Coupled Mediterranean ecomodel of the phosphorus and nitrogen cycles

Abstract

Assessment of the Mediterranean marine environment, in terms of nutrient depletion and easterly decreasing trophic gradients, requires full exploit of the knowledge and processes of the ecosystem dynamics. The three-dimensional model here proposed takes into account the phosphorus and nitrogen cycles coupled with the temporal and spatial hydrodynamical evolution. The food web considers, as primary producers, small autotrophs and large autotrophs uptaking on a multi-nutrient environment characterized by differential remineralizations of the detrital matter components. Grazing effects are modulated by herbivorous zooplankton response. The model is compared with the OCEAN-calibrated chlorophyll averages of the 1979–1985 Coastal Zone Color Scanner (CZCS) satellite data. This procedure validates the model biomass in the Western Mediterranean, which is 0.24 mg Chl m −3 when averaged yearly, with CZCS excess of about 10%, and in the Eastern Mediterranean, where the mean is 0.09 mg Chl m −3, with CZCS excess of 25%. The time series of the modelled results are well correlated with the CZCS estimates, with a higher value in the western than in the eastern basin. The temporal and spatial evolution of the biochemical patterns are studied. The effects of the implemented design on phosphorus and nitrogen cycles throughout the food chain are quantitatively followed and compared with results coming from specific cruises. Results about the vertically integrated biomasses are reported, giving an average of 2.7 g C m −2 for the total phytoplankton in the western basin and of 1.6 g C m −2 in the eastern one. The zooplanktonic biomass attains a mean value of 1.0 g C m −2 in the western basin and of 0.3 g C m −2 in the eastern one. Average productions accord with plankton abundances, with western primary and secondary productions, respectively, two and three times higher than the eastern ones.