THE INFLUENCE OF FRESH-WATER 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 invest igated. The seasonal and spatial distribution of total particulate mat erial (TPM), total particulate organic material. (TPOM), chlorophyll a , particulate protein and carbohydrate were compared in two hydrodynam ically contrasting systems, the Kariega and the Great Fish estuaries, which differ markedly in respect of their freshwater inflow. As a resu lt of high, sustained freshwater inflow, seston levels in the Great Fi sh estuary were of an order of magnitude higher than in the Kariega es tuary which receives minimal inflow. Chlorophyll a measurements showed that the Great Fish estuary is predominantly eutrophic/mesotrophic wh ereas the Kariega estuary is classified as oligotrophic. Correlation a nalyses indicated that the organic fraction of the seston was dominate d by detritus in the Kariega estuary and by phytoplankton in the Great Fish estuary. Although the quality of organic seston was higher in th e Great Fish estuary, the 'masking' effect of suspended inorganic part icles associated with freshwater inflow was much greater. In both estu aries, the majority of seston occurred in the nanoplanktonic size rang e. The seasonal and spatial patterns of seston distribution differed m arkedly under conditions of contrasting freshwater inflow. Suspended p articulates were more uniformly distributed along the length of the Ka riega estuary and were seasonally elevated during the warmer months. B y contrast, a pronounced spatial gradient nas evident in the Great Fis h 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 episodi cally following high freshwater inflow. This was partly the result of an accumulation of phytoplankton of riverine origin by hydrodynamic tr apping, however, species tolerant of brackish conditions responded to the enriched conditions by remaining viable and active for considerabl e periods within the estuary.