Limiting nutrients drive mountain stream ecosystem processes along an elevation gradient

Concentrations of N and P are important drivers of stream ecosystem processes, such as primary production and invertebrate nutrient recycling. However, studies have rarely investigated how these processes respond to nutrient additions along gradients of covarying environmental drivers. We conducted experiments in 8 streams to quantify how algal accrual rates, algal nutrient limitation, and invertebrate excretion rates changed along a natural elevation gradient (∼2000–3200 m a.s.l.) of increasing instream N and decreasing temperature. We also investigated how algal accrual rates and algal nutrient limitation changed with season (summer to autumn) as discharge and temperature decreased. First, we confirmed previous findings that algal biomass was primarily N limited in the watershed. As expected, algal N-limitation effect size decreased and invertebrate excretion N∶P molar ratios increased with elevation. Correlated environmental drivers had opposing directional effects on some response variables, allowing us to gain novel insights about ecosystem processes. For example, as elevation increased, the positive effect of instream N concentrations outweighed the negative effect of cooling water temperature on algal accrual and invertebrate mass-specific N-excretion rates. Also, algal accrual increased from summer to autumn in association with increasing N∶P resource ratios and decreasing discharge despite decreasing temperatures. These results demonstrated that resources were stronger drivers than temperature on ecosystem processes in these streams. This and other studies on covarying environmental drivers over space and time can help advance our ability to predict ecosystem responses of streams to future changes in temperature, hydrology, and nutrient loading.