Fluctuations of physicochemical characteristics in sediments and overlying water during an anoxic event: a case study from Lesina lagoon (SE Italy)

Abstract

Coastal lagoons are vulnerable systems often affected by severe anoxic events, also as direct consequence of excessive accumulation of organic matter. During these events, also in function of their location and geomorphology, they are characterized by large temporal and spatial fluctuations of physical and chemical conditions. 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 short-term dynamics of physicochemical characteristics of both sediment and overlaying water, inside and outside the critical area of the anoxic event. Nine sampling campaigns were carried out on a weekly basis between the first week of July and the second week of September 2008. The water sampling was performed at three sites located in the western side (inside and outside area interested by crisis) and in the eastern zone of the lagoon, close to the communicating channel with the sea, while the sediment samples were collected in four stations, including two sites within crisis area and two outside. 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. In relation to the water, wide temporal fluctuations of physico-chemical parameters were observed for the site inside the anoxic area, which exhibited the highest concentrations of chl a at the beginning of the observation period, with a peak of biomass of 180 µg.L -1 followed by saturation of oxygen higher than 100%. A strong significant difference among sampling sites was observed in relation to nutrients; in particular, the site within the anoxic area exhibited broader concentration ranges of both SRP and TP than the other sites, while TN was in general higher in the site outside. Significant differences related to the sedimentary variables were also observed between anoxic and reference area, except for TP which was homogeneously distributed during the observation period in all the sites. Disturbed sites showed the highest contents of TOC at the beginning of the observation period (July 2008), TN contents almost two times lower and mean contents of TFe higher than those obtained in control sites. All sites were characterized by highly reducing conditions despite the well oxygenated conditions of the overlying waters. These findings highlight wider time-related fluctuations of physico-chemical parameters (for both sediments and overlying water) within anoxic area than in control area, but, at the same time, stress the significant role of the factor space (anoxic sites and control sites) for the observed physico-chemical diferences. Also, this integrated approach between sediments and water indicates sediment-related source of nutrients in the anoxic overlying water, suggesting that quick mineralization processes and nutrient release in mid-term happened mainly in disturbed area. The high biomass production observed along the overlying water within the anoxic area at the beginning of the observation period was related to this greater availability of nutrients in disturbed area.