Abundance and biomass of mesozooplankton along north-south transects (165°E and 165°W) in summer in the North Pacific: an analysis with an optical plankton counter

Zooplankton abundance and biomass along 165°E and 165°W in the North Pacific were estimated using an optical plankton counter on preserved samples collected with a plankton net of 335 μm mesh-size from 0–150 m at 89 stations from the subarctic to subtropical regions during the summers of 2003–2006; the stations were grouped into subarctic (SA), transitional (TR) and subtropical (ST) regions for latitudinal comparisons. The two-way ANOVA and a post hoc test showed that total abundance and biomass were significantly larger in SA and smaller in ST stations, respectively, than in the other regions, but were not significantly different between 165°E and 165°W. Total ranges of abundance and biomass were: 34×10^3–65×10^3 indiv. m^−2 and 2.9–7.9 g dry mass m^−2, respectively. The variation in abundance was mainly governed by the variation of 0.34–1.00 mm equivalent spherical diameter (ESD) size class zooplankton, but total biomass variations were due mainly to the 2.00–3.00 mm ESD size class, which corresponds to the calanoid copepod Neocalanus spp. copepodid stage 5 (C5) that had a greater abundance in SA and TR than in ST. Despite possibly higher abundances of Neocalanus C5 in TR than in SA, abundance and biomass of the 2.00–3.00 mm ESD size class were not significantly different between the two regions. Size reduction of individuals due to higher temperatures in TR than in SA may be a possible explanation