Copepod grazing in the equatorial Pacific

The vertical distribution of copepod (>64 pm) grazing was mcasurcd on the equator at 14O”W during March/ April (19 d) and October 1992 (21 d). El NiAo 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 B2.0 pm 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-pm copcpod fraction during both time series (0.023 and 0.028 dI for March/April and October) but was higher in October (0.114 d-l) as compared to March/April (0.040 d-l) for >200-pm 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 copcpod grazing rates during this advective event. Grazing rates calculated by assuming that copepods capture only >2-pm phytoplankton (0.25 d-’ in March/April; 0.88 d-’ in October) suggest that copepod grazing could limit the production of >2-pm phytoplankton, primarily diatoms, in October but not during the El NiHo 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 arc in agreement with previous calculations, which suggest that most of the carbon consumed by copepods in the equatorial Pacific is from protozoa. Upwelling in the central equatorial Pacific may support up to 2 gigatons of new production per year (Chavez and Barber 1987) or approximately half of the estimated annual global new production (Eppley and Peterson 1979). A paradox in understanding the ecology of this important oceanic region is the presence ,of relatively high concentrations of inorganic macronutrients accompanied with low chlorophyll concentrations (Cullen 1991). Therefore, other factors such as micronutrients (i.e. iron; Martin et al. 1989), grazing (Walsh 1976), or both (Landry et al. 1997) probably limit the amount of phytoplankton production. A central objective of the U.S. Joint Global Ocean Flux Study (JGOFS) in the equatorial Pacific was to determine the factors that limit primary production and the resultant flux of carbon from the euphotic zone. Thus the program examined both grazing and iron limitation of primary production (Murray et al. 1994). Copepods were once thought to be the major grazers of phytoplankton in the ocean. However, there now is increasing evidence that phytoplankton are primarily consumed by protozoa (Capriulo et al. 1991; Landry et al. 1993). Protozoa can efficiently ingest the small (<2 pm) phytoplankton that usually dominate oceanic waters and have growth rates that Acknowledgments We thank the captain and crew of the RV Thompson for their help during the cruise. This research was part of the U.S. JGOFS EqPac program and was supported by National Science Foundation grants OCE 90-24381 (M.R.R.) and OCE go-9022418 (D.A.M.). Mari Butler, Hans Dam, Susan Kadar, and Juanita Urban helped with cruise preparation, sample collection, and data analysis. Jim Murray planned and coordinated the EqPac scientific study. Pat Wheeler made available new production data. Hans Dam, Jacques White, and Xinsheng Zhang made helpful comments on an earlier version of this manuscript. Contribution No. 2769 from CEES, University of Maryland, JGOFS Contribution No. 235. are similar to phytoplankton. In a review of the controls of phytoplankton production, Banse (1992) suggested that the most important limiting factor of phytoplankton production is grazing (mostly by protozoa), even in HNLC areas such as the equatorial Pacific (Dam et al. 1995; Landry et al. 1997). In upwelling areas new production is often dominated by diatoms. Thus in the upwelling zone of equatorial Pacific, diatoms are important components of the phytoplankton community (Chavez et al. 1990; Iriarte and Fryxell 1995; Kaczmarska and Fryxell 1994; Latasa et al. 1997). In contrast to the smaller autotrophs such as chroococcoid cyanobacteria and prochlorophytes, diatoms can be efficiently grazed by copepods (Frost 1972; Nival and Nival 1976; Berggreen et al. 1988). Thus there is the potential for copepods to exhibit grazing control over diatoms and new production. Although copepods may not be the primary grazers that control phytoplankton production, they can be important in controlling the export flux. Through their production of rapidly sinking fecal pellets, copepods contribute substantially to the flux of biogenic material (e.g. Fowler and Knauer 1986; Small et al. 1989; Altabet and Small 1990). Because over some time scale, new production has to balance export flux, copepods could indirectly influence the overall level of new production. More than 90% of zooplankton abundance and biomass during the JGOFS equatorial Pacific study (EqPac) was contributed by copepods (Roman et al. 1995). Thus, in the subsequent description of our methods and results we refer to copepod biomass, filtration rates, and grazing rates rather than the more general mesozooplankton. In this paper we present data on the vertical distribution of copepod grazing on the equator at 14O”W during March/April (19 d) and Oc-