From rainfall to seafloor: Tracing riverine and urban stormwater litter transport in Mediterranean fishing grounds through systematic meteorological station testing.

Precipitation can mobilize terrestrial litter into coastal waters through urban stormwater pathways, potentially including combined sewer outflows (CSOs), but establishing quantitative relationships between rainfall and benthic litter accumulation has proven challenging. River discharge adds complexity by potentially washing litter away from fishing grounds. We tested 54 meteorological station-fishing ground combinations across nine ports in the northwestern Mediterranean (2019-2024, 236 trawl hauls), examining precipitation windows from 2 to 20 weeks and river discharge effects at six connected sites. Precipitation served as a proxy for urban stormwater input, including CSO activation, which was not directly measured. Station selection critically determined detected effects: within Vilanova i la Geltrú alone, station choice produced estimates ranging from -62% to +88% per 10 mm precipitation, showing that proximity alone does not guarantee hydrological connectivity. Two port types emerged: Type A, a positive precipitation response (n = 7, 5 significant) consistent with urban stormwater delivery, and Type B, river washout (n = 2, negative effects). Rivers predominantly removed rather than delivered litter: five of six connected ports showed washout effects (-13.1% to -100% per 100 m3/s discharge). Only Vilanova i la Geltrú showed positive river effects (+12%), reflecting downstream accumulation from Barcelona via the Llobregat River plus a strong local precipitation response (+88%). Temporal signatures differed for most ports: positive precipitation effects typically peaked within 2-5 weeks (4 of 5 significant ports) versus 12-18 weeks for rivers, with L'Ametlla de Mar (16 weeks) as an exception; the specific mechanism was not directly confirmed. Management strategies should be location-specific: urban stormwater infrastructure for Type A ports, upstream river management for Type B, and coordinated approaches for multi-source zones. Urban precipitation-associated inputs, plausibly linked to CSO activity, emerge as the primary correlate of benthic litter delivery.