The importance of the river-estuary interface (REI) zone in estuaries

Abstract

A multidisciplinary and multi-institutional research programme studied the influence of river flow rate on salinity distribution and response of the biota in three Eastern Cape estuaries, South Africa. Elevated flow rates increased the size of the river estuary interface (REI) zone (< 10 g·l-1) both longitudinally and in volume. In the Gamtoos Estuary, a flow rate of between 0.8 and 1.2 m3·s-1 produced a maximum phytoplankton biomass, both as concentration and total within the estuary. There was no clear relationship between measurable mineral nutrient content in the water and phytoplankton biomass, presumably because minerals are taken up rapidly by microalgae and are therefore not reflected in the water analyses. The rate of water flow and the size of the REI were shown to affect the distribution of invertebrates. Pelagic and benthic invertebrates showed distinct species assemblages along the longitudinal salinity gradient, with filter-feeding forms dominating the benthic community in the REI region. The effect of the REI zone on fish was examined in the freshwater-rich Great Fish Estuary and in the freshwater-deprived Kariega Estuary. Estuarine associated fishes responded strongly to river flow in the Great Fish Estuary but a number of these taxa were limited or absent from the Kariega Estuary. These findings are discussed in relation to the determination of the quantity and quality of water required to sustain ecologically sound estuarine ecosystems in terms of the South African National Water Act (36) of 1998. WaterSA Vol.28(3) 2002: 271-280