Metridium senile, the frilled sea anemone, and Modiolus modiolus, the northern horse mussel, are important members of benthic suspension-feeding assemblages at several rocky hard-bottom subtidal (30-35 m) sites in the Gulf of Maine. Measurements of food availability, rates of food capture, absorption efficiencies, and standard metabolic costs show that inshore populations of Metridium senile have a significantly lower scope for growth than offshore populations, despite higher mean concentrations of particulate organic matter inshore. Similar measurements and calculations for Modiolus modiolus reveal the opposite pattern. These differences persisted at both of these sites during two summers, 1989 and 1990, when differences in mean temperature were not physiologically significant. Thus temperature is precluded as the primary effect on metabolism and growth. We suggest that these physiological differences reflect a response to flow regime and food availability that appears to be manifested differently for Metridium senile and for Modiolus modiolus, a passive and an active suspension feeder, respectively. Results from a reciprocal transplant experiment, to measure growth rates, carried out over a one-year period support the calculated scope for growth during the season when maximum growth rates would be expected. The flux of seston appears to be an important factor affecting the organismal performance of the passive suspension feeder (M. senile), whereas the concentration of seston is more important for the active suspension feeder (M. modiolus).