A nitrogen isoscape of phytoplankton in the western North Pacific created with a marine nitrogen isotope model

• Published in: Frontiers in Marine Science
• Main Authors: Yoshikawa, Chisato, Shigemitsu, Masahito, Yamamoto, Akitomo, Oka, Akira, Ohkouchi, Naohiko
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Frontiers Media SA 2024
• Subjects: Ocean Engineering; Water Science and Technology; Aquatic Science; Global and Planetary Change; Oceanography; Bering Strait; Oyashio; Pacific; Subarctic
• Online Access: http://dx.doi.org/10.3389/fmars.2024.1294608; https://www.frontiersin.org/articles/10.3389/fmars.2024.1294608/full

The nitrogen isotopic composition (δ 15 N) of phytoplankton varies substantially in the ocean reflecting biogeochemical processes such as N 2 fixation, denitrification, and nitrate assimilation by phytoplankton. The δ 15 N values of zooplankton or fish inherit the values of the phytoplankton on which they feed. Combining δ 15 N values of marine organisms with a map of δ 15 N values (i.e., a nitrogen isoscape) of phytoplankton can reveal the habitat of marine organisms. Remarkable progress has been made in reconstructing time-series of δ 15 N values of migratory fish from various tissues, such as otoliths, fish scales, vertebrae, and eye lenses. However, there are no accurate nitrogen isoscapes of phytoplankton due to observational heterogeneity, preventing improvement in the accuracy of estimating migratory routes using the fish δ 15 N values. Here we present a nitrogen isoscape of phytoplankton in the western North Pacific created with a nitrogen isotope model. The simulated phytoplankton is relatively depleted in 15 N at the subtropical site (annual average δ 15 N value of phytoplankton of 0.6‰), where N 2 fixation occurs, and at the subarctic site (2.1‰), where nitrate assimilation by phytoplankton is low due to iron limitation. The simulated phytoplankton is enriched in 15 N at the Kuroshio–Oyashio transition site (3.9‰), where nitrate utilization is high, and in the region around the Bering Strait site (6.7‰), where partial nitrification and benthic denitrification occur. The simulated δ 15 N distributions of nitrate, phytoplankton, and particulate organic nitrogen are consistent with δ 15 N observations in the western North Pacific. The seamless nitrogen isoscapes created in this study can be used to improve our understanding of the habitat of marine organisms or fish migration in the western North Pacific.