LAKE DWELLING TRICLADS
17
margin, but the calcium content was surprisingly low compared with
surrounding nearby lakes, e.g. The Mere, Ellesmere with 32 mg/litre
calcium. Gorham (1957) who recorded 7 - 4 mg/litre of calcium suggested
geological and hydrological reasons for tthe low mineral content of this
lake. It is probable that plant and animal material from the terrestrial
zone increases the productivity of the littoral zone so that it is higher
than would be judged on water chemistry alone. Lakes Lurgan, Marlacoo, and Lowry (Fig. 4, B, c, D ) all in N. Ireland also support an exceptionally high population; indeed the Irish lakes nearly all tend to
have a high population relative to the other areas especially Scotland,
suggesting that it is perhaps unrealistic t o group them together (Reynoldson, 1958a). However, even within the Irish area, these three lakes have
higher than average numbers of triclads, especially measured against
calcium. The first, of these was undoubtedly enriched by organic waste
from the township of Lurgan, while the other two had farm buildings
near the shore from which nutrient-rich water drained into the lakes
without noticeably increasing calcium or T.D.M. ; increased productivity
from such sources is well known (Hasler, 1947; Mundie, 1957; Berg,
1958). The reasons for exceptionally low populations at high calcium
and T.D.M. levels are not obvicus, but the two most striking examples
(Fig. 5, E, F) are both in the Scottish area surrounded by land which was
noL intensively farmed.
When the main areas are considered separately in the present context
they all show highly significant relationships between the standing crop
of triclads and calcium, T.D.M. and altitude (Table 111). There is of
course an obvious reason for the relationship with altitude since high
ground usually means hard and insoluble rocks (Mackereth, 1957). This
is made clear in Table IIIb where the regression coefficients between
T.D.M. and altitude are shown to be negative and highly significant
except for the N. Wales area. The latter result is explained by the circumstance that most of the lakes a t lower altitudes occur in narrow
valleys flanked by high ground and have a resultant low T.D.M. It will
be difficult to separate the influence of two such intimately connected
factors as altitude and water chemistry upon the distribution and abundance of triclads, but the results from the low-lying lakes of Islay showed
that calcium and T.D.M. per se can be related to triclad abundance
(Reynoldson, 1958a). The adverse effect of altitude in the presence of
high calcium was indicated by the relatively small triclad population restricted to P. nigra, found in Malham tarn (W. Yorks) at 1 2 5 0 ft
(381 m) with 48.8 mg/litre calcium.
ii. The Populations of the Individual Species. 117 analysing the changes
in population size of the individual species, the 68 lakes have been
grouped into categories based on calcium content and the population of
17
margin, but the calcium content was surprisingly low compared with
surrounding nearby lakes, e.g. The Mere, Ellesmere with 32 mg/litre
calcium. Gorham (1957) who recorded 7 - 4 mg/litre of calcium suggested
geological and hydrological reasons for tthe low mineral content of this
lake. It is probable that plant and animal material from the terrestrial
zone increases the productivity of the littoral zone so that it is higher
than would be judged on water chemistry alone. Lakes Lurgan, Marlacoo, and Lowry (Fig. 4, B, c, D ) all in N. Ireland also support an exceptionally high population; indeed the Irish lakes nearly all tend to
have a high population relative to the other areas especially Scotland,
suggesting that it is perhaps unrealistic t o group them together (Reynoldson, 1958a). However, even within the Irish area, these three lakes have
higher than average numbers of triclads, especially measured against
calcium. The first, of these was undoubtedly enriched by organic waste
from the township of Lurgan, while the other two had farm buildings
near the shore from which nutrient-rich water drained into the lakes
without noticeably increasing calcium or T.D.M. ; increased productivity
from such sources is well known (Hasler, 1947; Mundie, 1957; Berg,
1958). The reasons for exceptionally low populations at high calcium
and T.D.M. levels are not obvicus, but the two most striking examples
(Fig. 5, E, F) are both in the Scottish area surrounded by land which was
noL intensively farmed.
When the main areas are considered separately in the present context
they all show highly significant relationships between the standing crop
of triclads and calcium, T.D.M. and altitude (Table 111). There is of
course an obvious reason for the relationship with altitude since high
ground usually means hard and insoluble rocks (Mackereth, 1957). This
is made clear in Table IIIb where the regression coefficients between
T.D.M. and altitude are shown to be negative and highly significant
except for the N. Wales area. The latter result is explained by the circumstance that most of the lakes a t lower altitudes occur in narrow
valleys flanked by high ground and have a resultant low T.D.M. It will
be difficult to separate the influence of two such intimately connected
factors as altitude and water chemistry upon the distribution and abundance of triclads, but the results from the low-lying lakes of Islay showed
that calcium and T.D.M. per se can be related to triclad abundance
(Reynoldson, 1958a). The adverse effect of altitude in the presence of
high calcium was indicated by the relatively small triclad population restricted to P. nigra, found in Malham tarn (W. Yorks) at 1 2 5 0 ft
(381 m) with 48.8 mg/litre calcium.
ii. The Populations of the Individual Species. 117 analysing the changes
in population size of the individual species, the 68 lakes have been
grouped into categories based on calcium content and the population of