A comparative study of zooplankton dynamics in two subtropical temporarily open/closed estuaries, South Africa |
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Authors: | I Kibirige R Perissinotto X Thwala |
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Institution: | (1) School of Biological and Conservation Sciences, University of KwaZulu-Natal, Howard College Campus, G. Campbell Building, 4041 Durban, South Africa;(2) Present address: School of Education, Discipline of Mathematics, Science and Technology Education (DMSTE) Old-R Building, University of Limpopo, Private Bag X1106, Sovenga, South Africa |
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Abstract: | This study aims at comparing the zooplankton abundance/biomass of two South African estuaries affected to a different degree
by sewage pollution. In particular the hypothesis that microzooplankton may increase relative to mesozooplankton, as eutrophication
increases, is tested for the first time in African estuaries. The survey was carried out during a whole-year cycle, including
both open and closed phases. Results show that zooplankton abundance and biomass are higher in the Mdloti than in the Mhlanga,
during both the closed (U=1, P<0.001, U=16, P<0.001) and the open phase (U=101, P<0.05, U=88, P<0.01). There were no significant differences in abundance/biomass between the different reaches of each estuary, during either
their open or closed phase (1-way ANOVA, P>0.05). The different levels of dominance of the calanoid copepod Pseudodiaptomus hessei, which accounted for 53–64 and 86–97% of the total abundance in the Mdloti and the Mhlanga, respectively, suggests a shift
in the zooplankton community structure. There was a lower microzooplankton abundance/biomass contribution to the total zooplankton
in the Mhlanga (59.6–15.8%), compared to the Mdloti (99.1–96.5%). The highest microzooplankton contribution in the Mdloti
was observed during the closed phase, while in the Mhlanga this occurred during the open phase. This suggests that eutrophication
in the Mhlanga may impact negatively on the microzooplankton community. On the other hand the less affected Mdloti exhibits
an opposite trend with an increased microzooplankton component. |
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