OLIGOTROPHIC AND DYSTROPHIC LAKES
Few aquatic birds frequent large oligotrophic
lakes, but with increasing nutrient
status both the numbers and variety of species increase. The most characteristic
species of breeding duck of large, nutrient-poor lakes are goosander Mergus
merganser and red-breasted merganser M. serrator, the latter on lakes nearer the
sea; and small numbers of mallard Anas platyrhynchos and teal A. crecca, which
are equally characteristic of peaty dubh lochans. Divers also breed on oligotrophic
lakes but are restricted to Scotland. The red-throated diver Gavia stellala is found
especially on small peaty lochans, mostly within flighting distance of the sea, where
the adults prefer to feed, while the black-throated diver G. arctica is generally found
on larger lochs with islands, often far inland. The common gull Larus canus is a
characteristic breeding species on barren, stony-shored oligotrophic lakes and
their islands in the north, but usually with only a few pairs on each water-body. In
contrast, quite large colonies of black-headed gulls L. ridibundus are found on
oligotrophic lakes throughout Britain, especially on peaty lakes amongst emergent
Carex rostrata. The rare Slavonian grebe Podiceps auritus breeds in the marginal
sedge swamp around rather oligotrophic lochs in the Highlands. Common
sandpipers Tringa hypoleucos and dippers Cinclus cinclus are common and
widespread around the margins of stony-shored lakes, but the greenshank T.
nebularia which frequents similar habitats is confined as a breeder to the Highlands
of Scotland. Certain species which are principally marine in habitat will also
frequent oligotrophic lakes in small numbers. Of these the cormorant Phalacrocorax
carbo is the most common. Herons Ardea cinerea are widespread throughout
Britain and will feed on most types of water including the most nutrient-poor of lakes.
The indigenous breeding population of
the greylag Anser anser is found mainly on
the shallow rocky oligotrophic lakes of the Outer Hebrides where they breed on
islands and feed on the rich calcareous grasslands of the nearby machair. Smaller
numbers occur on the mainland of the northern Highlands either on large lochs, or
the smaller tarns and dubh lochans of the great flows in east Sutherland and
Caithness. Feral populations of this goose, in contrast, will breed on a wide variety
of water-bodies including highly eutrophic lowland lakes. Common scoter Melanitta
nigra breed on a small number of peaty hill lochans and larger lochs in Scotland;
while the osprey Pandion haliaetus has recolonised the Highlands where it nests on
trees in several widely scattered localities and fishes on the oligotrophic and
mesotrophic lakes and rivers. Wigeon Anas penelope are fairly frequent breeders
on oligotrophic lakes but also nest by mesotrophic and eutrophic waters.
Few species of aquatic mammals are found
in oligotrophic lakes in Britain. The
otter, which has declined in numbers in lowland Britain, is still a common species
around the lakes and rivers of the upland areas, while the water shrew is found in all
types of waters, but is absent from most of the Highlands of Scotland. As with birds,
some marine animals are occasionally seen in the freshwater lochs of Scotland.
For instance, grey seals are frequently encountered in Loch Shiel which is
connected to the sea by a short river.
MESOTROPHIC LAKES
In mesotrophic lakes, as the nutrient
content of the water and the productivity of the
lake increases, the numbers of both breeding and wintering wildfowl follow suit.
Diversity of species also increases, and birds such as the coot Fulica atra and
mute swan Cygnus olor which are generally absent from oligotrophic waters, may
appear.
EUTROPHIC AND MARL LAKES
It is only on eutrophic waters, generally
large shallow lakes, that large
concentrations of wintering waterfowl occur. These wildfowl concentrations are of
two distinct groups of birds: those which use the lake merely as a roosting site, and
flight out to feed in terrestrial or other aquatic habitats; and those birds which
actually feed from the lake. The former group includes such geese as the pink-
footed Anser brachyrhynchus which favours large lakes as a roost, and the greylag,
more commonly found on smaller water-bodies. Several of the dabbling ducks,
especially mallard, and some of the swans, roost by day and flight out at night to
feed on surrounding land or water. Many of these lowland lakes are also used as
roosts at night by huge numbers of gulls. These large concentrations of roosting
birds are usually absent from oligotrophic water, presumably because extensive
areas of farmland or estuarine water, which offer feeding to these birds, are not
generally found in their vicinity. Roosting birds can have a major influence on the
water chemistry of the smaller lakes by introducing large quantities of allochthonous
nutrients in their droppings. This process of nutrient enrichment has been termed
guano-trophy. (Rostherne Mere is considered to be a guanotrophic lake.) Species
which feed mostly within the lake include diving ducks such as goldeneye
Bucephala clangula, pochard Aythya ferina and tufted duck Aythya fuligula,
dabblers such as the shoveler Anas clypeata (which is principally an invertebrate
feeder), coot and mute swan which feed on aquatic macrophytes, and fish eaters
such as great crested grebe Podiceps cristata and goosander. These species are
more common on eutrophic waters since the production of food organisms is high
compared with oligotrophic lakes. The latter are generally deep and therefore most
of the bottom is inaccessible to wildfowl, which do not dive below about 8 m, while
eutrophic waters holding large numbers of feeding wildfowl are generally shallow.
Some wintering birds such as whooper and Bewick's swans (Cygnus cygnus and
C. bevnckii) feed on plants both from the lake and from surrounding farmland.
Of the breeding duck, pochard, tufted
duck, gadwall Anas strepera, shoveler, pintail
A. acuta, and garganey A. querquedula are associated with the richer lakes, whilst
mallard, teal and wigeon which breed in higher concentrations around such waters
also breed around oligotrophic lakes. Some of these ducks nest in swamp or close
to the water edge but others breed well away from the lake. The great crested
grebe and little grebe Podiceps ruficollis breed mainly on mesotrophic to eutrophic
lakes amongst emergent or floating vegetation, and the rare black-necked grebe P.
nigricollis appears to be confined to the richer lakes. The red-necked phalarope
Phalaropus lobatus is confined as a breeder to small eutrophic lochs in the extreme
north and west of Scotland.
As well as the truly aquatic birds, a
large number of other bird species are
associated with lowland eutrophic lakes. These include widespread fish-eating
species such as the heron and the kingfisher Alcedo atthis. The swift Apus apus
and members of the swallow family, Hirundinidae, may feed extensively on aquatic
insects, especially chironomid midges, hatching from the surface of lakes. A
number of species associated with the reed fringe surrounding many eutrophic
lakes include the water rail Rallus aquaticus, reed bunting Emberisa schoeniclus,
sedge warbler Acrocephalus schoena-baenus, and rarer species such as the
bittern, Botaurusstellaris, bearded tit Panurus biarmicus, and marsh harrier, Circus
aeruginosus, which are mostly confined to the extensive Phragmites beds of East
Anglia and south-east England and to one outpost in Lancashire. The more
exposed shores of eutrophic lakes where vegetation is sparse offer feeding to
wagtails, pipits and a variety of waders, the latter mostly as passage migrants. In
lowland England the little ringed plover Charadrius dubius has nested inland in
recent years on unvegetated gravel beside gravel pits and reservoirs.
One species of aquatic mammal confined
in Britain to eutrophic waters is the
introduced coypu Myocastor coypus, which is restricted to the Norfolk Broads and
dykes in East Anglia where it causes extensive damage by burrowing into the
banks and may cause flooding. Their numbers have been effectively controlled by
trapping. Water voles Arvicola amphibius and water shrews are also found in
eutrophic lakes but both these species are more frequently associated with running
waters.
