Watershed discharge modulates relationships between landscape components and nutrient ratios in stream seston

In order to understand the environmental controls of the elemental composition of suspended particles in streams and rivers, we examined relationships between the stoichiometry of suspended particles and catchment characteristics (e.g., topography and land cover) under contrasting hydrological conditions in streams from the Upper Peninsula of Michigan, USA. Particulate C:N, N:P, and C:P ratios varied considerably among streams on each of 10 separate sampling dates and within most streams through time. When averaged across the study period by stream, particulate C:N:P ratios were strongly related to several catchment properties. For example, particulate C:P and N:P ratios were negatively related to the percentage of catchment used for agriculture but positively related to the percentage of catchment found as wetlands. These relationships reflected, in part, variable concentrations of mineral particles, which were strongly related to stream length, agriculture, and upstream lake area. In addition, the strength of the relationships between particulate elemental composition and catchment properties changed depending upon hydrologic condition of the Ontonagon watershed. For example, periods of wetness, as indicated by high river discharge, were found to increase the strength of seston C:N ratio-percentage of wetlands relationships but to reduce the strength of seston C:P ratios-percentage of wetlands relationships. Our results thus demonstrate that the balance of elements fluxing through forested streams is strongly affected by catchment properties but that these effects are sensitive to watershed discharge. At times of high discharge, there are strong connections to catchment sources while in-stream processes appear to predominate at time of low discharge.