increase in phytoplankton growth after protist grazing is only about 30%, which must
go to mesozooplankton, sinking and mixing export. Landry et al. claim that viral lysis
of phytoplankton is “invisible” to dilution experiments, although, if substantial, it is a
transfer directly to heterotrophic prokaryotes. The grazing of 70–100% of each day’s
phytoplankton production does not eliminate the availability of produced organic
matter for export to depth. Grazers defecate some; some is transferred up several
trophic steps to migratory zooplankton and nekton that carry it down; and some
eventually reaches whales that die and sink to the seafloor.
(^) Landry et al. (2009) performed an essentially similar study in the more eutrophic
California Current and close inshore off Point Conception during the upwelling
season. A difference was attaching the incubation containers for eight, two-point
dilution experiments to a weighted line suspended to 140 m from a Langrangian float
with a drogue at 15 m. There were two float deployments in waters with chlorophyll
concentration greater than 1 μg liter−1 in each of May 2006 and April 2007. New
incubations were attached to the suspended line three to five times in each
deployment, producing a good deal of averaging. Overall, phytoplankton growth and
microzooplankton grazing rates, again measured from changes in chlorophyll
abundance, were integrated after “weighting the mean rates in each depth stratum to
the proportion of the total water-column Chl-a that it represented”. These rates,
averaged among days and integrated vertically (Fig. 9.7) show that microzooplankton
grazed 43% and 80% of phytoplankton growth in the two 2006 observation cycles and
55% and 58% in the two 2007 cycles. The mesozooplankton grazing rates also shown
(Fig. 9.7) are approximations from measured gut-content chlorophyll of bulk
zooplankton coupled with approximate turnover times from a temperature-
dependence relationship from Dam and Peterson (1988: fractional gut Chl reduction
≈0.0124 exp[0.077 T(°C)] min−1). Landry et al. also listed mixed-layer phytoplankton
growth rates that were two- to three-fold greater than the rates when integrated to 140
m (growth at depth is slow, while chlorophyll concentration can remain substantial).
Fig. 9.7 Weighted (by chlorophyll concentration of each depth stratum) average rates
of change of chlorophyll concentration due to separable processes in dilution
experiments suspended from drifters down through the euphotic zone of the upwelling
system offshore of Southern California. The averages include three to five repeated
trials with each drifter (work cycles). Also shown are estimates of change due to
mesozooplankton grazing and of net changes both observed in the water column and
from differences of the average rate estimates. Results are shown for four selected
cycles at different positions offshore and different chlorophyll standing stocks
(surface concentrations shown in μg liter−1.
(^) (After Landry et al. 2009.)