The Valle Smarlacca Lagoon is located on the north-western Adriatic coast of Italy, in the Emilia-Romagna region (Figure 1).

Figure 1.  Map and location of the Valle Smarlacca Lagoon.

The lagoon is part of the Valli di Comacchio lagoonal system (44.58°N, 12.23°E), a wide complex of shallow water impoundments covering 115 km² (see the Valli di Comacchio, this report).  The Valle Smarlacca Lagoon is located in the south-east corner of Valli di Comacchio, close to the Reno River.  It has surface area of 2 km² and a mean water depth of 0.8 m.  The surficial sediment is mainly composed of organic-rich silts.  This organic layer is 10-20 cm thick and overlies a deeper clay layer.  The salinity is relatively stable (22 to 24 psu) but can rise to 25-30 psu in summer due to evaporation.   The lagoon is surrounded by embankments and is completely separated from the other lagoons of the Valli di Comacchio system.  The Valle Smarlacca receives freshwater and nutrient inputs from the adjacent Reno River through artificially-regulated sluices and from a well by which groundwater is pumped into the lagoon.

The lagoon is exploited for fish farming of european seabass (Dicentrarchus labrus) and gilthead seabream (Sparus auratus).  The aquatic phanerogam Ruppia cirrhosa forms patchy meadow, alternating between areas of dense canopy and areas devoid of plants.

The lagoon is regularly subjected to dystrophic crises during summer, a phenomenon widely described in other European shallow water lagoons.  During the warmest summer months, the emerging Ruppia fronds become covered by dense tufts of epiphytic algae, whose decomposition leads to a significant oxygen uptake as well as sulphide accumulation in the water column.

The climate is mediterranean with some continental influence.  Precipitation is approximately 600 mm per year, with late spring and autumn peaks.  However, this pattern is undergoing significant changes with an increase of short-term intense events.

The data set used is from the year 1997 and was obtained during the ELOISE Projects “NICE-nitrogen cycling in estuaries” and “ROBUST: the role of buffering capacities in stabilising coastal lagoon ecosystems”.

From January to December 1997, water samples were collected in the central part of the lagoon and analysed for nitrate, nitrite, ammonium, dissolved reactive phosphorus by standard procedures (Dalsgaard et al. 2000).  Macrophyte biomasses, primary production, benthic fluxes of oxygen and nutrients, denitrification rates and sulphur, phosphorus, iron and nitrogen were also investigated (Hejis et al. 2000; Azzoni et al. 2001; Bartoli et al, 2001).

Air temperature and wet deposition data were obtained from the meteorological network of the Province of Ferrara.  Hydrochemical data for the Reno River were directly measured and for Adriatic Sea stations are reported in ARPA-DAPHNE (1998).

Water and salt balance

The water budget of the Valle Smarlacca Lagoon was calculated on a seasonal basis, using a single box–single layer model since this lagoon is small and shallow.

The lagoon exchanges water with the Reno River through a pumping station, which is artificially regulated depending on aquaculture requirements.  Every February the water level of the lagoon is lowered by 50-60 cm to force the fish to move to the deepest point of the lagoon for harvesting.  Following fish collection, the pumping station is activated and water is pumped from the Reno River into the lagoon to restore the water level.  The pumping station is activated mainly at high tide when the salt wedge in the Reno River reaches the canal which connects the river to the lagoon.   The water collected by the pumping station is normally brackish since the salinity of the water moving from the sea is higher than the freshwater of the Reno River, which is heavily polluted.  From spring to summer the lagoon is isolated from the Reno River, with only occasional inputs from the pumping station to compensate for evaporation losses.   In October the water level is lowered again for the second annual fish harvest and subsequently the normal water level is restored by pumping brackish water from the Reno River in the lagoon.  Due to these constraints, it is not possible to calculate a water budget based on salinity variations in the lagoon water.

The bottom of the lagoon is composed of a thick layer of clay which is not permeable to groundwater, but in late autumn and in winter when the temperature is lowest, “warm” groundwater (14°C) is pumped from a well into the intensive fish farming ponds and it then flows into the lagoon.  This groundwater is rich in dissolved inorganic P (54 µM) and ammonium (1,250 µM) of fossil origin and is an important source of nutrient input, inducing intense blooms of phytoplankton in the following spring (chlorophyll-a up to 330 µg L^-1).

Precipitation data (V_P) for 1997 were obtained from the Meteorological Station and evaporation (V_E) was calculated by the Hargreaves equation.  V_E and V_P were similar on an annual basis even evaporation exceed precipitation in spring and summer while precipitation dominated in autumn.

Since all the water fluxes except evaporation and precipitation are completely artificially regulated some assumptions were made.  Firstly, the water volume input and output of the lagoon in the fishing periods were calculated by multiplying the water level changes by the lagoonal surface area.  Secondly, since the water inputs and the water outputs are temporally separated and no exchanges of water occurs between the River and the lagoon, the mixing volume (V_X) was considered to be zero.  In the budgets, V_Q indicates the brackish water input from the Reno River, V_G the groundwater input from the well and V_R the water export from the lagoon via the pumping station.   For these reasons the salt budget was not calculated.

The seasonal water budgets are summarised in Table 1.  Mean daily water inputs and outputs were extremely small, compared to the volume of the lagoon and consequently the estimated water exchange time (t) was very long, more than a year.  On a seasonal basis, t is very long in spring when the pump does not operate.  A lower water exchange time was estimated for summer due to higher evaporation rates and the water pumped in from the river to compensate for evaporation losses.  In winter and autumn water exchange time was approximated 4 months.

Table 1. Seasonal water budgets of the Valle Smarlacca lagoon in 1997.   Mixing volume (Vx) was considered zero.

 Budgets of nonconservative materials

DIP balance

Since no data on dry deposition was available, atmospheric DIP inputs were assumed to be zero.  On an annual basis, the DIP exchanges were very low and nonconservative flux of DIP (DDIP) averaged -0.01 mmol m^-2 d^-1.  The main DIP sources were the groundwater inputs pumped from the well, which were about 7 times higher than the surficial inputs.  In spring and in summer, DIP concentrations in the water column were below the detection limit of the method (0.1 µM) for most of the time (Table 2).  This is in agreement with the low inputs reported for these periods and with the high metabolic activity of the biotic community composed mainly of rooted phanerogams, epiphytes, macroalgae and phytoplankton which need P for growth.  DDIP was very low, close to zero in spring and summer, and reached values of -0.02 mmol m^-2 d^-1 in winter and autumn when DIP inputs were close to 40 mol d^-1.   DDIP was negative for the whole period investigated, indicating that the lagoon acts as a net sink of DIP.  The seasonal dissolved inorganic phosphorus (DIP) budgets are reported in Table 3.

Table 2. Nutrient concentrations for the Reno River, groundwater and lagoon.

Table 3. Seasonal DIP budgets of the Valle Smarlacca Lagoon.

DIN balance

The seasonal dissolved inorganic nitrogen (DIN) budgets are reported in Table 4.  The main DIN inputs to the lagoon were the brackish water inputs from the Reno River, groundwater from the well and precipitation.  In contrast to the DIP budget, surficial DIN inputs dominated over groundwater inputs on an annual basis even if they were of the same order of magnitude.  DIN inputs were dominated by ammonium in both the surficial and ground water sources.   Ammonium was also the dominant nitrogen species in the water column of the lagoon (60-90%) during the investigated period.  DIN peaks (up to 67 µM) were measured in winter and autumn when DIN inputs were approximately 2x10³ mol d^-1.  As with DIP, DIN concentrations in the water column of the lagoon decreased in spring and summer attaining values of 3 and 6 µM for nitrate and ammonium, respectively.  DDIN was negative for the whole period investigated, especially in autumn and winter, indicating that the lagoon acted as a net sink for DIN.

Table 4.  Seasonal DIN budgets of the Valle Smarlacca Lagoon.

Stoichiometric calculations of aspects of net system metabolism

On an annual basis, the lagoon can be considered a net autotrophic system since the negative DDIP values calculated can be considered as an estimate of net DIP assimilation for organic matter production, as indicated in the LOICZ procedure.  The annual average of the net ecosystem metabolism (NEM) taken as the difference between ecosystem production and respiration (p-r), was +1 mmol C m^-2 d^-1 in 1997, assuming production of organic matter with a Redfield C:N:P ratio.  Results of the seasonal budgets are summarised in Table 5.  NEM values of +2 mmol C m^-2 d^-1 were estimated for the autumn and winter months when nutrient inputs were high, while negligible values were estimated for the spring and summer periods.  In the spring and summer months, the lagoon is almost completely isolated, water column DIP and DIN concentrations are very low and biological activity is driven by the rapid recycling of nutrients and internal nutrient sources.   Thus, whilst imports and exports of material are practically zero, there is an extremely high level of biological activity within the lagoon and large movements of nutrients between the ecosystem compartments.  The low observed NEM values indicated good balance between production and respiration in the system which would agree with the high coupling between P-regeneration and primary production.

Table 5. Seasonal variation of DDIN_exp, (nfix-denit) and net ecosystem metabolism (p-r) in the Valle Smarlacca in 1997.

In these months, the net production of organic matter, which was dependent on internal nutrient sources and the primary producers’ internal nutrient reserves, if calculated using the LOICZ model was below +0.1 mmol C m^-2 d^-1.   However, estimates based on DIP concentrations in the water column can hardly be considered significant for budgeting, as these concentrations were below the detection limits of the method for much of this period.   This is due to the strong coupling between P-regeneration rates and primary production, as sediment to water column fluxes of phosphate can be significant (Heijs et al. 2000).

The DDIN_exp values indicated in Table 5 were calculated by multiplying the DDIP by the Redfield N:P ratio and (nfix-denit) was calculated from the difference between observed and expected DDIN.  In the Valle Smarlacca lagoon, losses via denitrification appear to be dominant since (nfix-denit) was always negative (Table 5).  Nitrogen fixation seems was not a quantitatively important process in the N-budget of this lagoon since (nfix-denit) is in relatively good agreement with denitrification rates measured in the same year at a single station in the lagoon, which ranged between 0.05 and 0.25 mmol m^-2 d^-1 (Bartoli et al. 2001).   The latter result is somewhat unexpected since the rooted phanerogam meadows generally exhibit high nitrogen fixation rates and net N-inputs (Welsh 2000).  However, this result should be considered with caution, particularly in spring and in summer, since it is based on low DIP inputs, on a Redfield NP ratio which can not be representative of the heterogeneous organic matter produced and decomposed in the lagoon (phanerogams, epiphytes, macroalgae, plankton, fish food) and on denitrification rates measured at a single station and extrapolated to the whole lagoon.  Moreover the patchy distribution of rooted phanerogam meadows has to be considered.

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Last Updated 21 May 2006 by DPS