10:00 〜 10:15
[AHW27-05] Analysis of carbon and nitrogen dynamics in a eutrophic farm pond using stable isotopic signature
キーワード：ため池、安定同位体、炭素、窒素
Farm ponds, which are mainly built for irrigation, are estimated to store around 150 TgC of organic carbon per year on a global scale and the most active sites of biogeochemical cycle in the biosphere (Downing et al., 2008). In addition, farm ponds are expected to improve water quality by removing suspended solids through sedimentation, as well as by removing nitrogen and phosphorous through macrophyte absorption (Casanova et al., 1997; Okubo, 1998). Eutrophication and loss of macrophytes in ponds are an ongoing issue, especially in Hyogo Prefecture, where the largest number of farm ponds exist in Japan (Ishii et al., 2005). Cyanobacterial blooms are common in eutrophic ponds with little macrophytes, where intense photosynthesis and nitrogen fixation by cyanobacteria can lead to increase in organic carbon and nitrogen in pond water and outflowing water. This study aims to elucidate the seasonal changes in quantity and quality of carbon and nitrogen, and the biogeochemical processes behind them.
Nunoike (34°46’30.8“N 134°53’36”E) is a farm pond located in Kakogawa city, Hyogo. The water depth and surface area during irrigation period (from June to October) are about 1.6 m and 5.9 ha respectively. The catchment area is composed of paddy field (47.7 ha) and residential areas (23 ha). There is little macrophyte within the pond and cyanobacterial blooms are observed in summer. Water samples were collected approximately once or twice per month from April 2022 to October 2023 (31 times in total). Samples were collected at inflow and near outflow in pond. Each water sample was filtered using Whatman GF/F filter, and concentrations of dissolved organic carbon and nitrogen (DOC and DON) and dissolved inorganic nitrogen (DIN) in filtrates were measured. Concentrations of particulate organic carbon and nitrogen (POC and PON) on filters and their stable isotope ratios (δ13C-POC and δ15N-PON) were also measured. Chlorophyll a and b (Chl-a and Chl-b) concentrations were determined by spectroscopy. Concentrations of dissolved CO2 and pH were also measured on site.
Both DOC and POC in pond water were usually higher than those in the inflow water. POC in pond water increased from summer to autumn (23.2 ± 9.2 mgC/ L at pond surface) and accounted for more than half of total organic carbon. δ13C-POC of pond water was high (about -20 ‰) in summer and autumn. δ13C-POC and Chl-a exhibited similar seasonal variations and were positively correlated (r = 0.65, p < 0.001). The increase in δ13C-POC from summer to autumn could be attributed to two factors. First, reduction in photosynthetic isotope fractionation due to depletion of dissolved CO2 concentration in water, (i.e., phytoplankton preferentially utilizes 12CO2 over 13CO2). Second, phytoplankton came to utilize more HCO3-, which had higher δ13C than dissolved CO2. Total nitrogen (DIN + DON + PON) in pond water increased from summer to autumn (5.0 ± 2.4 mgN/ L at pond surface), which was nearly twice or more than those of inflow water. PON accounted for a significant portion of total nitrogen while DIN was little in pond water in summer and autumn. In contrast, DIN accounted for a fairly large portion of total nitrogen in inflow throughout the sampling period. δ15N-PON of pond water was lowest in summer (approximately 0 ‰). δ15N-PON and Chl-a: Chl-b ratio were negatively correlated (r = -0.71, p < 0.001). This could be explained by a summer phytoplankton bloom and fixation of atmospheric nitrogen (δ15N = 0 ‰) by cyanobacteria, which is supported by the Chl-aand /Chl-b ratio. Our study suggests that photosynthesis and N2-fixation especially by the cyanobacterial blooms may contribute to increase in organic carbon and nitrogen in eutrophic ponds and, therefore, in outflow water to the downstream areas.
Nunoike (34°46’30.8“N 134°53’36”E) is a farm pond located in Kakogawa city, Hyogo. The water depth and surface area during irrigation period (from June to October) are about 1.6 m and 5.9 ha respectively. The catchment area is composed of paddy field (47.7 ha) and residential areas (23 ha). There is little macrophyte within the pond and cyanobacterial blooms are observed in summer. Water samples were collected approximately once or twice per month from April 2022 to October 2023 (31 times in total). Samples were collected at inflow and near outflow in pond. Each water sample was filtered using Whatman GF/F filter, and concentrations of dissolved organic carbon and nitrogen (DOC and DON) and dissolved inorganic nitrogen (DIN) in filtrates were measured. Concentrations of particulate organic carbon and nitrogen (POC and PON) on filters and their stable isotope ratios (δ13C-POC and δ15N-PON) were also measured. Chlorophyll a and b (Chl-a and Chl-b) concentrations were determined by spectroscopy. Concentrations of dissolved CO2 and pH were also measured on site.
Both DOC and POC in pond water were usually higher than those in the inflow water. POC in pond water increased from summer to autumn (23.2 ± 9.2 mgC/ L at pond surface) and accounted for more than half of total organic carbon. δ13C-POC of pond water was high (about -20 ‰) in summer and autumn. δ13C-POC and Chl-a exhibited similar seasonal variations and were positively correlated (r = 0.65, p < 0.001). The increase in δ13C-POC from summer to autumn could be attributed to two factors. First, reduction in photosynthetic isotope fractionation due to depletion of dissolved CO2 concentration in water, (i.e., phytoplankton preferentially utilizes 12CO2 over 13CO2). Second, phytoplankton came to utilize more HCO3-, which had higher δ13C than dissolved CO2. Total nitrogen (DIN + DON + PON) in pond water increased from summer to autumn (5.0 ± 2.4 mgN/ L at pond surface), which was nearly twice or more than those of inflow water. PON accounted for a significant portion of total nitrogen while DIN was little in pond water in summer and autumn. In contrast, DIN accounted for a fairly large portion of total nitrogen in inflow throughout the sampling period. δ15N-PON of pond water was lowest in summer (approximately 0 ‰). δ15N-PON and Chl-a: Chl-b ratio were negatively correlated (r = -0.71, p < 0.001). This could be explained by a summer phytoplankton bloom and fixation of atmospheric nitrogen (δ15N = 0 ‰) by cyanobacteria, which is supported by the Chl-aand /Chl-b ratio. Our study suggests that photosynthesis and N2-fixation especially by the cyanobacterial blooms may contribute to increase in organic carbon and nitrogen in eutrophic ponds and, therefore, in outflow water to the downstream areas.