ABSTRACT: Nutrient addition experiments are one way of testing the importance of nutrients in ecological systems and can be undertaken relatively easily in soft sediments. We added nutrients (Osmocote^®) at 2 sites in Port Phillip Bay,Australia, which differed in proximity to sources of nutrient loading. Changes in faunal assemblages were assessed over an 18 wk experimental period in 1998. At the Sand Island site, nitrate levels were elevated in the high dose treatments and 3 taxashowed responses to nutrient additions, with differences in densities between control and dosed plots of between 50 and 600%. Assemblage-level measures of total abundance and diversity were approximately 20 to 80% greater in dosed plots relative tocontrols at the end of the experimental period. In contrast, at the more enriched Western Treatment Plant site, 3 of the 10 most dominant taxa were affected by the nutrient additions, with changes of between 50 and 200% observed during the course of theexperiment. At this site none of the assemblage level measures of abundance, biomass and diversity reflected an effect of the dose treatments. These results are broadly consistent with other studies that suggest that in areas of low ambient nutrientavailability diversity is likely to increase, while in areas with high background nutrient loadings there is likely to be a decrease. At Sand Island, changing the nutrient status of the area is liable to result in rapid changes in several deposit-feedingpopulations. At the Western Treatment Plant, the same absolute change in N may have less impact on the invertebrate fauna.