Copepod grazing in the equatorial Pacific

The vertical distribution of copepod (>64 μm) grazing was measured on the equator at 140°W during March/April (19 d) and October 1992 (21 d). El Nino conditions prevailed during the March/April time series. Hydrographic variables were similar to the climatological mean during the October time series. We found higher weight-specific copepod filtration rates in October when mixed-layer temperatures were generally 3°C cooler; there were more phytoplankton >2.0 μm and more calanoid copepods than in March/April. The grazing impact of the copepod community (biomass x weight-specific filtration rate) was similar for the 64-200-μm copepod fraction during both time series (0.023 and 0.028 d^-1 for March/April and October) but was higher in October (0.114 d^-1) as compared to March/April (0.040 d^-1) for >200-μm copepods. Within-cruise variability was highest in October when a tropical instability wave advected colder water with higher nitrate and chlorophyll through the study area. We observed over six-fold increases in copepod biomass, weight-specific filtration rates, and copepod grazing rates during this advective event. Grazing rates calculated by assuming that copepods capture only >2-μm phytoplankton (0.25 d^-1 in March/April; 0.88 d^-1 in October) suggest that copepod grazing could limit the production of >2-μm phytoplankton, primarily diatoms, in October but not during the El Nino conditions of March/April when there was less copepod biomass as well as a greater dominance of cyclopoid copepods. Estimates of fecal pellet production compared with export flux measurements suggest that most copepod fecal pellets produced in the euphotic zone decompose or are ingested by other zooplankton. Our estimates of copepod grazing rates on phytoplankton and protozoa are in agreement with previous calculations, which suggest that most of the carbon consumed by copepods in the equatorial Pacific is from protozoa.