Phytoplankton dynamics and their equilibrium phases in the Yanghe Reservoir, China

Abstract

Phytoplankton species composition, seasonal dynamics, and spatial distribution were studied during 2009 along with key physical and chemical variables in Yanghe Reservoir, a temperate eutrophic reservoir with a long water residence time of 284 days. In the northern part of the reservoir, Microcystis wesenbergii dominated throughout the summer but no steady-state phases were found, as periods of equilibrium lasted for only two weeks each. In the southern part, the first steady-state phase occurred in spring, lasted for four weeks, and was dominated by Cryptomonas erosa. The second phase was dominated by M. wesenbergii in late summer and persisted for four weeks. The results suggest that steady-state phases establish more readily in deep areas with longer residence times and more dominant species compared to shallow areas. Thermal stratification in the deep area and fluctuation of nitrogen and phosphorus concentrations in the shallow area were considered to be responsible for such differences. Canonical correspondence analysis indicated that M. wesenbergii was more strongly related to NOx–N than to temperature in both areas of the reservoir. We conclude that in eutrophic lakes, a high concentration of nutrients such as NOx–N is more important than temperature in the establishment of a steady state.