Long-term data reveal highly-variable metabolism and transitions in trophic status in a montane stream

In streams, gross primary production (GPP) and ecosystem respiration (ER) (i.e., stream metabolism) control the transport and fate of nutrients and organic carbon and vice versa. The importance of short-term and local factors in driving these processes is well known in the literature. However, little information exists regarding the extent of temporal variability of stream metabolism and how both local physicochemical and broad-scale climatic drivers affect this variability. We used 7 years of field data from an open-canopy headwater stream ecosystem in the southwestern United States to quantify the extent of seasonal and inter-annual variability in stream metabolism (GPP, ER, and net ecosystem production [NEP]) and to assess if temporal variation in these processes was related to the magnitude of snowmelt runoff. In spring, seasonal mean ER ( p 5 0.025, r² 5 0.67) and NEP ( p 5 0.004, r² 5 0.83) were more strongly related to discharge (Q) than GPP ( p 5 0.19, r² 5 0.32), potentially because of an increased influx of nutrients and organic carbon during years with higher snowmelt runoff. There were no strong relationships between seasonal mean GPP and Q, light, temperature, turbidity, and specific conductance ( p ≥ 0.27, r² ≤ 0.18). Our long-term data revealed unanticipated shifts from autotrophic to heterotrophic status within and across years. However, this variability was not strongly associated with environmental factors at either local (i.e., Q or photosynthetically-active radiation) or global (i.e., El Niño-Southern Oscillation) scales. Previous paradigms hold that local attributes dictated by geographic and landscape positioning (e.g., light and temperature regimes) control the trophic status of streams, but our findings suggest that complex combinations of spatiotemporally-variable factors, such as snow accumulation and melting, and their role in connecting terrestrial and aquatic ecosystems can lead to substantial within-stream variation in autotrophic or heterotrophic status.