A large-scale assessment of lake bacterial communities reveals pervasive impacts of human activities

Lakes play a pivotal role in ecological and biogeochemical processes and have been described as ‘sentinels’ of environmental change. Assessing ‘lake health’ across large geographic scales is critical to predict the stability of their ecosystem services and their vulnerability to anthropogenic disturbances. The LakePulse research network is tasked with the assessment of lake health across gradients of land use on a continental scale. Bacterial communities are an integral and rapidly responding component of lake ecosystems, yet large-scale responses to anthropogenic activity remain elusive. Here, we assess the ecological impact of land use on bacterial communities from 220 lakes covering more than 660 000 km 2 across Eastern Canada. Alteration of communities was found on every level examined including richness, community composition, community network structure and indicator taxa of high or low lake water quality. Specifically, increasing anthropogenic impact within the watershed lowered richness mediated by changes in salinity. Likewise, community composition was significantly correlated with agriculture and urban development within a watershed. Interaction networks showed decreasing complexity and fewer keystone taxa in impacted lakes. Together, these findings point to vast bacterial community changes of largely unknown consequences induced by human activity within lake watersheds. Significance Statement Lakes play central roles in Earth’s ecosystems and are sentinels of climate change and other watershed alterations. Assessing lake health across large geographic scales is therefore critical to predict ecosystem stability and lake vulnerability to anthropogenic disturbances. In this context, the LakePulse research network is tasked with a large-scale assessment of lake health across Canada. Bacterial communities are an integral and rapidly responding component of lake ecosystems, yet their large-scale responses to anthropogenic activity remain unknown. Here, we assessed the anthropogenic impact on bacterial communities of over 200 lakes located across large environmental gradients. We found communities to be impacted on every level investigated, indicating that human activities within watersheds cause vast bacterial community changes of largely unknown consequences.