In Britain these are not an important
category of water-body as the great excess of
rainfall over evaporation ensures that only very small, shallow bodies of water ever
dry out regularly. Temporary pools are more frequent in the low rainfall areas of the
south and east where the rocks are mostly calcareous and they therefore tend to be
mainly eutrophic in status. The principal feature of the flora and fauna of temporary
waters is their ability to withstand periods of desiccation or their ability to colonise
new habitats rapidly.
The macrophytic flora is characterised
by species such as Glyceria fluitans, G.
plicata, Agrostis stolonifera, Polygonum amphibium, Oenanthe fistulosa and, in less
rich situations where the soil is of gravel or sand, Littorella uniflora. The bare soil left
when the water of a pool recedes may be rapidly covered by common terrestrial
weeds, while freshly created water is often quickly invaded by floating thallatc plants
such as Lemna spp. and Riccia fluitans. Algae too are successful colonisers of
temporary pools which are frequently very turbid with planktonic forms, particularly
Cyanophyta and Chlorophyta, while the bottom may be covered with filamentous
algae, e.g. Spirogyra and Anabaena.
Triclads, mayflies, stoneflies and malacostracans
are all absent from temporary
pools but a number of molluscs are able to aestivate and are highly resistant to
desiccation. Lymnaea (Galba) glabra, L. (Galba) truncatula, Aplexa hypnorum,
Planorbis (Anisus) leucostomus, Pisidium persona-turn and P. casertanum are
typical and may be very abundant in small pools in Britain. The amphibious leech
Dina lineata is the only species generally found in such waters. The two rather
uncommon crustaceans, the fairy shrimp Cheirocephalus diaphanus and Triops
cancriformis, are restricted to small, rather turbid puddles in the south of England;
these species have eggs resistant to desiccation. A large number of copepods and
ostracods are directly resistant to drying out as adults while, under hostile
conditions, the Cladocera lay resistant ephippial eggs. Moina rectirostris and
Daphnia obtusa are characteristic and in the absence of many predators often
abound in small pools. Among the insects a number of species have adapted to
living in temporary pools by laying eggs which diapause throughout the summer
and only hatch in the autumn. The larvae then feed and grow in the winter while the
pool is full and the adults emerge in the spring before the pool dries out. These
insects include the dragonfly Lestes sponsa and beetles such as Agabus labiatus.
A number of other beetles, e.g. Helophorus brevipalpis which is a ready flier, utilise
such pools as adults but may not be able to breed there. Adult corixids are also
often found in large numbers in temporary pools but since the flightless nymphs
grow during the dry summer months they are unlikely to be successful breeders. A
number of opportunist Diptera, especially Culicidae and Chironomi-dae, will breed
in small pools which dry out, the success of these groups being attributable to their
very short life-cycle time. In the smallest and most ephemeral of pools which are
likely to form and then dry out again in a matter of hours the fauna may be restricted
to a few highly resistant protozoa, rotifers, harpacticoids and tardigrades.
