Benthic nutrient recycling on the northeastern shelf of the Gulf of Cádiz (SW Iberian Peninsula)

Abstract

Benthic fluxes of nutrients and dissolved nitrous oxide (N2O) were determined by ben- thic chamber incubations in 2 regions on the northeastern near-shore continental shelf of the Gulf of Cadiz (depth range: 8 to 34 m), one off the Guadalquivir River mouth and the other in the Bay of Cadiz and its neighboring shelf. In total, 25 in situ benthic chamber incubations were performed dur- ing June 2006, November 2006, and February 2007 to investigate the main factors controlling benthic nutrient recycling and its significance at the study site. Nutrient benthic fluxes (in m -2 d -1 ) varied between -0.9 and 1.0 mmol NO3 - , -0.3 and 0.3 mmol NO2 - , -1.2 and 3.5 mmol NH4 + , -0.04 and 0.22 mmol HPO4 2- , and 0.2 and 2.9 mmol Si(OH)4. Benthic fluxes of N2O, ranging from -1.2 to 10.4 µmol N-N2O m -2 d -1 , accounted on average for <1% of total inorganic nitrogen fluxes. The regeneration of NH4 + and Si(OH)4 was mainly influenced by organic carbon oxidation rates and bulk organic carbon in surface sediments, whereas HPO4 2- regeneration was mostly influenced by bottom water oxygen concentration and water depth. Denitrification was estimated to account for between 9 and 13% of organic matter remineralization and for a loss of between 57 and 67% of potentially recy- clable nitrogen. Overall, benthic remineralization was estimated to degrade about 47% of overlying primary production, supplying about 15 and 12% of the phytoplankton N and P requirements, respectively. This highlights the importance of benthic organic matter remineralization on the bio- geochemistry of the northeastern continental shelf of the Gulf of Cadiz.