Fecal microbial pollution in river and coastal environments: Influence of weather conditions

Coastal systems are under increasing threat from human activity and climate-driven disturbances, yet the specific impact of different weather conditions on microbial contamination remains insufficiently resolved. It was hypothesized that precipitation regimes, intensified by climate change, act as primary drivers of fecal microbial pollution and nutrient mobilization in riverine and coastal systems. To test this hypothesis, the Arzilla river (located in the western Adriatic Sea, Italy) and its adjacent coastal waters were investigated using an event-based spatial sampling design. Nutrients and total suspended matter were analysed, and the occurrence of Escherichia coli and intestinal enterococci were examined under contrasting weather conditions (ranging from drought to heavy rainfall). Despite its small size, the Arzilla watershed represents a model of a Mediterranean system where geomorphological, hydrological, and anthropogenic factors converge, making it representative for ecological and management considerations at a larger scale. Rainfall intensity has been demonstrated to exert a significant influence on the expression of microbial responses. An extreme precipitation event with levels in excess of 100 mm, resulted in elevated microbial loads exceeding regulatory thresholds at both river and marine stations. The range of microbial loads at the river stations was from 20 to >30,000 CFU/100 mL, while at the marine stations it was from <10 to >140,000 CFU/100 mL. These findings underscore, the vulnerability of coastal waters to climate change-related disturbances. An increase of nutrients and TSM concentrations during periods of wet conditions was observed, indicating an enhancement in runoff-driven transport. Enterococci exhibited greater persistence and delayed peaks compared to E. coli. The findings of this study provide mechanistic insight into the short-term dynamics linking precipitation regimes, microbial pollution, and nutrient mobilization at the land-sea interface. The findings emphasize the vulnerability of small Mediterranean catchments to climate-driven disturbances and underscore the need to integrate adaptive, event-based monitoring methodologies into European management frameworks.