Increasing trends in faecal pollution revealed over a decade in the central Adriatic Sea (Italy).

Faecal contamination of the coastal sea poses widespread hazard to human and environmental health and is predicted to rise in response to global change and human pressure. For better management and risk reduction it is thus imperative to clarify and predict trends of faecal pollution over spatial and temporal scales, and to assess links with climate and other variables. Here, we investigated the spatio-temporal variation in the Faecal Indicator Bacteria (FIB) Escherichia coli and enterococci, over a time frame spanning 11 years (2011-2021) along a coastal area covering approximately 40 km and 59 bathing sites in the Marche region (Adriatic Sea, Italy), characterized by intense beach tourism, high riverine inputs, resident population, maritime traffic and industrial activities. Our analysis, that considers 5,183 measurements during the bathing season (April to October), shows that FIB abundance varied significantly among years. A general, although not significant, increase over time of both FIB was observed, mainly due to a general reduction of structural zeros (i.e., zeros originated from the actual absence of the response variable) over the examined time period. FIB abundances displayed their maxima and minima in different years according to the municipality, with overall peaks recorded in different months (May-June or September), whereas the lowest values were always observed in October. FIB levels were not significantly related neither to rainfalls nor to river discharge, but the activation of combined sewer overflows (CSOs), typically occurring after intense rainfall events, appeared as a necessary condition for the high faecal contamination levels. Considering climate change scenarios predicting significant increases in extreme weather events, our findings support the usefulness of analysing long-term trends to identify pollution sources, and the prioritization of control strategies to better manage the release of microbial pollutants from combined sewer overflows in coastal waters to reduce human risks.