Drivers of anoxia in a eutrophic lagoon with clams farming (Sacca di Goro Lagoon, Italy): risk evaluation via multivariate statistics and timescales analysis

The anoxia risk in a shallow coastal lagoon (Sacca di Goro (SG), Italy) experiencing multiple pressures (clam aquaculture, anthropogenic nutrient inputs, climate change) was assessed by combining timescales of anoxia onset, benthic nutrient turnover, horizontal and vertical transport, multivariate statistics, and Geographic Information System (GIS). The biogeochemical and transport timescales were obtained by experimental measurements and physical modeling. To this purpose, daily oxygen (O₂) and nutrient fluxes [dissolved inorganic phosphorus (DIP), ammonium (NH₄⁺), dissolved reactive silica (SiO₂)] were measured in the benthic and pelagic compartments at six areas of the lagoon catching its complex spatial heterogeneity due to riverine and marine inputs, clams farming, depths, flushing and vertical exchange times, amount and quality of organic matter. Water quality, sedimentary features and fluxes and pore water chemistry were also measured. This approach allowed us to estimate the contribution of the benthic and pelagic compartments to the onset of anoxia, as well as to determine the main drivers that may lead to anoxia. The results show that the anoxia onset in the SG is mainly determined by the clam biomass. This is due to shorter timescales of both anoxia onset and benthic nutrient turnover times (faster rates of O₂ consumption and nutrient replenishment) in farming than in non-farming areas. However, the anoxia onset in the deepest part of the lagoon is also favored by water column respiration. On the other hand, vertical exchange times shorter than flushing times suggest that vertical mixing is the dominant physical factor in controlling the O₂ replenishment in the SG lagoon, counteracting the anoxia risk. The timescales analysis is a simple and effective method to understand the interplay between O₂ consumption and physical processes in a complex shallow eutrophic lagoon, which is a key issue in the environmental management of coastal lagoons farmed areas.