Apparent phytoplankton bloom due to island mass effect
Introduction
The Kuroshio is oligotrophic (Takahashi et al., 1985, Kaneko et al., 1998) due to strong stratification; however, it is known that this low-nutrient flow path is chosen as major spawning grounds by numerous small pelagic fish species (Watanabe et al., 1997). Flow perturbation by an island or a seamount has been suspected as one of the important processes for the ecosystem of the Kuroshio, because the Kuroshio always passes the Nansei Islands and the Izu–Ogasawara Ridge (Fig. 1A). The enhanced biological productivity in the vicinity of an island is known as the “island mass effect” (Doty and Oguri, 1956). Numerous studies have shown enhanced biological productivity and sometimes with evidence of upwelling, such as a doming of isopycnals (e.g. Heywood et al., 1990, Coutis and Middleton, 1999), a cold water formation, and increased levels of the nutrient content in the lee of islands in the Kuroshio (e.g. Takahashi et al., 1980, Kimura et al., 1994). Previous studies conjectured that the nutrification of the euphotic zone leads to an enhanced primary productivity (Takahashi et al., 1985, Furuya et al., 1986, Simpson and Tett, 1986, Heywood et al., 1990, Odate and Furuya, 1998, Signorini et al., 1999) and increased levels of zooplankton (copepods) concentrations (Toda, 1989). However, these physical events associated with the “island mass effect” are understood only qualitatively, due to the complexities of turbulent flows around islands, as well as observational difficulties. Consequently, the rate of contributions to the biological enhancement of the ecosystem is not clearly quantified.
Satellite data are useful for showing the elevated level of production around an isolated island for an extensive area. However, the information is limited to the surface features. In this study, we conducted a survey around an island in the Kuroshio with a free fall instrument to investigate the distribution of phytoplankton in subsurface layers.
Section snippets
Survey and instrumentations
We made a field survey in the vicinity of Nakano-shima from the Research and Training Vessel R.T.V. Umitaka-maru. The island is located to the south of Kyusyu Island, Japan (Fig. 1). The size of Nakano-shima is approximately 5.5 × 11 km. During the experiment, the Kuroshio was flowing directly at Nakano-shima so that the “island mass effect” could be expected.
CTD (FSI, Micro-CTD2) and fluorometer (WET Labs, FLF300) measured fine structure hydrography. These sensors were mounted on a free fall
Current
During the experiment, the Kuroshio was flowing northeast across a relatively shallow region between the edge of the continental shelf and the Nansei Islands. The current turned eastwards at the northern edge of the Nansei Islands then hit several islands (e.g. Kuchino-shima and Nakano-shima, see Fig. 1). The incident current was blocked by the islands and accelerated at the flanks of the islands (Fig. 2A).
Current distribution from the ADCP data shows that the flow was accelerated at the flanks
Conclusions
At the lee of Nakano-shima island in the Kuroshio, we observed cold water formation with high Chl-a concentration that suggests a conventional “island mass effect” phenomena. However, observed features revealed “apparent phytoplankton bloom” due to the island mass effect in the region. The cooling of the surface waters and the elevated level of the SCC at the lee of the island can be explained by the mixing of the water column and the diffusion of SCM, due to strong turbulence caused by the
Acknowledgments
We thank the crew of the R.T.V. Umitaka-maru for support during the experiment. N. Horimoto and Y. Matsumoto provided technical support for calibration of the fluorometers. J. Yoshida, Y. Tanaka, Y. Kitade and T. Masuda provided valuable discussion on this manuscript. Also, we thank two reviewers for their constructive suggestions. The bathymetric data were obtained from NOAA's National Geophysical Data Center (http://www.ngdc.noaa.gov/ngdc.html) and the Japan Oceanographic Data Center (//www.jodc.go.jp/
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