Water quality status and phytoplankton composition in Soetendalvlei, Voëlvlei and Waskraalsvlei, three shallow wetlands on the Agulhas Plain, South Africa

Abstract

Three wetlands on the Agulhas Plain, for which no limnological information was available, were investigated in order to provide baseline data on their present water quality and phytoplankton community structure. Physicochemical variables were assessed and phytoplankton biomass and community analyses were conducted seasonally in 2007. The water in all wetlands was well mixed with no thermal stratification at the time of sampling. Soluble reactive phosphorus levels corresponded to the eutrophic to hypertrophic category. Only Voëlvlei showed very high microalgal chlorophyll a levels (>400 μg l^–1) due to the dominance of cyanobacteria. In the other wetlands, flagellates, chlorophytes and diatoms were the main microalgal groups. A high rainfall event in July–August 2007 considerably altered the phytoplankton community structure in Voëlvlei when cyanobacteria were completely replaced by chlorophytes. During August and November 2007 phytoplankton biomass was low in all the wetlands, and eutrophication signs were observed only in Voëlvlei. However, dense stands of both submerged and emergent macrophytes were present in two of the wetlands, the decomposition of which could lead to water quality changes. Voëlvlei had only large stands of emergent macrophytes (Phragmites and Schoenoplectus).

Keywords::

• biomass
• community
• cyanobacteria
• diatoms
• microalgae