The biotic community in the streams and rivers is quite different from that in the ponds and lakes.
The differences in the community are largely due to differences in the physical and chemical conditions of their environments.
Important basic characteristics of a stream are as follows:
(a) Current:
The presence of definite and continuous current is the main characteristic of a stream. The current velocity, which may vary from 0.5 to 2 meter per second or more, depends on the stream gradient. The greater the discharge (volume/time), the greater the current velocity (distance/time) and also the amount of suspended material that is transported.
The rate of flow, in turn, influences a number of physical and chemical factors such as the temperature and dissolved oxygen concentration, which act directly on the biota. The current is a major limiting factor in streams. It might be assumed that plankton would be absent from fast- flowing streams, since such organisms are largely at the mercy of the current.
Plankton in small streams, if present, originates in lakes, ponds, or backwaters connected with streams and is soon destroyed as it passes through rapids of streams. Only in slow moving parts of streams and in large rivers, plankton is able to multiply and become an integral part of the community.
(b) Land-Water Interchange:
Streams are usually relatively shallow and, therefore, have a large surface compared to their depth. Land-water interchange is relatively more extensive in streams, resulting in a more open ecosystem. This means that the streams are more intimately connected with the surrounding land than are most lakes and ponds. For this reason a significant portion of the stream’s nutrients falls into them from their banks in the form of terrestrial leaves, grasses and other debris.
In fact many of the primary consumers living in a stream feed on detritus and allochthonous material (i.e., outside the system) rather than on green aquatic vegetation (autochthonous production). The producers of a stream can supply only a fraction of the energy required by its animal consumers.
(c) Oxygen:
The third important characteristic of a stream, which is also a difference between a stream and a lake, is the high amount of dissolved oxygen. The reasons for this are many: flowing water, the relative shallowness of the ecosystem, and the large surface area exposed to the atmosphere. For these reasons the waters of a shallow, fast-moving stream will have higher oxygen levels than deep, sluggish rivers. In a stream the photosynthetic production of oxygen is not as important as it may be in a lake or pond.
Because of turbulence, the stream water is usually well aerated and problem of oxygen depletion, as seen in the hypolimnion of a deep lake, is almost absent. But stream animals are very sensitive to decreases in the oxygen content of the water. If a stream becomes polluted with excessive load of oxygen-demanding organic matter, such as the domestic sewage or the industrial waste, the resulting oxygen-depletion may cause serious problems such as a massive fish kill.