LAKE DWELLING TRICLADS
15
2. The Abundance of l’riclads
A stricter selection of lakes was necessary to maintain comparability
of collecting sites. This reduced the original number of 107 to 68 of
which 22 were located in Mid-Scotland, 6 in N. Scotland extending to
the Highland Fault, 18 from N. Ireland, 16 from Snowdonia and 6 from
Shropshire. The samples measured the standing crop of triclads, and
since the Total Dissolved Matter (abbreviated later to T.D.M.) is a more
reliable indication of productivity (Larkin and Northcote, 1958; Rawson,
1960) than calcium, this parameter has also been related to triclad
abundance. Data for T.D.M. were available for only 44 of the 68 lakes
which were about equally divided between Mid-Scotland, N. Wales and
N. Ireland. The ranges of the physico-chemical properties of the lakes
used here were similar to those already outlined in considering distribution.
i. The Total Population. Scatter diagrams in which total numbers of
triclads collected per hour are related to calcium and T.D.M. are shown in
Fig. 4 and 5. The regression coefficients measuring both relationships
are highly significant (P < 0-OOl), nevertheless it is apparent that these
chemical factors only account for a part of the variance (especially with
calcium) and other factors apart from sampling errors, are important.
This is not surprising in view of the complex of factors involved. These
may be regarded as operating at three levels, basic physico-chemical
factors, primary production and secondary production, all of which
can be expected to influence organisms like triclads which are at or near
the apex of a food chain. While T.D.M. will be usually closely related
to productivity of the littoral zone there may be exceptions since as
Margalef (1958) pointed out, it is phosphate and nitrate which normally
determine primary productivity and these form only a small part of
T.D.M. There is also some evidence that very dystrophic conditions
are related to low triclad populations independently of calcium and
T.D.M. Consideration of some of the more striking exceptions to the
trends shown in Fig. 4 and 5 indicates some of the contributory factors.
For example, there is a group of lakes at the lower calcium and T.D.M.
levels which either do not support a triclad population at all or only
R relatively small one. These are mainly lakes at the higher altitudes,
nnd clearly such adverse factors as low winter and spring temperature
and a short summer are reinforcing the effect of low productivity.
Newton Mere (Fig. 4 ~ )
is also exceptional not only in supporting a
relatively high populat,ion but also in containing Dugesia lugubris and
Dendrocoelum lacteum although the calcium content is only 5 mg/litre.
This lake, on slightly higher ground than the others, was surrounded by
good agricultural land with a thin belt of deciduous trees along one
15
2. The Abundance of l’riclads
A stricter selection of lakes was necessary to maintain comparability
of collecting sites. This reduced the original number of 107 to 68 of
which 22 were located in Mid-Scotland, 6 in N. Scotland extending to
the Highland Fault, 18 from N. Ireland, 16 from Snowdonia and 6 from
Shropshire. The samples measured the standing crop of triclads, and
since the Total Dissolved Matter (abbreviated later to T.D.M.) is a more
reliable indication of productivity (Larkin and Northcote, 1958; Rawson,
1960) than calcium, this parameter has also been related to triclad
abundance. Data for T.D.M. were available for only 44 of the 68 lakes
which were about equally divided between Mid-Scotland, N. Wales and
N. Ireland. The ranges of the physico-chemical properties of the lakes
used here were similar to those already outlined in considering distribution.
i. The Total Population. Scatter diagrams in which total numbers of
triclads collected per hour are related to calcium and T.D.M. are shown in
Fig. 4 and 5. The regression coefficients measuring both relationships
are highly significant (P < 0-OOl), nevertheless it is apparent that these
chemical factors only account for a part of the variance (especially with
calcium) and other factors apart from sampling errors, are important.
This is not surprising in view of the complex of factors involved. These
may be regarded as operating at three levels, basic physico-chemical
factors, primary production and secondary production, all of which
can be expected to influence organisms like triclads which are at or near
the apex of a food chain. While T.D.M. will be usually closely related
to productivity of the littoral zone there may be exceptions since as
Margalef (1958) pointed out, it is phosphate and nitrate which normally
determine primary productivity and these form only a small part of
T.D.M. There is also some evidence that very dystrophic conditions
are related to low triclad populations independently of calcium and
T.D.M. Consideration of some of the more striking exceptions to the
trends shown in Fig. 4 and 5 indicates some of the contributory factors.
For example, there is a group of lakes at the lower calcium and T.D.M.
levels which either do not support a triclad population at all or only
R relatively small one. These are mainly lakes at the higher altitudes,
nnd clearly such adverse factors as low winter and spring temperature
and a short summer are reinforcing the effect of low productivity.
Newton Mere (Fig. 4 ~ )
is also exceptional not only in supporting a
relatively high populat,ion but also in containing Dugesia lugubris and
Dendrocoelum lacteum although the calcium content is only 5 mg/litre.
This lake, on slightly higher ground than the others, was surrounded by
good agricultural land with a thin belt of deciduous trees along one