Benthic nutrient regeneration and denitrification on the Washington continental shelf

Abstract

Benthic nutrient regeneration on the Washington continental shelf was investigated using vertical profiles of pore-water nutrient concentrations and whole sediment sulfate reduction rates. In August, carbon oxidation rates by sulfate reduction frequently exceeded those calculated from previously reported oxygen consumption rates. Total carbon oxidation rates averaged 18.9 pmol CO 2 cm −2 s −1, and using the 106 C: 16 N atomic ratio, 2.86 pmol N cm −2 s −1 would have been regenerated. Nutrient fluxes were calculated from pore-water profiles using diagenetic equations containing terms for vertical molecular diffusion, sedimentation, adsorption and macrobenthic irrigation which was estimated by modeling published radon distributions. In August, the calculated nitrate influx was 0.81 pmol N cm −2 s −1. The outward ammonium flux of 0.53 pmol N cm −2 s −1, calculated from pore-water ammonium profiles, was only 19% of the N regeneration rate. The missing ammonium could be accounted for by coupled nitrification and denitrification (1.16 pmol N cm −2 s −1) and by consumption of organic matter with high C:N ratios (9.8 by atoms). A comparison of annual regeneration and burial rates indicated that 84% of the deposited carbon is regenerated. For nitrogen, the annual outward ammonium flux was about half of the sum of the organic N burial rate and nitrate influx suggesting that these shelf sediments were a net sink for nitrogen. If the annual nitrate influx (0.57 pmol NO 3 − cm −2 s −1) to this shelf approximated that in other shelves, the global denitrification rate in continental shelf sediments wound be 69 Tg N y −1, a value similar to the oceanic water-column denitrifications rate.