Differential accumulation of enterococci and arsenic in pelagic Sargassum and seagrass wrack on South Florida beaches

Wrack, composed of organic debris like seagrass and the brown macroalga, Sargassum, accumulates on beaches and when present in overwhelming quantities can impact ecosystem function and public health through microbial contamination, trace element accumulation, and toxic gas emissions. Although studies have started to evaluate the impacts to the microbial quality from different types of wrack, few evaluate the impacts of trace elements. This study assessed enterococci and arsenic levels in beach environments across five South Florida beaches with varying wrack types (Sargassum vs. seagrass) and management practices. Enterococci levels did not significantly differ between wrack types (p = 0.30), with a maximum of 9,600 CFU/g. However, Sargassum exhibited significantly higher arsenic concentrations (up to 64.3 mg/kg) compared to seagrass (2.18 mg/kg) (p < 0.001). In sand, arsenic levels were statistically higher (4.92 mg/kg) when Sargassum was managed through integration. These findings emphasize the need to consider wrack composition when assessing arsenic impacts and can inform beach management strategies to minimize environmental and public health risks associated with excessively large Sargassum strandings.