Underestimation of primary productivity on continental shelves: evidence from maximum size of extant surfclam (Spisula solidissima) populations

Atlantic surfclams (Spisula solidissima), among the largest extant non-symbiotic clam species in the world, live in dense aggregations along the Middle Atlantic Bight (MAB) continental shelf. The food resources that support these populations are poorly understood. An individual-based model that simulates the growth of post-settlement surfclams was used to investigate the quantity of food needed to maintain existing surfclam populations along the MAB continental shelf. Food inputs to the model were based on measured near-bottom water-column chlorophyll concentrations. Simulations showed that these water-column food sources supported only 65% of the observed body mass of a standard large surfclam (160-mm shell length). Additional simulations using benthic food sources to supplement water-column food sources by 20% generated surfclams that grew to observed size and biomass and exhibited spawn timing consistent with the known surfclam spawning season. The simulation results suggest that measured water-column chlorophyll concentrations may underestimate the food available to the continental shelf benthos. Large continental shelf bivalves are an essential resource for fisheries and higher trophic level consumers. Understanding available and utilized food resources is important for predicting long-term impacts of climate change on benthic secondary production and fishery yield on the continental shelf.