Porewater concentrations and benthic fluxes of dissolved inorganic carbon (DIC), sulfate (SO42−), calcium (Ca2 ), manganese (Mn), iron (Fe), ammonia (NH4 ), and soluble reactive phosphorus (DIP) were measured in the sediments of Albufera d'Es Grau (Minorca Island), a mesohaline lagoon, in Spring/early Summer. In this period, DIC and NH4 were produced in sediments by the decomposition of organic matter through aerobic pathways in the upper 1 cm and below this depth through anaerobic pathways mainly sulfate reduction. In March, the average flux of DIC into the water was 21.45 ± 26.05 mmol m−2 d−1, more than 50% of which came from anaerobic respiration below 1 cm sediment depth. In June, the DIC flux increased to 55.29 ± 24.38 mmol m−2 d−1, but the rates of production through sulfate reduction decreased drastically to less than 10% of the total flux. NH4 -flux was also relatively low, decreasing from 1000 μmol m−2 d−1 in March to 200 μmol m−2 d−1 in June. Differences between stoichiometric relationships in porewater and in measured fluxes indicated immobilization of nitrogen in sediments due to macrophyte uptake and nitrification, this immobilization mainly occurring in early summer. DIP release into the water was always below 15 μmol m−2 d−1, these low values being attributed to three main factors: a) more than 40% of sedimentary phosphorus in Es Grau sediments occurred in non-reactive forms such as detrital apatite or refractory-P; b) accumulation of Fe-oxides at the sediment surface acts as a trap for DIP produced through organic matter decomposition in March; and c) phosphate released from Fe-bound P in June during the period of increased oxygen consumption was effectively trapped as fulvic-P complexes and as microbial phosphate. Differences between fluxes directly measured and estimated from porewater concentrations also indicated that, at some sites, bioturbation contributed to the total flux of NH4 and Mn and phosphate immobilization.
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Vol. 24 • No. 4