Benthic flux of biogenic elements on the Southeastern US continental shelf: influence of pore water advective transport and benthic microalgae

In situ, paired light and dark benthic flux chamber incubations were used t o estimate the exchange of nutrients, oxygen and inorganic carbon across th e sediment - water interface of the South Atlantic Eight (SAB) continental shelf. The results indicate that physically forced non-diffusive pore water transport and benthic primary production (EPP) by sea floor microalgae exe rt a major influence on benthic exchange rates on the mid- and outer-contin ental shelf (depths of 14-40 m). Light fluxes to the sea floor and sediment photosynthetic pigment distributions determined on two, widely spaced cros s-shelf transects suggest that BPP may occur over 84% of the SAB continenta l shelf area. Microalgal gross BPP rates at all study sites averaged 400 +/ - 260 mg C m(-2) d(-1) between May and September 1996 while water column pr imary productivity averaged 682 +/- 176 mg C m(-2) d(-1), implying a total primary productivity for this region of approximately 1100 mg C m(-2) d(-1) (1.6 times the water column productivity alone). The results are also cons istent with the advective transport of pore waters. Benthic flux chambers a ppear to retard this exchange, affecting the accuracy of derived net fluxes . Given our inability to relate pore water gradients to fluxes in non-diffu sive regimes and to mimic natural advective transport in intact core incuba tions, traditional techniques such as pore water gradient diffusion calcula tions or shipboard core incubations also may not provide accurate flux esti mates. Because of these limitations, fundamental questions remain concernin g the processes that control nutrient inventories in pore waters and the ma gnitude of the net benthic flux of nutrients on the sandy SAB shelf. (C) 19 99 Elsevier Science Ltd. All rights reserved.