Sediment metabolism in a transitional continental - marine area: The Albufera of Majorca (Balearic Islands, Spain)

Abstract

The concentrations of nutrients in sediment pore water and the fluxes of nutrients at the water-sediment interface were measured in a channel that joins continental and marine areas in the Albufera of Majorca in order to evaluate the role of sediments in the nutrient dynamics in this system.

Upstream, surficial pore water presented lower values of Eh, which became negative in summer, whereas downstream Eh remained positive. Nutrient concentrations were especially high upstream, reaching 1000 mol L-1 of NH4 and 75 μmol L^-1 of PO₄ during summer.

In summer, measured fluxes showed intense respiration upstream, with an oxygen consumption of 130 mg m^-2 h^-1 and a respiratory quotient near 4, which indicates dominance of anaerobic respiration. Total CO₂ efflux and nutrient fluxes were also high, reaching 30.50 mmol m^-2 h^-1 for CO₂, >2000 μmol m^-2 h^-1 for NH₄ and 58 μmol m^-2 h^-1 for PO₄. A substantial amount of the total CO₂ efflux (14 mmol m^-2 h^-1) was due to calcium carbonate redissolution. Downstream, oxygen consumption, respiratory quotient and ammonium fluxes were lower (around 70 mg m^-2 h^-1, between 2 and 3, and <20 μmol m^-2 h^-1, respectively), which indicates a moderate rate of decomposition activity and suggests denitrification as the main respiratoy process. Differences between fluxes measured '^{in situ}' and those calculated from pore-water concentrations indicated non-diffusive fluxes upstream and suggest substantial denitrifying activity downstream.

Extra keywords: benthic chambers, sediment fluxes, pore water, ammonium, phosphate.