Mr. Landry et al., MICROZOOPLANKTON GRAZING IN THE CENTRAL EQUATORIAL PACIFIC DURING FEBRUARY AND AUGUST, 1992, Deep-sea research. Part 2. Topical studies in oceanography, 42(2-3), 1995, pp. 657-671
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.