COMMUNITY METABOLISM AND NUTRIENT CYCLING AT GRAYS REEF, A HARD BOTTOM HABITAT IN THE GEORGIA BIGHT

Benthic and pelagic metabolism and nutrient fluxes were measured during summer on a hard bottom on the continental shelf of the Georgia Bight, USA. Internal pools of organic matter and nutrients in the sediments and water column were also measured and compared with physical transport associated with ocean currents. Gray's Reef is a heterogeneous system consisting of a mosaic of bare sand regions, hard bottom regions thinly veneered with shifting sands, and exposed rock outcrops. Sediment organic carbon content increased from < 0.2 % of dry weight in sandy regions to an epifaunal biomass > 77 g C m-2 on rock outcrops. Sponges and corals accounted for a large percentage of macrofaunal biomass when sands were shallow. Benthic metabolism and nutrient regeneration were positively related to the spatial distribution of epifaunal biomass. In regions of high epifaunal biomass respiration exceeded 133 mg C m-2 d-1 and nutrient flux amounted to 23 mmol inorganic N, 1.8 mmol inorganic P, and 24 and 0.4 mmol m-2 d-1 dissolved organic N and P, respectively. The benthic community of Gray's Reef was markedly heterotrophic, consuming almost twice as much organic matter as was produced on the bottom (production:respiration = 0.52:1). Community respiration in the water column exceeded by ca 30 % a fairly high level of pelagic primary production of 2.15 g C m-2 d-1. The overall Gray's Reef system was heterotrophic and strongly dependent on allochthonous organic carbon for support of almost one third of its total respiratory requirements (production:respiration = 0.68:1). A large percentage (> 50 %) of system biomass and respiration was attributable to filter-feeding organisms which are largely ungrazed, including corals and sponges. A net effect of this hard bottom system was the capture and removal of organic matter produced in the water column. As most of the macrofauna is ungrazed, this organic matter becomes unavailable for support of food chains leading to the production of commercially important fishes. We present some thoughts on the relationship between hard bottoms, in general, and the anomalously low level of fishery production in the Georgia Bight as a whole.