Abstract: The microbial food web is a dynamic component of the ecosystem. Feeding interactions between microorganisms and their grazers range from broad and general to very specific. In streams and many rivers the microbial biomass and activity found in sediments greatly exceeds that of plankton communities. Sediment contaminant toxicity has been estimated by extending the equilibrium partitioning approach and plankton toxic criteria to the benthic habitat, estimating pore water concentrations from the octanol-water partitioning coefficient for the compound and the organic carbon content of the sediments. This approach assumes that organisms acquire toxicants from pore waters, and thus the pore water concentration of the contaminant in question is what determines toxicity and/or bioaccumulation. Recent research underlines the importance of considering sediment quality beyond organic carbon content when evaluating the availability of toxicants to the sediment and epibenthic biota. In addition, an appreciation of feeding relationships between bacteria, algae and invertebrate grazers provides further refinement to our understanding of how toxicants enter the food web.