Macrozoobenthic Diversity along an Oxygen Gradient in the Deep Trough of the Gulf of St. Lawrence (Canada)

Abstract

In 2015, we studied the macrozoobenthic community composition along a dissolved oxygen gradient in the deep trough of the Gulf of St. Lawrence (Canada). We sampled the seabed at nine stations using box corers (three replicates per station), starting in the outer Gulf and ending in the Lower St. Lawrence River Estuary. We found four different communities dominated by polychaetes, crustaceans, and molluscs, with the emphasis shifting from mollusc to polychaete communities as oxygen saturation decreased. Contrary to our expectations, the stations furthest upstream in the estuary with the lowest oxygen saturation levels had the highest species diversity, and also the highest density and biomass values. Key genera of the hypoxic zone included bivalves (Thyasira), cumaceans (Diastylis), amphipods (Harpinia), and polychaetes such as Ampharete, Ceratocephale, Galathowenia, and Trochochaeta. We attribute this to the stability of the environmental conditions and the absence of stress, where the constant supply of oxygen, even at low concentrations, seems to be more important than the absolute oxygen concentration.