Enhanced nutrient regeneration at commercial hard clam (Mercenaria mercenaria) beds and the role of macroalgae

High densities of bivalves found in aquaculture can exert 'top-down' control on primary production through feeding while simultaneously influencing local 'bottom-up' effects on production by enhancing nutrient recycling. Thus bivalves may decrease or increase localized eutrophication (sensu Nixon), depending on environmental conditions and specific culture practices. This study investigates hard clam aquaculture influence on benthic nutrient regeneration and metabolism, seasonally using in situ incubations. Effects of macroalgae, which proliferate on predator-exclusion nets at cultivation sites, are also investigated. Ammonium (NH4+) and phosphate effluxes averaged 154 and 100 times higher, respectively, at clam beds compared to reference sediments. Macroalgae decreased NH4+ efflux from clam beds by 20 to 77%, while having no significant effect on phosphate. Nutrient release from clam beds to the water column supports macroalgal growth, supplying nitrogen in excess of macroalgal demand in spring and fall and 58% of demand in summer, suggesting N recycling in the benthos is sufficient to support macroalgal production. As a bio-extractive practice, clam aquaculture is a net sink for nutrients in aquatic systems. However, our data suggest clam cultivation may influence eutrophication locally by facilitating increased macroalgal production due to increased benthic nutrient recycling. Given the high capacity for macroalgae to temporarily sequester nutrients released from the clam beds, macroalgal harvest may be an effective means to negate these effects of the clams and remove unwanted nutrients from the ecosystem.