Effects of prolonged elevated water salinity on submerged macrophyte and waterbird communities in Swartvlei Lake, South Africa

Abstract

Large fluxes in the physico-chemical characteristics of estuarine lakes can have profound effects on biota and processes. Where salinity in Swartvlei Lake usually ranges between 5 and 12, extended open conditions post–2007 floods, coupled with reduced freshwater inflows due to drought, resulted in salinity exceeding the upper tolerance of dominant submerged macrophytes. A resulting die-back of macrophytes equated to a 99% decline in standing crop, and was followed by a 95% decline in the biomass of waterbirds. Significant positive correlations exist between the biomass of macrophytes and both piscivorous and herbivorous waterbirds. Whereas Swartvlei Lake is expected to, in the short term, revert to its former (pre–2007 flood) state, inevitable environmental changes such as global warming and resulting changes in local climatic and marine conditions, along with increased freshwater abstraction from feeder rivers, could cause the observed large fluctuation in the abundance of aquatic biota to become more frequent.

Keywords: Swartvlei Lake, estuarine lake, drought, flood, macrophyte senescence, biotic interactions, environmental change, global warming