THE INFLUENCE OF RIVER IMPOUNDMENT ON THE ESTUARINE SEAGRASS ZOSTERA-CAPENSIS SETCHELL

Photogrammatic methods spanning a 47-year period were used in conjunction with field studies to document the influence of altered catchment hydrology on the dynamics of Zostera capensis Setchell in the Kromme estuary, South Africa. Reduced freshwater inflow and associated lack of sedimentary disturbances, stable salinities and reduced turbidities are considered to have contributed to the increased Z. capensis population. Over the last nine years there has been a fourfold increase in the total standing biomass of Z. capensis. Biomass density (g m-2) increased by a factor of 2.4 and areal cover by a factor of 1.6. The effect of all floods smaller than the 1-in-30 year flood has been reduced by the construction of a recent dam, the Charlie Malan Dam, four km upstream of the estuary. A moderate flood in November 1989 failed to alter the Z. capensis population, in contrast with the 50% reduction recorded, following a flood of similar dimensions prior to the construction of the Charlie Malan Dam.