Coastal lagoons provide important ecosystem services like biodiversity enhancement, nutrient cycling, and climate change mitigation. Despite their natural variability, lagoons are sensitive to climatic and human pressures, including eutrophication. Excess nutrient loads cause algal blooms, oxygen depletion and mass mortalities, occasionally resulting in dystrophic crises. Sedimentary organic matter (OM) attributes and meiofaunal communities can either detect environmental change or assess lagoons health. We investigated seasonal changes in meiofauna and sedimentary OM characteristics also intercepting the occurrence of a summer dystrophic event in the S'Ena Arrubia Lagoon (Western Sardinia). Three areas along a salinity–trophic gradient, each comprising two stations, were sampled seasonally and immediately after a dystrophic event caused by high temperatures and excess organic loads. Physical-chemical parameters were measured, and three sediment cores collected for the analysis of meiofauna and sedimentary OM quantity, biochemical composition, and degradation rates. The results revealed a response gradient across the lagoon, driven either by seasonality or the dystrophic event. The innermost lagoon was the most affected by the dystrophic event, with meiofaunal density and diversity dropped by 53% and a nematode-dominated community. OM contents and protein turnover time increased, indicating enhanced primary production and reduced microbial activity. After the event, meiofaunal abundance and diversity partly recovered, with pioneer taxa appearing, whereas OM composition shifted toward a more refractory composition. The outer lagoon area, less diverse, was much less affected. Our findings suggest that the dystrophic event made the innermost lagoon more vulnerable to future anthropogenic and climatic stressors, and highlight the need for prosecuting the lagoon monitoring.

This study has been conducted within the framework of the project e.INS www.einsardinia.eu (Next Generation EU- PNRR - M4 C2 I1.5 CUP F53C22000430001) and attending PhD in Life, Environmental and Drug Sciences (UniCa, Cycle XXXVIII), funded by DM 352/2022 (Next Generation EU-PNRR) and IMC Foundation).