Benthic metabolism and nutrient regeneration in hydrographically different regions on the inner continental shelf of Southern New England

We examined the effect of hydrography on benthic-pelagic coupling in a transitional inner continental shelf area. From Oct 2009 to Jul 2012, we measured sediment oxygen demand (SOD), benthic inorganic nutrient fluxes, and sediment characteristics (e.g. chl a and phaeopigment content, grain size, etc.) in two regions of the Southern New England continental shelf: a relatively well mixed ecosystem (Block Island Sound, BIS), and an adjacent, seasonally stratified ecosystem (Rhode Island Sound, RIS). Despite a higher rate of euphotic zone primary production in BIS, benthic metabolism (measured as SOD) was not significantly different between the two areas (BIS = 953.8 ± 88.2 μmol m−2 h−1; RIS = 912.2 ± 69.1 μmol m−2 h−1). We speculate that the similarity of SOD at these two sites was due to differences in water column hydrography between the Sounds, where the energetic water column mixing in BIS could potentially resuspend organic matter back to the water column to be decomposed before reaching the benthos. Additionally, we think that the seasonal presence of a strong pycnocline in RIS prevented mixing of regenerated DIN and DIP to surface waters for use by phytoplankton. Apparent differences in benthic macrofaunal abundance between Block Island Sound and Rhode Island Sound translated to differences in dissolved inorganic nutrient fluxes between the two areas. Excretion and irrigation activities by the dense amphipod communities in BIS likely caused higher effluxes of DIN (NH4+ = 36.9 ± 7.7 μmol m−2 h−1; NOX = 23.5 ± 3.4 μmol m−2 h−1) and DIP (7.2 ± 1.4 μmol m−2 h−1) compared to fluxes in RIS (NH4+ = 22.8 ± 4.5 μmol m−2 h−1; NOX = 11.1 ± 5.5 μmol m−2 h−1; DIP = 3.2 ± 0.8 μmol m−2 h−1). These findings indicate that the hydrographic regime of the water column may exert a strong influence on benthic-pelagic coupling dynamics on the Southern New England shelf and in other inner continental shelf ecosystems.