Estuaries are biogeochemical reactors able to modulate the transfer of energy and matter from the watershed to the coastal zones and to retain or remove large amounts of terrestrially generated nutrients. However, they may switch from nutrient sink to source depending upon interannual variability of the nutrient supply and internal processes driving whole system metabolism (e.g., net autotrophic or heterotrophic). We tested this hypothesis in the Curonian Lagoon, a hypertrophic estuary located in the south east Baltic Sea, following the budget approach developed in the Land-Ocean Interactions in the Coastal Zone (LOICZ) project. Annual budgets for nitrogen (N), phosphorus (P), and silica (Si) were calculated for the 2013–2015 period. The lagoon was divided in a flushed, nutrient loaded area, and in a confined, less loaded area. The lagoon was always a sink for dissolved inorganic Si and P whereas it was a N sink in the confined area, dominated by denitrification, and a N source in the flushed area, due to dinitrogen (N₂) fixation. The net ecosystem metabolism (NEM) indicated that the Curonian Lagoon was mainly autotrophic because of high primary production rates. In this turbid system, low N:P ratio, high summer temperatures, and calm weather conditions support high production of N₂-fixing cyanobacteria, suppressing the estuarine N-sink role.

Microbial respiration determines the accumulation of reduced solutes and negative redox potential in organic sediments, favoring the mobilization of dissolved inorganic phosphorus (DIP), generally coprecipitated with Fe oxyhydroxides. Macrophytes releasing oxygen from the roots can contrast DIP mobility via the oxidation of anaerobic metabolism end-products. In this work, the submerged macrophyte Vallisneria spiralis was transplanted into laboratory microcosms containing sieved and homogenized organic sediments collected from a contaminated wetland. Sediments with and without plants were incubated under light and dark conditions for oxygen and DIP fluxes measurements and pore water characterization (pH, oxidation-reduction potential, DIP, dissolved Mn, and Fe). Bare sediments were net DIP sources whereas sediments with V. spiralis were weak DIP sources in the dark and large sinks in light. V. spiralis radial oxygen loss led to less negative redox potential and lower Fe, Mn, and DIP concentrations in pore water. Roots were coated by reddish plaques with large amounts of Fe, Mn, and P, exceeding internal content. The results demonstrated that at laboratory scale, the transplant of V. spiralis into polluted organic sediments, mitigates the mobility of DIP and metals through both direct and indirect effects. This, in turn, may favor sediment colonization by less-tolerant aquatic plants. Further in situ investigations, coupled with economic analyses, can evaluate this potential application as a nature-based solution to contrast eutrophication.

In the summer of 2022, the Po River Delta (Northern Italy), a eutrophication hotspot, was severely affected by high temperatures, exceptional lack of rainfall and saline water intrusion. The effect of saline intrusion on benthic nitrogen dynamics, and in particular the N removal capacity, was investigated during extreme drought conditions. Laboratory incubations of intact sediment cores were used to determine denitrification and DNRA rates at three sites along a salinity gradient in the Po di Goro, an arm of the Po River Delta. Denitrification was found to be the main process responsible for nitrate reduction in freshwater and slightly saline sites, whereas DNRA predominated in the most saline site, highlighting a switch in N cycling between removal and recycling. These results provide evidence that salinity is a key factor in regulating benthic N metabolism in transitional environments. In a climate change scenario, salinity intrusion, resulting from long periods of low river discharge, may become an unrecognized driver of coastal eutrophication by promoting the dissimilatory nitrate reduction to ammonium and N recycling of bioactive nitrogen within the ecosystem, rather than its permanent removal by denitrification.