Study on heterogeneous distribution of plankters at oceanic fronts. Lecture of the member awarded the Okada prize of the oceanographical society of Japan for 1989.:Study on Heterogeneous Distribution of Plankters at Oceanic Fronts
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In this paper, I review my study on heterogeneous distribution of plankters at oceanic fronts, taking advantage of an opportunity awarded the Okada Prize 1989 of the Oceanographical Society of Japan. The main focus is on the formation and retention mechanisms of phytoplankton peak abundance at the Kuroshio Front, and the events observed at other oceanic fronts, such as the Oyashio Front, the Antarctic Polar Front and the Subtropical Convergence in the southern hemisphere are compared to those found at the Kuroshio Front.<BR>Phytoplankton standing stock was hign in the inner cold belt of the Kuroshio Front. Since the inner cold belt form from water masses entrained into the front from coastal and/or the Oyashio areas, one of the characteristics of the phytoplankton community is high contribution of microplankton fraction. Rising of isopleths of temperature and nutrient salts etc. at the front suggested that upwelling event occurred along the front. Incubation experiments with nutrient addition showed it would flourish the entrained species but oceanic ones when upwelling occurred at the front.<BR>Zooplankton was strongly aggregated at the Kuroshio Warm-Core Ring Front, and the peak of abundance was spatially separated in each individual. I thought that biological processes, such as motility and prey-predator interaction, play an important role to control the aggregated patterns.<BR>Since the cabbeling event is expected at the Oyashio Front where the relatively cold and low-salinity Oyashio water faces the relatively warm and saline water of the perturbed area, it appears that the phytoplankton species having larger density than that of sea water sink down there. The mixture of sea waters on both sides of the front, however, may accelerate their growth by supply of some deficient element(s).<BR>I consider that raise of temperature is the most plausible factor to make phytoplankton peak abundance at the Polar Front. Experiments on board showed that raising temperature activates photosynthesis of antarctic phytoplankton. Phytoplankton standing stock and productivity were also high at the Subtropical Convergence.<BR>The primary object of this study is the biological role of oceanic fronts for fish ecology. I consider that concentrated phyto-and zooplankton communities at oceanic fronts support the energy of migrating fishes, and also support the survival of juvenile fishes just after hatching which are transported from coastal areas.
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