## 4 Discussion
### Implications for Carbon Outgassing Estimates and Future Research Needs  
This study found CO\({}_{2}\) outgassing rates significantly lower during daylight hours in spring and summer, resulting in a large underestimation of daily outgassing rates under varying flow and temperature regimes. As GPP is generally highest in the Mississippi River in autumn and winter months [33], we would anticipate a similar diel trend throughout these seasons. Though diel variation may not be as drastic in autumn and winter due to lower temperatures, shorter day-light lengths, and potential nutrient limitations [58]. These findings raise questions about the current global estimation of CO\({}_{2}\) outgassed from rivers. Assuming a 34% under-estimation of a total flux of 650 Tg C yr\({}^{-1}\), as recently calculated by [PERSON] and others [7], would result in an under-estimation of 221 Tg C outgassed from streams and rivers to the atmosphere, annually. However, it should be noted that many large tropical rivers that contribute significantly to fresh-water CO\({}_{2}\) emissions [59; 60] may not experience this large of a diel cycle due to limited light attenuation and low nutrient levels limiting GPP [61; 62], limiting the applications of this analysis. Nonetheless, several major world rivers have biogeochemical trends similar to the Mississippi River and diel variability may be even higher in other rivers depending on climate, nutrient, and organic matter availability, and varying day-night length ratios across geographic regions. Therefore, we argue that future high-resolution sampling studies analyzing diel \(p\)CO\({}_{2}\) and CO\({}_{2}\) outgassing dynamics in rivers at differing orders and varying latitudes would be beneficial for constraining the relationship between diel variability and the former environmental factors. Ideally, these relationships could be used to develop a collective day-night \(p\)CO\({}_{2}\) curve that could be used to incorporate diel variability in global CO\({}_{2}\) outgassing estimations.  
The large range of values in global outgassing estimates highlights the need of high-resolution in-stream samples. High resolution sampling would not only constrain the diel variation found in this study, but also incorporate seasonal and spatial variation found in several past studies [12; 13; 14; 45; 15; 16]. It is clear that the estimation of CO\({}_{2}\) outgassing has continued to increase as more data has been collected across rivers at varying orders and geographic locations, shifting the paradigm of the river's role in the carbon cycle. Estimates by [PERSON] and others [3] was the only study to include first-order streams, but was responsible for noting the very high \(p\)CO\({}_{2}\) values with low data resolution, which likely skewed their estimates of 1800 Tg yr\({}^{-1}\) Therefore, conducting high-resolution sampling similar to that conducted in this study in varying geographic regions, as well as in lower order streams and along spatial gradients in large river systems, could help constrain concerns regarding the uncertainties in \(p\)CO\({}_{2}\) measurements driving variability in outgassing calculations from aquatic systems.