Predation and Food Webs - Freshwater Ecology: Concepts and Environmental Applications - page 405

A B

800

1.5

600

1.0

400

0.5

200

0 1 2 3 4 0 1 2 3 4

90

D

C

70

150

50

100

30

50

10

0

0 1 2 3 4

0 1 2 3 4

Year

Year

FIGURE 19.9 Effects of biomanipulation on fish, chlorophyll, Daphnia, and macrophyte
cover in four shallow eutrophic lakes in The Netherlands. Year 0 is before biomanipulation.
Points are means from four lakes and error bars equal 1 SD (data from Meijer et al., 1994).

heterotrophic nanoflagellates (very small protozoa) during the summer.
Pace and Cole (1994) suggested that there was little influence of Daphnia
populations on bacteria. Simek et al. (1998) described a lake in which a
filtering cladoceran controlled bacterial populations in one basin and a cil-
iate was the primary bactivore in another basin. Jeppesen et al. (1998)
found a weak link between the microbial loop and zooplankton abundance
in an 18-year time series from a hypereutrophic lake.
Consideration of temporal and spatial scale can also alter the response
of food webs to manipulation. For example, pulses of nutrients can have
different effects depending on food web structure and their timing (Cot-
tingham and Schindler, 2000). If zooplankton have time to respond to nu-
trient enhancement of phytoplankton growth, further pulses of nutrients
will have little influence. If predation on zooplankton is high, they may not
be able to suppress ephemeral phytoplankton blooms in response to nutri-
ent pulses (Strauss et al., 1994).
Top-down effects can also occur in benthic habitats of lakes. Snails re-
move littoral periphyton and are susceptible to predation by sunfish. When
sunfish are excluded, algal biomass decreases significantly (Brönmark et al.,
1992). Since sunfish are prey for large piscivores, it can be argued that a
high biomass of large piscivores will lead to a decreased biomass of peri-
phyton through the tropic cascade. Fish were removed from eutrophic
Lake Ringsjön, Sweden, including the littoral benthic feeding bream
Abramis brama, in an attempt to use biomanipulation to improve water
quality. Water clarity improved in the pelagic zone and there was an unin-
tended increase in benthic invertebrate populations and concurrent in-
creases in staging waterfowl abundance (Bergman et al., 1999).