Fluctuations of physicochemical characteristics in sediments and overlaying water during an anoxic event: a case study from Lesina lagoon, Italy

The results presented in this work are related to an anoxic event occurred in Lesina lagoon (SE Italy) in summer 2008. The main aim is to describe the temporal dynamics of physicochemical characteristics of both sediment and overlaying water inside and outside the critical area. Nine sampling campaigns were carried out on a weekly basis. At each occasion T, S, pH, DO, nutrients and chl a were measured in water and TOC, TN, TP, TFe and ORP were performed on superficial sediments. Multivariate analysis was used in order to evaluate differences between inner and outer sites and assess the relationships between sediments and overlaying water. The factor "location" was significant for the observed physicochemical differences, as also tested by Anosim test one-way which supported evident differences in sediments (R = 0.599, p = 0.01%) and water (R = 0.381, p = 0.01%) between inner and outer sites. Wide fluctuations of oxygen and chlorophyll a were observed in the sites located within the anoxic area, with a peak of biomass of 180 µg.L-1 followed by saturation of oxygen higher than 100%. PCA analysis on sediment variables indicated a clear separation between inner and outer sites along the first axis (PC1) strongly associated with TOC, TN and ORP. A relationship between the content of TP in sediments and TP in water, followed by a phytoplanktonic bloom, was observed and confirmed by statistical correlation methods based on Spearman rank (Rho = 0,569, significance level of 0.01%).