Flowing Water (Lotic)
There have been a number of attempts to classify the longitudinal zones of a stream. Since specific ecological entities are not uniquely identifiable, a complete system has not been successfully designed.
Generally, however, researchers have divided the lotic ecosystem into the spring region, the spring brook, the fast stream to small river where the mean temperature rises to 20° C, and the slow-large rivers where the temperature rises to over 20° C and the bottom is mainly sand and mud.
The character of a stream is dominated by the velocity of the current. This velocity varies from stream to stream and within the stream itself. It also depends upon the size, shape and steepness of the stream channel, the roughness of the bottom, the depth and the rainfall.
The velocity of flow influences the degree of silt deposition and the nature of the bottom. Flowing water also transports nutrients and carries waste products away from many aquatic organisms and may even sweep them away.
Balancing this depletion of bottom fauna, the current continuously reintroduces bottom fauna from areas upstream. Similarly, as nutrients are washed downstream, more nutrients are carried in from above.
The transport and removal action of flowing water benefits such continuous processes as photosynthesis. Because of this, the productivity of primary producers in streams is 6 to 30 times more than producers in standing water.
The temperature of the small stream is not constant. In general, small shallow streams tend to follow, but lag behind, air temperatures. Streams with large areas exposed to direct sunlight are warmer than those shaded by trees and shrubs or with high steep banks. This is ecologically important because temperature affects the composition of the stream communities.
The constant swirling and churning of stream water over riffles and falls results in greater contact with the atmosphere, thus the oxygen content of the stream water is high at all levels and often is near saturation point for the existing temperature. Only in deep holes or in polluted waters does dissolved oxygen show any significant decline.
Other gases, free carbon dioxide, in the stream is in equilibrium with that of the atmosphere. Most carbon dioxide in flowing water occurs as carbonate and bicarbonate salts.
Streams that are fed by ground water from limestone springs receive the greatest amount of carbonates in solution. The amount of bound carbon dioxide is influenced by the nature of the surrounding terrain and decomposition taking place in the pools of still water.
Because of a coating of algae and ooze on the bottom, little calcium carbonate is added by the action of carbonic acid on limestone streambeds. The degree of acidity and alkalinity or pH of the water reflects the carbon dioxide content as well as the presence of organic acids and pollution.
Thus, the higher the pH of stream water, the richer the natural waters generally are in carbonates, bicarbonates and associated salts. Such streams support more abundant aquatic life and larger trout populations than streams with acid waters which are generally lower in nutrients.
Most streams receive a part of their basic energy supply from the land as well as from connected ponds and backwaters. Many of the first level consumer organisms in the streams are detritus feeders. Much of their food comes from terrestrial vegetation along the banks.