Open waters include both fresh waters
and those brackish waters in which the
benthic fauna consists largely of insects and oligochaetes as opposed to marine
and brackish conditions where crustaceans, molluscs and polychaetes
predominate. Both running waters and standing waters are included, i.e. rivers,
streams, canals, lakes, ponds and small pools. Open waters are certainly the most
widespread of the natural or semi-natural habitats in Britain, although they cover
only about 1 percent of the land surface. The deeper parts of some large
oligotrophic lakes in the north arc among the few habitats in Britain virtually
unaffected by man's activities. In many areas nature conservation interests in open
waters are increasingly threatened by water-based recreational activities, water
abstraction, river regulation, drainage, hydro-electric schemes and pollution,
including eutrophication, all of which may adversely affect wildlife. These threats are
only partially offset by the creation of new open waters in the form of reservoirs and
gravel pits. Hence the urgent need for the survey and description of intact open
waters in Britain, and the selection of a series of sites to represent adequately the
range of variation.
The most fundamental division of freshwater
ecosystems is into standing and
running waters. Although the difference between lakes and rivers is generally
obvious, in a few running waters, such as the Fenland drains, current velocities may
be so low in the summer months that they are effectively lakes, with a predominantly
lacustrine flora and fauna. Conversely, physical conditions on the exposed wave-
washed shores of large lakes may approach those in fast flowing rivers and
components of a typical riverine flora and fauna may be present.
The major difference between non-tidal
running waters and standing waters is that,
in the former, currents are induced by gravity and, although variable in velocity, are
constant in direction, while in the latter, currents are mainly induced by wind action
and are very variable both in velocity and direction. The constancy of river currents
assures a steady supply of paniculate matter carried downstream in the drift, upon
which a large assemblage of specialised filter-feeding invertebrates depends.
These are absent from standing waters. The steady downstream current also
prevents the growth of a true zooplankton and phytoplankton in many of our rivers.
However, if a river is long enough or its flow slow enough a true plankton does
develop. Running waters tend to be more aerated and to have a more constant
temperature than neighbouring standing waters, so that a number of organisms
dependent on low summer temperatures and high oxygen tensions are confined to
them, while other species requiring high summer temperatures are restricted to
small standing waters.
