9:45 AM - 10:00 AM
[AHW27-04] Diel and Seasonal Variation of Partial Pressure of Carbon Dioxide in Surface Water of an Agricultural Reservoir
Keywords:agricultural reservoir, carbon dioxide, diel variaion, photosynthesis, respiration
Lakes and reservoirs are known as both sources of carbon dioxide (CO2) (e.g., Canadell and Monteiro, 2021) and sinks of organic carbon (e.g., Tranvik et al., 2009) and are considered important components in the carbon cycle. Dissolved CO2 concentration (CO2 conc) and CO2 partial pressure (pCO2) in inland waters are known to exhibit diel variation (e.g., Martinez-Cruz et al., 2020). However, since this conclusion has been drawn from several observations spanning only a few days, the controlling factors of diel variation and long-term trend of it remain unclear. Because pCO2 can fluctuate severalfold depending on local time, it is necessary to account for this variation in order to accurately estimate CO2 emissions from inland waters. The objective of this study was to determine the diel variation and the seasonal variation of dissolved CO2 concentration and pCO2 in an agricultural reservoir in Hyogo Prefecture, in which the largest number of agricultural ponds exist in Japan.
The survey was conducted from September 2022 to August 2023 at Uwaike in Himeji City, Hyogo Prefecture (flooded area: approximately 45,000 m², Mean water depth: 4.9 m). Continuous monitoring was conducted on meteorological data (e.g., photosynthetically active radiation: PAR) and the physical environment of the reservoir (e.g., surface water temperature: WT) at a pier located about 50 m downstream from the inflow point of stream into the reservoir (at a water depth of approximately 1.5 m). Interval (every 30 minutes) and snapshot observations (both 3–5 days per week) of CO2 conc were also performed using a diaphragm CO2 meter, along with experimental analyses of collected water samples (e.g., spectrophotometric analysis of chlorophyll). pCO2 was calculated based on the obtained CO2 conc and WT. Using pCO2 from interval observations, we calculated the mean diel variation monthly and then averaged it by 3 months to examine the mean diel variation seasonally.
Snapshot observations confirmed that higher chlorophyll concentrations corresponded to lower pCO2 levels, indicating that pCO2 variation was regulated by photosynthesis. The interval observations yielded results indicating that the time at which pCO2 reached its daily maximum (pCO2 max) and minimum (pCO2 min) was the earliest in summer (maximum at 5:00–5:30 AM and minimum at 1:00–1:30 PM) and the latest in winter (maximum at 9:00–9:30 AM and minimum at 5:00–5:30 PM). pCO2 max or pCO2 min showed different relationships to daily mean values of WT and/or PAR in different seasons. pCO2 max showed 1) no significant relationships to both factors in autumn, 2) positive relationships to both factors in winter, 3) a negative relationship to WT in spring, and 4) a negative relationship to WT and a positive relationship to PAR in summer. pCO2 min 1) negatively related to both factors in autumn, 2) positively related to WT in winter, and 3) negatively related to WT in both spring and summer. Additionally, as for the relationships between the mean diel variation of 30-minute change in CO2 conc (ΔCO2), which was averaged by season, and WT and/or PAR of the corresponding time, ΔCO2 showed 1) negative relationships to both factors in autumn, 2) a negative relationship to PAR in winter, and 3) a positive relationship to WT and negative relationship to PAR in spring and summer. pCO2 max or pCO2 min and ΔCO2 were differentially affected by WT in spring and summer, suggesting that WT elevation has a negative effect on CO2 conc in seasonal variation, while it has a positive effect on CO2 conc in diel variation. Furthermore, negative relationships to WT and PAR suggest the contribution of CO2 fixation in photosynthesis, while positive relationships to WT indicate the contribution of CO2 production by respiration, and positive relationships to PAR may indicate either photoinhibition of photosynthesis (e.g., Neale and Richerson, 1987) or photodegradation of organic matter (e.g., Granéli et al., 1998).
The survey was conducted from September 2022 to August 2023 at Uwaike in Himeji City, Hyogo Prefecture (flooded area: approximately 45,000 m², Mean water depth: 4.9 m). Continuous monitoring was conducted on meteorological data (e.g., photosynthetically active radiation: PAR) and the physical environment of the reservoir (e.g., surface water temperature: WT) at a pier located about 50 m downstream from the inflow point of stream into the reservoir (at a water depth of approximately 1.5 m). Interval (every 30 minutes) and snapshot observations (both 3–5 days per week) of CO2 conc were also performed using a diaphragm CO2 meter, along with experimental analyses of collected water samples (e.g., spectrophotometric analysis of chlorophyll). pCO2 was calculated based on the obtained CO2 conc and WT. Using pCO2 from interval observations, we calculated the mean diel variation monthly and then averaged it by 3 months to examine the mean diel variation seasonally.
Snapshot observations confirmed that higher chlorophyll concentrations corresponded to lower pCO2 levels, indicating that pCO2 variation was regulated by photosynthesis. The interval observations yielded results indicating that the time at which pCO2 reached its daily maximum (pCO2 max) and minimum (pCO2 min) was the earliest in summer (maximum at 5:00–5:30 AM and minimum at 1:00–1:30 PM) and the latest in winter (maximum at 9:00–9:30 AM and minimum at 5:00–5:30 PM). pCO2 max or pCO2 min showed different relationships to daily mean values of WT and/or PAR in different seasons. pCO2 max showed 1) no significant relationships to both factors in autumn, 2) positive relationships to both factors in winter, 3) a negative relationship to WT in spring, and 4) a negative relationship to WT and a positive relationship to PAR in summer. pCO2 min 1) negatively related to both factors in autumn, 2) positively related to WT in winter, and 3) negatively related to WT in both spring and summer. Additionally, as for the relationships between the mean diel variation of 30-minute change in CO2 conc (ΔCO2), which was averaged by season, and WT and/or PAR of the corresponding time, ΔCO2 showed 1) negative relationships to both factors in autumn, 2) a negative relationship to PAR in winter, and 3) a positive relationship to WT and negative relationship to PAR in spring and summer. pCO2 max or pCO2 min and ΔCO2 were differentially affected by WT in spring and summer, suggesting that WT elevation has a negative effect on CO2 conc in seasonal variation, while it has a positive effect on CO2 conc in diel variation. Furthermore, negative relationships to WT and PAR suggest the contribution of CO2 fixation in photosynthesis, while positive relationships to WT indicate the contribution of CO2 production by respiration, and positive relationships to PAR may indicate either photoinhibition of photosynthesis (e.g., Neale and Richerson, 1987) or photodegradation of organic matter (e.g., Granéli et al., 1998).