Phytoplankton population dynamics of a small reservoir: physical/biological coupling and the time scales of community change

Abstract

This paper describes the day-to-day changes in species composition that resulted from intermittent wind mixing of the surface waters of a small reservoir. Two major scales of community change were detected: a short-term (>1 day) scale associated with the redistribution of cells within the basin, and a 5–14-day scale associated with growth responses. The physical scales of change were found to be almost identical to the biological scales: wind stress caused changes in the temperature gradient of surface waters at scales of a day, and major vertical mixing events occurred at scales of 10–14 days. The presence of buoyant species ensured that rapid advection of populations followed wind events. Community change was a function of both advection and growth, so that both real and apparent changes in abundance occurred. The observed seasonal succession was both a true succession and a changing sequence of populations dependent on horizontal advection of water within the basin. Consistent stratification was present throughout the summer period and phytoplankton diversity was low. Even so, the seasonal succession was best described as a series of allogenic perturbations followed by biological restructuring of the community. Daily sampling was necessary to document fully the mechanisms driving the seasonal succession of species.