# Multi-Year Monitoring of Ecosystem Metabolism in Two Branches of a Cold-Water Stream
## 4 Discussion
This resulted in the North Branch site having a mean bias in GPP (GPP uncorrected for groundwater/GPP corrected for groundwater) of 0.57, indicating GPP estimates may be lower than if corrected for groundwater input. For ER, the bias was 1.12, indicating that our measures of ER may be higher than if corrected for groundwater input. Both GPP and ER were only measured in the fall in the 2015 study. However, the groundwater input in the North Branch were fairly steady throughout the 2017-2019 period with a slight decrease in 2019. Applying the 0.57 bias for GPP brings the levels of GPP up, but all were still lower than the level of GPP in the South Branch. For ER, applying the 1.17 bias does not affect the ranking of ER between years and locations. There may be unaccounted for changes in DO concentrations in the aquifer, which, in addition to changes in groundwater input, could influence changes in the measures of DO in this study. Water table level, flooding, and rainfall inputs can all influence subsurface DO levels resulting in seasonal and diel changes in groundwater DO levels ranging from hypoxic to 91% saturation [66]. Even when GPP levels in the North Branch were corrected for groundwater input, the South Branch had higher GPP levels, which was unexpected given the colder temperature of this stream. [PERSON] et al. [46] found similar results for the fall of 2015 with GPP higher in the South Branch. They also found that periphyton production was higher in the North Branch than the South Branch, which was also unexpected given the higher GPP in the South Branch. They found that nutrient additions, especially of P, increased periphyton production in both streams, suggesting it may be a limiting nutrient. The levels of N in both branches are considered relatively high (>1 mg/L), while phosphorus levels are considered low (<0.1 mg/L) [67]. Nitrogen levels were higher in the South Branch, which might support greater GPP. The lower N levels in the North Branch may be due to denitrification occurring in the wetlands associated with Lake Edith at the head of this branch. While nutrient levels may not be greatly important in influencing GPP, nitrate-N was the third most important predictor of GPP after temperature and light [68]. However, even though nutrient levels within a river are often poorly correlated with GPP, nutrient supplies, rather than concentrations, often limit GPP [68]. The land use in the South Branch contains more cultivated crops and hay/pasture land than the North Branch [69], and thus, nutrient supplies may be greater in the South Branch supporting higher GPP. Another possibility is that there are differences in the types of primary producers in the two branches. Autotrophs from cold-water streams can have higher photosynthetic rates than those from warmer streams and which could lead to higher GPP in cold-water streams [70]. Several studies have shown that there are both seasonal and interannual variation in ecosystem metabolism. There are two major types of seasonal variability in U.S. rivers: a summer peak or a spring peak [71]. Spring peak rivers had daily patterns of GPP that were only weakly correlated with water temperature, discharge, or PAR and tend to be associated with smaller watersheds such as headwater streams. In this study, GPP was higher in the spring and corresponded to a time when light levels were greater in the forested area near the sampling sites. Seasonal patterns of ecosystem metabolism can be greatly affected by leaf phenology, with higher rates of GPP rates and NEP approaching autotrophy under an open canopy in the spring [44; 56]. Models with season as a main effect were the best predictors of GPP, ER, and NEP [72], with approximately 50% of the variation in GPP and ER related to season [43]. The sag in percent oxygen in the North Branch in May and August of 2018 could be due to inputs of organic material from the surrounding wetland. This corresponded to peaks of ER during these months.