Response of zooplankton communities to altered water quality and seasonal flow changes in selected river dominated estuaries in KwaZulu-Natal, South Africa.

Abstract

Globally, estuaries are among the most productive ecosystems with many threatened by anthropogenic activities. Zooplankton is a bioindicator of ecosystem integrity. The spatial and temporal composition of zooplankton communities were quantified and compared within and between three estuaries (uMvoti, Thukela and aMatikulu/Nyoni estuaries) with different levels of human pressure in KwaZulu-Natal, South Africa. Additional effects of some physico-chemical variables and seasonal flow patterns to zooplankton community structuring were analyzed. The aMatikulu/Nyoni Estuary was selected as a reference site due to its good ecological state. Sampling dates represented high flow (March and April) and low flow (August and September) from 2014 to 2016. Following aMatikulu/Nyoni, highest abundance was recorded in Thukela and then uMvoti Estuary with copepod Pseudodiaptomus hessei and Acartia natalensis dominating the three estuaries. Highest abundance was recorded during low flow in the uMvoti and Thukela estuaries. Redundancy analysis revealed higher salinity and oxygen as environmental determinants of zooplankton community structure in the aMatikulu/Nyoni while turbidity and pH were the determinants of zooplankton community structures in uMvoti and Thukela estuaries. Elevated concentrations of DIN in the Thukela Estuary during high flow identifies the Thukela River as an important source of nitrogen to this estuary. Our findings suggest that these estuaries be managed to ensure sufficient freshwater supply which controls primary production. Although the three estuaries were from the same biogeographical region with a similar river dominated function, high variability in their zooplankton communities could be explained by differing water quality due to differing human pressure in their catchments.