Nontarget Chemical Composition of Surface Waters May Reflect Ecosystem Processes More than Discrete Source Contributions

We investigated environmental, landscape, and microbialfactorsthat could structure the spatiotemporal variability in the nontargetchemical composition of four riverine systems in the Oregon CoastRange, USA. We hypothesized that the nontarget chemical compositionin river water would be structured by broad-scale landscape gradientsin each watershed. Instead, only a weak relationship existed betweenthe nontarget chemical composition and land cover gradients. Overall,the effects of microbial communities and environmental variables onchemical composition were nearly twice as large as those of the landscape,and much of the influence of environmental variables on the chemicalcomposition was mediated through the microbial community (i.e., environmentaffects microbes, which affect chemicals). Therefore, we found littleevidence to support our hypothesis that chemical spatiotemporal variabilitywas related to broad-scale landscape gradients. Instead, we foundqualitative and quantitative evidence to suggest that chemical spatiotemporalvariability of these rivers is controlled by changes in microbialand seasonal hydrologic processes. While the contributions of discretechemical sources are undeniable, water chemistry is undoubtedly impactedby broad-scale continuous sources. Our results suggest that diagnosticchemical signatures can be developed to monitor ecosystem processes,which are otherwise challenging or impossible to study with existingoff-the-shelf sensors. The overallchemical composition in local rivers is structuredmore by seasonal variations in aquatic bacteria and rainfall insteadof land cover. Therefore, ecosystem processes are important contributorsto aquatic chemistry.