Experimental manipulation of shade, silt, nutrients and salinity on the temperate reef sponge Cymbastela concentrica

Abstract

Discharge of sewage effluent into the ocean has been shown to cause changes in the structure and distribution of a range of biological assemblages, including those dominated by sponges. To date, the underlying mechanisms by which exposure to sewage alters such assemblages is unclear, although a number of potential models have been proposed. Here, a series of manipulative field experiments were done using the phototrophic sponge Cymbastela concentrica. Hypotheses from the general models that increased shade, silt, nutrients or salinity gradients were tested to find a cause for observed declines in populations exposed to sewage. Changes in the variables examined (i.e. growth and reproductive status of C. concentrica and concentrations of chl a associated with symbiotic micro-algae in C. concentrica) strongly supported the models showing that shading and siltation were a cause for decline. Nutrients did not affect any of the variables that were measured, whereas a decreasing salinity gradient caused a decline in growth, reproductive status and symbiotic algae (as measured by the concentration of chl a). This work makes a significant contribution to our understanding of the mechanisms that underpin the changes in patterns observed when sponges are exposed to physical factors associated with a sewage plume. © Inter-Research 2006.