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Abstract
The Berg Estuary is permanently open in the Mediterranean-type climate of the Western Cape Province, South Africa. Construction of the Berg River Dam was completed in 2007 and is expected to influence the quantity and quality of river water entering the estuary at the head. This study examined the distribution of phytoplanktonic and benthic microalgal communities throughout the estuary and the results are used to predict changes in the microalgal biomass in response to reduced flow. Salinity profiles measured in August and November 2005 indicated distinct differences in flow between winter and summer. River water was the greatest source of nutrients in August and previous research has shown that the source switches to marine water during periods of low flow. Low residence time, as a result of strong river and tidal flow, appeared to be the major factor preventing phytoplankton chlorophyll a (chl a) from exceeding 8 μg/1. Benthic ch1 a was significantly higher in August compared to November and the pattern of distribution was dissimilar to other South African estuaries. Flagellates were the dominant group of phytoplankton at most times during the study. Particularly high densities of phytoplankton were present 10.1 km from the mouth in August, dominated by flagellates (77% of total cell numbers) and 28 km from the mouth in November, dominated by diatoms (86% of total cell numbers). The predicted response of microalgae to reduced river inputs is discussed in detail. Two sampling sessions, coinciding with low- and high-flow conditions as outlined in the intermediate Resource Directed Measures (RDM) procedure of the Department of Water Affairs and Forestry (DWAF), were insufficient to accurately determine changes in microalgal biomass and community structure. Instead, predictions had to be made based on data from other South African estuaries.