Influence of suspended scallop cages and mussel lines on pelagic and benthic biogeochemical fluxes in Havre-aux-Maisons Lagoon, Îles-de-la-Madeleine (Quebec, Canada)

An in situ experiment was done in July 2004 to test and compare the influence of suspended bivalve cultures (1- and 2-year-old blue mussels (Mytilus edulis) and sea scallops (Placopecten magellanicus)) on biogeochemical fluxes in the water column and at the benthic interface in Havre-aux-Maisons Lagoon (Quebec, Canada). Aquaculture structures increased the pelagic macrofaunal biomass (PMB) and acted as an oxygen sink and nutrient source in the water column under dark conditions. Although PMB was lower in scallop culture, the influence of scallop cages on pelagic fluxes was similar to or greater (nitrate and nitrite) than that of mussel lines. Sediments were organically enriched, and benthic macrofaunal abundances were decreased in mussel culture zones relative to the control zone, but such an effect was not observed in the scallop zone. Nevertheless, benthic oxygen demand did not vary among culture types and control zones. Benthic nutrient fluxes were greatest beneath aquaculture structures. Both pelagic and benthic interfaces may modify oxygen and nutrient pools in culture zones in Havre-aux-Maisons Lagoon. The contribution of aquaculture structures to oxygen, ammonium, and phosphate pools may be a function of PMB and type. While aquaculture structures had an important role on nitrate and nitrite cycling, silicate turnover was mainly driven by benthic mineralization of biodeposits.