The effects of a single freshwater release into the Kromme estuary Part 2: Microalgal response

Abstract

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-1, prior to the release, to a maximum of 3 856 cells·ml-1 by the end of the first spring tidal cycle. The average number of flagellates increased from 1 903 cells·ml-1, before the release, to a maximum of 3 300 cells·ml-1 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-2, before the release, to 63.3 mg·m-2 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.

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