By Ron Divelbiss
Lakes and ponds are complex and delicate ecosystems, where balances change with the seasons, climatic variations, and water levels. Pollution can destroy these fragile balances all too easily. Sewage and fertilizers artificially boost nutrient levels, thus encouraging blue-green algae growth, ultimately leading to oxygen starvation and the death of many life-forms.
Lakes grow old and die because they eventually fill up with sediments and wastes. The lifetime of a lake is controlled by its origin, size, and shape, together with the climate and, most importantly, the area drained by the lake (its watershed). The natural aging process that affects lakes is called eutrophication. Although the process is gradual and continuous, it can be divided into stages according to the amount of nourishment the lake offers any potential life. At first, when the lake is "young," there is oligotrophy, when nourishment is sparse; next, mesotrophy, when there is moderate nourishment; and finally eutrophy, or abundant nourishment.
Young lakes have, at all depths, clear oxygenated water that is poor in the nutrients essential for plants to grow, such as nitrogen and phosphorus. This lack of fertility limits the growth of phytoplankton, the tiny plant organisms that form the basis of the food chain. As a result, relatively few plants and animals grow and live here. Rapid replacement of the water in a lake slows eutrophication, because the discharged water carries the phytoplankton away with it.
Algae and phytoplankton require 21 different elements in order to flourish. In general, small amounts of these elements are carried into the lake by rain and snow, but the main source is the lake's watershed. As a lake becomes older, its fertility increases at an accelerating rate. In nutrient rich lakes, plants grow luxuriantly near the shore; mats of algae cover the surface in green slime. As the plants die, season after season, the lake becomes smaller and smaller. Eventually the lake turns into a marsh, which in turn may become a bog and then a meadow.
A lake has three main zones: a pelagic zone (deep, open water in the center), a littoral zone (the area on the gently sloping sides), and a benthic zone (the bottom below the littoral zone). Each zone has a different variety of life-forms, often overlapping each other. Ultimately, all depend on photosynthetic algae growth.
Large algae and plankton colonies grow in the still, pelagic water. Most plankton eat algae, including some that rapidly multiply in sunny weather, when algae is abundant. Plankton are a vital food link between algae and larger creatures; growth rates of perch closely correlate to sunshine and plentiful plankton (their food). A lake's waters provide birds and fish with all their food. Lakes are also a night refuge for birds from land predators such as raccoons and skunks.
Benthic animals living on or in the mud mostly eat organic debris from above, though there are also benthic algae. Larger animals include worms, larvae, and sometimes mollusks. Phantom midges live by day on the bottom, but feed on the surface at night, when they are invisible in the dark. Animals at depths have breathing systems to cope with low oxygen levels. As a lake ages, it grows more fertile and its ecosystem changes, favoring species that can cope with less oxygen and more congested conditions.
Large plants (macrophytes) in the littoral zone help create the lake's most complex animal community. Cattails give shelter from wind and waves, and trap sediment, home for fragile mud organisms. Slime made of algae and other microorganisms adheres to underwater plant parts, and is a major food source of pond snails. Underwater plants give cover to crayfish and other crustaceans.
The water surface hosts many insects, including pond striders, which use surface tension to avoid sinking. Some air-breathing insects and spiders store air to dive in the water. Further down, various nymphs and larvae extract oxygen from the water. The sunfish is one of many littoral fish; its feeding creates plant debris subsequently eaten by invertebrates. The camouflaged bass ambushes prey, starting with larvae in its first year and graduating to tadpoles, young fish, and finally large fish, birds, or frogs.