17:15 〜 18:45
[BCG04-P02] Seasonal changes in methane emission from an agricultural pond with controlled water level
キーワード：水圏、渦相関法、溶存酸素、メタン、水位
Methane (CH4) emission from aquatic ecosystems play an important role in the global CH4 budget, and it is expected to increase in the future due to anthropogenic disturbances such as eutrophication along with positive climate feedback. In Japan, there are many agricultural ponds in areas with low rainfall including Hyogo Prefecture. Such agricultural ponds are often shallow and eutrophic, and are expected to release large amounts of CH4 produced in anaerobic sediment. In aquatic ecosystems, CH4 is released via three main pathways: diffusion, ebullition and plant-mediated transport. In particular, ebullition varies in time and space, thus CH4 flux from pond can have high spatial and temporal variability. Micrometeorological techniques represented by the eddy covariance (EC) method can measure gas exchanges between water surface and the atmosphere continuously with spatial representativeness. However, few studies have measured CH4 emission from agricultural ponds by the EC method. In this study, CH4 flux from a shallow agricultural pond in Hyogo prefecture, Japan was measured by the EC method. The factor controlling seasonal variations in CH4 flux was investigated by analyzing the relationship between CH4 flux and environmental data such as water temperature, water level and dissolved oxygen concentration.
The site was an agricultural pond in Kakogawa-city of Hyogo prefecture, Japan (34°77’N, 134°89’E), and the pond is mainly surrounded by paddy fields and nutrient rich. Emergent plants do not exist in the pond, and it has shores that are partly covered with a bank made of concrete. The water depth was approximately 1.6 m during the farming season and the water was drained during agricultural off-season. Instruments for the EC and meteorological measurements were mounted on a small tower at the edge of the pond. CH4 and CO2/H2O fluxes were measured with an ultrasonic anemometer (CSAT3B, Campbell Scientific) and open-path CH4 (LI-7700, LI-COR) and CO2/H2O analyzers (LI-7500, LI-COR). Water temperature and dissolved oxygen were measured at three depths. The measurement period was from April 1, 2021 to November 21, 2023. Flux data was selected only when winds were coming from the pond and the 70 % of flux footprint was situated above the pond.
We divided the data of each year into three periods: water level rising period (P1, from April 1 to April 30), high water level period (P2, from May 1 to September 30), and water level falling period (P3, from October 1 to October 31). Water level was kept around 1.6 m in P2. CH4 emission rates were low in P1 even if water level was already high. CH4 emission rates increased exponentially with water temeperature increase in P2. Then CH4 emission rates decreased as temperature and water level decreased in P3. CH4 emission rates in P3 were higher than P1 even at lower water level. In other period, CH4 flux was almost zero. Dissolved oxygen concentration at the bottom of the pond was low in P2 and P3, thus CH4 production was considered to occur actively in the sediment in these periods. In addition, it was considered that the dead plankton which can be easily decomposed substrate for CH4 production and the lower water level led to relatively high CH4 emission in P3. Seasonal changes in CH4 emission from the pond were strongly influenced by water temperature, and also affected by artificially managed water levels and high CH4 emission rates were observed not only in summer (P2) but also in autumn (P3).
The site was an agricultural pond in Kakogawa-city of Hyogo prefecture, Japan (34°77’N, 134°89’E), and the pond is mainly surrounded by paddy fields and nutrient rich. Emergent plants do not exist in the pond, and it has shores that are partly covered with a bank made of concrete. The water depth was approximately 1.6 m during the farming season and the water was drained during agricultural off-season. Instruments for the EC and meteorological measurements were mounted on a small tower at the edge of the pond. CH4 and CO2/H2O fluxes were measured with an ultrasonic anemometer (CSAT3B, Campbell Scientific) and open-path CH4 (LI-7700, LI-COR) and CO2/H2O analyzers (LI-7500, LI-COR). Water temperature and dissolved oxygen were measured at three depths. The measurement period was from April 1, 2021 to November 21, 2023. Flux data was selected only when winds were coming from the pond and the 70 % of flux footprint was situated above the pond.
We divided the data of each year into three periods: water level rising period (P1, from April 1 to April 30), high water level period (P2, from May 1 to September 30), and water level falling period (P3, from October 1 to October 31). Water level was kept around 1.6 m in P2. CH4 emission rates were low in P1 even if water level was already high. CH4 emission rates increased exponentially with water temeperature increase in P2. Then CH4 emission rates decreased as temperature and water level decreased in P3. CH4 emission rates in P3 were higher than P1 even at lower water level. In other period, CH4 flux was almost zero. Dissolved oxygen concentration at the bottom of the pond was low in P2 and P3, thus CH4 production was considered to occur actively in the sediment in these periods. In addition, it was considered that the dead plankton which can be easily decomposed substrate for CH4 production and the lower water level led to relatively high CH4 emission in P3. Seasonal changes in CH4 emission from the pond were strongly influenced by water temperature, and also affected by artificially managed water levels and high CH4 emission rates were observed not only in summer (P2) but also in autumn (P3).