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