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 (M-g) 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 similar to 1700 km north-south transect ranged between the seasonal average of 0.2 mu g Chl a L-1 to 1.8 mu g Chl a L-1 in an extensive Chl a-rich patch in the center of the transect. Limiting (< 20 mu mol kg(-1) O-2) DO concentrations were encountered as shallow as 24 m. Biomass of M-g in waters above the upper oxycline (UO) ranged between 5.6 mu g C L-1 and 36.6 mu g C L-1, with highest M-g biomass observed in locations with highest Chl a. Heterotrophic dinoflagellates contributed most, on average, to M-g biomass (41% to 53%), followed by aloricate spirotrich ciliates (24% to 29%) and heterotrophic nanoflagellates (11% to 33%). Biomass of M-g decreased, on average, over 96% in waters below the UO, but this decrease did not appear to be regulated by DO; M-g 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% M-g biomass variability, whereas Chl a and temperature combined accounted for 84%. In two M-g grazing experiments we found that M-g removed 33% and 108% of surface primary production in the upper mixed layer. These estimates of M-g grazing, while limited in scope, fall within estimates from other regions of the equatorial Pacific Ocean, and help reinforce the paradigm that M-g are influential in regulating organic carbon dynamics in the eastern tropical Pacific. A primary finding from this study was that observations of M-g 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 M-g community. (C) 2013 Elsevier Ltd. All rights reserved.