Rivers, streams and related environment are known as ‘lotic ecosystems’. Lotic ecosystems have been studied to a lesser extent as compared to ponds, lakes etc. which are known as ‘lentic ecosystems’. Streams and rivers are variable ranging in size from Ganga, Yamuna, Sutlej, Narmada, Gomti, Mahanadi, Krishna, Godavari, and Kaveri.
The water currents in streams and rivers are none of a controlling and limiting factor and velocity of a current varies greatly in different parts of the same stream. The stream forms an open ecosystem and in general oxygen tension is more uniform. It can be divided into two types, on the basis of the velocity and rate of flow of water body.
This is defined as the portion of the stream in which the flow of water is both rapid and turbulent. In this case physical parameters like shear force and rate of water movement tend to be quite different on top of a rock fragment, between rock fragments as well as beneath the rock fragments. The rarely occurring phytoplankton of rapid zone of streams include- blue green algae.
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Some animals of rapidly flowing water streams live among the mosses and flowering plants. Animals inhabiting these plants have a torpedo like body which enables them to offer minimum resistance to the current. If the current is sufficiently slow, certain swimming organisms such as fish will be present. The fishes of fast water ecosystems have small body, size and stream land body. Trout are capable of moving against the current or water fall by muscular effect. Majority of torrential fishes, however, live on among rocks and boulders. Over which water flows very surly.
The physico-chemical characteristics of fast water ecosystems resemble with cold and deep lakes. Temperature of water tends to be quite low. The productivity is quite low, but diversity is high. In the fast water ecosystem, the main control of productivity is the current, which limits the amount and type of autotrophs.
In this case the flow of water is slower and likely to be laminar. The power of the stream is greatly reduced and smaller sediments particles such as salt and decaying organic debris are deposited on the bottom. The streams have a higher temperature and accordingly, the plankton organisms and the protozoan’s occur in large number in this ecosystem.
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The life of the plant is abundant in slow water ecosystems. It includes rooted vascular plants such as ponds, weeds and grasses. Motile algae such as diatoms and flagellates may be in the open water. The primary productivity of slow water ecosystems is higher than that of the rapid water ecosystem and the community is relatively less dependent on food materials from outside.
In slow water streams, oxygen concentration remains main limiting factor. The high level of animal activity can withdraw a large amount of oxygen from water of a slowly moving stream. The low level of turbulent means that less oxygen is incorporated into the water at the surface. Thus, the dissolved oxygen content of a slowly moving stream is likely to be much lower than that of fast moving streams. Carps and cat fishes are the most common fishes in slow water while species with high oxygen demands such as trout are found in fast moving water.