5:15 PM - 6:30 PM
[AHW22-P08] The influence of vertical water mixing on inter-annual nitrogen dynamics in Lake Biwa
Keywords:Lake Biwa, Nitrogen dynamics, Inter-annual changes, Nitrogen stable isotope ratio
Introduction
Nitrogen is a key nutrient in aquatic environments, but an excessive supply of nitrogen leads to deterioration of water quality, so it is important to maintain an appropriate amount of nitrogen compounds in the lake. To control the amount of nitrogen compound in the lake, it is necessary to clarify the dynamics of nitrogen, and for this purpose, it is necessary to measure both external nitrogen inflow and the internal nitrogen cycle. In recent years, the stratification structure of various lakes around the world has been changing due to global warming, and it has been reported that changes in stratification structure also affect nutrient dynamics. In Lake Biwa, the vertical water convection to the bottom has not happened in recent years. So we need to monitor the whole layer data and inter-annual data to clarify the effects of differences in water mixing conditions on nitrogen dynamics. Although many studies have measured the seasonal variation of nitrogen dynamics, a few studies have measured the medium to long-term variation, and there are very few studies on nitrogen dynamics in all layers. Therefore, we aimed to clarify nitrogen dynamics in Lake Biwa using the middle to long-term monitoring data and to elucidate how it is affected by water mixing conditions in this study.
Material and method
In this study, lake water samples were collected every 1-2 months from 2015 to 2020 at 14 depths in the basin of Lake Biwa's North Lake (Depth is about 90 m), and water temperature, dissolved oxygen, and chl.a were measured by CTD probe. We measured the concentrations of nitrogen compounds, nitrogen stable isotope ratio (δ15N) of nitrogen compounds (δ15NDN, δ15NNO3, and δ15NPN). From the measured nitrogen compound concentrations, d15N, and the area of Lake Biwa by depth calculated from the map, the amount of nitrogen compound and δ15N in the whole lake water were calculated.
Results and discussion
The amount of dissolved nitrogen (DN) was shown to be decreasing in the long term. Since nitrate-nitrogen (NO3-) and dissolved organic nitrogen (DON) account for most of the DN content, it is important to examine the fluctuations of NO3- and DON. NO3- increased from autumn to winter and decreased from spring to autumn, while DON decreased from autumn to winter and increased from spring to autumn.
The factors that increase or decrease DN is considered to be the inflow from rivers as an external inflow process and the exchanging from the lake bottom as an internal load. Both factors increase the nitrogen compound amount in the lake. Since this cannot be explained by the fact that NO3- and DON are fluctuating at the same time, we will discuss the effect of the internal nitrogen cycle. The increase of NO3- and decrease of DON from autumn to winter can be attributed to nitrification.
The autumn to winter nitrification event started times vary from year to year. The timing of this nitrification event happened was fast in 2018, slow in 2016, 2020 and no nitrification event has happened in 2017, 2019. Vertical water convection was late in 2016, and no vertical water convection of bottom was in 2019,2020. Thus we conclude the difference in the timing of this event might be due to the strength of the stratification and timing of vertical water convection.
Nitrogen is a key nutrient in aquatic environments, but an excessive supply of nitrogen leads to deterioration of water quality, so it is important to maintain an appropriate amount of nitrogen compounds in the lake. To control the amount of nitrogen compound in the lake, it is necessary to clarify the dynamics of nitrogen, and for this purpose, it is necessary to measure both external nitrogen inflow and the internal nitrogen cycle. In recent years, the stratification structure of various lakes around the world has been changing due to global warming, and it has been reported that changes in stratification structure also affect nutrient dynamics. In Lake Biwa, the vertical water convection to the bottom has not happened in recent years. So we need to monitor the whole layer data and inter-annual data to clarify the effects of differences in water mixing conditions on nitrogen dynamics. Although many studies have measured the seasonal variation of nitrogen dynamics, a few studies have measured the medium to long-term variation, and there are very few studies on nitrogen dynamics in all layers. Therefore, we aimed to clarify nitrogen dynamics in Lake Biwa using the middle to long-term monitoring data and to elucidate how it is affected by water mixing conditions in this study.
Material and method
In this study, lake water samples were collected every 1-2 months from 2015 to 2020 at 14 depths in the basin of Lake Biwa's North Lake (Depth is about 90 m), and water temperature, dissolved oxygen, and chl.a were measured by CTD probe. We measured the concentrations of nitrogen compounds, nitrogen stable isotope ratio (δ15N) of nitrogen compounds (δ15NDN, δ15NNO3, and δ15NPN). From the measured nitrogen compound concentrations, d15N, and the area of Lake Biwa by depth calculated from the map, the amount of nitrogen compound and δ15N in the whole lake water were calculated.
Results and discussion
The amount of dissolved nitrogen (DN) was shown to be decreasing in the long term. Since nitrate-nitrogen (NO3-) and dissolved organic nitrogen (DON) account for most of the DN content, it is important to examine the fluctuations of NO3- and DON. NO3- increased from autumn to winter and decreased from spring to autumn, while DON decreased from autumn to winter and increased from spring to autumn.
The factors that increase or decrease DN is considered to be the inflow from rivers as an external inflow process and the exchanging from the lake bottom as an internal load. Both factors increase the nitrogen compound amount in the lake. Since this cannot be explained by the fact that NO3- and DON are fluctuating at the same time, we will discuss the effect of the internal nitrogen cycle. The increase of NO3- and decrease of DON from autumn to winter can be attributed to nitrification.
The autumn to winter nitrification event started times vary from year to year. The timing of this nitrification event happened was fast in 2018, slow in 2016, 2020 and no nitrification event has happened in 2017, 2019. Vertical water convection was late in 2016, and no vertical water convection of bottom was in 2019,2020. Thus we conclude the difference in the timing of this event might be due to the strength of the stratification and timing of vertical water convection.