A modelling assessment of hydrodynamics and biogeochemistry of the northern Adriatic Sea, and effects on clam dynamics in Barbamarco Lagoon, Italy

[Truncated abstract] Barbamarco Lagoon is a small lagoon in the Po River Delta on the western margin of the Northern Adriatic Sea. It is the site of a commercially valuable clam (Tapes philippinarum) fishery. The primary aim of this study was to determine the interactions of river inflows, climate forcing, water column structure, nutrient distributions and primary production in the Northern Adriatic Sea, subsequent impacts on phytoplankton supply to clam fisheries in Barbamarco Lagoon, and to develop strategies for optimisation of clam yields. This research utilised an interdisciplinary, dynamically coupled three-dimensional (3D) hydrodynamic and ecological model adapted to incorporate a population dynamics model for clams. The model was validated with in situ field and remote sensing data. The model was used to derive integrated mass balances for the region, quantify the interactions of hydrodynamics and ecological processes and their impacts on clam populations, as well as to gain insight into the coupling of small and large scale systems. Investigation of the effects of Po River inflows, particularly nutrient inputs, seasonal stratification and meteorological forcing on phytoplankton concentrations and distributions, and basin-wide nutrient fluxes, indicated a close coupling of physical and biochemical processes over a range of space and time scales in the Northern Adriatic Sea. Nutrient fluxes due to advection and particulate organic matter sedimentation were strongly influenced by variations in nutrient loadings from riverine inflows. ... The impacts of clam grazing and waste production on lagoon nutrient concentrations and phytoplankton biomass were mostly localised, with minimal impact on lagoon-wide concentrations at current clam stocking levels due to strong tidal flushing. However, clam populations were found to alter the cycling of nutrients in the system, causing the lagoon to become a net sink for particulate organic matter and to export dissolved organic matter to the adjacent sea via tidal flushing. The impact of clam grazing on phytoplankton dynamics in Barbamarco Lagoon under different hydrodynamic regimes and nutrient loadings were also assessed in terms of carrying capacity. Several environmental indicators, in addition to bottomwater DO concentrations, were used to quantify the ecological carrying capacity of the system under different scenarios. Production carrying capacity is also assessed for different clam rearing strategies and lagoon morphologies using harvest and net growth to seeding ratios, total harvest value and clam satiety. Increasing exchange with the Northern Adriatic Sea or increasing freshwater input into Barbamarco Lagoon was found to improve clam food supply, clam growth rates and hence commercial harvest, whilst also decreasing negative effects on water quality due to clam respiration and waste production. Seeding the entire lagoon with clams exceeded both production and ecological carrying capacities but indicated wellflushed areas deeper than 1.5 m near the lagoon mouths were best suited to clam cultivation. An integrated understanding of the spatial variability and interactions between tidal flushing, nutrient cycling, primary production and clam dynamics is crucial for the long term sustainable management and potential expansion of the fishery.