Apparent phytoplankton bloom due to island mass effect

doi:10.1016/j.jmarsys.2006.04.019

Journal of Marine Systems

Volume 69, Issues 3–4, February 2008, Pages 238-246

https://doi.org/10.1016/j.jmarsys.2006.04.019 Get rights and content

Abstract

A continuous monitoring of temperature and chlorophyll-a (Chl-a) concentration from a surface water monitoring system and a towed free fall instrument (MVP) around a small island in the Kuroshio showed low sea surface temperature (SST) and high surface Chl-a concentration (SCC) distribution in the lee of the island that indicates typical “island mass effect” phenomena. When the observed Chl-a profiles (0 to 250 m) were integrated, the total amounts in the lee side data were slightly smaller than those of the upstream side of the island. The difference was statistically significant at the 95% confidence level. The cross section diagram of Chl-a indicated the diffusion of subsurface Chl-a maximum (SCM) from the upstream to the downstream flanks of the island. The diffusivity of SCM and the change of potential energy require the same level of strong turbulent dissipation rate at the flanks of the island. That is consistent with our previous direct measurement in a similar hydrodynamic condition. Therefore, the observed high SCC is due to turbulent diffusion of SCM, and clearly showed that high SCC does not require any new production. Although a high fluorescence field behind an isolated island in a strong flow is often visible from satellite images, the images do not necessarily indicate an enhanced primary production at that moment.

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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Apparent phytoplankton bloom due to island mass effect

Abstract

Introduction

Section snippets

Survey and instrumentations

Current

Conclusions

Acknowledgments

Deep-Sea Res. I

J. Exp. Mar. Biol. Ecol.

Deep-Sea Res. I

Deep-Sea Res. I

J. Exp. Mar. Biol. Ecol.

The island mass effect

J. Cons. - Cons. Int. Explor. Mer

Diapycnal mixing in the thermocline: a review

J. Geophys. Res.

How islands stir and fertilize the upper ocean

Geophys. Res. Lett.

The effects of flow disturbance by an oceanic island

J. Mar. Res.

Evaluation of coastal up-welling effects on phytoplankton growth by simulated culture experiments

Mar. Biol.