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SUMMARY
Swartvlei is an ectogenic meromictic coastal lake connected to the sea via an estuary which closes periodically due to the formation of a sand bar across its mouth. The inflow of high-salinity water water from the sea into the lake while the mouth is open has a marked effect on the distribution of the estuarine zooplankton in the lake. There appear to be two zooplankton communities in the lake during daytime, namely a mixolimnion community occurring mainly under aerobic conditions, and a monimolimnion community occurring mostly under anaerobic conditions and in the presence of H2S. The mixolimnion community is dominated by the copepod Acartia natalensis, lamellibranch veligers (probably those of Musculus virgiliae) and calanoid nauplii, whilst the monimolimnion community is dominated by cyclopoid copepods (mainly Halicyclops species), cyclopoid nauplii, and polychaete larvae and juveniles. The mixolimnion zooplankton organisms concentrate in the halocline during daytime, where they probably feed mostly on detritus and bacteria. At night a general upward migration of the more abundant zooplankton organisms was observed. Marine zooplankton was encountered in the lake and estuary when the system was open to the sea.