Coccolithophore assemblages and morphotypes of Emiliania huxleyi in the boundary zone between the cold Oyashio and warm Kuroshio currents off the coast of Japan

Abstract

Living coccolithophore assemblages were studied off the Japanese coast in the boundary zone between the cold Oyashio and the warm Kuroshio Currents as well as in adjacent water masses. A total of 64 coccolithophore taxa were identified. Ten taxa that comprised more than 10% of at least one sample were designated major taxa. Morphological observations and morphometric measurements enabled us to sub-divide the Emiliania huxleyi into four morphotypes: Type A, Type B, Type B/C, and Type B/C-2. Based on the composition of major taxa and morphotypes of Emiliania huxleyi, four coccolithophore assemblages were recognized, corresponding to distinct water masses: Oyashio Current, Tsugaru Warm Current, Kuroshio Current, and Western North Pacific Central Water (WNPCW). In the Oyashio Current regime, the standing crop of coccolithophores was high, although diversity was low. The floral assemblage was dominated by E. huxleyi Type B and consistently contained Coccolithus pelagicus ssp. pelagicus HOL, Calciopappus caudatus, and Syracosphaera orbiculus. In the Tsugaru Warm Current regime, the flora was characterized by abundant E. huxleyi Type B/C-2 and Gephyrocapsa oceanica as well as by relatively common Braarudosphaera bigelowii. In the Kuroshio Current regime, E. huxleyi Type A, Gephyrocapsa ericsonii, or G. oceanica were abundant. The coccolithophore flora in the WNPCW was characterized by abundant E. huxleyi Type A, Discosphaera tubifera, and Umbellosphaera tenuis Type IV. Large Emiliania huxleyi specimens (≥4.2 μm in length) only occurred north of the Oyashio Front, while to the south, E. huxleyi were consistently ≤4.1 μm long, regardless of the morphotype. Large E. huxleyi can be a useful paleoenvironmental indicator for reconstructing past migrations of the Kuroshio and Oyashio Currents in the North Pacific Ocean.