Broad scale patterns in mesozooplankton biomass and grazing in the eastern equatorial Pacific

We investigated biomass distributions and grazing rates of mesozooplankton in the eastern equatorial Pacific between 110°-140°W and 4°S-4°N during cruises in December 2004 (EB04) and September 2005 (EB05). Median (±SE) euphotic zone estimates of zooplankton biomass, collected with a 200-μm mesh net, varied from 2.27±0.24 g dry weight m−2 during EB04 to 3.13±0.22 g dry weight m−2 for EB05 (however, when stations from overlapping regions were compared, no significant differences were found between years). Trends in gut fluorescence estimates of mesozooplankton grazing followed biomass, with significantly higher median rate estimates during EB05 (3.39±0.32 mg pigment m−2 d−1) than during EB04 (2.31±0.34 mg pigment m−2 d−1). Spatial gradients in mesozooplankton biomass and grazing on meridional transects sampled at 110°W in 2004 and 140°W in 2005 could be interpreted as either in situ growth/grazing responses or downstream advective flows relative to spatial patterns in phytoplankton. The present zooplankton biomass estimates for the equatorial Pacific are 80–90% higher than those from similar measurements made by the US Joint Global Ocean Flux Studies EqPac Program in 1992. Our grazing rates similarly exceed EqPac estimates by a factor of 2 or 3, in absolute terms and as percent of phytoplankton biomass consumed daily (11% - EB04; 14% - EB05). Although the equatorial region has not been regularly sampled between EqPac and the present study, both the magnitude and the direction of the observed changes are consistent with the documented decadal increase in mesozooplankton biomass in the adjacent North Pacific Subtropical Gyre based on monthly sampling at Stn. ALOHA, as well as an increase in the strength of the trade winds. These results may be indicative of a general shift up in productivity or community size structure and role of mesozooplankton in the open-ocean tropical/subtropical Pacific, and they provide important time points for validating the performance of ecosystem models of the region.