The environmental management of anthropogenic inputs of organic materials to the sea requires a knowledge of the effects of different intensities and frequencies of input in relation to the nature of the receiving assemblages of organisms. A microcosm experiment was carried out to study whether the response of nematode communities to 3 different quantities of organic matter (high/medium/low) given once at the beginning of the experiment differs from the response to the same quantities given in several smaller doses during the experimental period. Communities from an organic-poor sandy estuary and an organic-rich muddy estuary were compared. Marked changes in community structure were revealed, depending on the total quantity of organic matter, the number of doses being administered during the experimental period and the environment where the communities had developed naturally. In the sand microcosms most univariate measures, including diversity and species richness, decreased significantly with increasing level of organic enrichment, whereas evenness increased in some cases. Nematode assemblages from the muddy estuary remained unaffected in the low additions; medium and high additions of organic matter resulted in significant decreases of most univariate measures. Results from multivariate analyses of the species abundance data revealed significant differences between undisturbed controls and organically enriched treatments for both nematode assemblages. In the sand microcosms, characteristic nematode communities developed which reflected different levels of organic enrichment. Changes in community structure were not as extreme in the mud microcosms. An increasing amount of organic matter led to a decrease of diversity caused by declining abundances of dominant nematode species. Both univariate and multivariate analyses of the data showed that the same amount of organic matter administered in many small doses had a milder effect on community structure than when administered in fewer but larger doses.