High temporal resolution vessel transects reveal systematic patterns of floating marine debris in the Baía de Todos os Santos, South Atlantic

Monitoring floating marine debris (FMD) is a global priority, emphasized in international initiatives and increasingly incorporated into marine policy frameworks, including many Marine Strategy Framework Directives. Despite this relevance, systematic monitoring of FMD remains virtually absent across the South Atlantic Ocean basin. Throughout 2025, we monitored weekly FMD using a fixed and temporally consistent vessel-based transport route across Baía de Todos os Santos, Brazil, with high temporal granularity encompassing tidal stages, weeks, and months, based on standardized visual observations from the vessel deck. Plastic items accounted for over 99% of all recorded debris, and no transect yielded zero detections, indicating the pervasive occurrence of floating litter along the monitored route. Tidal phase, surface current direction, and velocity were associated with distinct patterns of FMD accumulation and transport throughout the year. The terminal sections of the route exhibited the highest cumulative densities, likely driven by retention under lower current velocities compared with the faster-flowing mid-route section. Both mean and maximum FMD densities per km 2 were among the higher values reported in vessel-based observational studies, although direct comparisons should be interpreted cautiously due to methodological heterogeneity among monitoring designs. This study provides the first systematic application of this monitoring approach along the Brazilian coast and suggests fixed-route, high-frequency vessel monitoring may offer a replicable framework for improving floating marine debris assessment in other South Atlantic coastal systems.