Ecosystem response of an anthropogenically impacted lagoon to induced climate-driven water stagnation: an enclosure experiment.

Few manipulative field studies have investigated lagoon ecosystem responses to climate-driven environmental stressors. To address these knowledge gaps, we investigated the response of structural and functional variables in the mercury-contaminated Grado Lagoon (northern Adriatic Sea) to water stagnation, i.e., a realistic condition potentially expected in the near future. To induce water stagnation, in June 2024 we positioned 18 mesocosms (∼0.8 m3 each), in the most contaminated and confined lagoon area, and compared the results with natural external conditions. The short-term (T1, 4 days) and long-term (T2, 10 days) isolation-driven effects on the ecosystem functioning were assessed by simultaneously investigating physical-chemical features of water and sediments, pelagic and benthic microbial communities' biomass and structure, and the main biological processes. Water stagnation mimicked increased residence time in confined lagoon areas during summer and progressively triggered cascading effects, including oxygen reduction (T1, -52.9 ± 7.4%), reduced primary production (T2, -64.6 ± 18.0%), and dominance of all heterotrophic processes, compared to external autotrophic conditions. Our results show that even short-term stagnation and moderate oxygen reduction can trigger significant cascading effects on biological and biogeochemical processes, altering benthic-pelagic coupling. The rapid biomass increase of the microphytobenthic community under early stress conditions (T1, +86.5 ± 42.2% vs outside), likely enhanced ecosystem resistance. Although hypoxic conditions were not reached, our results obtained in a single confined lagoon area/season, indicate that water stagnation significantly altered ecosystem functioning, suggesting that such conditions might be precursors to disrupting effects on the ecosystem and, if persistent or recurrent, could lead to dystrophic events in early summer.