Assessment of spatial and temporal variability in fecal contamination patterns within urbanized bay shellfish growing...

Fecal contamination poses substantial risks to public health and aquaculture in marine shellfish growing areas. This study evaluated microbial contamination in Tahe Bay using fecal indicator bacteria quantification and 16S rRNA gene sequencing. Samples were collected over 1 year on a monthly basis from seawater at six sites within the shellfish growing area and one site at a beach area used for bathing. Additionally, various environmental and biological samples were collected along the Tahe Bay coastline, including sewage from three outfalls, beach sand, rainwater runoff, and fecal samples from humans, dogs, and birds. Source contributions were determined using the SourceTracker program. Enterococci were detected in 76% of seawater samples, indicating widespread fecal contamination, with sewage outfalls originating from a marina, seafood-processing plant, and domestic sewage of an adjacent town contributing 39, 11, and 28% of the contamination. Spatial analysis revealed increased abundances of Arcobacter and Thiothrix nearshore, in close proximity to these outfalls. Although contributions from humans, dogs, birds, and beach sand were generally minor, increased levels of human fecal microbes were observed at the bathing beach site. Seasonal shifts in microbial community composition were primarily driven by temperature and salinity, with key taxa, including Vibrio, Marinobacterium, and Synechococcus, exhibiting environmental sensitivity. Precipitation and tidal fluctuations further modulated bacterial distribution, while the predominance of genera such as Sulfitobacter and Leucothrix indicated nutrient enrichment and potential eutrophication. The findings demonstrate that community-based microbial source tracking is an effective tool for monitoring fecal pollution in shellfish growing areas.