Standing waters occur either where the
drainage is impeded, e.g. by glacial
deposition or deepening in valleys, by the build-up of peat in mire systems, where
water accumulates in kettle-holes and other depressions such as the
Shropshire/Cheshire meres and the Norfolk Broads, or where man has created
artificial dams or barriers of various kinds.
The overwhelming factor affecting the
flora and fauna of standing waters is the
chemical content of the water. A large number of elements and compounds are
found dissolved in natural waters, and include trace elements and certain organic
compounds which limit plant growth under culture conditions, but about which little is
known in the field. In general, however, the primary production (and hence in most
cases also the secondary production) of a waterbody is related to its alkalinity. This
relationship is partially explained by the direct relationship between alkalinity and
the concentration of dissolved major plant nutrients, nitrogen and phosphorus.
Phosphate is commonly the factor limiting the algal primary production in fresh
water in Britain. Alkalinity is a more convenient and constant measure of productive
level, however, as the dissolved plant nutrients (phosphate, nitrate, nitrites and
ammonia) vary considerably in their concentrations over the seasons, depending
upon their uptake by plants, sedimentation in organic form, and release from the
sediment. Nitrogen and phosphorus also occur in organic and inorganic states in
many different forms, thereby adding to the difficulties of chemical analysis and
interpretation. Water pH has often been used as a measure of the productive level
of water-bodies, but this can vary greatly according to photosynthetic activity,
particularly during the summer months.
Summer pHs are not a reliable guide to
the trophic status of standing water.
Because of the low alkalinity of most oligotrophic lakes, any substantial algal growth
will raise the pH far above 7; indeed 10 would be quite possible. The very reasons
which permit wide fluctuations in pH in oligotrophic systems during the summer
tend to damp down the pH fluctuations in eutrophic systems and there are few
eutrophic bodies of water in which the pH is raised as high as 11 by photosynthesis.
Bodies of standing water have generally
been classified into unproductive and
productive types, to which the terms oligotrophic and eutrophic, respectively, have
become applied. A third category, which has an intermediate alkalinity and
productivity has been termed mesotrophic. Further categories are required to
accommodate those systems where, because of special chemical conditions, the
relationship between alkalinity and nutrient content does not apply. Sodium rather
than calcium may be the prevalent cation in some lakes, particularly in coastal
regions, causing a high alkalinity, not necessarily accompanied by a high nutrient
content. Those bodies of standing water with a sodium concentration higher than
that of calcium and a total ionic content and conductivity intermediate between that
of fresh water and sea water, are termed brackish. In limestone regions, lakes have
high alkalinities but, since phosphorus is generally present only in insoluble form,
primary production in the form of phytoplankton is generally very low. The
phosphorus is precipitated together with a deposit of calcium carbonate, known as
marl, which coats the bottom substrate, and hence these examples are often
termed marl lakes. A final category of of bodies of standing water distinguishes
those where dissolved humic acids derived from acidic peat lower the pH to well
below neutrality. Such systems are termed dystrophic and are generally extremely
unproductive, although the nutrient content may be relatively high.
Although there is general acceptance of
these six categories, the divisions
between them are arbitrary and there is no general agreement as to their precise
definition.
Within each of these major divisions lakes
may be divided into further categories
using other physical attributes, including depth, area, nature of shoreline, altitude
and latitude. These attributes are all to some extent interdependent (for instance
large lakes in northern Britain tend to be oligotrophic, deep and have stony
shorelines, while eutrophic bodies of water in southern Britain tend to be shallow
and have organic shorelines) so that the theoretically large number of possible lake
categories can be reduced in practice.
In shallow waters, the depth to which
light penetration enables plant growth to
continue, the photic zone, may extend down to the bottom over the entire area. The
exact depth of the photic zone depends on the clarity of the water, being greater for
oligotrophic and marl lakes than for dystrophic and eutrophic waters. In general, the
shallower the water the more important are the benthic plant communities and the
animal communities dependent on them, whereas in deeper systems the major
producers are plankton and dependent animal communities.
Depth is also important in relation to
the establishment of thermal stratification. In
deep lakes, a narrow transitional zone of relatively rapid temperature change
occurs just below the depth to which the lake water is normally stirred by wind
action. A density gradient is formed at this level which effectively prevents the
transfer of water between the lower hypolimnion and the upper epilimnion until
thermal stratification is broken down when the upper lake waters cool in autumn.
This barrier, called the thermocline, forms at depths ranging from 5-25 m
depending on the size of lake, and in large shallow lakes stable stratification does
not occur. In shallower systems, wave action can extend down to the bottom,
bringing the substrate into suspension and so preventing the establishment of an
undisturbed sediment. In the deeper lakes, there is a progression of substrate
types from eroded shorelines to soft muds in deeper water. The depth at which the
transition from erosion to deposition takes place depends on the size of the lake
and its exposure to the wind.
In large lakes, wind action becomes an
increasingly important ecological factor,
with its major effect upon the shallow littoral zone. Small, sheltered bodies of water
generally have shorelines on which fine organic or mineral sediments are
deposited, and here marginal vegetation may flourish. With increasing size and
exposure, the shorelines become more eroding and thus more inhospitable for
plant colonisation. With extreme wave action the shorelines may consist of barren
storm beaches of unstable stones or gravel, or wave-washed stable bedrock.
Mean wind speeds increase both with increasing
altitude and latitude, so that a
small area of water in lowland southern Britain may have a depositing muddy
shoreline with extensive reed-swamp development, while a body of standing watear
of the same area and shape in the uplands of the north may be bounded by barren,
stony, wave-washed shores. Mean water temperatures decrease with increasing
altitude and latitude, and many species of aquatic plants and animals are restricted
in their British distribution by their particular temperature requirements. In high-
altitude lakes, ice scour at the time of melting restricts the colonisation of the
shallow littoral by both plants and animals.
Within a deep lake there are four main
habitat zones.
1. The open water, all depths inhabited
by the plankton.
2. The profundal benthic zone which is
in deep water beyond the depth of light
penetration, where the bottom consists generally of fine organic mud, and where no
plant production occurs.
3. The sublittoral benthic zone which
is in shallower water and is the zone in which
most macrophytes grow, the substrate being generally of fine sediments such as
silt or mud.
4. The littoral benthic zone which is
in the shallowest water at the margin of the lake
where the effects of wave action are strong and the substrate consists of eroding
mineral sediments such as sand, gravel, stones or boulders.
These divisions obviously grade into one
another, but it is convenient to discuss the
range of variation of the vegetation and invertebrate populations in terms of these
zones, as each has its own distinctive communities. The depth at which the
transition between the three benthic zones takes place is variable and depends on
a number of factors such as water clarity, exposure and lake size. In shallow lakes
the profundal may be absent, while in small sheltered waters where wave action is
negligible, the sublittoral zone may extend up to the shoreline.
Because of the importance of depth and
size to the physical characteristics of
standing waters, it is practical to subdivide the above chemically based primary
divisions into water-bodies with mean depths above and below 5 m, and with areas
above and below 10 ha.
