Spatial and Seasonal Variations of Stable Isotope Ratios of Particulate Organic Carbon and Nitrogen in the Surface Water of the Kuroshio

Abstract

Bioavailable nitrogen and carbon amount are increasing in the ocean. Their stable isotope ratios are good indicators for understanding what happens in the sea. We measured the nitrogen and carbon stable isotope ratio of organic particles which is mainly consisted of living phytoplankton but also consisted of zooplankton, bacteria, and so on in the Kuroshio, flowing in the southern part of Japan. The Kuroshio is one of the key areas not only for fisheries in East Asia countries, but also for Pan-Pacific countries, but we do not know the supply source of nitrogen and carbon which is essential for living resources. Our main finding is that nitrogen sources are different between areas and seasons. That is, deep-sea water is an important source of nitrogen between Japan and the Kuroshio throughout the seasons, but nitrogen fixation by plankton and rain is an important source in the offshore area during summer. KODAMA ET AL.

Variations in the Ratio of Carbon and Nitrogen Stable Isotopes of Particulate Organic Matter in the Southern Sea of Japan

“…After shipboard filtration, the samples were frozen (< -20°C) until pretreatment. Onshore laboratory analyses followed the methods described by Kodama et al [2021]. The samples were exposed to HCl fumes for more than two hours to remove carbonate salt, dried in an oven (60°C) overnight, and kept in a desiccator until measuring the isotope ratios.…”

“…Generalized linear models (GLMs) were applied to assess the variables that predicted the relationships between the environmental parameters and δ 13 C POM and δ 15 N POM as in Kodama et al[2021]. We assumed that the error distributions of δ 13 C POM and δ15N POM were normally distributed with a linear link function in the GLMs.…”

“…In the southwestern East China Sea, the stable carbon isotopic values of POM increased with an increase in phytoplankton abundance, while the nitrogen isotopic values decreased with freshwater input and coastal upwelling in summer [Ho et al , 2021]. In the Kuroshio area, active nitrogen fixation decreases the nitrogen isotopic ratio of POM in offshore waters during the summer, while the nitrogen isotopic ratio of POM remains high in coastal waters [Kodama et al , 2021].…”

“…The variation in the stable isotope ratio of POM is necessary because the stable isotope ratios of higher trophic organisms in this sea are not always reflected in the stable isotope ratio values of POM [Nakamura et al , 2022]. In addition, repeated observations indicate that the carbon and nitrogen stable isotope ratio of POM is unstable in the marginal seas of the western North Pacific [Ho et al , 2021;Kodama et al , 2021]. In the southwestern East China Sea, the stable carbon isotopic values of POM increased with an increase in phytoplankton abundance, while the nitrogen isotopic values decreased with freshwater input and coastal upwelling in summer [Ho et al , 2021].…”

Mesoscale movement patterns of common bottlenose dolphins along Kuroshio Current and its bifurcation

“…Marine apex predators such as cetaceans are expected to be affected directly or indirectly by the variability in oceanography and ecosystems linked with variations in the KC path patterns. Since 2017, significant meandering in the KC has been frequently observed (Kodama et al, 2021) but the effect of such meander patterns on the habitat of cetaceans has not been sufficiently investigated. During shipboard sighting surveys, encounters of Risso's dolphin ( Grampus griseus ) schools were highly concentrated on the waters of the CSKC (Kanaji & Gerrodette, 2020).…”