ABSTRACT: In the Natal Bioregion of eastern South Africa, biomass of marine subtidal filter feeders is particularly high and makes a central contribution to distinguishing this bioregion from adjacent ones. We analysed the trophic role of riverine suspended particulate organic matter (POM) and the extent to which subsidies from rivers may explain this high filter-feeder biomass. Using carbon, nitrogen and sulphur isotopes, and a 3-end-member Bayesian-mixing model, we determined (1) the proportions of various primary producers contributing to the inshore POM pool and (2) the relative amounts of marine seaweed, pelagic POM and riverine POM assimilated by filter-feeder communities at various distances from 4 river mouths during austral summer and winter. Of the inshore POM pool available to filter feeders, riverine POM contributed 17 to 62%, pelagic POM 18 to 77% and seaweed 6 to 53%. The contributions of riverine POM to inshore POM declined significantly with distance from river mouth, but were unaffected by season or river size. Most material assimilated by filter feeders was of marine origin, notably seaweed detritus (39-62%), but with a noteworthy uptake of riverine POM (9-33%). Only small seasonal differences (<10%) and no biologically meaningful spatial trends were detected in the proportional assimilation of the 3 food sources by filter feeders. Although important, the trophic contribution of riverine POM may be subordinate to other factors such as turbidity and productivity in explaining the high biomass of filter feeders. Collectively, however, these river-associated factors are likely to explain the contrasts in trophic organisation among marine bioregions.