The aquatic flora is limited more by the
habitat requirements of the individual
species than by any geographical restriction of range. Most species are found
almost throughout Britain wherever suitable habitats occur, and the restriction of
some species to certain parts of the country reflects the localisation of suitable
habitats.
In terms of their distribution in the
British Isles the aquatic flora can be divided into
five main groups; widespread & common; widespread & local; northern species;
southern species; and rare species.
The Review gives special prominence to
the vertebrates particularly in relation to
their associations with the main nutrient types of standing water.
Open waters are more likely to be altered
by man's activity than completely
destroyed, and few open water sites in Britain have disappeared entirely. The
modification of open waters can be so profound however that they become
valueless to nature conservation, and in extreme cases (e.g. heavily polluted rivers)
they may be virtually lifeless. Pollution, including eutrophication, is the major
modifying factor affecting open waters, and chemical and biotic tests for different
forms of pollution play a part in the comparative evaluation of sites. Since most
sources of pollution originate at points within the catchment remote from the
waterbody, naturalness of the catchment, as well as intactness of the water-body
itself, is important. In the case of oligo-trophic sites the effects of slight pollution or
enrichment are more drastic and the degree of intactness demanded of the
catchment is much greater than that demanded of eutrophic sites. Fortunately most
oligotrophic waters are in the relatively unpopulated uplands, but aerial applications
of fertilisers may pose a threat to them.
The hydrology of open waters may also
be affected by man's activities. Many
natural lakes have been converted to storage reservoirs in which rapid and large-
scale fluctuations in water level cause severe impoverishment of the littoral and
sublittoral fauna and flora. The greater and more rapid the drawdown in a reservoir
the less is it likely to be of value for conservation, and in this respect hydro-electric
reservoirs are usually of less value than water supply reservoirs. In rivers, the
hydrology is modified by the construction of locks, weirs and dams, and by
regulation of discharge, embankment and straightening as well as water
abstraction and effluent discharge. Most of these modifications lead to a
deterioration in the structural and biological diversity of river systems.
In lowland Britain increased recreational
activity on many open waters is destroying
much aquatic wildlife and the presence of the more destructive forms of recreation
on a water-body has counted against that site in the overall assessment.
Natural water-bodies are normally preferable
to those artificially constructed,
particularly in relation to the stratigraphical history depicted in their sediments, and
artificial water-bodies have only been chosen where no natural alternative exists.
However, some types of water-body such as the shallow fenland meres (e.g. the
former Whittlesea Mere) are no longer found in a natural state in Britain and, since
many of the interesting invertebrate species are now extinct in this country, this
ecosystem is irretrievably lost and artificial waters now form the nearest alternative.
In many parts of lowland Britain, particularly in the chalk and limestone areas, there
are very few natural standing water-bodies, and artificial types, such as Tring
Reservoirs and the Cotswold Water Park gravel pits, are habitats not previously
found in Britain, yet have high conservation value. Many rivers, particularly the chalk
streams of southern England, which are often thought of as a natural part of the
British scene, are highly modified and only maintained in their present form by
intensive management involving dredging, weed-cutting, removal of coarse fish,
and stocking with trout. They nevertheless represent a diverse habitat not found in a
natural state in Britain.
In spite of the modifications to many
British freshwater stystems, some of these,
including a high proportion of Scottish Highland lochs and streams, are probably
the habitats least changed by man in inland Britain.
Structural diversity
This is an extremely important criterion
in the assessment of open waters, since,
probably more than for any other formation, chemistry and morphology are the most
critical factors controlling the flora and fauna. Diversity of shoreline is particularly
important in the case of lakes and, since the coarser sediments only occur in
exposed situations, larger lakes are more diverse than smaller ones. Similarly, the
presence of deep water gives additional diversity.
