11:30 AM - 11:45 AM
[MIS14-10] Recent changes in the nitrogen cycle of Lake Inawashiro, Japan
Keywords:nitrogen cycle, triple oxygen isotope, lake, assimilation, nitrification
Lake Inawashiro in Fukushima prefecture, Japan, had been classified to an acid trophic lake with low pH until 1990s due to the acidic inflow eluted from the adjacent volcanic area, so that the primary production in the water column has been limited despite nitrogen nutrients such as nitrate were abundant in the water column. However, over the past 20 years, the pH has gradually increased and become more neutral; the current pH is around 7.3 in the water column. As a result, biological activities, including primary production, are expected to increase within the lake, leading to significant temporal changes in the nitrogen cycle as well, in which microbes play significant roles. Therefore, in order to clarify the changes in the nitrogen cycle, we conducted some lake observations in 2022-2023 to quantify the nitrogen cycle such as gross nitrification rates and gross assimilation rates in the lake, and compared it with the results from 2014-2015.
From 2022 to 2023, water samples were collected twice in each year, in June and September, from each layer of the water column at the deepest point of the lake, together with those of major inflows and an outflow. The isotopic compositions of nitrate in each water sample was determined by using continuous flow mass spectrometer subsequent to the chemical conversion to N2O. Both gross assimilation rates and gross nitrification rates were calculated using the method developed by Tsunogai et al. 2018. The total amount of NO3- in the water column in June decreased from 81.6 Mmol in 2015 to 49.2 Mmol in 2023. Since the changes in the Delta-17O were small, from +3.3 permil on the average during 2014-2015 to +3.1 permil on the average during 2022-2023, we concluded that the increase in the nitrification rates were insignificant during the period. On the other hand, the increase in the assimilation rate was significant and most of the assimilated nitrogen has been accumulated on to the sediments in the form of organic nitrogen.
From 2022 to 2023, water samples were collected twice in each year, in June and September, from each layer of the water column at the deepest point of the lake, together with those of major inflows and an outflow. The isotopic compositions of nitrate in each water sample was determined by using continuous flow mass spectrometer subsequent to the chemical conversion to N2O. Both gross assimilation rates and gross nitrification rates were calculated using the method developed by Tsunogai et al. 2018. The total amount of NO3- in the water column in June decreased from 81.6 Mmol in 2015 to 49.2 Mmol in 2023. Since the changes in the Delta-17O were small, from +3.3 permil on the average during 2014-2015 to +3.1 permil on the average during 2022-2023, we concluded that the increase in the nitrification rates were insignificant during the period. On the other hand, the increase in the assimilation rate was significant and most of the assimilated nitrogen has been accumulated on to the sediments in the form of organic nitrogen.