## 3 Results  
### Hourly River Discharge, Water Conditions, and Atmospheric Measurements  
During the study period, discharge in the Mississippi River at Baton Rouge (USGS #0774000) was three times greater in May (24,530 std. \(\pm\) 1996 m\({}^{3}\) s\({}^{-1}\)) than in August (8912 \(\pm\) 57 m\({}^{3}\) s\({}^{-1}\)), which is representative of the typical long-term hydrograph of the river. Gathering samples over such a wide range of flow conditions allowed us to compare diel \(p\)CO\({}_{2}\) dynamics under high and low flow regimes. In spring, river discharge marginally decreased over the diel period (Figure 2a). In summer river discharge remained constant over the 24-h samples. Figure 2: Diel variation of river discharge (**a**), water temperature (**b**), pH (**c**), dissolved oxygen (**d**), turbidity (**e**), atmospheric temperature (**f**), solar radiation (**g**), and wind speed (**h**) during the study period. Hollow data points represent samples collected during daylight hours (i.e., solar radiation >0.00 kW m\({}^{-2}\)). The daily hours are Central Standard Time of the United States. The magnitude of some ambient water parameters varied greatly between the seasons, however, diel variability was consistent between spring and summer. Water temperature was nearly 8 \({}^{\circ}\)C warmer in summer than spring, averaging 29.3 (\(\pm\)0.4) and 21.4 \({}^{\circ}\)C (\(\pm\)1.5), respectively. The pattern and degree of variability in water temperature was similar between all samples, with temperatures rising during daylight hours until 15:00 CST, gradually decreasing over the remainder of daylight, then remaining constant over evening hours (Figure 2b). Water temperatures on average varied by 0.9 \({}^{\circ}\)C (\(\pm\)0.1) over the 24-h periods, but demonstrated no significant difference between daylight and darkness hours. pH was also higher in summer than spring, averaging 7.75 (\(\pm\)0.03) in spring and 8.25 (\(\pm\)0.04) in summer. Diel variation in pH was similar to water temperature, with maximum measurements at 15:00 CST and lowest measurements during darkness hours (Figure 2c). pH varied on average by 0.09 (\(\pm\)0.01), resulting in measurements during daylight being significantly higher than during darkness (\(p\) < 0.0001). DO averaged 8.64 (\(\pm\)1.11) in spring and 8.41 mg L\({}^{-1}\) (\(\pm\)1.25) in summer.). The diel variability of DO was greater in summer, however, no clear diel pattern could be discerned (Figure 2d). Water turbidity was slightly higher in spring than summer, averaging 37 (\(\pm\)6) and 24 NTUs (\(\pm\)14), respectively. During the majority of sample periods, turbidity was highest during daylight hours (Figure 2e), likely due to daytime boat traffic in the river. Meteorological parameters showed a minimal difference between spring and summer, but similar diel patterns between seasons. Air temperature averaged 25.7 \({}^{\circ}\)C (\(\pm\)4.8) in spring and 26.9 \(\pm\) 4.3 \({}^{\circ}\)C in summer. Diel variation in air temperature was consistent between samples, rising from sunrise until 15:00, where it then gradually decreased through to 6:00 the following day (Figure 2f). This distinct diel pattern resulted in an average diel temperature range of 11.3 \({}^{\circ}\)C (\(\pm\)0.8) and air temperatures significantly (\(p\) < 0.0001) greater by 7.2 \({}^{\circ}\)C (\(\pm\)2.7) during daylight hours. Solar radiation did not vary between seasons, averaging 0.23 kW m\({}^{-2}\) (\(\pm\)0.09) in spring and 0.24 kW m\({}^{-2}\) (\(\pm\)0.13) in summer. During all sample days, solar radiation rose until 12:00 or 15:00 then decreased to 0 kW m\({}^{-2}\) by 21:00 (Figure 2g). Maximum solar radiation readings ranged between 0.27-0.38 kW m\({}^{-2}\). Average wind speed was the only atmospheric parameter to reflect a large difference between seasons, with wind speeds on average two times higher in spring than summer (9.5 \(\pm\) 6.2 vs. 4.6 \(\pm\) 4.6 km hr\({}^{-1}\).