Zooplankton community structure and biomass in a southern African temporarily open/closed estuary

The zooplankton community structure and biomass in the upper, middle and lower reaches of the temporarily open/closed Kasouga estuary were investigated monthly over a period of one year. Multiple range tests performed after ANOVA indicated that there were no distinct spatial patterns in the physico-chemical and biological variables during the study (P > 0:05). Total chlorophyll-a (chl-a) concentration and zooplankton biomass during the investigation ranged between 0.91 and 5.92 mg chl-am 3 and between 19.51 and 103.50 mg Dwtm 3, respectively. The highest levels (O3 mg chl-am 3; O45 mg Dwtm 3) were consistently recorded during summer and the lowest (!2.5 mg chl-am 3; !40 mg Dwtm 3) during winter and during those periods when the estuary breached. Zooplankton community structure was strongly linked to the presence/absence of a link to the marine environment. In the absence of any link to the sea, the zooplankton community was almost entirely dominated by copepods (mainly Pseudodiaptomus hessei, Acartia longipatella and Halicyclops spp.), which comprised between 72 and 97% of the total zooplankton biomass. The inflow of seawater into the estuary following overtopping or breaching coincided with an increase in the contribution of mysids, amphipods and fish larvae to the total zooplankton biomass. Hierarchical cluster analysis identified three distinct zooplankton groupings during the study. These groupings corresponded to the summer and winter communities and a community associated with overtopping and breaching events. SIMPER analysis indicated that the differences between the summer and winter communities reflect changes in the relative contributions of the dominants as opposed to the presence/absence of species. The overtopping/breaching group was characterized by the presence of larvae of marine breeding invertebrates and fish. These data indicate that the zooplankton community structure and biomass in the temporarily open/closed Kasouga estuary are determined by the interactive effects of freshwater inflow, water temperature and mouth status.