Linking watershed and reach-scale controls of dissolved and particulate nutrients in a stream network

Nutrient concentration and flux in streams are regulated by both local and watershed-scale processes, but these processes are usually studied separately. We examined dynamics of dissolved and particulate nutrients throughout a stream network ranging from headwater springs to small rivers in northern California. Dissolved inorganic nutrients were extremely low and showed little spatial variation in the watershed. In contrast, particulate and dissolved organic species were more varied. Dissolved organic phosphorus (P) decreased with watershed size, while particulate P, dissolved organic carbon (DOC), and nitrogen (N) increased between 2 to 6 fold from small headwater streams to mid sized rivers. Analyses of DOC sources showed that DOC was derived from upland soils and trees in the headwaters but was more influenced by microbial sources within the channel and riparian zone at downstream sites. With increasing watershed size, combined effects of changes to flow paths through riparian zones and increases in aquatic N fixation by cyanobacteria appear to elevate DON inputs to rivers. As a result of opposite trends for dissolved P and DON concentration with stream size, dissolved N:P increased downstream by 30. This increase indicates a spatial shift from N to P limitation of algal production as stream size increased in the stream network. Although headwater streams are thought to exert a stronger influence on nutrient cycling than larger channels, our results indicate the opposite. Under summer baseflow conditions, it appears that downstream changes in channel geomorphology lead to a strong influence of larger channels on nutrient dynamics in the stream network.