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 benthic chamber incubations in 2 regions on the northeastern near-shore continental shelf of the Gulf of Cádiz (depth range: 8 to 34 m), one off the Guadalquivir River mouth and the other in the Bay of Cádiz and its neighboring shelf. In total, 25 in situ benthic chamber incubations were performed during 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 HPO42–, 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 HPO42– 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 recyclable 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 biogeochemistry of the northeastern continental shelf of the Gulf of Cádiz.