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

Abstract

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 mm mesh-size from 0-150 m at 89 stations from the subarctic to subtropical regions during the summers of [2003][2004][2005][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: 34ϫ10 3 -65ϫ10 3 indiv. m Ϫ2 and 2.9-7.9 g dry mass m Ϫ2 , 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.

Interannual and latitudinal changes in zooplankton abundance, biomass and size composition along a central North Pacific transect during summer: analyses with an Optical Plankton Counter

“…4). In previous studies, zooplankton biomass in the North Pacific estimated by OPC measurements on formalin preserved samples was reported to be correlated with directly measured mass by a factor of 0.97-1.16 (Yokoi et al 2008, Matsuno et al 2009, Matsuno & Yamaguchi 2010. The factor calculated in the present study (1.176) is close to these previously reported values.…”

“…In this region, day-night differences in abundance have been reported for copepod Metridia pacifica C6F (Saito et al 2011). However, significant day-night differences in OPC-derived total plankton abundance and biomass were not detected in any of the regions (Matsuno & Yamaguchi 2010). This may partly be because the broader size range surveyable using the OPC (0.250-20 mm) may mask the day-night differences that were observed for a relatively narrower size range.…”

“…have also been reported (Batten et al 2003). During spring to summer, latitudinal changes in zooplankton abundance, biomass and community structure in the North Pacific have been reported for the western (165°E) and eastern (165°W) regions (Matsuno & Yamaguchi 2010) and the western (155°E) region (Yokoi et al 2008). Common to these studies, the highest biomass and lowest abundances have been reported for the transitional region (TR) (Yokoi et al 2008, Matsuno & Yamaguchi 2010.…”

“…This may partly be because the broader size range surveyable using the OPC (0.250-20 mm) may mask the day-night differences that were observed for a relatively narrower size range. Because the regional differences in abundance were greater than the daynight differences in this region (Matsuno & Yamaguchi 2010), we made no conversion according to the diel regime.…”

“…During spring to summer, latitudinal changes in zooplankton abundance, biomass and community structure in the North Pacific have been reported for the western (165°E) and eastern (165°W) regions (Matsuno & Yamaguchi 2010) and the western (155°E) region (Yokoi et al 2008). Common to these studies, the highest biomass and lowest abundances have been reported for the transitional region (TR) (Yokoi et al 2008, Matsuno & Yamaguchi 2010. It is not known whether this pattern is common across the whole North Pacific, as there is little information on zooplankton community structure throughout the region.…”