Geographic Variation of Particle Size Distribution in the Kuroshio Region: Possible Causes in the Upper Water Column

Toshi Nagata,Yosuke Yamada,Yu Umezawa,Hideki Fukuda

doi:10.3389/fmars.2021.768766

Abstract

Particle size distribution (PSD) in the ocean is a fundamental property that influences carbon export and food webs; however, PSD variation and its causes in oligotrophic oceans are not entirely clear. Here, we used Laser In-Situ Scattering and Transmissometry to investigate PSD (size range 5.2–119 μm) and related variables at 11 stations in the surface layer (0–20 m) of the Kuroshio region of the western North Pacific, where strong current causes dynamic hydrographic and ecological conditions. PSD slopes (range –3.2 to –4.2), derived from the power law model, were steeper at onshore stations and flatter at oligotrophic stations located offshore and at lower latitudes. Notably, slopes tended to become steeper with increasing chlorophyll a concentration, opposing the generally observed relationship between the two variables, whereas they became flatter with increasing transparent exopolymer particle (TEP) concentration. Possible explanations of the above results are localized occurrence of nanophytoplankton and TEP facilitation of particle aggregation. The results support the hypothesis that PSD slopes are controlled by a multitude of factors, including phytoplankton community dynamics and aggregation processes. To determine whether TEP-induced particle aggregation enhances or suppresses carbon export, we need a better understanding of the nature (porosity, density, and sinking velocity) of aggregates in oligotrophic oceans.

Highlights

Particle size distribution (PSD) in the ocean is a fundamental property that influences oceanic carbon export mediated by sinking particles, which in turn affects the oceanic uptake of atmospheric CO2 and the earth’s climate (Kwon et al, 2009)
Because depth resolution differed among PSD and environmental variables (Supplementary Table 1), depth-averaged values of these variables were calculated for each station and used to examine geographic variation of these variables and relationships among them
At Station ALOHA in the North Pacific Subtropical Gyre, Barone et al (2015) reported that the PSD slope varied in the range of –4.2 to –5.9, steeper than those obtained in the Kuroshio region

Summary

Introduction

Particle size distribution (PSD) in the ocean is a fundamental property that influences oceanic carbon export mediated by sinking particles (biological carbon pump; Guidi et al, 2008; Cael and White, 2020; Maerz et al, 2020; Omand et al, 2020), which in turn affects the oceanic uptake of atmospheric CO2 and the earth’s climate (Kwon et al, 2009). Understanding PSD variation is essential for a better understanding of ocean biogeochemical cycles and food webs; yet its causes and controls are not well understood. This knowledge gap represents a major obstacle. Because linear dimensions of the major phytoplankton groups (e.g., diatoms, coccoliths, haptophytes, and dinoflagellates), except for picocyanobacteria and picoeukaryotes, are within this size range, PSD determined by LISST provides information concerning phytoplankton size composition (Barone et al, 2015; White et al, 2015), the particles detected by LISST contain detrital particles and aggregates (Leroux et al, 2018)

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Results

Discussion

Conclusion