SEDIMENT AND INTERSTITIAL WATER CHEMISTRY OF THE ORBETELLO LAGOON (GROSSETO, ITALY) - NUTRIENT DIFFUSION ACROSS THE WATER-SEDIMENT INTERFACE

The Orbetello lagoon is now highly eutrophic and has experienced increasing incidence of anoxia causing serious economic damage. A multidisciplinary study was commenced in March 1987 to investigate the part played by the lagoon sediments and interstitial water in recycling nutrients and contributing to the observed anoxia. Eleven undisturbed cores were collected and sub-sampled at 2 cm intervals. Interstitial water was obtained by centrifugation and analyzed for pH, Eh, nutrients, major and some minor elements. Differential fluxes from the sediments of 0.2-10-mu-g cm-2 day-1 of ammonia and 0.02-0.7-mu-g cm-2 day-1 of orthophosphate were obtained depending on the season, temperature of the sediments and the sampling location. A highly significant linear correlation (r2 = 0.86) was found between bicarbonate and ammonia concentrations in the interstitial waters, due to the release of these compounds during the degradation organic matter. A diagenetic model was developed to predict the alkalinity of interstitial water from the theoretical reactions involved in the decomposition of organic matter. The predicted values for bicarbonate and ammonia agreed well with the experimental results.