Freshwater community consists of an array of organisms depending on the physico-chemical and biological characteristics of the freshwater environment.
Freshwater habitats are divided into two major categories, lotic (lotus = washed, or running water), and lentic (lenis = calm, or standing water) habitats.
Lotic habitats are those existing in relatively fast running streams, springs, rivers and brooks. Lentic habitats are represented by the lakes, ponds, and swamps.
The above classification of the freshwater environments is based on two conditions: currents and the ratio of the depth to surface area. Since lakes and ponds often contain currents or at least wave action and since streams often harbour quiet pools or calm backwaters, the difference between lotic and lentic waters is not very precise. However, temperature, light, currents, amount of respiratory gases, and concentration of biogenic salts are important limiting factors influencing the organisms of all freshwater habitats.
Lentic Community:
Lentic waters are generally divided into three zones or sub-habitats: littoral, limnetic, and pro-fundal. A small pond may consist entirely of littoral zone. However, a deep lake with an abruptly sloping basin may possess an extremely reduced littoral zone.
Lake Zonation:
The three major zones of a lake described as follows (Fig. 4.9).
(a) Littoral zone:
The littoral zone adjoins the shore (and is thus the home of rooted plants) and extends down to a point called the light compensation level, or the depth at which the rate of photosynthesis equals the rate of respiration. Within the littoral zone producers are of two main types: rooted or benthic plants, and phytoplankton (plant plankton) or floating green plants, which are mostly algae.
The littoral zone is the home of greater variety of consumers than are the other zones. The zooplankton (animal plankton) of the littoral zone is rather characteristic and differs from that of the limnetic zone in preponderance of heavier, less buoyant crustacea which often cling to plants or rest on the bottom when not actively moving their appendages. Important groups of littoral zooplankton are large, weak-swimming species of Daphia and Simocephalus, some species of copepods, many families of ostracods and some rotifers.
The nekton of littoral zone is often rich in species and numbers. Adult and larval diving beetles and various adult Hemipetra are conspicuous. Various Diptera larvae and pupae remain suspended in the water, often near the surface. Pond fish, frogs, turtles, and water snakes are almost exclusively the members of the littoral zone community. Tadpoles of the frogs are important primary consumers, feeding on algae and other plant material.
Periphyton of the littoral zone exhibits a zonation paralleling that of the rooted plants, but many species occur almost throughout the littoral zone. Among the periphyton forms, for example, pond snails, damselfly nymphs and climbing dragonfly nymphs, rotifers, flatworms, bryozoa, hydra, and midge larvae rest on, or are attached to stems and leaves of the plants.
Another group containing both primary and secondary consumers may be found resting or moving on the bottom or beneath silt or plant debris— for example, sprawling odonata nymphs (which have flattened rather than cylindrical bodies), crayfish, isopods, and certain mayfly nymphs. Descending more deeply into the bottom mud are burrowing odonata and ephemeroptera, clams, true worms, snails, chironomids (midges), and other diptera larvae.
(b) Limnetic Zone:
The limnetic zone includes all the waters beyond the littoral zone and down to the light compensation level. The limnetic zone derives its oxygen content from the photosynthetic activity of phytoplankton and from the atmosphere immediately over the lake’s surface. The atmospheric source of oxygen becomes significant primarily when there is some surface disturbance of water caused by wind action or human activity. The community of the limnetic zone is composed only of plankton, nekton, and sometimes neuston (organisms resting or swimming on the surface).
Phytoplankton producers consist of diatoms, green algae, blue- green algae, and algae- like green flagellates, chiefly the dinoflagellates. The limnetic zooplankton consists of few species but the number of individuals may be large. Copepods, cladocerans, and rotifers are generally of first importance; but their species are largely different from those found in the littoral zone. The limnetic nekton consists almost entirely offish. In ponds, the fish of the limnetic zone are the same as those of the littoral zone, but in large bodies of water a few species may be restricted to the limnetic zone.
(C) Profundal Zone:
The bottom and deep water area of a lake, which is beyond the depth of effective light penetration is called the pro-fundal zone. In north-temperate latitudes, where winters are long and severe, this zone has the warmest water (4°C) in the lake in winter and coldest water in summer.
The major community consists of bacteria and fungi and three groups of animal consumers:
(a) Blood worms, or haemoglobin containing chironomid larvae and annelids,
(b) Small clams, and
(c) Phantom larvae, or Chaoborus (corethra).
The first two groups are benthic forms, the last are plankton that regularly move up into the limnetic zone at night and down to the bottom during the day. All the animals of the pro-fundal zone are adapted to withstand periods of low oxygen concentration, whereas many bacteria are anaerobic. Large numbers of bacteria in the bottom ooze constantly bring about decomposition of the organic matter (plant debris, animal remains, and excreta) that accumulates on the bottom.
Eventually the organic sediments are mineralized and nitrogen and phosphorus are put back into circulation in the form of soluble salts. In this way, the pro-fundal zone provides rejuvenated nutrients, which are carried by currents and swimming animals to other zones.