14
T. B. REYNOLDSON
limit recorded was 1.6 mg/litre whilst only 5% of total occurrences
were below 5 mgllitre compared with 22% for P. nigra. The distributions of Dugesia lugubris and Dendrocoelum lacteum were so similar that
they can be considered together. Both species were absent from lakes
low in calcium. Only a single occurrence was found below 5 mg/litre, at
4.8 mg/ in Newton Mere, Shropshire, and this may be exaggerated since
Gorham (1957) recorded 7.4 mg/litre. Eighty per cent of the records
were from lakes with more than 10 mg/litre; comparable figures for
P. nigra and P. tenuis were 30% and 40% respectively. When the distribution of these triclads was examined separately in the four main areas
they showed the same pattern except that each presented a less complete
picture (Reynoldson, 1958b). For example, most of the Snowdonia lakes
fell into the range with a calcium value less than 5 mg/litre while the
Shropshire meres lay a t the upper end of the range.
Table I1 presents the data in summary form from a slightly different
approach, showing the proportion of lakes occupied by the various species in particular calcium ranges. Polycelis nigra is seen to occupy a
gradually increasing proportion of lakes reaching a peak of 94% in the
calcium range 5-1-10.0 mg/litre but above this its incidence declines and
it occurs in only 72% of the lakes in which calcium exceeds 20 mg. In
contrast, P. tenuis although starting off at a lower level of incidence
continues to increase until in the richest lakes it is universally present.
Dugesia lugubris and Dendrocoelum lacteum both tend to occur in a
higher proportion of lakes as the calcium increases but their maximal
incidence is about 70%, i.e. neither is quite so well dispersed as the
Polycelis species, even in the most favourable lakes.
TABLE I1
Percentage Distribution of Triclads in Relation to Ca,lciunz Concentration
of Water
Species
Absent
Phagocata vitta
Polycelis nigra
Polycelis tenuis
Dugeaia lugubris
Dendrocoelum lacteum
Total lakes
<2.5
26%
26%
44%
6%
0%
0%
33
2.6-5.0
36 %
0%
64%
27%
9%
9%
12
5*1-10.0 10.1-20.0
0%
0%
0 Y O
0%
94%
82%
82%
91%
29%
36%
24%
36%
20
12
> 20.0
0
0%
72%
looyo
69 %
66%
30
Calcium mg/litre
Total
lakes
13
9
68
58
30
28
107
Calcium has been selected as a convenient framework within which
to describe the distribution of these triclads, but an equally convincing
pattern also emerges in relation to ttotal dissolved matter.
T. B. REYNOLDSON
limit recorded was 1.6 mg/litre whilst only 5% of total occurrences
were below 5 mgllitre compared with 22% for P. nigra. The distributions of Dugesia lugubris and Dendrocoelum lacteum were so similar that
they can be considered together. Both species were absent from lakes
low in calcium. Only a single occurrence was found below 5 mg/litre, at
4.8 mg/ in Newton Mere, Shropshire, and this may be exaggerated since
Gorham (1957) recorded 7.4 mg/litre. Eighty per cent of the records
were from lakes with more than 10 mg/litre; comparable figures for
P. nigra and P. tenuis were 30% and 40% respectively. When the distribution of these triclads was examined separately in the four main areas
they showed the same pattern except that each presented a less complete
picture (Reynoldson, 1958b). For example, most of the Snowdonia lakes
fell into the range with a calcium value less than 5 mg/litre while the
Shropshire meres lay a t the upper end of the range.
Table I1 presents the data in summary form from a slightly different
approach, showing the proportion of lakes occupied by the various species in particular calcium ranges. Polycelis nigra is seen to occupy a
gradually increasing proportion of lakes reaching a peak of 94% in the
calcium range 5-1-10.0 mg/litre but above this its incidence declines and
it occurs in only 72% of the lakes in which calcium exceeds 20 mg. In
contrast, P. tenuis although starting off at a lower level of incidence
continues to increase until in the richest lakes it is universally present.
Dugesia lugubris and Dendrocoelum lacteum both tend to occur in a
higher proportion of lakes as the calcium increases but their maximal
incidence is about 70%, i.e. neither is quite so well dispersed as the
Polycelis species, even in the most favourable lakes.
TABLE I1
Percentage Distribution of Triclads in Relation to Ca,lciunz Concentration
of Water
Species
Absent
Phagocata vitta
Polycelis nigra
Polycelis tenuis
Dugeaia lugubris
Dendrocoelum lacteum
Total lakes
<2.5
26%
26%
44%
6%
0%
0%
33
2.6-5.0
36 %
0%
64%
27%
9%
9%
12
5*1-10.0 10.1-20.0
0%
0%
0 Y O
0%
94%
82%
82%
91%
29%
36%
24%
36%
20
12
> 20.0
0
0%
72%
looyo
69 %
66%
30
Calcium mg/litre
Total
lakes
13
9
68
58
30
28
107
Calcium has been selected as a convenient framework within which
to describe the distribution of these triclads, but an equally convincing
pattern also emerges in relation to ttotal dissolved matter.