北海道周辺4海域（道東・道南太平洋，北部日本海および南部オホーツク海）における動物プランクトンバイオマスおよび種組成の季節変化と年変動

抄録

To evaluate the seasonal/annual changes and regional variations in biomass structure, 175 zooplankton samples were collected from the 0–150 m layer and the 0–500 m or the 0–300 m layer using a modified NORPAC net at four areas around Hokkaido (Doto and the Donan area of the North Pacific, the northern Japan Sea and the southern Okhotsk Sea) every one-three months during February 2008 to December 2011. Seasonal zooplankton biomass peaks were found from April to June every year, with the exception of the southern Okhotsk Sea. Large cold water copepods (Neocalanus spp., Eucalanus bungii and Metridia spp.) dominated the biomass throughout the areas, composing ca. 40–50% of the total zooplankton biomass in terms of wet weight. The most dominant species varied with area; i.e. E. bungii (Doto and Donan area), N. plumchrus/flemingeri (northern Japan Sea) and M. pacifica/okhotensis (southern Okhotsk Sea). The biomass of large copepods, especially N. plumchrus/flemingeri, showed significant year-to-year changes. In the northern Japan Sea(Tsushima Warm Current area), N. plumchrus/flemingeri were abundant during spring 2010, when the current was less warm and high chlorophyll-a values were observed. In the Doto and Donan area of the North Pacific, N. plumchrus/flemingeri were less abundant during spring 2009, when their was less Oyashio water and lower chlorophyll-a concentrations were found. Our results suggest that the biomass of large cold-water copepods fluctuates year-to-year, being influenced by the physical/biological environment-especially in spring.