Presentation Information
[AOS25-P08]Seasonal variations of Nitrogen Nutrients from Eastern Hokkaido to the Waters off Tohoku, Japan
*Hibiki Otsuka1, Sayaka Yasunaka1, Takako Masuda2, Daisuke Hasegawa2, Hiroshi Kuroda3, Takuya Nakanowatari2, Yukiko Taniuchi2, Tamaha Yamaguchi2 (1.Tohoku University, 2.Japan Fisheries Research and Education Agency, 3.Hokkaido University)
Keywords:
Biogeochemistry,Nitrogen Cycle,Subsurface,Nitrification
Inorganic nitrogen nutrients are essential elements for phytoplankton growth. Phytoplankton assimilates inorganic nitrogen (nitrate (NO3-), nitrite (NO2-), and ammonium (NH4+)) into its biomass and produces organic matter through photosynthesis. NO3- is consumed in the surface layer by phytoplankton uptake, while it is abundant in the deeper layers due to the remineralization of the sinking organic nitrogen in the biological pump. NO2- and NH4+ are also depleted in the surface layer. But their concentrations reach maxima in the subsurface layer and decrease rapidly at greater depth. Many studies have investigated seasonal variations of NO3-, but few have examined the seasonal changes of NO2-. In addition, NH4+ data are quite limited. Understanding the dynamics of these inorganic nitrogen nutrients in the ocean is important for understanding marine biological productivity, especially primary production processes. In this study, we used ship-based observational data along the A (Akkeshi off) line which has been conducted by the Japan Fisheries Research and Education Agency (FRA), and examined the seasonal variations of inorganic nitrogen from eastern Hokkaido to the waters off Tohoku.
Following Kusaka (2016), we classified the water into Coastal Oyashio Water, Oyashio Water, and Warm Water, using salinity thresholds of 33.0 and 33.7. In the surface Coastal Oyashio Water and Oyashio Water, chlorophyll (Chl) concentration increased in spring and autumn, and NO3- was depleted in summer. NH4+ concentration in the subsurface layer increased after the bloom and had a low concentration in autumn. In Warm Water, although the seasonal variations in Chl, NO3-, and NO2- were similar to those in Oyashio Water, their concentrations are relatively low and the bloom occurred only once in spring. NH4+ showed no apparent seasonal variation. NO2- does not show significant vertical concentration difference in winter. In contrast, in summer, particularly in the Warm Water and Oyashio Water, NO2- concentration is high in the subsurface layer and low at the surface and in deeper layers. The decrease of surface NO3- and the increase in subsurface NH4+ after the bloom are thought to be due to uptake by phytoplankton and the decomposition of organic matter produced by photosynthesis. NO2- is produced by NH4+ oxidation (nitrification), but this process is inhibited by light. Therefore, production is suppressed at the surface. In contrast, low-light conditions in the subsurface favor the nitrification of NH4+, leading to the increase in NO2- concentration. In deeper layers, the concentration decreases as NO2- is further oxidized to NO3-. The vertical differences in NO2- concentration are hidden by vertical mixing during winter, but become apparent in summer due to enhanced stratification, resulting in the observed NO2- subsurface high concentrations.
NH4+ observations along the A-line were much less than those of other variables and only in the limited seasons. To ensure data continuity and consistency, we conducted NH4+ measurements on frozen water samples collected during five A-line research cruises from May 2024 to March 2025. The results will be reported in the presentation.
Following Kusaka (2016), we classified the water into Coastal Oyashio Water, Oyashio Water, and Warm Water, using salinity thresholds of 33.0 and 33.7. In the surface Coastal Oyashio Water and Oyashio Water, chlorophyll (Chl) concentration increased in spring and autumn, and NO3- was depleted in summer. NH4+ concentration in the subsurface layer increased after the bloom and had a low concentration in autumn. In Warm Water, although the seasonal variations in Chl, NO3-, and NO2- were similar to those in Oyashio Water, their concentrations are relatively low and the bloom occurred only once in spring. NH4+ showed no apparent seasonal variation. NO2- does not show significant vertical concentration difference in winter. In contrast, in summer, particularly in the Warm Water and Oyashio Water, NO2- concentration is high in the subsurface layer and low at the surface and in deeper layers. The decrease of surface NO3- and the increase in subsurface NH4+ after the bloom are thought to be due to uptake by phytoplankton and the decomposition of organic matter produced by photosynthesis. NO2- is produced by NH4+ oxidation (nitrification), but this process is inhibited by light. Therefore, production is suppressed at the surface. In contrast, low-light conditions in the subsurface favor the nitrification of NH4+, leading to the increase in NO2- concentration. In deeper layers, the concentration decreases as NO2- is further oxidized to NO3-. The vertical differences in NO2- concentration are hidden by vertical mixing during winter, but become apparent in summer due to enhanced stratification, resulting in the observed NO2- subsurface high concentrations.
NH4+ observations along the A-line were much less than those of other variables and only in the limited seasons. To ensure data continuity and consistency, we conducted NH4+ measurements on frozen water samples collected during five A-line research cruises from May 2024 to March 2025. The results will be reported in the presentation.