Annual characterization of the nutrients and trophic state in a Mediterranean coastal lagoon: The Marano and Grado Lagoon (northern Adriatic Sea)

Abstract

A 1-year cycle of physico-chemical measurements was conducted in the Marano and Grado Lagoon (Adriatic Sea, Italy) with the aim to assess its trophic state. Some selected variables (nutrients, chlorophyll a, dissolved oxygen, temperature and salinity) were used to describe the system within the application of the Water Framework Directive (WFD) and to compare it with other Mediterranean environments.From an ecological point of view the Marano and Grado Lagoon is one of the best conserved wetlands in the whole Mediterranean area and is protected by several European environmental pieces of legislation. However, the neighbouring mainland hosts several socio-economic activities which can affect its water quality. The results obtained in this work highlighted considerable spatial and temporal variability of physico-chemical parameters. Specifically, a large amount of anthropogenic nutrients, mainly in form of nitrogen (N), was carried from inland through river discharges in the Marano basin (western sector). However, these inputs were offset by the water exchange with the adjacent open sea, which exerted a dynamic dilution effect, and induced low water residence time thus limiting nutrients enrichment and their accumulation for long time periods. The Molar Redfield ratio (DIN/SRP) was always higher than the optimum value of 16 (in some cases it was up to 2000), thus suggesting that the system is phosphorus (P) limited, as confirmed by low chlorophyll a content, rare algal blooms and good water oxygenation. Trophic state indices and threshold values (EEA method, Carlson Trophic Index, TRIX and ASSETS) were applied. A broad range of classes was found (from oligotrophic to hypertrophic depending on water bodies, method applied and period of the year), thus suggesting that an over simplification of this complex ecosystem should be avoided. Further studies will be conducted in order to define the Lagoon’s ecological status sensu WFD and the adequate management implications.