## 4 Discussion
### Biological Processes Influencing Diel pCO\({}_{2}\) Variation  
This is the first study to document significant diel variation of riverine \(p\)CO\({}_{2}\) and CO\({}_{2}\) outgassing utilizing actual in-stream measurements in the Lower Mississippi River. These findings are in coincidence with studies that have found a similar pattern of a reduction of atmospheric CO\({}_{2}\) over large water bodies during daylight hours [45; 46] This variation is apparently driven by autotrophic processes, resulting in an increase in net ecosystem production (NEP = gross primary production (GPP)--community respiration (CR)) [47] over daylight hours. During all samples across both seasons, \(p\)CO\({}_{2}\) values were highest during the dark, early morning hours and lowest between 15-18:00 CST, following the peak of daylight (Figure 3a,b). The large variation suggests a diel change in the balance of processes producing and removing \(p\)CO\({}_{2}\) in the water column. Reduction in \(p\)CO\({}_{2}\) over daylight corresponded with a diel increase in water temperature and solar radiation (Figure 2b,g), both of which have shown to increase aquatic metabolism [48; 49]. The decrease in \(p\)CO\({}_{2}\) also paired with an increase in pH over daylight hours (Figure 2c). This relationship suggests a diel increase in photosynthesis may be driving the decrease in \(p\)CO\({}_{2}\) as in-situ photosynthesis would drive up pH values. Our in-stream measurements confirm results of a past study of diel \(p\)CO\({}_{2}\) dynamics and stream metabolism in two acidic lower-order peatland streams by [PERSON] et al. [50], which found lowest \(p\)CO\({}_{2}\) levels during early afternoon hours, highest values in the early morning, and a diel increase in NEP, especially during warmer months. In a separate study of aquatic metabolism, [PERSON] and others [47] also documented a diel increase in NEP in many of the streams studied due to a daily increase in GPP, but fairly consistent CR rates. The authors concluded the shift in NEP was driven primarily by \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline  & \(p\)CO\({}_{2}\) & \(\mathbf{T_{w}}\) & \(\mathbf{DO}\) & pH & N & \(\mathbf{T_{a}}\) & \(\mathbf{u}\) & SR & \(\mathbf{Q}\) \\ \hline \(p\)CO\({}_{2}\) & \(1.00\) & \(-0.92\) & - & \(-0.98\) & \(0.46\) & - & - & - & \(0.93\) \\ T\({}_{w}\) & & \(1.00\) & - & \(0.96\) & \(-0.46\) & - & - & - & \(-0.99\) \\ DO & & & \(1.00\) & - & - & - & - & - & - \\ pH & & & & \(1.00\) & \(-0.47\) & - & - & 0.76 & \(-0.97\) \\ N & & & & & \(1.00\) & - & 0.48 & - & 0.49 \\ T\({}_{a}\) & & & & & & \(1.00\) & 0.63 & 0.86 & - \\ u & & & & & & & \(1.00\) & 0.60 & 0.40 \\ SR & & & & & & & & \(1.00\) & - \\ Q & & & & & & & & & \(1.00\) \\ \hline \end{tabular}
\end{table} 
Table 2: Pearson correlation coefficients among \(p\)CO\({}_{2}\), water temperature (T\({}_{w}\)), dissolved oxygen (DO), pH, turbidity (N), atmospheric temperature (T\({}_{a}\)), wind speed (u), solar radiation (SR), and river discharge (Q). Only significant correlations at \(\alpha\) = 0.05 are shown. solar radiation rather than water temperature. We hypothesize that the diel variation in \(p\)CO\({}_{2}\) in the Mississippi River can be explained by a similar shift in NEP, likely controlled by solar radiation driving up photosynthesis rates, as recent studies in the Mississippi and Chattahoochee Rivers have found that water temperature is not a strong predictor of GPP in either river [33]. Even though \(p\)CO\({}_{2}\) values were significantly lower during daylight hours in this study, water temperature showed minimal difference between daylight and darkness, suggesting that a minor shift in water temperature is not the main driver of diel variability. The diel decrease in \(p\)CO\({}_{2}\) was in coincidence with a diel increase in average wind speed during all sample events (Figure 2h).