LAKE DWELLING TRICLADS
7
productivity, soft rocks to chemically rich water and lakes of high productivity (Mackereth, 1957) (Plate 1, b, c). This broad relationship is
modified by other factors such as shape, depth, aspect (Gorham, 1958),
and often by human activities which always complicate interpretation of
field work.
The preliminary work showed that all the triclad species required
resting sites providing shade. They were also most numerous in shallow
water, scarce or absent when the edge of the lake was undercut and
leading directly into deep water. Sandy shores were unfavourable,
probably due to abrasion. The basic requirements are best met either
by a shore with semi-emergent vegetation such as Yellow Flag (Iris
pseudacorus, Linn.), Potamogeton or Water Lily (Nymphaea sp.) etc.
which would normally be found growing on a soft substratum or by a
stony shore. While the presence of vegetation usually means a sheltered
habitat, a stony shore may be either exposed or sheltered to a varying
degree. Exposure is unsuitable for lake-dwelling triclads because of the
danger of crushing by shifting of the substratum under wave action
(Beauchamp, 1932); their food supply is limited in a similar way. The
effect of shelter and exposure upon these triclads was shown clearly in
a! large lake such as Windermere where an exposed stony shore yielded
only single specimens of two species, while a sheltered stony shore
harboured the four main species in large numbers. In some small lakes
the effects of exposure can be very apparent. For example, in Shaw
Lough (County Armagh, N. Ireland) triclads were absent from the stony
shore exposed to the prevailing wind, whilst a sheltered shore with
stones yielded a population of 282 collected per hour comprising three
species (Reynoldson, 1968a). Such examples could be multiplied.
In deciding between a shore of emergent vegetation and a stony shore
for routine sampling, the most important criterion was amenability to
quantitative sampling, but availability of shores was also important.
Stony shores of comparable type are characteristic of oligotrophic lakes
and are to be found in about half of the eutrophic lakes, the opposite is
roughly true of plant dominated shores. In amenability to sampling,
stony shores are superior. The species composition and the concentration of plants vary and the vegetation cannot be so easily searched,
features which complicate any standard sampling technique; also such
shores are usually less accessible. For these reasons a moderately sheltered, stony shore was preferred as the standard type for routine sampling (Plate la).
Before finally deciding on a stony shore the habitat had to be tested
to see whether it would bias the data in favour of a particular species.
During the preliminary field work both types of shore were sampled in
a number of lakes. The results showed that none of the four species
7
productivity, soft rocks to chemically rich water and lakes of high productivity (Mackereth, 1957) (Plate 1, b, c). This broad relationship is
modified by other factors such as shape, depth, aspect (Gorham, 1958),
and often by human activities which always complicate interpretation of
field work.
The preliminary work showed that all the triclad species required
resting sites providing shade. They were also most numerous in shallow
water, scarce or absent when the edge of the lake was undercut and
leading directly into deep water. Sandy shores were unfavourable,
probably due to abrasion. The basic requirements are best met either
by a shore with semi-emergent vegetation such as Yellow Flag (Iris
pseudacorus, Linn.), Potamogeton or Water Lily (Nymphaea sp.) etc.
which would normally be found growing on a soft substratum or by a
stony shore. While the presence of vegetation usually means a sheltered
habitat, a stony shore may be either exposed or sheltered to a varying
degree. Exposure is unsuitable for lake-dwelling triclads because of the
danger of crushing by shifting of the substratum under wave action
(Beauchamp, 1932); their food supply is limited in a similar way. The
effect of shelter and exposure upon these triclads was shown clearly in
a! large lake such as Windermere where an exposed stony shore yielded
only single specimens of two species, while a sheltered stony shore
harboured the four main species in large numbers. In some small lakes
the effects of exposure can be very apparent. For example, in Shaw
Lough (County Armagh, N. Ireland) triclads were absent from the stony
shore exposed to the prevailing wind, whilst a sheltered shore with
stones yielded a population of 282 collected per hour comprising three
species (Reynoldson, 1968a). Such examples could be multiplied.
In deciding between a shore of emergent vegetation and a stony shore
for routine sampling, the most important criterion was amenability to
quantitative sampling, but availability of shores was also important.
Stony shores of comparable type are characteristic of oligotrophic lakes
and are to be found in about half of the eutrophic lakes, the opposite is
roughly true of plant dominated shores. In amenability to sampling,
stony shores are superior. The species composition and the concentration of plants vary and the vegetation cannot be so easily searched,
features which complicate any standard sampling technique; also such
shores are usually less accessible. For these reasons a moderately sheltered, stony shore was preferred as the standard type for routine sampling (Plate la).
Before finally deciding on a stony shore the habitat had to be tested
to see whether it would bias the data in favour of a particular species.
During the preliminary field work both types of shore were sampled in
a number of lakes. The results showed that none of the four species