Intensified Anthropogenic Disturbances Impair Planktonic Algae in an Urban River

Anthropogenic activities have posed significant threats to water quality and aquatic organisms in urban rivers, yet the understanding of how planktonic and sedimentary algal communities respond to different levels of anthropogenic disturbances remains limited. To address this knowledge gap, we conducted a systematic investigation on the effects of three anthropogenic-disturbance levels (i.e., low in the upstream, moderate in the midstream, and high in the downstream reach) on algal communities in water and sediment of the Bahe River. Our results revealed that planktonic rather than sedimentary algae were more vulnerable to anthropogenic activities in the urban river. Notably, we found the lowest biodiversity and niche breadth of planktonic algae in the downstream subjected to the high anthropogenic stress. Furthermore, the lowest complexity and stability of co-occurrence patterns of planktonic algae as well as planktonic algae-bacteria were witnessed under the highest human disturbance. Anthropogenic activities, followed by physicochemical variables, geographical patterns, and bacteria co-occurring with algae, played essential roles in shaping the niche differentiation and community assembly of planktonic algae. This study suggested that anthropogenic disturbances (represented by nighttime light, gross domestic product, and impervious surface) can alter physiochemical conditions (such as nitrogen and phosphorus contents) and thereby damage algal compositions along the urban river, highlighting the importance of effective measures for mitigating the impacts of anthropogenic activities and protecting the ecological health of river systems.