Sources and dynamics of fecal pollution and antibiotic resistance across a Mediterranean land-sea continuum.

A comprehensive understanding of fecal pollution dynamics and associated antibiotic resistance is critical for effective water quality management in coastal watersheds exposed to strong hydrological variability. This study aimed to identify the relative contribution of human and animal fecal sources, assess the spatial and temporal distribution of contamination, and evaluate the occurrence of selected antibiotic resistance genes (ARGs) across a Mediterranean land-sea continuum (central Adriatic Sea, Italy) under contrasting rainfall conditions. Surface water and sediment samples were collected during the bathing season and analyzed using host-associated microbial source tracking (MST) markers and selected ARGs. Human fecal contamination was the dominant source, as indicated by the consistent detection of HF183, which significantly increased during moderate and extreme rainfall events, indicating inputs from runoff and sewer overflows. The presence of human adenovirus (HAdV) further supported sewage-derived contamination, whereas animal-associated markers showed lower detection frequencies and more heterogeneous spatial patterns. ARGs, including tetracycline resistance genes tetM, were widely detected, with generally higher concentrations at freshwater sites. While ARG distribution was influenced by hydrological variability, tetM showed lower temporal variability and remained relatively abundant under drought conditions, suggesting longer-term environmental persistence. Rainfall emerged as a major driver shaping marker distribution, with water showing high temporal variability and sediments acting as secondary reservoirs enriched during heavy rainfall events. These findings provide relevant insights for improving water quality strategies and support targeted management actions to mitigate fecal pollution in Mediterranean coastal systems.