General description of seasonal variations in nutrients, chlorophyll a, and netplankton biomass along the A-line transect, western subarctic Pacific, from 1990 to 1994

Abstract

General seasonal variations of biological parameters, such as chlorophyll alpha, nutrients and biomass of netplankton, along the A-line transect, which crosses the Oyashio current off southeastern coast of Hokkaido, Japan, were described based on the data obtained from 1990 to 1994. Nitrate concentration was highest in March when the water temperature was lowest. Increase of chlorophyll-alpha concentration was observed n April and May, but intensive blooming (5 mg m(-3)) were only observed at shelf/slope and the first branch of Oyasshio current regions. In other area, chlorophyll-alpha concentration lower than 3 mg m(-3) was usually observed during this period. Fall blooms (1-3 mg m(-3)) were also observed at shelf/slope regions. Annual minima in nitrate were observed during the period between July and October. However, macronutrients were not depleted throughout a year except in the transitional area (south of the subarctic front). These show the depletion of macronutrients is not the entire cause of the termination of spring phytoplankton bloom. Netplankton minima were observed in January and maxima were observed in May or June. Magnitudes of seasonal fluctuations in nutrient concentrations and plankton biomass are larger than one in alaskan Gyre, eastern subarctic Pacific