Abstract: When studying river ecosystems, we often compare ecosystem states and processes among watersheds to understand variation in ecosystem functioning. In the context of river restoration, however, variation among sites within a watershed can give rise to different levels of ecosystem response. To restore major fish migrations, governments and private owners are increasingly removing dams or otherwise facilitating fish passage, creating new sources of nutrient and energy subsidies that may vary widely among sites within the upstream river network. In anticipation of such a restoration action, we wanted to map within-network variation in leaf litter breakdown and the nutrient limitation of algal growth. Nutrient diffusing substrate experiments revealed wide variation in algal accrual rates and nutrient limitation, sometimes with nitrogen suppression, which were associated with variation in dissolved nutrient concentrations. Leaf litter breakdown rates were also heterogeneous across the river network, and variation in these rates strongly co-varies with nutrient concentration. This spatial heterogeneity in nutrient and carbon dynamics suggests that ecosystem responses to nutrient subsidies from restored fish migrations will depend greatly on where fish choose to spawn within a watershed. As river connectivity restoration continues to expand, it will be important to account for within-watershed variation in ecosystem context when predicting or evaluating restoration outcomes.
