[A-OS13] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS13-11] Integrated trophic positions of zooplankton communities as a new indicator of marine ecosystem structure

*Naoto F. Ishikawa¹, Kazuaki Tadokoro², Chisato Yoshikawa¹, Nanako O. Ogawa¹, Naohiko Ohkouchi¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.Japan Fisheries Research and Education Agency)

Keywords:Compound-specific nitrogen isotope analysis of amino acids (CSIA-AA), Ecosystem metabolism, iTP, Oyashio and Kuroshio, Size distribution, Time-series analysis

To design a sustainable Earth ecosystem in the Anthropocene, it is necessary to understand the role of biodiversity in structuring marine ecosystems. However, integrating biodiversity and ecosystem structure is still challenging because no sound index is available to date. To address this issue, the integrated trophic position (iTP), defined as a biomass-weighted average trophic position (TP) of a given food web, would be useful because the iTP can be measured using compound-specific nitrogen isotope (δ¹⁵N) analysis of amino acids (Ishikawa et al. 2023, 2024). In this presentation, we show spatiotemporal variations in amino acid δ¹⁵N and iTP values of marine zooplankton communities collected from two sites on the Oyashio and Kuroshio currents during 2012, 2014, 2017, 2019, and 2021. We observed considerable variations in δ¹⁵N values of phenylalanine (a proxy for δ¹⁵N of nitrate, from -2.31 to +6.08‰) as well as iTP values (from 2.04 to 3.01) in space and time. These variations are likely controlled by local oceanography, diversity in zooplankton species and size, and top-down force made by higher consumers such as fish. We also found a negative correlation between iTP and ecosystem metabolisms, which suggests the more efficient the trophic energy transfer, the less metabolic energy lost to the environment. The results suggest that the iTP value of zooplankton communities is a promising indicator of marine ecosystem structure.

References:
Ishikawa, N. F., Tadokoro, K., Matsubayashi, J., & Ohkouchi, N. (2023). Biomass pyramids of marine mesozooplankton communities as inferred from their integrated trophic positions. Ecosystems, 26(1), 217-231.
Ishikawa, N. F., Takashima, A., Maruoka, H., & Kondoh, M. (2024). Integrated trophic position as a proxy for food-web complexity. Methods in Ecology and Evolution, 15(1), 164-177.