Apparent phytoplankton bloom due to island mass effect

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.