While the effects of abiotic parameters on microbial tidal biofilms are relatively well-documented, the effects of grazing and/or bioturbation by meiofauna are poorly understood. We investigated the impact of a natural nematode assemblage on the biomass and microbial community structure of a multispecies diatom biofilm. Nematodes stimulated diatom biomass accumulation of the biofilm and caused a shift in diatom community structure. Higher diatom biomass accumulation in the presence of nematodes could be the result of increased diatom biomass production through nutrient regeneration resulting from grazing or bioturbation, and/or through shifts in interspecific interactions between diatoms (e.g. competition) through selective grazing. Alternatively, lower biomass in the controls may be due to higher secretion of diatom production in the form of bound extracellular polymeric substances (EPS). Our observations underscore that meiobenthos, and especially nematodes, are important for the structure and production of tidal biofilms.