Assessment of environmental management effects in a shallow water basin using mass-balance models

Abstract

Comparative analysis of trophic networks was carried out to evaluate environmental management actions aimed at countering an environmental crisis in Orbetello Lagoon, Italy. Two mass-balance models of this shallow water coastal system were constructed, for 1995 and 1996. During this period, there was an observed change in the composition of the submerged vegetation that indicated a significant improvement in the lagoon’s ecology. Mass-balance models were built using the Ecopath modelling software in order to explain the energy transfer through the trophic levels (TLs) of the lagoon’s ecosystem. Comparative analysis of the two trophic networks allowed a complete description of the lagoon, and gave clear indications regarding the ‘eutrophication level’ sensu Nixon and the stage of system maturity sensu Odum of the ecosystem for the 2 years. The turnover rate (production/biomass ratio) of pleustophytes was proposed as an indicator of “environmental health” at the ecosystem level because it allows positioning the submerged vegetation as a whole along an axis of adaptation strategies to natural selection. All the indices of ecosystem maturity and stability examined showed that the lagoon was in a more stable condition in 1996 than in 1995, although the system was still in a condition of stress. The effects of management actions carried out in the system were quantified by estimating the changes in primary production and accumulation of detritus in the system. Management of the lagoon as an extensive aquaculture operation was assessed by analysing the fishery catch, the transfer efficiencies at different trophic levels and the impact of cormorants, and it was found to contribute to system stability.