erosion is called cultural eutrophication . Much debate and research has been spent on how to slow down or control this process. One interesting management tool is called biomanipulation, in which piscivorous (fish-eating) fish are added to lakes to consume planktivorous (plankton-eating) fish. Because the planktivores are visual feeders on the largest prey, this allows higher numbers of large zooplankton to thrive in the water, which consume more phytoplankton, particularly non-toxic blue-green algae. A more practical approach to controlling cultural eutrophication is by limiting the nutrient loading into our bodies of water. Although this isn't easy, we must consider ways of limiting excessive uses of fertilizers, both at home and on farms, as well as more effectively regulating the release of treated sewage into rivers and lakes, particularly those which are vulnerable to eutrophication. Another lake management tool is to aerate the bottom (hypolimnetic) water so that it remains oxygenated. This keeps iron in the oxidized state (Fe⁺³), which chemically binds with phosphate (PO₄) and prevents it from being available for algal uptake. Lakes that have anoxic bottom water keep iron in the reduced state (Fe⁺²), and phosphate is released from the sediment into the water. Fall and spring turnover then returns this limiting nutrient to the photic zone, promoting high algal growth. When these organisms eventually die and sink to the bottom, decomposers use up more oxygen, and we get a "snow ball" effect. Thus, productive lakes can become more eutrophic with time, and may eventually develop into hypertrophic (overly productive) systems.

Lotic ecosystems differ from lakes and ponds in that currents are more of a factor and primary production inputs are generally external (allochthonous) instead of internal (autochthonous). Thus, a river or stream is considered a heterotrophic ecosystem along most of its length. Gradients in physical and chemical parameters also tend to be more horizontal than vertical in running water habitats and organisms living in lotic ecosystems are specially adapted for surviving in these conditions. For example, trout require higher amounts of dissolved oxygen , and are primarily found in relatively cold, fast-moving water with low nutrient input. Carp are able to tolerate warmer, slower, more productive bodies of running water. Darters are fish that quickly dart back and forth behind rocks in the bottom of fast-moving streams and rivers as they feed on aquatic insects. Many of these insects are shredders and detritivores on the organic material like leaves that enter the water. Other groups specialize by scraping algae and bacteria off rocks in the water.

Recently, ecologists have begun to take a greater interest in studying fresh water wetlands. These areas are defined as being inundated or saturated by surface or ground water for most of the year, and therefore having characteristic wetland vegetation. Although some people consider these areas a nuisance and prefer them being drained, ecologists realize that they are valuable habitats for migrating water fowl. They also serve as major "adsorptive" areas for nutrients, which are particularly useful around sewage lagoons. We must therefore take greater care in preserving these habitats.

[John Korstad ]

RESOURCES

BOOKS

Horne, A. J., and C. R. Goldman. Limnology. New York: McGraw Hill, 1994.

Hutchinson, G.E. A Treatise on Limnology. vol. 1: Geography, Physics, Chemistry ; vol. 2: Introduction to Lake Biology and Limnoplankton ; vol. 3: Limnological Botany ; vol. 4: The Zoobenthos. New York: Wiley, 1993.

Smith, R. L. Ecology and Field Biology. 4th ed. New York: Harper Collins, 1996.

Wetzel, R. Limnology. 2nd ed. Philadelphia: Saunders, 1983.