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 spongeCymbastela 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.