from separate, unamended dilutions, and the data-set included 392 such pairs of m and
μ that are plotted in Fig. 9.4.
Fig. 9.4 Scatter plot of paired rate estimates from dilution experiments for
phytoplankton grazing mortality, m, and phytoplankton growth rate, μ. Data are all
from experiments in which mortality was from nutrient-enriched dilutions and growth
from unamended dilutions. The solid line is a Model II linear regression (r = 0.6). The
dashed line represents a 1 : 1 relationship
(^) (After Calbet & Landry 2004.)
The implication of the fitted line is that, on a global and all-season average, 57% of
phytoplankton growth is consumed by microzooplankton on the day that it occurs.
(^) While the regression in Fig. 9.4 is statistically significant, the scatter represents
variability occurring and expected in the pico- to microphytoplankton and the
community of grazers feeding on them. A preponderance of μ values near or even
below 1 d−1 still represents reasonably rapid growth, especially in cold regions.
Values >1.4 d−1 (>2 doublings d−1) are very high, and the modest number exceeding
that are likely to represent the rapid cell cycles of very small phytoplankton. A
substantial number of points near the 1 : 1 line are typical of oligotrophic habitats
where pico- and nano-sized autotrophs and heterotrophic flagellates sustain a close
balance between autotroph increase and mortality. The points both well above and
well below the 1 : 1 line mostly (apart from unrealistic experimental outcomes)
represent short-term measures from longer-term shifts: ups and downs in small
autotroph abundance over periods of many days like those illustrated in Figs. 11.5a
and 11.5b. Those are likely to occur in all oceanic systems, with lower amplitude
where conditions are more oligotrophic.
(^) Calbet and Landry (2004) also summarized dilution results for different ocean
habitats (Table 9.4) in terms of proportions of primary production grazed daily by
microzooplankton. With the exception of estuaries, that fraction is on the order of