ABSTRACT: This study investigated the impacts of nutrient enrichment on intertidal biofilm microbial communities and on larval metamorphosis of invertebrates. We modulated the nutrient levels in the water column during biofilm development and examined the resulting biofilms for bacterial and diatom community composition using 2 DNA fingerprinting techniques and microscopic examination, respectively. Results of both terminal-restriction fragment length polymorphism (T-RFLP) and denaturing gradient gel electrophoresis (DGGE) analyses indicated that the nutrient enrichment treatment altered the biofilm bacterial communities relative to the control. Biofilms that had different microbial compositions were tested for their effects on larval metamorphosis of the polychaete Hydroides elegans, the barnacle Balanus amphitrite, and the slipper limpet Crepidula onyx. We showed that nutrient enrichment in the water column effectively changed the biofilm composition in ways that increased its inductiveness for larval metamorphosis of C. onyx and B. amphitrite, but not of H. elegans. This study highlights the apparent importance of nutrient enrichment in influencing the intertidal biofilm composition directly and the invertebrates indirectly.