9780521861724htl 1..2

all fish were removed from the Danish lakes. Presence of fish may explain

why the body weight of total zooplankton and cladocerans, as well as the

zooplankton:phytoplankton ratio, are higher overall in the fish-free Greenland

lakes than in the Danish lakes after the fish removal. It is noteworthy that the

reduction in chlorophyll a, total phosphorus and total nitrogen occurred

in Lake Engelsholm despite the absence of submerged macrophytes

(Jeppesenet al., 1998), indicating that it is largely the effect of the fish removal

that triggers the abrupt changes observed.

Figure 7.3Average of summer means (1 May–1 Oct) of selected variables

for the period before (2–6 years) and after (9–13 years) a mass removal of plankti-

benthivorous fish by biomanipulation (Lake Bastrup, BAS, and Lake Engelsholm,

ENG), and in Lake Arreskov (ARS) in addition by mass fish kill. The three lakes are all

eutrophic.

Table 7.1Morphometric data and annual mean hydraulic retention time in the

epilimnion of the three lakes in which the fish stock was reduced.

Surface area

(km^2 )

Mean depth

(m)

Maximum

depth (m)

Retention

time (days)

Lake Engelsholm 0.44 2.6 6.1 65–88

Lake Arreskov 3.17 1.9 3.7 510–770

Lake Bastrup 0.32 3.5 7.0 600–2200

126 J. I. JONES AND E. JEPPESEN