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

Abstract

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, NH 4 +-flux to the water and Ca-flux toward sediments increased (NH 4 +-flux: 5000–3000 μmol m −2 d −1 in seawater and 600/250 μmol m −2 d −1 in brackish water; Ca-flux: −40/−76 meq m −2 d −1 at S=37 and −13/−10 meq m −2 d −1 at S=10); however, later NH 4 +-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=10 (55/45 mmol m −2 d −1), increased during the experiment at S=37 (from ∼30 mmol m −2 d −1 immediately after salinity increase to ∼60 mmol m −2 d −1 after 17 days). In brackish conditions, NH 4 + and Ca 2+ fluxes were consistent with a single diffusion-reaction model that assumes a zero-order reaction for NH 4 + production and a first-order reaction for Ca 2+ production. In seawater, this model explained the Ca-flux observed, but did not account for the high initial flux of NH 4 +. The mass balance for 17 days indicated a higher retention of NH 4 + in porewater in the littoral station in seawater conditions (9.5 mmol m −2 at S=37 and 1.6 mmol m −2 at S=10) and a significant reduction in the water consumption at both sites (5 mmol m −2 at S=37; 35/23 mmol m −2 at S=10). In contrast, accumulation of dissolved inorganic carbon in porewater was lower in seawater incubations (−10/−1 meq m −2 at S=37; 50/90 meq m −2 at S=10) and was linked to a higher efflux of CO 2 to the atmosphere, because of calcium carbonate precipitation in water (675/500 meq m −2). These results indicate that increased salinity in shallow coastal waters could play a major role in the global carbon cycle.