Abstract
The number of Vibrio-related infections in humans, e.g., by Vibrio vulnificus, has increased along the coasts of the Baltic Sea. Due to climate change, vibriosis risk is expected to increase. It is, therefore, pertinent to design a strategy for mitigation of the vibriosis threat in the Baltic Sea area, but a prerequisite is to identify the environmental conditions promoting the occurrence of pathogenic Vibrio spp., like V. vulnificus. To address this, we sampled three coastal Baltic sites in Finland, Germany, and Denmark with salinities between 6 and 21 from May to October 2022. The absolute and relative abundances of Vibrio spp. and V. vulnificus in water were compared to environmental conditions, including the presence of the eelgrass Zostera marina, which has been suggested to reduce pathogenic Vibrio species abundance. In the water column, V. vulnificus only occurred at the German station between July and August at salinity 8.1-11.2. Temperature and phosphate (PO43-) were identified as the most influencing factors for Vibrio spp. and V. vulnificus. The accumulation of Vibrio spp. in the sediment and the co-occurrence with sediment bacteria in the water column indicate that sediment resuspension contributed to V. vulnificus abundance. Interestingly, V. vulnificus co-occurred with specific cyanobacteria taxa, as well as specific bacteria associated with cyanobacteria. Although we found no reduction in Vibrio spp. or V. vulnificus associated with eelgrass beds, our study underscores the importance of extended heatwaves and sediment resuspension, which may elevate the availability of PO43-, for Vibrio species levels at intermediate salinities in the Baltic Sea. Importance: Elevated sea surface temperatures are increasing the prevalence of pathogenic Vibrio at higher latitudes. The recent increase in Vibrio-related wound infections and deaths along the Baltic coasts is, therefore, of serious health concern. We used culture-independent data generated from three Baltic coastal sites in Denmark, Germany, and Finland from May to October (2022), with a special focus on Vibrio vulnificus, and combined it with environmental data. Our temporal model shows that temperature, combined with sediment resuspension, drives the prevalence of V. vulnificus at intermediate salinities in the coastal Baltic Sea.