Introduction

It is conventional to classify freshwater habitats as lakes, ponds, and streams. That will serve well here, because many of the specific problems being attacked are quite different, and many of the techniques used in conducting experiments in these three kinds of habitats are different. The separate conditions have resulted in experimental designs specific to the habitat, even when the problems have been much the same in principle.

In the case of bodies of water large enough to be considered lakes, most ecological thinking has concentrated on the plankton. This is the general term designating drifting algae and small animals whose vertical movements may be important, but whose horizontal movements are insignificant from the standpoint of their ecology. Although fish predation on zooplankton is important in many lakes, the ecology of the plankton frequently has been studied independent of the fish and other large animals in the lake, and a number of experiments were carried out on the assumption that the important external influence was the supply of nutrients to the system. The edges and bottoms of lakes have entered limnological thought largely through the bacterial mineralization of nutrients that arrive at the bottom in the form of dead organisms. These nutrients are recycled when the lake becomes isothermal and the water can be mixed by the wind. Mixing is largely prevented in summer because the upper level of water becomes warmer and lighter than the deeper water and tends to be circulated by the wind as the mostly independent epilimnion.