The Effect of Wind on the Distribution of Chlorophyll A and Crustacean Plankton in a Shallow Eutrophic Reservoir

Abstract

The horizontal and vertical distribution patterns of chlorophyll a and crustacean zooplankton in a shallow reservoir were studied in relation to wind-induced water movements. Flow measurements using drogues suggested that horizontal rotation currents were not well developed and that regions of downwelling water occurred downwind. Plankton distribution patterns could be explained in terms of an interaction between the vertical movement of the organisms and mechanical transport. When green algae or diatoms were dominant chlorophyll remained homogeneously distributed vertically and horizontally. Buoyant blue-green algae and positively phototactic crustacea tended to accumulate downwind. Local concentrations of blue-green algae only appeared at wind speeds below 400 cm/s, zooplankton patches were capable of forming even under high winds. Using the ratio of mean crowding to mean density (Lloyd 1967) as an index of vertical and horizontal heterogeneity, the formation of horizontal aggregations in both the blue-green algae and the zooplankton could be predicted from a knowledge of their vertical distribution. Some indication of blue-green algal patchiness could be obtained directly from wind data. The appearance of systematic concentration gradients in the blue-green algae provided a basis for a crude model of algal accumulation. A series of empirical relationships was derived and used to forecast conditions under which undesirable downwind accumulations of blue-green algae could occur in the reservoir.