Predation and Food Webs - Freshwater Ecology: Concepts and Environmental Applications - page 404

The trophic cascade can extend to physical and chemical aspects of the
lake. Low zooplankton grazing rates can lead to greater influx of atmospheric
CO 2 related to increased algal biomass with high photosynthetic demand
(Schindler et al., 1997). Predation by zooplanktivorous fish can increase nu-
trient supply by excretion, enhancing algal production both by indirectly low-
ering grazing pressure and by directly providing nutrients (Persson, 1997;
Vanni et al., 1997; Vanni and Layne, 1997). Mazumder et al. (1990) demon-
strated that lakes with higher abundance of planktivorous fish had lower zoo-
plankton, higher phytoplankton, and lower light penetration. The lower light
penetration led to less heating of the deeper water in the spring, which led to
a shallower epilimnion depth. Similar results occurred both in 15-m-deep,
8-m-diameter enclosures in which the plankton was manipulated and in 27
small Ontario lakes with varied levels of planktivorous fish. These results
demonstrated a clear local effect of organisms on heat content and physical
structure of the lake as mediated by the food chain.
The idea of the trophic cascade in lakes requires simplification of food
webs into food chains with discrete trophic levels. Several examples illus-
trate how community complexity can alter the intended effects of food
chain manipulation. In shallow eutrophic ponds, very low rates of zoo-
plankton grazing resulting from intense predation can lead to blooms of
unwanted cyanobacteria (Spencer and King, 1984). In the same ponds
without fish, zooplankton flourish, phytoplankton decrease drastically, and
macrophytes and periphyton growths can reach nuisance levels. Similar re-
sults were seen in shallow eutrophic lakes in The Netherlands. In this case,
fish biomass was lowered, zooplankton initially flourished, phytoplankton
decreased, and then macrophytes filled the lakes, leading to suppression of
both zooplankton and phytoplankton numbers (Fig. 19.9). Another exam-
ple of how complexity of food webs alters efficacy of biomanipulation is
the fact that large fish reproduce well when their numbers are increased for
trophic control of zooplankton. The young of the year fish that are pro-
duced are zooplanktivorous and can reverse the effects of the biomanipu-
lation, at least temporarily (Hansson et al., 1998).
In warm reservoirs in the United States, gizzard shad (Dorosoma ce-
pedianum) can be the most abundant fish. As young of the year, these fish
consume individual zooplankton. After growing to several centimeters in
length, they develop a long gut that includes a grinding chamber and also
a filtering structure on their gills. This allows them to consume and thrive
on phytoplankton, zooplankton, and detritus. This fish feeds on several
trophic levels at once and confounds the use of a food chain model to de-
scribe trophic dynamics in these reservoirs (Stein et al., 1995). In other
shallow lakes, the greatest effect of fish on algae may not be through the
food web but through increased sediment suspension (Havens, 1991).
Another example in which the idea of a food web rather than a food
chain more acccurately characterizes aquatic trophic relations is the link
between the trophic cascade and the microbial loop. This link is equivocal;
some investigators have established the presence of a top-down effect on
the microbial loop, and others have not. For example, Pace et al. (1998)
demonstrated variable effects on heterotrophic flagellates but increased
populations of ciliates and rotifers in lakes with low Daphnia populations.
Gasol et al. (1995) showed that cladocerans have the greatest effects on