Protist control of bacterial stocks is implied by time-series data from a fjord (Fig.
5.10) (Anderson & Sorensen 1986). Counts of both heterotrophic flagellates and
bacteria showed complementary oscillations, strongly implying classical predator–
prey population cycling and, thus, a predator–prey relationship. A further
demonstration of the predator–prey relations among very small organisms, in this case
photosynthetic protists, is provided by an experiment with filters of different sizes
performed by Calbet and Landry (1999, 2004). Net growth rates (μ = increase −
grazing) of autotrophic Prochlorococcus with cells of ∼1 μm diameter and of
heterotrophic bacteria mostly <1 μm were determined in incubations of whole
seawater, and of filtrates from 1,2, 5, 8, and 20 μm filters. Results were as follows:
Fig. 5.10 Time-series of bacterioplankton and bactivorous nanoflagellates in a
shallow-marine fjord (Limfjorden, Denmark). Points are means of counts from 1 and
2 m depths. This appears to be a classic predator–prey oscillation.
(After Anderson & Sorensen 1986.)
REMAINING SIZESNET GROWTH RATE DAY−1NET GROWTH RATE DAY−1
PROCHLOROCOCCUS HETEROTROPHIC BACTERIA
Control +0.20 −0.04
<20 μm +0.15 −0.06
<8 −0.11 −0.10
<5 −0.40 −0.18
<2 +0.04 −0.06
<1 +0.09 −0.03(^) Keep in mind that only the growth rates of the smallest components, i.e. those
passing the 1 μm filter, were measured. The 20 μm and 8 μm filters apparently
removed predators on the grazers of smallest cells, so the burgeoning grazers
increased, reducing the net increase rate. The 2 μm and 5 μm filters removed those