EFFECTS OF FLOW AND SESTON AVAILABILITY ON SCOPE FOR GROWTH OF BENTHIC SUSPENSION-FEEDING INVERTEBRATES FROM THE GULF OF MAINE

Metridium senile, the frilled sea anemone, and Modiolus modiolus, the northern horse mussel, are important members of benthic suspension-fee ding assemblages at several rocky hard-bottom subtidal (30-35 m) sites in the Gulf of Maine. Measurements of food availability, rates of foo d capture, absorption efficiencies, and standard metabolic costs show that inshore populations of Metridium senile have a significantly lowe r scope for growth than offshore populations, despite higher mean conc entrations of particulate organic matter inshore. Similar measurements and calculations for Modiolus modiolus reveal the opposite pattern. T hese differences persisted at both of these sites during two summers, 1989 and 1990, when differences in mean temperature were not physiolog ically significant Thus temperature is precluded as the primary effect on metabolism and growth. We suggest that these physiological differe nces reflect a response to now regime and food availability that appea rs to be manifested differently for Metridium senile and for Modiolus modiolus, a passive and an active suspension feeder, respectively. Res ults from a reciprocal transplant experiment, to measure growth rates, carried out over a one-year period support the calculated scope for g rowth during the season when maximum growth rates would be expected. T he flux of seston appears to be an important factor affecting the orga nismal performance of the passive suspension feeder (M. senile), where as the concentration of seston is more important for the active suspen sion feeder (M. modiolus).