Pregi e limiti del LOICZ-BM: l'esempio della Sacca di Goro

J. M. Zaldívar
European Commission, Joint Research Center,
Institute for Environment and Sustainability
Inland and Marine Waters Unit, TP 280
21020 Ispra (VA), Italy

The application of the LOICZ methodology will be illustrated by using the Sacca di Goro lagoon. Emphasis will be placed on the problems one can encounter when analysing several data sets, i.e. from the catchment, from the lagoon and from the Sea that have different sampling frequencies. Advantages/disadvantages of several approaches will be discussed and simulation results will be compared with biochemical budgets.
The Sacca di Goro is an eutrophic lagoon located along the North Adriatic Coast of Italy. It is the estuary of the southernmost branch of the Po river delta. This lagoon has been investigated using monthly averaged data from the last decade. Water, Salt and nutrient budgets were estimated using LOICZ methodology with a single box single layer approach.
Concerning water and salt budgets a mean freshwater residence time, i.e. the ratio between Sacca di Goro total volume and fresh water flow, of 24.5 days was obtained, whereas the total water exchange time, which is given by the ratio between Sacca di Goro total volume and fresh water flow plus the exchanged flow with the Adriatic Sea has a mean value of 1.5 days or 3.4 days depending on the data and the model used. Furthermore, the value has oscillated strongly during the last twenty years due to the morphological changes that the Sacca di Goro mouth has experienced in the last decades.
Non-steady state DIP and DIN (Nitrates+nitrites and ammonium) budgets have been established at seasonal intervals between 1989 and 1998. Whereas the mean DDIP values follow several yearly trends depending on the seasonal values for macroalgal biomass, riverine inputs and lagoon concentrations; DDIN has been predominantly negative during the last decade indicating than the Sacca di Goro has been a sink of inorganic nitrogen, however at the beginning of 87 and at the end of 97 the Sacca di Goro has acted as a source, with positive DDIN. More data is necessary to see if there is a reversal in the general trend.
Stoichiometric calculations were performed considering both the Redfield ratio (C:N:P=101:16:1) and the ratio for macroalgae (C:N:P=335:35:1) reported by Atkinson & Smith (1983). The latter seems more suitable for the Sacca di Goro, which is affected by huge macroalgal blooms. The data on net ecosystem metabolism are probably not correct due to the fact that during these years Sacca di Goro has experienced anoxic crises and redox-mediated phosphorous desorption has occurred.