Summer Dystrophic Criticalities of Non-Tidal Lagoons: The Case Study of a Mediterranean Lagoon

Eutrophication determines algal blooms and the subsequent accumulation of organic matter in sediments, which, in turn, results in the dominance of anaerobic respiratory processes that release toxic gases. Dystrophy is a final dissipative moment that reduces the organic load in the sediment. A case of dystrophy occurring in the Burano lagoon (Tuscany, Italy) in 2021 is reported. The study examined the weather, physico-chemistry of the water, submerged vegetation and sediment labile organic matter. In spring, dissolved oxygen (DO) and pH showed high values, in an abundance of submerged vegetation, while low values had ammonium, nitrate and orthophosphate. In mid-August, as warm and moist sea breezes prevailed, hydrogen sulfide releases were produced, preceded by a sharp rise in ammonium and orthophosphate concentrations, which remained high until November. During dystrophy, DO varied between anoxia and oversaturation, the latter in Cyanobacteria blooms. Dystrophic waters evolved gradually due to microphytes blooms, which changed from Cyanobacteria, in August, to the Dinophyta Alexandrium tamarense, in September, and Bacillariophyta, in November. Sediment labile organic matter varied between 3% and 7%. Ruppia spiralis meadows suffered the total detachment of fronds and stems during the dystrophy and proved to be areas of accumulation of organic detritus, themselves sources of dystrophic phenomena.