Dystrophic standing waters
These have not been well covered in this
review and, in general, it has been
assumed that a representative range of small dystrophic waters is included within
peatland and upland sites. This is particularly the case with dystrophic waters on
upland blanket mire, but in the case of the richer lowland dystrophic waters a series
of open water sites has been selected which represents the most outstanding
examples of this type of habitat. One very large dystrophic water, Loch Laidon, has
also been chosen as no other comparable dystrophic water is known to occur in
sites selected for their peatland or upland interest.
Dystrophic waters are rare in southern
Britain where they are confined to areas of
base-poor rocks and to areas of impeded drainage where topogenous mire
systems have developed. The main concentration of such mires is on the Tertiary
sands which extend from Surrey through the New Forest in Hampshire to Dorset.
Most of these mires however contain very little open water and only in some of the
New Forest Valley Mires, such as Cranesmoor is there extensive pool
development. These pools are similar to those found on many patterned blanket
mires in the north west of Britain and contain a similar range of flora and fauna. The
range of variation of such pools has not however been adequately covered in this
review and they are regarded as bonus to the peatland interest. Woolmer Pond,
Hampshire, also lying on the Tertiary sands, is a much larger dystrophic water
which may be an ancient peat-cutting. This very shallow lake of 25 ha is of interest
on account of its rich invertebrate fauna associated with dense beds of submerged
bryophytes, particularly Drepanocladus fluitans.
East Anglia is geologically mostly calcareous,
and natural dystrophic waters are
only represented by tiny pools in a few oligotrophic mire systems (e.g. Dersingham
Bog) which occur along the narrow belt of non-calcareous Green-sand in west
Norfolk. One of the Norfolk Broads, Calthorpe Broad, has recently and dramatically
become highly dystrophic, at least seasonally. Prior to 1970 this Broad was highly
calcareous and was one of the few Broad-land sites which retained the
characteristic flora and fauna of the area. In that year, however, a drop in the water
table resulted in temporary chemical changes which brought about a fall in pH to pH
3.0-3.4.
In the upland areas of south-west England
and south and north Wales dystrophic
waters are represented by pools and small lakes in areas of blanket mire on sites
such as North Dartmoor, Cors Goch, Radnorshire, and Y Berwyn, where they are of
bonus interest. Raised mires such as Shapwick Heath, Somerset, Cors Fochno,
Cardiganshire and Cors Goch glan Teifi, Cardiganshire, generally contain very little
natural standing open water, but peat-cuttings may contain pools which are
chemically richer than those on upland blanket mire and have a different
invertebrate fauna.
Within the Cheshire—Shropshire-Staffordshire
plain many kettle-holes have
developed into basin mires surrounding areas of dystrophic open water, which
typically become cut-oft from the surrounding drainage and are very acidic and
base-poor. The Gull Pool on the basin mire of Abbots Moss, Cheshire, has a thick
carpet of the moss Drepanocladus fluitans and a fauna similar to that of Woolmer
Pond, while the pools on the south of the Moss have a more limited fauna. Other
very acidic peat-stained pools formed within a basin mire schwingmoor are found
on Chartley Moss, Staffordshire. These have an interesting aquatic fauna
containing a number of rare caddis species, and are chosen as the grade 1
example of an acidic basin mire open water site. At Clarepool Moss the open
water formed at the periphery of the mire is influenced by outside drainage and is
therefore both base-rich and dystrophic. Despite the extremely acidic dystrophic
condition, the invertebrate fauna contains a large proportion of species associated
with eutrophic conditions as well as carnivores typical of peat pools. Because of
this interesting dual nature the area is given grade 1 status as an open water site.
At Sweat Mere, Shropshire, the drainage of the mire has been affected by the
cutting of ditches draining from the nearby larger eutrophic lake, Crose Mere, and
the pool is now base-rich (alkalinity = 160 p.p.m. CaCO3) and neutral, with a fauna
typical of a base-rich pond. This site, in association with Crose Mere, is rated
grade i mostly on account of the well-developed hydroseral succession. A series of
partially interconnected ponds at Brown Moss, Shropshire, show a range of trophic
conditions ranging from acidic, base-poor pools to eutrophic ponds with a wide
range of associated flora and fauna. They are of great potential research interest
and deserve grade 2 status.
From northern England northwards, blanket
mire becomes an increasingly common
habitat in upland areas and with the increasing rainfall in the west, peat pools and
dystrophic tarns and lochans become more abundant, reaching their greatest
concentration in the extreme northwest of Scotland where they cover a high
proportion of the land surface. Dystrophic pools, tarns and lochans occur as a
bonus on most of the grade I blanket mire peatland and upland sites in northern
England and Scotland, e.g. on Irthinghead Mires, Moor House, Silver Flowe,
Rannoch Moor and Inverpolly. Larger dystrophic waters are more uncommon and
the largest known example in Britain of a dystrophic lake is Loch Laidon,
Perthshire/Argyll, which adjoins Rannoch Moor and is one of the most unproductive
lakes in Britain.
Oligotrophic standing waters
These are by far the most abundant type
of standing water-body in Britain and
information is available only for a small proportion of them. Fortunately, however,
the range of variation of oligotrophic waters is more limited than that of richer
waters, and the commonest types, with their very limited and uniform flora and
fauna, are already well represented in sites selected for their upland interest. In the
selection of oligotrophic lakes, therefore, only those showing extreme
characteristics (e.g. Arctic-alpine lakes, very large deep lakes and isolated lowland
lakes) have been included «s key open water sites.
Oligotrophic lakes are virtually confined
to the high rainfall, base-poor upland areas
of north Wales, the Lake District, Galloway and the Scottish Highlands, where
intense erosion during the last glaciation was the major factor in their formation.
The only known oligotrophic lake in lowland England is Oakmere in Cheshire (gr. 1)
which is a kettle-hole occupying a pocket of base-poor glacial drift. Despite its very
low alkalinity it contains calcicoles such as Typha angustifolia and Asellus
meridianus alongside a flora and fauna more typical of oligotrophic conditions. In
south-west England the upland areas of Dartmoor, Exmoor and Bodmin Moor
contain very few standing open waters since they lie south of the area of glaciation.
A few small pools do occur, however, the largest of these being Dozmary Pool on
Bodmin Moor which is not, however, sufficiently distinctive for inclusion within the
series.
The most southerly areas of glacial erosion
occurred in south Wales where small
examples of lakes occupying glacial troughs and corries are found. Llyn y Fan Fawr
(gr. 2) is the largest and most southerly of the corrie lakes. In north Wales these
lakes are more abundant (although some are polluted by drainage from old copper
mines) and a number are represented on existing National Nature Reserves
(NNRs), e.g. Llyn Cau on Cader Idris and Llyn
Glaslyn (gr. 2) below Y Wyddfa. The latter
is a high-altitude example showing some
affinities to those in the Cairngorms in its very sparse fauna and absence of aquatic
angiosperms. Llyn Idwal on the northern side of Glyder Fawr represents the
mesotrophic end of this series of upland corrie lakes and has a diverse relict
aquatic flora for which it is given grade 1 status. A few large oligotrophic glacial
trough lakes are also found in north Wales. Llyn Tegid (gr. 1), Merioneth, represents
the richer end of the spectrum and contains the white fish Coregonuslavaretus
pennanti, while Llyn Cwellyn (gr. 2), Caernarvonshire, is more barren and
oligotrophic, and contains the Welsh race of the charr Salvelinus alpinus. Llyn
Padarn is similar to Llyn Cwellyn and also contains charr, but is less intact.
In northern England natural oligotrophic
lakes are confined to the Lake District:
although there are a number of artificial oligotrophic reservoirs in the Millstone Grit
areas of the Pennines, none of these is of great conservation value. The two
extreme ends of the range of variation of the larger Lake District lakes are
represented by Wastwater (gr. 1) which is in most respects the most oligotrophic,
and Esthwaite Water (OW.48, gr. 1*) which is the most productive and is
mesotrophic. Buttermere (gr. 2) is similar to Wastwater, but is a less extreme case
in that it is smaller and shallower and has a somewhat richer fauna. Many of the
lakes of nutrient status intermediate between that of Esthwaite and Wastwater are
now more or less modified, either by enrichment, e.g. Windermere, or by water
abstraction, e.g. Ullswater, Ennerdale and Thirlmere. Smaller lakes in the Lake
District are represented by high-altitude corrie lakes and lower-lying small glacial
trough lakes and tarns. Among the former, Red Tarn lies within the upland site of
Helvellyn and Fairfield and contains an isolated population of skelly Coregonus
lavaretus, while Blea Water (gr. 2) is an extremely deep, barren type. Among the
lowland tarns, Blelham Tarn (gr. 1) and the tarns on Claife Heights which are
included within the Esthwaite Water site are selected primarily for the considerable
research already invested in them by the Freshwater Biological Association.
In Galloway, the larger oligotrophic fjord
lochs such as Loch Doon and Loch Ken
have been converted into reservoirs and have very little conservation value although
the marshes surrounding the latter are important for wildfowl, particularly bean
geese (gr. 1). There are also numerous smaller shallow lochs in this area, but none
are considered sufficiently distinct from examples in the Highlands for inclusion
within the series of key sites. A few examples (Loch Enoch and Loch Neldricken),
however, lie within the upland site of the Merrick-Kells. Further north in the Southern
Uplands the high-altitude corrie loch, Loch Skene, lies within the Moffat Hills upland
site (gr. 1), but has only been rated grade 3. Loch Lomond (gr. 1*) is the largest
oligotrophic lake in Britain and has been selected because of its unique dual
trophic nature. The deep, narrow northern end is morphologically oligotrophic
(although chemically it is at the lower end of the mesotrophic scale) whereas to the
south of the Highland Boundary Fault the loch broadens and becomes shallower
and chemically much richer. It has an exceptionally diverse fish population and is
also a research site for Glasgow University.
In the Highlands and Islands, oligotrophic
lochs abound, and in the intensely
glaciated, high rainfall areas of gneiss in the extreme north-west of Scotland and
the Outer Hebrides the land surface is densely studded with a multitude of lochs
and lochans which in places occupy much of the land surface. Many of these lochs
are included in areas selected for their upland interest as at Inverpolly, Rhum and
Cairngorms. Thus, in the Highlands, only oligotrophic lochs showing outstanding
features have been selected as key sites. The highest-altitude lakes in Britain are
found in the Cairngorms where lakes such as Loch Etchachan (altitude 930 m) and
Loch Coire an Lochain (altitude 995 m) are of an Arctic-alpine character. In these
clear-water lakes ice cover may last for six months and the summer temperature of
the epilimnion rarely exceeds 10 °C. The shores are very steep and consist of large
stones and boulders worn smooth by the action of the ice and remaining relatively
free from algae. There are no aquatic angiosperms or fish and the flora consists of
a few species of bryophyte while the scanty invertebrate fauna has a few adaptable
or high-altitude species. Because of their unique qualities, their isolation from other
Arctic-alpine lakes in Europe, and their location within a high-grade upland area,
they are graded as of international importance (gr. 1*). The most extreme
oligotrophic lake in Britain is Loch Morar (gr. 1*) which is not only the deepest lake
in Britain, maximum depth 310 m, but is seventeenth deepest in the world. For a
large lake it also has an extremely low dissolved mineral content and very clear
water. Many of the other large, deep oligotrophic lochs of Scotland have been
severely modified by hydro-electric schemes, but Loch Shiel, is still relatively intact
and is a grade 2 alternative for Loch Morar. It is not as deep nor as chemically poor
as the latter, however, and its catchment has been partially afforested. Loch Stack
(gr. 2) is a typical shallower, slightly richer oligotrophic fjord loch, somewhat similar
to Llyn Tegid in north Wales.
Large, shallow, very exposed oligotrophic
lochs in northwest Scotland are
represented by Loch Druidibeg (gr. 1*) on South Uist, on whose numerous islands
the largest colony of indigenous greylag geese breeds. The influence of sea spray
on this site is reflected in the presence of brackish water invertebrates such as
Gammarus (Rivulo-gammarus) duebeni and Neomysis integer. The flora and fauna
of Druidibeg contrast strongly with the neighbouring calcareous machair lochs,
a'Mhachair and Stilligarry, which lie even nearer the sea and are influenced by
blown shell sand. On the mainland, Loch Sionascaig (gr. 1) on the Inverpolly NNR is
an example of an extremely barren, irregularly shaped loch in which large areas of
the bottom, even in deeper water, consist of bedrock or sand.
Mesotrophic standing waters
Such lakes are relatively infrequent in
the British Isles, and are mostly situated
along the margins of upland areas. No examples are known in south-east England
or East Anglia where the rocks are mainly too calcareous to produce lakes of this
trophic status, but a few examples are found in the areas of base-poor Tertiary
sands in southern England. Hatchet Pond (gr. 2) is the largest of the ponds in the
New Forest belonging to this category. It contains isolated populations of species
normally found in oligotrophic lakes in the north of Britain alongside a flora and
fauna typical of southern Britain. Little Sea Mere (gr. 1), on the Studland Heath NNR
in Dorset, is another isolated mesotrophic lake formed in recent times from a
coastal lagoon. Most coastal lagoons in this country develop on calcareous sand
and are consequently base-rich, e.g. Loch of Strathbeg, Kenfig Pool and Lochs
Spiggie and Brow. The development of such a system in an area of mineral-
leached sand to produce a mesotrophic lake is very unusual in Britain. The mere
contains species typical of mesotrophic lakes alongside species of richer waters
(e.g. Asellus) and has a very varied dragonfly population. Looe Pool in Cornwall is
another example of a coastal mesotrophic lake formed by the damming of a
drowned river valley by a shingle bar. This lake receives the effluent of the town of
Helston, and, although the flora and fauna is very diverse the site is not included in
the series of key sites on account of its lack of intactness. Nearby on the Lizard, the
tiny Ruan Pool is also mesotrophic, but it is so infilled with vegetation that it is best
regarded as a bonus to the heathland site.
Examples of kettle-hole mesotrophic lakes
are Llyn Ebyr in mid-Wales and Gormire
at the foot of the Hambledon Hills escarpment in the North Riding of Yorkshire, but
neither has been graded higher than 3. Esthwaite Water, which is contiguous with
North Fen NNR, is the richest of the larger Lake District lakes. It is the largest and
best example of a mesotrophic lake in England and Wales and has been the site of
extensive research by the Freshwater Biological Association especially on aspects
of lake enrichment. It is also one of the few sites in Britain for the two North
American plants Najas flexilis and Elodea nuttallii.
Gladhouse Reservoir (gr. 1) lying at the
foot of the Moorfoot Hills in Midlothian, is an
artificial mesotrophic lake whose main interest is the largest overwintering
population of pink-footed geese (approx 10 % of the world population). Loch
Lomond, where it broadens out south of the Highland Boundary Fault, changes from
a deep oligotrophic loch into a shallow mesotrophic one.
Loch Insh (gr. 1), an unusual lake for
Britain, lies within the course of the River Spey,
and has a very short retention time. It is an example of a more sandy type of
mesotrophic loch with abundant submerged vegetation. It is contiguous with the
Loch Insh Fens. Lochs Kinord (gr. 1) and Davan are two mesotrophic kettle-hole
lochs lying in the granite drift of the Dee valley. Despite the relatively low alkalinity
and nutrient content they have a diverse flora and extensive hydroseral communities
grading into fen, features which are very unusual for Highland lochs. Loch Eye (gr.
1) in east Ross represents the top end of the mesotrophic scale and compares with
Loch Leven prior to enrichment. Its diverse aquatic flora has components usually
associated with both base-rich and base-poor situations, a combination not found
in other lakes. The loch lies between two important wintering wildfowl sites, the
Lower Dornoch Firth and Cromarty Firth and is itself an important wildfowl roost.
Eutrophic standing waters
These are almost entirely confined to
the lowlands but, south of the area covered by
the last glaciation, natural examples are very uncommon. The Oppen Pits (gr. 1)
were formed by the deposition of coastal shingle at Dungeness, and show different
stages in the hydroseral succession. Elsewhere in south-east England eutrophic
standing waters are only represented by artificial water-bodies. These include
Stodmarsh in Kent which is of considerable ornithological and peatland interest. It
contains a lake too polluted by run-off from a coal tip and by inflow from the River
Stour for inclusion within the national series. Some of the Metropolitan Water Board
Reservoirs are also of considerable wildfowl value but are too artificial for inclusion
as key open water sites.
A series of natural eutrophic waters in
the Breckland of East Anglia were formed as
kettle-holes in the calcareous drift which largely covers this region. These waters,
known as the Breckland Meres ( gr. 1) are noted for irregular fluctuations in water
level which are not directly related to rainfall. They have a very rich aquatic flora, but
a fauna composed mainly of species resistant to desiccation. The large pump-
storage reservoir at Abberton (gr. 1) in Essex is of national importance for wintering
wildfowl.
Much of south Lincolnshire, Cambridgeshire
and west Norfolk were once covered
by extensive fens and shallow eutrophic lakes. In the seventeenth century, Dutch
engineers initiated large-scale drainage operations and the amounts of standing
water in these areas have since become progressively less. The habitat is now
represented only by a few situations, such as the Ouse Washes and Wicken Fen,
which are artificially maintained and contain only small relics of the old fenland open
waters. The greatest extent of fen and eutrophic open water in East Angliais found
in the Norfolk Broads which are derived from mediaeval peat-cuttings but, as a
result of pollution, increasing recreational activity and changes in land
management, the Broads are less interesting biologically than they were 50 years
ago. Some, such as Surlingham and Rockland, are so polluted that all submerged
vegetation has disappeared and the fauna is limited to a few pollution-resistant
species. These areas still retain considerable peatland interest however. The only
freshwater Broads still known to retain the characteristic Broadland fauna and flora
are the rather small and isolated Upton Broad (gr. 1) and Calthorpe Broad, which
complement each other. Even the latter, however, has undergone drastic and rapid
changes during the course of this review, and may no longer be a viable Broadland
habitat. Further survey is required to determine whether other intact fragments of
open water remain in this area. Reports indicate that Martham Broad is still
relatively intact, but this may be a brackish rather than a eutrophic site.
Elsewhere in East Anglia artificial fenland
conditions exist in the Ouse Washes ( gr.
1) where extensive alluvial meadows are inundated in winter to form large
impermanent lakes. In summer these largely dry out to leave only small pools and
dykes which nevertheless have a rich and diverse flora and fauna. The Ouse
Washes are of great ornithological importance both for wintering wildfowl and for
breeding marsh birds, and they also contain areas of running water interest.
In south-west England natural eutrophic
waters are rare, the only large example
being Slapton Ley in Devon ( gr. 1), which is a fine example of a lake impounded by
a coastal shingle beach. It is used for research and education by the nearby Field
Studies Centre and is selected as the most southerly example of a series of
maritime lakes in this category.
The tiny eutrophic pond known as Priddy
Pool, on the Mendips, is included mainly
for its research interest as one of the key freshwater sites for the study of biological
production during the International Biological Programme. Together with two
adjacent caves of running water interest, it is considered as part of a single
complex open water site (gr. 2). Chew Valley and Blagdon Reservoirs (gr. 2) at the
foot of the Mendips, are included as representatives of large eutrophic lakes which
do not naturally occur in this part of the country. They are also of considerable
interest as sites for the study of eutrophication in lowland lakes and as wildfowl
wintering areas.
During the last glaciation the ice sheet
reached its most southerly extent in Britain in
south Wales, and in the lower-lying areas of glacial deposition kettle-holes were
formed as the ice retreated. The largest of these is now occupied by Llyn Syfaddan
(gr. 1). It is now one of the most intact of the large British eutrophic lakes which so
far appears to have escaped the changes which have occurred elsewhere, and
which have been attributed to eutrophication, but it is threatened by recreational
activities.
Kenfig Pool (gr. 2) is a dune slack pool
lying south of the area of glaciation on the
Glamorgan coast. It has a very diverse flora and fauna and is similar to other dune-
system eutrophic lakes such as the machair lochs on the west coast of Scotland.
In north Wales, eutrophic lakes are found
mainly on the island of Anglesey where
they contrast strongly with the oligotrophic lakes of Snowdonia. Llyn Coron (gr. 2) is
a large, shallow example formed behind coastal dune systems and is comparable
with Loch of Strathbeg in Aberdeenshire. It is also an important wildfowl site.
The greatest concentration of natural
standing water-bodies in lowland Britain is
found in the Cheshire-Shropshire-Staffordshire plain where many kettle-holes,
known locally as meres, have developed in the thick deposits of glacial drift. Most
of these meres are eutrophic. One, Oakmere, is oligotrophic, while others such as
Clarepool Moss have become infilled with basin mire and are now dystrophic. Of
the eutrophic meres Rostherne Mere (gr. 1*), Cheshire, is the deepest, and one of
the very few lakes in Britain in which summer stratification leads to deoxygenation
of the hypolimnion. As a consequence the deep-water benthos is practically non-
existent. In addition this site is receiving natural nutrient enrichment from the
droppings of the large number of gulls and wildfowl which frequent it. Crose Mere is
more typical of the meres in general and also contains relict populations of
Gammarus (Rivulogammarus) lacustris and Nuphar pumila outside the main areas
of their range in Britain. This site is closely associated with the nearby dystrophic
Sweat Mere and the two are considered as one site. Some of the complex of pools
at Brown Moss are eutrophic while others are dystrophic, and the area shows a
great range of trophic status and flora and fauna. The Frees Branch of the
Shropshire Union Canal (gr. 1) has a range of alkalinity along its length although it
is eutrophic throughout. The site exhibits a complete successional sequence from
wet woodland at the more eutrophic closed end, to diverse open water submerged
plant communities at the less eutrophic end.
There are no eutrophic lakes in the Lake
District, and in northern England natural
examples are relatively few and scattered. Hornsea Mere is one of the largest
natural water-bodies in lowland England. Despite persistent algal blooms it has a
rich and varied submerged aquatic flora. Tubificids and snails abound in this lake
but there are indications that deleterious changes, resulting from nutrient over-
enrichment, may be taking place. Because of this lack of intactness, it is not of
national importance on limnological grounds, but when its considerable wildfowl
interest is taken into account it is regarded as a grade I site. In the Pennines,
Semer Water (gr. 1) is a limestone lake, with no marl formation, which experiences
great fluctuations in water level because of flash floods. These have the effect of
producing an element of a typical riverine fauna within the invertebrate fauna of the
stony shores, including at least six species of mayfly and crayfish - a phenomenon
not found in any other lake in Britain. Further north in the Pennines, Tarn Dub (gr. 2)
on the Upper Teesdale NNR is a fine example of a large temporary pool which
dries out regularly and has an interesting flora and fauna resistant to desiccation.
The tarn lies at the base of a Whin Sill escarpment on Carboniferous sediments
and is moderately eutrophic. A number of lakes occur in similar geological settings
in Northumberland; these include Broomlee Lough, Greenlee Lough, Grindon
Lough and Crag Lough, but none is considered sufficiently intact or distinctive for
inclusion within the national series.
In Scotland, eutrophic lochs are confined
to the central lowlands and the coastal
plain. A number of kettle-hole lochs occur on the plain bordering the north coast of
the Solway Firth, most of which are shallow, but Mill Loch, Lochmaben (gr. 1), is a
relatively deep example which is of particular interest as containing a relict
population of the vendace Coregonus albula, known elsewhere in Britain only in the
Lake District where a different race occurs. Farther west in Wigtownshire a number
of shallow lochs are important mainly for their wildfowl populations. Of these, the
White Loch of Lochinch (gr. 2) is particularly important as a roost for greylag geese.
Dud-dingston Loch (gr. 2) in Midlothian is also of interest as a large winter roost of
pochard, but it has little limnological value.
Between the Firth of Forth and the Highland
Boundary Fault numerous kettle-hole
lochs have formed in drift largely originating from the Highlands; the trophic status
of these lochs varies according to the nature of the drift, but most are eutrophic. The
largest is Loch Leven (gr. 1*), Kinross, which is also the largest natural eutrophic
water in Britain. This is a very important site both for breeding and wintering
wildfowl and has one of the densest nesting populations of tufted duck and mallard
recorded. It is also a main site for the study of the productivity of freshwater
ecosystems as part of the International Biological Programme. In recent years there
have been very many changes in the flora and fauna associated with increasing
nutrient content and this is now a key site for the study of eutrophication and
conservation management of fresh waters. Kilconquhar Loch (gr. 2), Fife, is a much
smaller eutrophic kettle-hole loch, also of some ornithological importance but,
unlike Loch Leven, it has extensive marginal and submerged vegetation.
A whole series of kettle-hole lochs occurs
in a line just to the south of the Highland
Boundary Fault from the Lake of Menteith in the south-west to Lochs Rescobie and
Bal-gavies in the north-east. In general, the productivity of these lochs increases
from west to east and Lochs Rescobie and Balgavies represent the most eutrophic
end of the series, but artificial enrichment has led to a loss of floristic diversity. They
are, however, important for wintering wildfowl and it is mainly for this interest that
they are given grade 2 status. A number of other kettle-hole lochs here have also
been selected mainly for their wildfowl interest, particularly as pink-footed and
greylag roosts, but little is known about the freshwater interest of these sites. These
lakes are Carsebreck Lochs (gr. 1), Drummond Pond (gr. 2), Loch of Kinnordy (gr.
2) and Loch of Lintrathen (gr. 2). The artificially constructed Dupplin Lochs in
Perthshire (gr. 1) also come into this category and are examples of lochs enriched
by bird droppings. They are, however, very poor in floristic and faunal diversity.
North of the Highland Boundary Fault eutrophic
lakes are restricted to the coast
where they are influenced by wind-blown calcareous sand, and to areas of base-
rich rocks in Caithness, Orkney and small isolated areas in the uplands. Loch of
Strathbeg (gr. 1) on the north-east coast of Aberdeenshire is the largest dune slack
lake in Britain with an area of 200 ha. It is very exposed and shallow and the flora
and invertebrate fauna are influenced by sea spray and contain a few brackish
water species. It is also of outstanding importance as a wintering wildfowl resort for
a wide range of species including particularly whooper swan and geese. The much
smaller Loch Spynie on the Moray coast is more sheltered, with more abundant
submerged vegetation and a completely freshwater fauna. It is an important greylag
roost but as the loch has been partially drained by a canal it is only given grade 2
status.
Farther north, on the Old Red Sandstone
of east Ross, Caithness and Orkney,
eutrophic lochs, mainly kettle-holes, are frequent. Loch Eye in east Ross (gr. 1) is
intermediate between mesotrophic and eutrophic conditions. The largest of these
northern eutrophic lochs is Loch Watten (gr. 1): a shallow exposed kettle-hole
which, as it has a largely non-agricultural catchment, is probably the least modified
large eutrophic lake in Britain. It has a very diverse submerged and marginal
aquatic flora, a prolific invertebrate fauna and is a well-known trout fishing loch. On
Orkney the northern end of the Loch of Harray (gr. 1) is eutrophic, very similar to
Loch Watten, but most of this loch and the neighbouring Loch of Stenness are
brackish. The Shetlands are composed mainly of base-poor rocks and eutrophic
lakes are mainly confined to coastal areas. The largest are Lochs of Spiggie and
Brow at the southern end of the Mainland where they are formed behind coastal
calcareous sand dunes. Because of their extreme isolation these lochs lack a rich
invertebrate fauna, but the maritime influence and low summer temperatures are
indicated by the presence of brackish and cold-water species. These lochs are
also important for wintering wildfowl, particularly whooper swan.
In the western Highlands and the Hebrides
eutrophic lochs are very rare. A few
examples occur in areas where the Durness Limestone outcrops, as at the
Inchnadamph NNR, where the small shallow Loch Mhaolach-coire (gr. 2) is found.
The limestone surrounding this loch is largely covered with blanket mire, giving a
highly alkaline loch with periodically peat-stained water which has a flora and fauna
very similar in composition to oligotrophic lakes. It has dense beds of submerged
vegetation, however, and the invertebrate fauna is very prolific and supports a
population of fast-growing brown trout.
Along the western seaboard of Scotland,
and particularly on the Outer Hebrides, a
coastal fringe of flat machair grassland (in places up to about 2 km wide) grows on
calcareous shell sand of marine origin, and contains a number of shallow eutrophic
lochs. On South Uist these lochs characteristically have a rich, submerged
vegetation and a brackish element in the invertebrate fauna, particularly
Potamopyrgus jeninksi, Neomysis integer and Gammarus duebeni. Loch Stilligarry
and Loch a'Mhachair on the Druidibeg NNR, are excellent examples. The former
has a well-developed reed-swamp and shows a good hydro-serai progression. The
Royal Society for the Protection of Birds' reserve at Balranald on North Uist
contains a small machair loch, Loch nam Feithean (gr. 2) which is very overgrown
with emergent vegetation and is important as a nesting site for red-necked
phalarope.
Marl lakes
This is the rarest category of standing
water in Britain and is confined to areas of
highly calcareous soluble limestone and chalk, or fluvio-glacial deposits derived
from these rocks. Since they are so rare and have a great floristic and faunal
diversity, a large proportion has been accorded key site status.
The Chalk of south and south-east England
is devoid of natural standing waters, but
the Tring Reservoirs (gr. 1) are situated at the foot of the Chalk escarpment of the
Chilterns. Of these, only Wilstone has been surveyed. It is a clear-water marl lake
with extensive growths of submerged macrophytes. The high ionic content of the
water coming from the Chalk may account for the presence of species such as
Notonecta viridis and Sigara concinna which are mainly of coastal distribution.
Natural standing water-bodies are also
absent from the permeable Jurassic
limestones of southern England, but excavation of fluvio-glacial deposits of gravel
derived from this rock in the upper Thames valley is producing a series of artificial
lakes known as the Cotswold Water Park which will constitute by far the largest
marl lakes in Britain. A lake of about 100 ha and a few smaller lakes, some of
which are still to be excavated, have been proposed as a nature reserve area. The
flora and fauna of neighbouring existing pits is already quite diverse and despite
much disturbance, the park attracts increasing numbers of wildfowl. It is likely that,
with suitable management, these could become one of the most important areas of
open water in southern England. On its potential value the Cotswold Water Park
has, therefore, been given grade i status. Four small artificial marl ponds and
associated limestone streams (gr. 2) are found within the Wychwood NNR. These
ponds show very different floristics but a similar fauna characteristic of calcareous
conditions. In south-west England, Chew Valley and Blagdon Reservoirs are fed
from the Carboniferous Limestone of the Mendips, but neither is a true marl lake,
and they are probably influenced by drainage from the Keuper Marl on which they
lie. Both have relatively high phosphate contents, and Chew Valley has dense algal
blooms on occasions.
The Carboniferous Limestone of south Wales
lies south of limits of glaciation and
has very few standing waters. Bosherston Lake: Llyn Hosier (gr. i1 occupies a
drowned limestone river valley which has become isolated from the sea by a
coastal sand dune. No other British marl lake has been formed in this manner.
Submerged vegetation is extremely profuse and shows a transition from a
community in which Chara hispida predominates, nearer the coast, to a community
dominated by Potamogeton spp., and finally to reed-swamp.
In north Wales the extensive Carboniferous
Limestone has natural lakes only on
Anglesey, where there are a number of limestone lakes now largely invaded by
valley mire. Llyn yr Wyth Eidion (gr. 2) lies within the valley mire of Cors Erddreiniog
and similar-sized but un-surveyed water-bodies occur in the nearby Cors Goch. The
Llyn, which lies on deposits of lacustrine shell mud, is deep for its size and unusual
for British marl lakes in that it stratifies in summer. Submerged vegetation is not
abundant in this lake which is made somewhat turbid by drainage from the
surrounding peatland.
In the Midlands there are no natural standing
waters on either the Jurassic
limestone or on the Carboniferous Limestone of the Peak District but, further north
in the Pennines, Malham Tarn ( gr. 1*), at an altitude of 380 m, is the highest and
largest marl lake in Britain. It shows an exceptional range of plant and animal
communities associated with the base-rich tarn, the fen at its inflow and the
marginal raised mire. The presence of a Field Studies Centre on the shores of the
lake make this the best known British marl lake. Sunbiggin Tarn ( gr. 1) is another
much smaller example on the limestone of the Pennines lying within a grade i
peatland site. The tarn itself has a diverse flora and fauna compared to an
impoverished nearby pool which is receiving nutrient enrichment from a colony of
black-headed gulls. Hawes Water (gr. 1) lies within thick lacustrine shell marl
deposits near the north Lancashire coast. It has well-developed marginal
vegetation which grades into a eutrophic flood-plain mire. Like Llyn yr Wyth Eidion,
it is deep and undergoes thermal stratification in summer.
No natural marl lakes are known in southern
Scotland and they are exceedingly rare
in the Highlands. None is known in the areas of Dalradian limestone in the eastern
Highlands but three small examples occur on this formation on the island of
Lismore in Argyll. These lochs (gr. 2) are similar to those at Durness for which they
are a substitute. The Durness Lochs (gr. 1*) in the extreme north-west of Scotland
are probably the finest examples of marl lakes in Britain. They are larger and more
diverse than those on Lismore and because of their extreme isolation from other
calcareous lakes have a fauna that contains only a few of the species associated
with base-rich conditions farther south. Loch Borralie is the only marl lake in Britain
now containing charr.
On the Scottish islands the only known
marl lake is a tiny pool on the limestone
peninsula of Whiteness on the Mainland of Shetland, but this does not merit
inclusion in the national series.
Brackish standing waters
Brackish lakes are rare in Britain since
the conditions required to produce such
water-bodies are rather precise. In England the best examples are found in the
Norfolk Broads where some slight tidal influence and seepage of sea water through
the low-lying soil combine to produce brackish water in a few of the Broads lying
near the sea. Of these, Hickling Broad and Horsey Mere (gr. 1*) are the most intact
and still preserve the unique Broadland flora and fauna which includes rare plants
such as Najas marina and breeding birds such as the bittern and marsh harrier.
Martham Broad is similar, but probably contains no features not found in Hickling
and Horsey.
Elsewhere in Britain brackish lakes occur
mainly on the islands of Scotland where
complex glacial basins connect to the sea via narrow tidal outflows which allow a
limited inflow of sea water at high tide. These lakes are very different from the
Broads in having rocky exposed shorelines. On Orkney, the inter-connected
brackish lochs of Harray and Stenness (OW.78) show a transition from sea water in
the lower end of Loch of Stenness to base-rich fresh water in the northern end of
Loch of Harray, with an associated gradient of fauna and flora. In the more
freshwater areas animals and plants typical of eutrophic lakes are found alongside
brackish water species. On the Outer Hebrides many brackish lochs grade from
sea water to oligotrophic freshwater conditions. The finest example is Loch an Duin
(gr. 1*), and Lochs Roag and Fada and the Howmore Estuary (part of the Grogarry
Lochs grade 1* site in South Uist) which extend from brackish conditions into
oligotrophic fresh water. These lakes are similar in floral and faunal composition, in
the more saline parts, to Loch of Stenness but are completely different in the more
freshwater areas where animals and plants typical of oligotrophic lakes are found
alongside brackish water species.
Lowland, sluggish eutrophic ditches and
rivers
These rivers are characteristic of lowland
England particularly East Anglia. The
river channels are usually artificial having been constructed during the draining of
the Fen-lands and other low-lying land. They are generally deep and steep sided
and are maintained by dredging, the spoil being used to form levees to prevent
flooding of the surrounding ground. The channels have often been straightened and
have a very uniform flow over the whole length which may cease entirely in summer.
In south-east England the dykes to the
east of the Lam-pern Wall on the Stodmarsh
NNR in Kent are rich botanic-ally but this type of habitat is better represented on
Pevensey Levels (gr. 1) and on the Romney Marsh, its grade 2 alternative. These
two sites, which were once extensive freshwater marshes, now consist of grazing
land intersected by numerous dykes which retain much of the flora and fauna of the
former marsh. Of the two, Pevensey Levels scores higher on its diversity of fauna,
especially water beetles, which include a number of rare species. The sites are
similar botanically but Pevensey has a number of rather local species. The main
drains at Romney Marsh suffer to some extent from spraying with herbicides to
control emergent vegetation and there are pressures from arable farmers for the
water table to be lowered.
The lowermost sections of a number of
rivers in southern England, including key
sites such as the River Avon just above Christchurch, are sluggish and resemble
Fenland drains. In south-west England extensive raised mires and intervening
alluvial flood-plain mires formerly occurred on the Somerset Levels, where the
Shapwick Heath NNR is a remnant, but many of the dykes now draining this area
are polluted and none shows any features not better represented in East Anglia.
The most extensive areas of fens and freshwater
marshes existing in historic times
were to the south of the Wash and in the coastal areas of East Anglia. These have
been almost completely drained and in the surviving fragments there is now very
little open water, but the rivers and dykes which drain these areas contain relics of
the former aquatic flora and fauna of the Fens.
The artificial rivers which flank the
Ouse Washes and flood them in winter are good
examples of Fenland drains. The New Bedford River on the east is brackish and
tidal, while the Old Bedford River and River Delph on the west are fresh water and
very slow flowing. The freshwater rivers are very clear, and have an abundance of
submerged plant species and a relatively intact invertebrate fauna. The lower
reaches of many of the chalk rivers of East Anglia are very slow flowing, a factor
which determines their ecology: for example the River Great Eau becomes a
weedy Fenland drain, but the lower reaches of the Lark are deep and turbid with
very little submerged vegetation.
The lower reaches of the Yare, Waveney,
Bure and Ant are very turbid, largely
devoid of submerged vegetation, and have very little nature conservation value. The
slightly brackish channels connecting Hickling Broad and Horsey Mere are intact
and biologically similar to the standing water areas and are included in the same
key site.
In northern Britain, areas of flat alluvial
marshland are rare and there are few
eutrophic sluggish rivers of any length. On Anglesey, the Afon Ffraw a short, slow-
flowing, weed-choked stream, comes into this category and is included within the
site boundary of Llyn Coron of which it is the outflow. Small dykes are present in the
extensive area of Phragmites swamp at Leighton Moss on the Lancashire coast
but are not sufficiently diverse for inclusion in the national series.
In Scotland, sluggish rivers are even
more infrequent. The lowermost section of the
River Endrick is a very slow-flowing lowland eutrophic river. The section of the
River Spey (gr. 1) flowing through the Loch Insh Fens does not fit easily into any
classification of river systems since, although it is very slow flowing and has the
appearance of an East Anglian river, it lies at an altitude of about 300 m and is
mesotrophic, with species characteristic of both base-poor and base-rich
conditions.
