Micro-grazer biomass, composition and distribution across prey resource and dissolved oxygen gradients in the far eastern tropical north Pacific Ocean

The ecology of micro-grazers (Mg) was investigated across prey and dissolved oxygen (DO) gradients in the eastern tropical north Pacific Ocean (ETNP) during October–November 2007. Surface (<200 m) chlorophyll a (Chl a) across a ∼1700 km north–south transect ranged between the seasonal average of 0.2 μg Chl a L−1 to 1.8 μg Chl a L−1 in an extensive Chl a-rich patch in the center of the transect. Limiting (<20 μmol kg−1 O2) DO concentrations were encountered as shallow as 24 m. Biomass of Mg in waters above the upper oxycline (UO) ranged between 5.6 μg C L−1 and 36.6 μg C L−1, with highest Mg biomass observed in locations with highest Chl a. Heterotrophic dinoflagellates contributed most, on average, to Mg biomass (41% to 53%), followed by aloricate spirotrich ciliates (24% to 29%) and heterotrophic nanoflagellates (11% to 33%). Biomass of Mg decreased, on average, over 96% in waters below the UO, but this decrease did not appear to be regulated by DO; Mg biomass more strongly correlated with Chl a (r=0.83, P<0.001) and temperature (r=0.76, P<0.001) at discrete depths than with DO (r=0.67, P<0.001). Using a multiple stepwise regression model, Chl a alone accounted for 68% Mg biomass variability, whereas Chl a and temperature combined accounted for 84%. In two Mg grazing experiments we found that Mg removed 33% and 108% of surface primary production in the upper mixed layer. These estimates of Mg grazing, while limited in scope, fall within estimates from other regions of the equatorial Pacific Ocean, and help reinforce the paradigm that Mg are influential in regulating organic carbon dynamics in the eastern tropical Pacific. A primary finding from this study was that observations of Mg biomass are higher than previously reported for the ETNP. This observation suggests that the region’s complex air–sea interactions and the resultant positive influence on primary production and phytoplankton biomass can episodically support high biomass of a diverse Mg community.