Dystrophic pools and lakes occur amongst
or on oligotrophic mires or where a lake
receives water draining largely from oligotrophic mire. The distribution of these
waters in Britain therefore follows that of such mires. The most widespread
oligotrophic mire type, blanket mire, is confined to the north and west of Britain, and
contains a range of dystrophic waters from moderate-sized lakes to minute pools
on patterned bogs. The mode of origin of small peat pools is still obscure, but many
of the larger dystrophic waters occupy former glacial troughs (e.g. Loch Laidon). In
the south and east of Britain dystrophic waters are far more scattered and many
occupy former peat-cuttings on raised and basin mires: others have been formed
by the encroachment of a schwingmoor on a eutrophic lake which thus becomes
isolated from the ground water by the surrounding peat.
Peat pools and dubh lochans on blanket
mire are the most widespread of
dystrophic waters in Britain. Typically, these are small, shallow bodies of water less
than a metre deep and with peat shorelines and bottoms. Littoral and profundal
zones do not therefore exist, and the whole pool may be considered as a sublittoral
habitat. The waters are generally acid (pH 4.0 to 6.0) and deficient in bases,
although a number show surprisingly high phosphate and organic nitrogen contents,
possibly resulting from the decay of allochthonous material carried into them. The
water is normally stained brown by dissolved humic acids, though not invariably at
all seasons.
The characteristic macrophytic plant species
found in small bog pools are
bryophytes such as Sphagnum sub-secundum var. auriculatum, S. subsec.undum
var. inundatum, S. cuspidatum and Drepanodadus fhritans, and the angio-sperms
Eriophorum angustifolium, Utricularia minor, Meny-anthes trifoliata and
Potamogeton polygonifolius. Other angiosperms such as Nymphaea alba
(including ssp. occi-dentalis), Sparganium minimum, Juncus bulbosus, Ekocharis
multicaulis and Carex limosa also frequently occur in such situations together with
certain leafy liverworts including Gymnocolea inflata and Cephalozia fluitans. The
bottom is often covered by a dense layer of algae of which Nostoc spp., Tribonema
minus, desmids, Zygamales and Microspora are characteristic, but phytoplankton
production is low.
In small peat pools there is no true zooplankton
but a number of species of
copepods and cladocerans are associated with the bottom peat and Sphagnum.
These include the cladocerans Streblocerus serricaudatus, Acantholeberis curvi-
rostris and Chydorus sphaericus, and the copepods Cyclops (Acanihocy clops)
venustus, Cyclops (Diacyclops) languidus, C. nanus and Bryocamptus weberi.
Peaty-shored dystrophic lakes and peat
pools have a very characteristic benthic
invertebrate fauna in which some groups, such as molluscs, are typically absent,
while others such as dragonflies, beetles and water bugs are very abundant. In
small pools the invertebrate fauna often contains a preponderance of carnivorous
species which must be largely dependent for food on terrestrial invertebrates that
fall into the water.
The fauna consists predominantly of insects,
and groups such as triclads, leeches,
molluscs, Malacostraca and Hydracarina are poorly represented or absent.
Molluscs are only found in the slightly richer sites when Lymnaea (Radix) peregra,
Pisidium casertanum, P. obtusale and P. personatum occasionally occur, and in
the larger dubh lochans Valvata piscinalis may be found in the deeper water.
Tubificid and naidid worms are often abundant in the bottom deposits and
vegetation of peaty pools, but leeches only occur in larger dubh lochans. The water
spider Argyroneta aquatica is often plentiful in small pools where it spins its net
amongst the submerged mosses and Utricularia. Mayflies and stoneflies are
typically absent from the small waters, but species such as Leptophlebia marginata
and L. vespertina, which are typical of oligotrophic lakes, are found in the larger
dubh lochans.
One of the characteristic groups found
in mire pools are the dragonflies which are
represented by a wide range of species. The most widespread and abundant
species found in such situations, but not necessarily confined to peat pools, are
Sympetrum danae, Aeshna juncea, Pyrrhosoma nymphula, Lestes sponsa and
Libellula quadrimaculata. Other widespread but rather uncommon mire species are
Leu-corrhinia dubia and Coenagrian puella while Coenagrion hastulatum,
Somatochlora arctica and Aeshna caerulea are confined to the north (where they
may all breed in dystrophic waters) but only A. caerulea is restricted to peat pools.
Somatochlora metallica has an odd distribution, being found both in acidic waters
in Scotland and in eutrophic waters in southern England, but not elsewhere.
Another characteristic group associated
with peatland pools are the water bugs.
The common species of acidic blanket mire pools in the north and west are
Callicorixa wollastoni, Hesperocorixa castanea, Arctocorisa carinata, Sigara scotti,
Notonecta obliqua, Gerris costai, G. odonto-gaster, G. gibbifer and, in the wet
Sphagnum at the sides of the pools, Hebrus ruficeps. Sigara lateraiis and S.
venusta are less frequently found in these pools. Glaenocorisa propinqua and
Arctocorisa germari are found in larger peat pools and dubh lochans. The water
beetles are represented by a large number of species of Hydroporus, including
particularly H. nigrita, plus Agabus spp., Ilybius aenescens, Rantus exsoletus, R.
bistriatus, Acilius sulcatus, A. canaliculatus and Gyrinus minutus and G. opacus.
The alder fly Sialis lutaria is found
in deeper dystrophic waters where there is a soft
organic sediment and caddis flies also occur in peaty pools (usually in small
numbers). Limnephilidae and Phryganeidae are characteristic, Agryp-nia varia, A.
obsoleta and Oligotricha striata being common and widespread, but the
Limnephilidae of these waters are not well known. Holocentropus spp. are
frequently found among thick moss in peat pools, but other Polycentropidae such
as Cyrnus flavidus and Polycentropus flavomaculatiis are only found in larger
bodies of water where the shorelines are stony.
Chironomidae larvae are often extremely
abundant, but little is known of the actual
species found. The phantom midges are represented by Chaoborus crystallinus, C.
obscuripes and Mochlonyx martinii and they may be extremely abundant in the
deeper pools. In most peat pools various biting midges (Ceratopogonidae) and
mosquitoes (Culex, Aedes, Anopheles and Theobaldia) also occur as larvae.
In lowland Britain, dystrophic waters
are mainly found as residual open water in
basin mires. Although acidic, they are usually richer than pools on blanket mire and
may have calcium contents as high as 30 p.p.m. Such pools are often deeper than
those on blanket mire and are invariably heavily peat-stained and plant growth is
confined to shallow water. Since they are usually bounded by floating rafts of
vegetation, a shallow littoral is absent, and submerged vegetation is restricted to a
narrow fringe of Sphagnum cuspidatum, S. subsecundum var. inundatum and on
occasions Utricularia minor. The marginal raft-forming vegetation generally
comprises species such as Potentilla palustris, Menyanthes trifoliata, and a wide
range of Carex spp. growing on a bed of Sphagnum spp., particularly S. recurvum.
Algal production is also limited by lack of light penetration so that very little
autochthonous primary production occurs in these waters. As a result, dissolved
oxygen concentrations may be low and organic nitrogen and phosphate derived
from the decay of allochthonous material may be present in relatively high
concentrations.
The invertebrate fauna of lowland mire
pools differs somewhat from that found in
upland blanket mire pools, but whether these differences result from the different
physical conditions within the pools, particularly water chemistry, or to restriction in
geographical range of some species is difficult to ascertain.
The fauna of lowland mire pools consists
primarily of insects, but non-insect groups
may be more abundant than in upland pools. In addition to those species of mollusc
found in blanket mire pools, soft-water species of gastropod such as Lymnaea
(Galba)palustris, Planorbis (Gyraulus) albus, P. (Armiger) crista and Potamopyrgus
jenkinsi may occur in the richer basin mire pools of the lowlands. Leeches such as
Helobdella stagnalis also live in these conditions and in the most eutrophic pools
the malacostracans Asellus meridianus and Crangonyx pseudogracilis may also
occur. Stoneflies and mayflies are typically absent, but Cloeon dipterum has been
found associated with the marginal vegetation of a few areas.
In addition to the widespread and abundant
dragonflies found in a variety of
dystrophic waters, a number of species are restricted to the south of Britain where
they occur in lowland peat pools, though not all of them are confined to base-poor
waters. These include Orthetrum coerulescens, O. cancellatum, Libellula Julva,
Ischnura pumilio and Coenagrion mercuriale, the last two being typical of
Sphagnum flushes. The lowland pools have a different assemblage of water bugs
from their upland counterparts and the common species are Hesperocorixa
castanea, H. linnei, H. sahlbergi, Notonecta glauca and Gerris odontogaster.
Hesperocorixa moesta, Ilyocoris cimicoides and Hebrus pusillus are found less
frequently and /. cimicoides occurs in dense submerged bryophytes and H. pusillus
in wet Sphagnum at the edges. Water beetles are usually abundant in lowland
pools and a similar range of species to that found in upland situations is usually
present, but a few species (e.g. Haliplus variegatus and Ilybius fenestratus) are
confined to lowland Britain. The rare caddis flies Oligotrichia clathrata and Rhadi-
coleptus alpestris are confined to a few lowland peat pools in the Midlands,
limnephilids such as Micropterna lateralis and Limnephilus centralis may be
numerous and are more widespread. The phantom midge larvae Chaoborus
flavicans and C. crystallinus are very characteristic of lowland dystrophic waters
and may be extremely abundant especially where low oxygen concentrations
prevail.
In larger dystrophic waters, as exposure
to wind and wave action increases with
increased fetch, there is a tendency for the peat shorelines to be replaced by
coarser inorganic sediments of sand, gravel, stone or boulders. Dystrophic lakes of
greater than 5 ha are seldom entirely bounded by shorelines of actively growing
peat, and in the more exposed northerly sites, much smaller pools may have
eroding mineral shorelines. The largest dystrophic lake found in this survey was
Loch Laidon (465 ha) which receives drainage from Rannoch Moor, and all larger
lakes in Britain appear to have a pH greater than 6.0. The vegetation of mineral
shored dystrophic lakes is similar to that of oligo-trophic lakes, but limited light
penetration restricts macro-phytic growth and phytoplankton production to a
shallower photic zone. Stones in shallow water are often covered by thick
mucilaginous growths of algae, of which Batracho-spermum spp. is often
conspicuous and these are not generally encountered in oligotrophic lakes.
The invertebrate fauna of the larger waters
in this category is similar in composition
to that of oligotrophic lakes. The extremely low primary production limits the
production of the invertebrate fauna which is therefore very sparse. Molluscs are
generally fewer than in typical oligotrophic lakes and only very adaptable species
such as Pisidium casertanum and Lymnaea (Radix) pereger are found.
