Effect of changes in water salinity on ammonium, calcium, dissolved inorganic carbon and influence on water/sediment dynamics

The effect of a sudden increase in salinity from 10 to 37 in porewater concentration and the benthic fluxes of ammonium, calcium and dissolved inorganic carbon were studied in sediments of a small coastal lagoon, the Albufera d’Es Grau (Minorca Island, Spain). The temporal effects of the changes in salinity were examined over 17 days using a single diffusion-reaction model and a mass-balance approach. After the salinity change, NH4 + -flux to the water and Ca-flux toward sediments increased (NH4 + -flux: 5000– 3000 lmolm 2 d 1 in seawater and 600/250 lmolm 2 d 1 in brackish water; Ca-flux: 40/ 76 meqm 2 d 1 at S ¼ 37 and 13/ 10 meqm 2 d 1 at S ¼ 10); however, later NH4 + -flux decreased in seawater, reaching values lower than in brackish water. In contrast, Ca-flux presented similar values in both conditions. The fluxes of dissolved inorganic carbon, which were constant at S ¼ 10 (55/45 mmolm 2 d 1), increased during the experiment at S ¼ 37 (from 30 mmolm 2 d 1 immediately after salinity increase to 60 mmolm 2 d 1 after 17 days). In brackish conditions, NH4 + and Ca2+ fluxes were consistent with a single diffusion-reaction model that assumes a zero-order reaction for NH4 + production and a first-order reaction for Ca2+ production. In seawater, this model explained the Ca-flux observed, but did not account for the high initial flux of NH4 + . The mass balance for 17 days indicated a higher retention of NH4 + in porewater in the littoral station in seawater conditions (9.5 mmolm 2 at S ¼ 37 and 1.6 mmolm 2 at S ¼ 10) and a significant reduction in the water consumption at both sites (5 mmolm 2 at S ¼ 37; 35/23 mmolm 2 at S ¼ 10). In contrast, accumulation of dissolved inorganic carbon in porewater was lower in seawater incubations ( 10/ 1 meqm 2 at S ¼ 37; 50/90 meqm 2 at S ¼ 10) and was linked to a higher efflux of CO2 to the atmosphere, because of calcium carbonate precipitation in water (675/500 meqm 2). These results indicate that in‘reased salinity in shallow coastal waters could play a major role in the global carbon cycle.