nutrient status and are sufficiently predictable to be used as a management tool
(Box7.2). Certain patterns in food-chain length and body-size distribution are
insufficient alone to predict trophic cascades; other factors must be involved.
Box 7.2 Trophic cascades, biomanipulation and body sizeMass removal of planktivorous fish (biomanipulation) is a method regularly
used in shallow lakes to reinforce improvements in lake water quality after a
nutrient loading reduction (Perrowet al., 1997 ; Hanssonet al., 1998 ; Jeppesen &
Sammalkorpi,2002 ). The idea is that removal of zooplanktivorous fish
induces increased grazing by zooplankton and higher water clarity, which,
in turn, stimulates the growth of submerged macrophytes. Via a number of
positive feedback mechanisms this will contribute to maintaining the clear-
water state (Moss,1990 ; Schefferet al., 1993 ). Analyses of data from before and
after the biomanipulation may provide evidence for the cascading effects of
fish in eutrophic lakes. Here, we present data from three Danish long-term
studies of the effects of reduction on zooplanktivorous fish. The lakes are all
eutrophic and relatively shallow (Table7.1). In Lake Engelsholm, 19 t cypri-
nids were removed by netting from April to September 1992, reducing the
calculated biomass of cyprinids from 675 to 150–300kg ha^1 (Møller,1998 ).
In Lake Arreskov, a major fish kill occurred during autumn and winter (1991).
An additional 4 t cyprinids were removed in 1995, and during 1993 and 1995
the lake was stocked with under-yearling pike (Esox lucius) to a density of
141 ind. ha^1. Cyprinid biomass was calculated as 172kg ha^1 in 1987 and
71kg ha^1 in 1995 (Sandby,1998 ). In Lake Bastrup, 7 t cyprinids were removed
during 1995–1997. The cyprinid biomass was calculated to 300–400kg ha^1
before biomanipulation and to 150kg ha^1 afterwards (Frederiksborg County,
2000 ).
The fish removals resulted in a major increase in cladoceran body weight
and the zooplankton:phytoplankton biomass ratio, a major reduction in
phytoplankton biomass expressed as chlorophyll a, and marked reductions
in both total phosphorus and total nitrogen (Fig.7.3, see also Jeppesenet al.,
1998 , 2004). These changes occurred in all three lakes despite different
nutrient levels. In contrast, no uniform response could be traced in the
body weight of copepods, rotifers or total zooplankton. When comparing
these results with those from oligotrophic Greenland lakes with and without
fish (Box7.1), we find a strong effect of fish on cladoceran body weight and
the zooplankton:phytoplankton ratio in both sets of lakes. However, in
contrast to the oligotrophic Greenland lakes the fish reduction in the eutro-
phic Danish lakes has major cascading effects on the phytoplankton and
even on the nutrient concentrations. This holds true even though far fromBODY SIZE AND TROPHIC CASCADES IN LAKES 125