Sulfide-induced release of phosphate from sediments of coastal lagoons andthe possible relation to the disappearance of Ruppia sp.

The production and consumption of sulfide and its influence on phosphorous cycling were studied in a hypertrophic coastal lagoon (Valle Smarlacca, Ita ly). Oxygen measurements revealed that the water phase was supersaturated e xcept for the layer directly overlying the sediment. This layer was devoid of oxygen and contained sulfide at all times. Maximal rates of sulfide prod uction, calculated from in situ profiles, were observed in the 0 to 2 cm se diment layer and the 1 cm water layer directly above. Sediment iron data su ggested a moderate chemical buffering capacity towards free sulfide; howeve r, the in situ buffering capacity was fully exploited. Stirring increased t he chemical buffer, indicating that, in situ, part of the iron did not cont ribute to the chemical buffer. The potential rate of biological sulfide oxi dation, estimated in sediment slurries amended with oxygen, was high; howev er, the actual rate was low due to a shortage of oxygen in the sediments an d the overlying water. Evidence was obtained for enhanced release of phosph ate caused by free sulfide. Under simulated natural conditions the release of phosphate exceeded the initial concentration of Fe-bound phosphate by an order of magnitude, indicating a significant contribution of non-iron-boun d phosphate. The observations in Valle Smarlacca were used to shed Light on the virtually complete disappearance of Ruppia sp, from other lagoons of t he Valli di Comacchio (Northern Italy).