The Influence of Freshwater Inflow on the Nature, Amount and Distribution of Seston in Estuaries of the Eastern Cape, South Africa

The influence of freshwater inflow in determining the particulate food resources for suspension-feeders in eastern Cape estuaries was investigated. The seasonal and spatial distribution of total particulate material (TPM), total particulate organic material (TPOM), chlorophylla, particulate protein and carbohydrate were compared in two hydrodynamically contrasting systems, the Kariega and the Great Fish estuaries, which differ markedly in respect of their freshwater inflow. As a result of high, sustained freshwater inflow, seston levels in the Great Fish estuary were of an order of magnitude higher than in the Kariega estuary which received minimal inflow. Chlorophyllameasurements showed that the Great Fish estuary is predominantly eutrophic/mesotrophic whereas the Kariega estuary is classified as oligotrophic. Correlation analyses indicated that the organic fraction of the seston was dominated by detritus in the Kariega estuary and by phytoplankton in the Great Fish estuary. Although the quality of organic seston was higher in the Great Fish estuary, the ‘ masking ’ effect of suspended inorganic particles associated with freshwater inflow was much greater. In both estuaries, the majority of seston occurred in the nanoplanktonic size range. The seasonal and spatial patterns of seston distribution differed markedly under conditions of contrasting freshwater inflow. Suspended particulates were more uniformly distributed along the length of the Kariega estuary and were seasonally elevated during the warmer months. By contrast, a pronounced spatial gradient was evident in the Great Fish estuary with highest values recorded towards the upper reaches. The nutrient status of the Great Fish estuary was considerably higher than that of the Kariega estuary and determined principally by freshwater inflow. High phytoplankton standing stocks generally developed episodically following high freshwater inflow. This was partly the result of an accumulation of phytoplankton of riverine origin by hydrodynamic trapping, however, species tolerant of brackish conditions responded to the enriched conditions by remaining viable and active for considerable periods within the estuary.