two-thirds. That may imply a greater role for mesozooplankton grazing in estuaries,
where a greater fraction of phytoplankton are usually of larger sizes and somewhat
less accessible to microherbivores (but see discussion of heterotrophic dinoflagellates
below). However, much of the consumption of estuarine phytoplankton is by benthic
filter-feeders, and a good portion is mixed away when flow carries it out to sea. The
remaining one-third of primary production (a very broad average) in oceanic areas can
be eaten by mesozooplankton, is lysed by viruses, and contributes to multi-day
increases in phytoplankton stock. It can also sink to depth, to be eaten on the way
down to or on the seafloor. Recheck the variability in Fig. 9.4, which shows that
grazing often exceeds growth; those events are crushed by the standard-error
calculation in Table 9.4, which should only be taken as measure of the validity of the
grand means as such, not a measure of how much the autotroph–micrograzer
interaction varies.
Table 9.4 Percentage of primary production grazed daily by microzooplankton, as
determined from dilution experiments. Values are mean percentages and standard
errors for subsets of the data in Fig. 9.4.
From Calbet and Landry (2004).
HABITAT PERCENT OF PRIMARY PRODUCTION
Oceanic 78.0 ± 1.8
Coastal 56.6 ± 2.9
Estuarine 38.6 ± 2.5
Tropical/subtropical71.3 ± 2.3
Temperate/subpolar68.8 ± 2.3
Polar 65.2 ± 3.7
(^) In a companion paper, Landry and Calbet (2004) discuss the implication of these
estimates, basically values of m/μ × 100, for the rates of secondary production by
grazing protists. Based on Straile (1997, discussed below) they assign an approximate
gross growth efficiency (GGE) of 30%. Thus, if the global average percentage of
primary production grazed by protists is ∼70%, their production should be on the
order of 21% of primary production. If GGE is somewhat higher (lower), then the
proportion is higher (lower). When there are multiple trophic levels among the
microheterotrophs, the total production is somewhat greater. For three levels, total
production would be:
(^) of primary production. More levels would add trivially more. If global primary
production is approximately 44 Gt C yr−1, then production of grazing protists (and
tiny metazoans; all heterotrophs left in dilution incubations) is likely about 10 Gt C yr
−1.