The effects of a single freshwater release into the Kromme Estuary. 2: Microalgal response

The release of freshwater from the Mpofu Dam in November 1998 resulted in a short-term (6 d) longitudinal and vertical gradient of both salinity and nitrate in the estuary. There was no significant increase in phytoplankton chlorophyll a during the period of the study. The phytoplankton communities were dominated (> 10%) by diatoms and flagellates throughout the study. The average number of diatoms increased significantly from 301 cells·ml, prior to the release, to a maximum of 3 856 cells·ml by the end of the first spring tidal cycle. The average number of flagellates increased from 1 903 cells·ml, before the release, to a maximum of 3 300 cells·ml after two spring tidal cycles. There was no change in subtidal benthic chlorophyll a biomass but intertidal benthic chlorophyll a increased from 35.6 mg·m, before the release, to 63.3 mg·m by the sixth day. The conclusion is that the amount of freshwater released was insufficient to increase the nutrient content of the water to a level that resulted in a significant increase in primary productivity. The length of time that the freshwater influence was present also prevented a significant increase in microalgal growth.