Why is &amp;lt;i&amp;gt;Trichodesmium&amp;lt;/i&amp;gt; abundant in the Kuroshio?

T Shiozaki,T Kodama,S Takeda,F Hashihama,X Liu,K Furuya,S Itoh

doi:10.5194/bg-12-6931-2015

Abstract

Abstract. The genus Trichodesmium is recognized as an abundant and major diazotroph in the Kuroshio, but the reason for this remains unclear. The present study investigated the abundance of Trichodesmium spp. and nitrogen fixation together with concentrations of dissolved iron and phosphate in the Kuroshio and its marginal seas. We performed the observations near the Miyako Islands, which form part of the Ryukyu Islands, situated along the Kuroshio, since our satellite analysis suggested that material transport could occur from the islands to the Kuroshio. Trichodesmium spp. bloomed (> 20 000 filaments L−1) near the Miyako Islands, abundance was high in the Kuroshio and the Kuroshio bifurcation region of the East China Sea, but was low in the Philippine Sea. The abundance of Trichodesmium spp. was significantly correlated with the total nitrogen fixation activity. The surface concentrations of dissolved iron (0.19–0.89 nM) and phosphate (< 3–36 nM) were similar for all of the study areas, indicating that the nutrient distribution could not explain the spatial differences in Trichodesmium spp. abundance and nitrogen fixation. Numerical particle-tracking experiments simulated the transportation of water around the Ryukyu Islands to the Kuroshio. Our results indicate that Trichodesmium growing around the Ryukyu Islands could be advected into the Kuroshio.

Highlights

The Kuroshio is a western boundary current in the North Pacific Ocean that originates in the North Equatorial Current and bifurcates to the east of the Philippines
Phosphate concentrations were consistently low within the Mixed layer depth (MLD) in all of the studied areas, and the maximum abundance of Trichodesmium spp. and total nitrogen fixation activity generally occurred near the surface, suggesting that the phosphate conditions for surface Trichodesmium spp. and other diazotrophs were similar among all of the areas
We hypothesize that the high abundance of Trichodesmium spp. and active nitrogen fixation in the Kuroshio were ascribable not to the unique nutrient environment, but rather to the supply of Trichodesmium spp. and other diazotrophs from the surrounding islands

Summary

Introduction

The Kuroshio is a western boundary current in the North Pacific Ocean that originates in the North Equatorial Current and bifurcates to the east of the Philippines. McGowan (1971) suggested that some plankton species are delivered by the Kuroshio to the north from the equatorial region. Because Trichodesmium is a major nitrogen fixer in the Kuroshio, it is believed to be the key genus for understanding the Kuroshio ecosystem (Chen et al, 2008, 2014; Shiozaki et al, 2014a). Marine nitrogen fixation is thought to be regulated by the supply of iron and phosphorus (Mahaffey et al, 2005), and Trichodesmium thrives in iron-rich oligotrophic regions (Moore et al, 2009; Shiozaki et al, 2010, 2014b).

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Conclusion