We investigated environmental, landscape, and microbial
factors
that could structure the spatiotemporal variability in the nontarget
chemical composition of four riverine systems in the Oregon Coast
Range, USA. We hypothesized that the nontarget chemical composition
in river water would be structured by broad-scale landscape gradients
in each watershed. Instead, only a weak relationship existed between
the nontarget chemical composition and land cover gradients. Overall,
the effects of microbial communities and environmental variables on
chemical composition were nearly twice as large as those of the landscape,
and much of the influence of environmental variables on the chemical
composition was mediated through the microbial community (i.e., environment
affects microbes, which affect chemicals). Therefore, we found little
evidence to support our hypothesis that chemical spatiotemporal variability
was related to broad-scale landscape gradients. Instead, we found
qualitative and quantitative evidence to suggest that chemical spatiotemporal
variability of these rivers is controlled by changes in microbial
and seasonal hydrologic processes. While the contributions of discrete
chemical sources are undeniable, water chemistry is undoubtedly impacted
by broad-scale continuous sources. Our results suggest that diagnostic
chemical signatures can be developed to monitor ecosystem processes,
which are otherwise challenging or impossible to study with existing
off-the-shelf sensors.