Importance of Aquatic Communities
Biological responses to changes in water quality are important because the aquatic community has the ability to:
(1) immediately change if lethal levels are exceeded,
(2) integrate or accumulate the effects of sublethal pollutants, and
(3) provide mechanisms for self-purification of the water system through biochemical and biological stabilization.
Also, we can gain valuable insight into the dynamic structure or ecological resiliency of various aquatic systems through observation of pollutant effects on the biological community. These approaches stress the pronounced interrelationships of the organisms to one another and to physiochemical features of the water system in which the plants and animals live. These interrelationships include harmful and beneficial interactions, as well as those involving only tolerance.
At equilibrium conditions, population density, the species composition or diversity of individuals can be used to characterize the stability of intricate relationships between food chains, predators and consumers. Further definition of the stability of the community can be expressed by the physiological conditions and metabolic rates of the organisms within the community.
As we can see, aquatic communities have evolved to be essentially self-sufficient and generally independent of other communities. However, within each community are found a number of minor communities.
Examples of these are: the benthos where the bottom organisms reside, the aufwuchs which are the organisms that are attached to or move upon a submersed substrate, and the water phase between the surface and the bottom that harbors the nekton and plankton.
Since each of these minor communities is surrounded by a characteristic set of physical-chemical environmental factors, each community will respond to environmental changes in its own way. Most of these responses will show up in the affected community as a reduction of species diversity, a shortening of the food chain and a decrease in community stability.
For a given aquatic habitat, a riffle community of an upland stream or an epilimnion community of a lake is composed of characteristic species populations. Each of these communities has gained individuality through evolutionary adaptation to conditions determined by the aquatic environment.
During evolution, mutual adjustment and harmonious interactions of the organisms have reached an equilibrium with the constraints of the environment. From this, we can see that a community possesses structural unity in so far as the species populations composing it are concerned.
However, this does not mean the community is static or fixed in times and space. Quite the contrary is the case. Community composition and organization change in response to annual, seasonal, and daily environmental cycles. Superimposed on these cycles are spatial and temporal variations in density of populations and the distribution of the individuals of a population.