Benthic fluxes and nitrogen cycling in sediments of the continental margin of the eastern North Pacific

Abstract

The exchange of O2, N2, NO3−, NH4+, Si(OH)4, and PO4−3 between the sediments and the overlying water (benthic flux) was determined at 18 locations on the Washington State continental margin using an in situ benthic tripod. Oxygen consumption by the sediments ranged from 21.2 pmole cm−2 s−1 on the shelf to 2.85 pmole cm−2 s−1 on the slope. Nitrogen gas fluxes were from the sediments to the overlying water. They varied 5.5 to 1.2 pmole-N cm−2 s−1 and were always greater than the corresponding NO3− flux into the sediments. A nitrogen mass balance indicated that the difference between the N2 flux out and the NO3− flux in could be accounted for by oxidation of NH4+ produced during aerobic and anaerobic carbon remineralization to NO3− and subsequent denitrification to N2. Comparison of the benthic fluxes of O2, NO3− and Si(OH)4 with the fluxes predicted from molecular diffusion across the sediment water interface showed that for all three solutes the benthic fluxes were up to three times greater than the molecular fluxes and indicated the importance of macrobenthic irrigation in these sediments. However, several existing empirical irrigation models were not able to describe all three solutes. The overall carbon oxidation rate, as estimated from the sum of the O2 flux, the N2 flux and the measured SO4= reduction rate, could be fit with a normalized power function; i.e., carbon oxidation rate (gC m−2 y−1) = 110 · (z/100)−0.91. The exponent describing the rate of attenuation with depth (−0.91) was similar to the carbon rain rate attenuation coefficient determined from sediment traps in the pelagic, eastern North Pacific