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SUMMARY
The usually natural closing and artificial opening of the Tow River mouth to the sea have a major influence on the physico-chemical environmental conditions in the coastal lake system formed by the Tow River, Eilandvlei, Langvlei and Rondevlei, as it affects the water-level and direction of water flow through the system. The lower reaches of the Tow River and the three coastal lakes are brackish and their open waters support an estuarine zooplankton fauna which, during 1976, was dominated in daytime mainly by lamellibranch veliger larvae (probably those of Musculus virgiliae), nauplius larvae of the copepods Acartia natalensis and Pseudodiaptomus hessei, and by the copepodites and adults of Acartia natalensis. Langvlei yielded the highest mean daytime zooplankton standing crop (17 mg dry mass m−3) for the year, followed by Rondevlei (15 mg m−3) and Eilandvlei (6 mg m−3), although the highest mean daytime numbers were obtained in Eilandvlei (14 641 organisms m−3). At night there was a general upward migration of zooplankton and benthic organisms in the water column, increasing the numbers in the surface layers considerably.