Dynamics of biogeochemical properties in temperate coastal areas of freshwater influence: Lessons from the Northern Adriatic Sea (Gulf of Trieste)

High spatial and temporal variabilities of biogeochemical properties are prominent features of regions under freshwater influence as a result of multiple factors. Understanding the ecological functioning of these ecosystems, which provide important services for humans, is challenging since it requires adequate observational strategies and efforts. Multi-years (1999e2006) continuous observations in the northernmost part of the Adriatic Sea (Gulf of Trieste) allowed us to compute a climatological description of seasonal dynamics of biogeochemical properties for three relevant sites: a coastal area directly influenced by a river, an off-shore area located in the centre of the Gulf and a coastal area located far from potential source of external nutrients. The analysis of the climatologies provides a quantitative corroboration of the conceptual scheme for biogeochemical and ecological seasonal dynamics of temperate coastal areas under freshwater influence already proposed in literature, highlighting the role of river input, lateral transport, stratification regime and interaction with bottom environment as driving factors. While all areas follow a common pattern of succession of ecological processes, spatial variability accounts for a significant decrease of the absolute trophic state, and for a phase delay in biogeochemical dynamics. Results show that spatial heterogeneity is an inherent structural feature of coastal ecosystems, suggesting that the evaluation of the quality status of coastal ecosystems should be made by using different reference terms for different sub-areas.