# Multi-Year Monitoring of Ecosystem Metabolism in Two Branches of a Cold-Water Stream
## 4 Discussion
In addition to seasonal variability, there can be interannual variability in ecosystem metabolism in streams attributable to a combination of light availability, water temperature, storm flow, and desiccation [44; 73]. In this study, interannual variation differed between the two locations. GPP was higher in the South Branch than the North Branch in each year of the study, even though water temperatures were colder and P levels were similar. However, the years in which GPP peaked differed between branches. In the North Branch, GPP was the highest in 2019, but in the South Branch, it was the highest in 2018. Interannual variation was different for ER, being the highest for the North Branch in 2018, with no significant difference between years for the South Branch. The North Branch was more heterotrophic than the South Branch with NEP greatest for the South Branch in 2018 and in 2019 for the North Branch with higher levels of GPP driving these trends. Since seasonal and interannual variability in ecosystem metabolism could be due to differences in temperature, PAR, and discharge, we examined the correlations between these factors and GPP, ER, and NEP. As expected [65; 74; 75], PAR was positively correlated with GPP in both locations. The slope for PAR versus GPP was greater in the North Branch than in the South Branch (0.6 versus 0.04 for the North and South branches, respectively) indicating a great impact of PAR on GPP in the North Branch. [PERSON] et al. [46] found that instream light levels were greater in the North Branch than the South Branch and that periphyton production, at least in the fall, was greater in the North Branch. This could account for the greater impact of light on GPP. Water temperature and PAR can interact synergistically with higher water temperature, enhancing the response of PAR on GPP [73], which was apparently the case in the warmer North Branch. However, the correlation between PAR and temperature makes disentangling the impacts of temperature and PAR difficult [76]. While there were significant interaction terms between PAR and location with both ER and NEP, there were no significant main effects (PAR only), indicating that most of the difference in the interaction term stemmed from the differences between the branches. Since ER in streams may be more influenced by allochthonous sources of carbon than autochthonous sources [75; 77], the lack of relationship between PAR and ER was not unexpected. NEP is correlated with PAR, but mainly through its influence on GPP and not ER [76]. Water temperature was positively correlated with GPP in both locations. ER on the other hand was positively correlated with temperature in the North Branch but negatively correlated in the South Branch. The slope for temperature versus GPP was 0.8 for the North Branch and 0.7 for ER, indicating that the impact of temperature was similar for both GPPand ER at this site. For the South Branch, the slope of the temperature versus GPP was 0.4, while the slope was \(-1.0\) for temperature versus ER, indicating differential impacts of temperature on GPP and ER. Unlike this study where there was either little difference in temperature effects between GPP and ER (North Branch) or where temperature had different impacts on GPP than ER (South Branch), others have found that temperature had a greater influence on ER than on GPP [78]. This was attributed to the higher temperature dependence of respiration compared to photosynthesis [78]. Others, however, have found that temperature sensitivities of GPP and ER at the ecosystem level may not follow the differential sensitivities to temperature at the cellular level [41], while still, others have found that GPP did not vary with temperature when adjusted for differences in autotrophic biomass [70]. The temperature dependence of ER could be influenced by the amount of dissolved organic matter (DOM) with ER increasing more with temperature with higher amounts of DOM [79].