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

Benthic fluxes of dissolved oxygen, total alkalinity, inorganic carbon and methane were determined in situ by benthic chamber deployments at 9 stations (depth range: 8 to 34 m) on the northeastern near-shore continental shelf of the Gulf of Cádiz (SW Iberian Peninsula) during 3 cruise periods (June 2006, November 2006 and February 2007). Sediment oxygen uptake and dissolved inorganic carbon fluxes, which ranged respectively from 9 to 27 mmol O2 m–2 d–1 and from 18 to 58 mmol C m–2 d–1, showed a good linear correlation with the organic carbon content of surface sediments (range: 0.25 to 1.18% dry wt). Benthic alkalinity fluxes were corrected for the influence of NH4 and NO3 + NO2 fluxes, and the upper and lower limits for carbon oxidation rates (Cox) were constrained by considering 2 possible scenarios: maximum and minimum contribution of CaCO3 dissolution to corrected alkalinity fluxes. The averages of the calculated minimum and maximum Cox values ranged from 14 to 46 mmol C m–2 d–1. Benthic fluxes of methane were very variable, ranging from 0.9 to 24 μmol CH4 m–2 d–1, and represented only about 0.02% of the mineralized carbon. Two regions were differentiated by sediment properties and location: one off the Guadalquivir River mouth, which showed higher carbon remineralization, and another in the Bay of Cádiz and neighboring shelf. The mean carbon oxidation rate estimated for the entire study site was 28 ± 8 mmol C m–2 d–1. The average ratio of carbon oxidation to oxygen uptake was 1.6 ± 0.3, indicating the importance of anaerobic mineralization processes and the storage of reduced metabolites.