MICROZOOPLANKTON GRAZING IN THE CENTRAL EQUATORIAL PACIFIC DURING FEBRUARY AND AUGUST, 1992

Dilution studies were conducted on EqPac cruises in the central equato rial Pacific (2 degrees N to 2 degrees S, 140 degrees W) during Februa ry-March and August-September 1992 to determine phytoplankton growth r ates and mortality rates attributable to microzooplankton grazing. Ins tantaneous growth rates (mu) based on bulk chlorophyll measurements we re highly variable from day-to-day, but averaged 0.83 day(-1) for the upper (10-20 m), 0.34 day(-1) for the mid (40-50 m) and 0.22 day(-1) ( 70-80 m) for the lower euphotic zone on the first cruise. Correspondin g rate estimates for microzooplankton grazing (m) were 0.72, 0.22 and 0.21 day(-1), respectively. During the second cruise, growth estimates strongly exceeded grazing estimates for the two upper (mu = 0.98 and 1.00 day(-1); m = 0.57 and 0.42 day(-1)), but not the lower depth stra ta (mu = 0.32 day(-1); m = 0.27 day(-1)). Grazing losses accounted for about 83% of depth-integrated phytoplankton growth in February/March experiments and only about 55% in August-September experiments. In add ition, growth rates in the presence of added nutrients (including iron ) showed evidence of limitation in February-March, which coincided wit h a major El Nino event, but not in August-September, which was more r epresentative of climatological mean conditions. Differences in growth rates, implied nutrient limitation, and the balance of phytoplankton growth and microzooplankton grazing were consistent with a greater abu ndance of large diatoms in August-September. Despite the disparity bet ween chlorophyll-based estimates of growth and grazing rates for this cruise, flowcytometric analyses of specific populations (Prochlorococc us, Synechococcus, and autotrophic nanoeukaryotes) in a subset of expe riments conducted in August demonstrated that microzooplankton grazing was still sufficient to balance growth rates of the smaller component s of the phytoplankton assemblage.