Vertebrate eDNA Profiles in Treated Sewage Effluent and Upstream River Water

Environmental DNA (eDNA) is a valuable tool for biodiversity assessment, yet its application in rivers is complicated by factors like anthropogenic inputs from Sewage Treatment Works (STWs). Such inputs can cause 'eDNA pollution', potentially misrepresenting local fauna. This study investigated the impact of treated sewage effluent on vertebrate eDNA signatures by comparing final effluent from seven UK STWs with paired upstream river water using 12S rRNA metabarcoding. Vertebrate eDNA profiles (Actinopteri, Amphibia, Aves, Mammalia, Hyperoatia) differed significantly between sources. Upstream samples were dominated by native fish DNA, consistent with expected local riverine communities. In stark contrast, effluent samples were overwhelmingly characterised by human DNA and DNA from terrestrial livestock (e.g., chicken, pig, cattle), reflecting inputs into the wastewater system. Alpha diversity metrics were significantly lower in effluent, and overall community composition was significantly different between effluent and upstream sites. These findings demonstrate that treated sewage effluent introduces a distinct, concentrated anthropogenic eDNA signature. This 'eDNA pollution' highlights a significant potential for monitoring bias in eDNA-based surveys downstream of WWTP discharges, underscoring the critical need to consider sampling locations carefully for accurate ecological assessments in regulated rivers.