This study assessed the influence of a freshwater pulse on selected physico-chemical and biological variables in a permanently open freshwater deprived southern African estuary. In the absence of the freshwater pulse a reverse gradient in salinity was evident with hypersaline (salinity > 40) conditions prevailing in the upper reaches of the estuary. Total chlorophyll-a (chl-a) concentration during this period ranged from 0.25 to 0.60μg·l-1. The mean total zooplankton abundance and biomass in the absence of freshwater during the daytime was 666 ind·m-3 (SD ± 196) and 12.4 mg·dwt·m-3 (SD± 3.3), respectively. During the night time the mean total zooplankton abundance was 3121 ind·m-3 (SD ± 1203) and the biomass 21.8 mg·dwt·m-3 (SD ± 196). The total zooplankton abundance during the dry season was numerically dominated by the copepod nauplii and the calanoid copepod, Pseudodiaptomus hessei, which contributed up to 76% of the total zooplankton counts. The freshwater pulse was associated with the establishment of a horizontal gradient in salinity along the length of the estuary and a significant increase in the total chl-a concentration (range from 0.74 to 11.75μg·l-1) and zooplankton biomass (range from 23.7 to 76.6 mg·dwt·m-3)(p < 0.05 in both cases). Additionally, there was a marked increase in the total zooplankton abundances and biomass within the estuary. A distinct shift in the zooplankton community composition was evident with the copepod, Acartia longipatella numerically dominating the zooplankton counts.
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