Response of the Plankton to a Fresh Water Pulse in a Fresh Water Deprived, Permanently Open South African Estuary

Pierre William Froneman, Paul Denzi Vorwerk

Coastal Research Group, Department of Zoology and Entomology, Rhodes University, Grahamstown, South Africa.
South African Environmental Observation Network (SAEON) Elwandle Node, Grahamstown, South Africa.

DOI: 10.4236/jwarp.2013.54040                

Abstract

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 0.60 μg·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 11.75 μg·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.

Keywords

Estuary Zooplankton Community; Freshwater Inflow

Conflicts of Interest

The authors declare no conflicts of interest.

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