Induced Trophic Cascades
(^) Meta-analysis of dilution experiments implies intense interactions among ocean
microheterotrophs and phytoplankton, and intense interactions can be revealed by
short-term manipulations that induce trophic cascades. That is the idea of size-fraction
grow-out experiments. Ocean water and the organisms that it contains are poured
gently through filters of several pore sizes to initiate incubations of progressively
smaller size fractions. Abundance of pico-autotrophs, and often heterotrophic bacteria
(Hbact), are determined before and after a 24-hour incubation in each filtrate and in
the original whole seawater (sometimes filtered through a 200 μm screen) and growth
rates (assumed exponential) are calculated. In the original Calbet and Landry (1999)
experiment in the North Pacific subtropical gyre, the pore sizes were 1, 2, 5, 8, and 20
μm. Each filter from 20 μm down to 5 μm decreased the apparent growth rates of
both Prochlorococcus and Hbact (text table, Chapter 5 page 111; Fig. 9.5). The
interpretation is that the coarser filters remove progressively more of the predators on
the grazers of bacteria-sized organisms, while the finer filters (2 and 1 μm) remove
both the predators and the grazers. Both the predators and grazers are primarily
heterotrophic flagellates (lately termed Hflag (Fig. 6.2) that move to and ingest even
smaller organisms.
Fig. 9.5 Net growth rates of Prochlorococcus and heterotrophic bacteria in the NPSG
after removal of organisms larger than the filter pore sizes along the abscissa. The
control rates are those in unfiltered water. Decreasing growth in water retaining
progressively smaller organisms, those >2 μm, and of increasing growth after removal
of organisms >2 μm implies a multi-step food-chain among organisms smaller than 20
μm. Vertical bars are standard errors of four replicates.
(^) (After Calbet & Landry 1999.)