Copepod succession in two South African estuaries

Abstract

The seasonal succession of copepod species was studied from November 1976 through October 1978 in the Swartkops and Sundays river estuaries. South Africa. Acartia natalensis appeared in the plankton in spring and reaches maximum abundance during summer and autumn. It was replaced by A. longipatella in late autumn which reached maximum abundance in winter and spring. Cycles of dominance are regulated by the interaction of temperature, salinity and competition between the two species. A natalensis a more tolerant of low salinity and the replacement of A. longipatella by A. natalensis starts in the upper estuary and spreads seawards. Maximum abundance of A. natalensis is attained in water of lower salinity than in the case of A. longipatella. In the Sundays estuary A. natalensis appeared briefly in the latter half of the study and in the upper and middle estuary only. In the lower estuary A. longipatella was present during all seasons. This was due to the absence of competition from A. natalensis, high salinity and lower summer temperatures due to marine influence. A third species of copepod, Pseudodiaptomus hessei functions as a pioneer species, exploiting “new water” after flooding or strong fresh water inflow. High abundance may therefore occur during any season and no competition between P. hessei and either species of Acartia was observed.