# Multi-Year Monitoring of Ecosystem Metabolism in Two Branches of a Cold-Water Stream
## 4 Discussion  
This study found differences in measures of ecosystem metabolism between the two branches of Valley Creek with differences between years. There were also significant impacts of temperature, PAR, and discharge for at least some measures of stream metabolism. Several factors can influence aquatic ecosystem metabolism. These include temperature, light intensity, hydrologic disturbance [42; 55; 56], fish spawning [57], nutrient input [58; 59],  
Figure 8: Effect of stream discharge on various measures of ecosystem metabolism (GPP, ER, and NEP). Lines are parametric best-fit lines, and shaded areas are 95% confidence limits. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Variable** & **Factor** & **F Ratio** & **Degrees of Freedom** & **Probability \textgreater{} F** \\ \hline \multirow{4}{*}{GPP} & In (PAR) & 45.0 & 1, 868 & \textless{}0.0001 \\  & Location & 83.1 & 1, 868 & \textless{}0.0001 \\  & In (PAR) * & 32.9 & 1, 868 & \textless{}0.0001 \\  & location & & & \\ \hline \multirow{4}{*}{ER} & In (PAR) & 0.1 & 1, 868 & 0.8 \\  & Location & 3.9 & 1, 868 & 0.05 \\  & In (PAR) * & 29.3 & 1, 868 & \textless{}0.0001 \\  & location & & & \\ \hline \multirow{4}{*}{NEP} & In (PAR) & 0.6 & 1, 868 & 0.4 \\  & Location & 107.2 & 1, 868 & \textless{}0.0001 \\ \cline{1-1}  & In (PAR) * & 28.4 & 1, 868 & \textless{}0.0001 \\ \cline{1-1}  & location & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: ANCOVA of In(PAR) on ln(GPP), ln(ER), and ln(NEP) for the North Branch and South Branch of Valley Creek, MN. and urban local inflows [60]. Land use and riparian canopy cover influence GPP, with agricultural, grassland, and urban streams having higher GPP than forested or desert streams, but little difference in ER between land-use types [61]. Changes in dissolved oxygen levels are used to calculate ecosystem metabolism [60]. Both biotic and abiotic factors can influence the oxygen content in stream systems. Biotic influences include primary production and respiration [62], which are reflected in measures of ecosystem metabolism. Abiotic factors include temperature, water velocity, and oxygen exchange with the atmosphere and groundwater [62; 63]. There were differences in oxygen levels between the two locations in this study and between years. [PERSON] et al. [46] found that the dissolved oxygen level in groundwater was lower than surface water in both of the locations in this study (10.8 versus 93% saturation in groundwater and surface water in the North Branch, respectively, and 23 vs. 95% saturation in groundwater and surface water in the South Branch, respectively). Groundwater input can have a substantial impact on the measurement of steam metabolism [64], although more research is needed into the role of groundwater input on these measures [65]. Some of the differences in the dissolved oxygen levels and measures of ecosystem metabolism between branches could be explained by the North Branch's greater groundwater input and its lower DO content. In 2019, the groundwater hydraulic head was lower in both streams, indicating less groundwater input into the streams. However, the DO levels in 2019 were not significantly higher than in either 2017 or 2018. In 2019, the weather was wetter and cooler than the other years in this study. This may have resulted in the greater stream water depth found, and if the groundwater level stayed the same then, the hydraulic head would be lower. Despite the greater water depth in 2019, it did not result in greater discharge. This could be a result of water depth being measured for the full sampling period, but discharge data are only available for July-November 5. Consequently, the impact of the differing groundwater inputs on DO between years is difficult to ascertain. [PERSON] et al. [46] found that the groundwater in both branches had low levels of oxygen (10.8 and 23.0% for the North and South Branches, respectively).