Relationships between the biomass of waterfowl and submerged macrophytes in a South African estuarine lake system

The Wilderness Lakes system, comprising three estuarine lakes (Eilandvlei, Langvlei and Rondevlei), supports a diverse waterbird community, which includes 12 duck species and the abundant Red-knobbed Coot Fulica cristata. Biannual counts of waterfowl (ducks and Red-knobbed Coot) and assessments of submerged macrophyte standing crop, undertaken between 1992 and 2005, were used to investigate relationships between the biomass of macrophytes and waterfowl. Large changes in macrophyte biomass occurred during most of the study period in Eilandvlei (decreases) and Rondevlei (increases), whereas changes in Langvlei were small with no apparent long-term trends. Significant positive correlations existed between duck biomass and macrophyte biomass in Rondevlei and Eilandvlei, and between Red-knobbed Coot biomass and macrophyte biomass in Eilandvlei. Trends in the biomass of Red-knobbed Coot and macrophyte biomass were similar in Rondevlei and Langvlei, although statistically significant correlations could not be demonstrated. Management of the lakes should include provisions to maintain or restore water quality, which is adequate to sustain submerged macrophytes and the abundant waterfowl that they support.