Impact of beach wrack on microorganisms associated with faecal pollution at the Baltic Sea Sandy beaches

We investigated whether higher quantities of faecal indicator bacteria (FIB) are in the areas with red algae-dominated wrack compared to areas without it and if the birds are the primary source of faecal pollution on sandy beaches of the Baltic Sea. Water, sand and wrack samples were collected during the recreational season, and abundances of FIB, HF183 (human faecal pollution) and GFD (bird faecal pollution) markers, as well as the presence of Salmonella and Campylobacter, were assessed. Significantly higher levels of Enterococcus spp. were found in the wrack accumulation areas in water and sand than in the areas without wrack when there was a faecal pollution event, which could be explained by entrapment and changed physico-chemical water conditions. Both faecal pollution markers were identified, however, with no apparent pattern. Campylobacter bacteria were identified in the wrack-affected water, sand, and beach wrack. While this research provides valuable insights into beach wrack serving as a reservoir for FIB, further investigations, including multi-day samplings, are necessary to gain a deeper understanding of the long-term dynamics of microbiota within red algae-dominated wrack. [Display omitted] •FIB enumeration, Salmonella and Campylobacter presence (PCR) and the source of faecal pollution (H183 and GFD markers) (qPCR) were analysed;•Enterococcus spp. levels were significantly higher in water and sand with wrack accumulations;•FIB levels were positively correlated with human faecal pollution in areas affected by wrack, while it varied in respect to bird pollution;•cDOM and turbidity were indicated as the primary drivers of FIB abundances in water with accumulated wrack;•Campylobacter were found in water and sand with wrack and wrack itself;