Marl lakes are confined to scattered areas
of soluble limestone and chalk where
precipitation of calcium carbonate takes place. This precipitate, termed marl, is
produced mainly as a result of the removal of carbon dioxide from the water by the
photosynthetic activity of aquatic plants, both algae and macrophytes, which results
in the shifting of the bicarbonate/carbonate equilibrium towards carbonate. As
calcium carbonate is very insoluble it is precipitated in those lakes where calcium
is by far the most abundant cation. The precipitation of marl also has other effects;
phosphorus is precipitated in the form of various insoluble compounds and
becomes unavailable to phytoplankton. The low levels of dissolved carbon dioxide
may also limit the phytoplankton production and organic matter in suspension
becomes bound to the marl and is sedimented out. As a result the water is the
clearest of any category of lake in Britain and has a very characteristic blue tint.
Although the phosphorus is unavailable to phytoplankton, rooted macrophytes are
able to obtain it from the sediment, and they may be extremely abundant to great
depths. The bottom sediment in deep water is generally very low in organic
material, being composed largely of calcium carbonate, so there is little available
food for benthic invertebrates. In shallower water in the macrophytc zone the marl
deposit is mixed with a proportion of organic material derived from the decay of the
plants and though this mud is generally anaerobic below the surface it may support
a relatively abundant invertebrate fauna. The most productive invertebrate
communities are those associated with the macrophytes and the littoral zone.
Unlike eutrophic lakes, marl lakes are found in both the upland and lowland areas of
Britain, those in the upland areas may be stony shored, steep-sided and deep,
thus, in morphometry, resembling oligotrophic lakes. Many marl lakes have been
formed by karst erosion while others occupy small glacial troughs and some of
these may be relatively deep. Shallower examples are found in areas of highly
calcareous sand or shell marl, while in southern England only artificial examples
occur. In many respects, although they are highly calcareous, marl lakes are
oligotrophic in nature.
The phytoplankton is very sparse, but
there are insufficient data to give figures for
biomass. Judging from American work, however, gross production is of the order of
0.5-5 g organic carbon/m2 of lake surface per year (i.e. at the lower end of the
range of oligotrophic lakes). The species composition of the phytoplankton is
similar to that of mesotrophic or eutrophic lakes.
The zooplankton is also generally very
sparse, but the species composition is
rather variable. In the northern marl lakes species typical of oligotrophic lakes may
be present (although not species such as Holopedium and Bythotrephes) while in
lakes in southern Britain, species typical of eutrophic conditions are found.
Bosmina coregoni has been found in Scottish marl lakes but B. longirostris is the
species found in the lakes of southern England and Wales.
Where the bottom mud consists primarily
of inorganic marl the biomass of
invertebrates in the profundal zone may be very low, but where aquatic
macrophytes contribute a significant quantity of organic material a relatively rich
invertebrate fauna exists. In the latter case some deoxygena-tion may take place in
the hypolimnion and species such as Chironomus spp., tubificids and Chaoborus
flavicans, characteristic of the profundal of eutrophic lakes, are present. In the less
productive marl lakes a profundal fauna similar to that of oligotrophic lakes is found,
but C. flavicans appears to be a constant member of the deep-water fauna.
Aquatic vegetation colonises to greater
depths than in lake types other than the
clearest oligotrophic, and some marl lakes may be the most productive of all
standing waters for higher plants. Fontinalis antipyretica has been recorded as
growing down to 12 m in one such British lake but, generally, aquatic macrophytes
are restricted to less than 6 m. Chara spp. attain their greatest importance and may
dominate the submerged communities from the shoreline down to the limit of plant
growth. A laige number of Chara spp. occur but none appears to be restricted to
this type of lake. Short-growing species such as C. aspera and C. deli-catula may
be found in the shallower water while in deeper water more robust species such as
C. aculeolata and C. hispida may dominate. As well as the Characeae a wide
range of submerged angiosperms may also be prolific, all the species found in
eutrophic lakes being encountered. Since many marl lakes are rather steeper
sided, and in the north more exposed and rocky than typical eutrophic lakes, the
extent of the shallow-water marginal vegetation is generally rather limited and the
rate of hydroseral progression is low. In extreme cases, as at the Durness lochs in
Sutherland, the shorelines are of wave-washed limestone rocks with only very
scattered growths of Eleocharis palus-tris, Equisetum fluviatile and Cinclodotus
fontinaloides. In more sheltered examples aquatic transition successions more
typical of eutrophic lakes occur, and in these situations a marl composed of
fragments of aquatic gastropod shells may contribute to or largely form the
accreted material on which plant colonisation takes place.
Associated with the rich growth of submerged
macro-phytes is an abundant and
diverse invertebrate fauna containing many of the species found in the sublittoral
zone of eutrophic lakes. In the isolated northern and upland marl lakes, however, the
fauna tends to be somewhat impoverished and certain groups may be absent or
represented by only a few adaptable species (e.g. mayflies in Malham Tarn and
molluscs in the Durness lochs). Molluscs are generally extremely abundant and are
represented by a wide range of species; Potamopyrgus jenkinsi, a recent maritime
colonist of fresh water, is nowadays found in its greatest abundance in association
with beds of Chara in marl lakes, where it may be present in enormous
concentrations almost to the exclusion of other invertebrates. The beetle Haliplus
confmis is another very typical associate of Chara in these conditions. The
exposed littoral is a more important component of marl lakes than of eutrophic
lakes, and whilst its stones are covered with encrustations of marl, they are usually
relatively free from silt. Encrusting algae such as Rivularia and Coleochaete, which
provide crevices and a relatively soft substrate for tunnel-forming invertebrates such
as psycho-myid caddises and orthoclad chironomids, are often prominent, as well
as filamentous algae species found in eutrophic waters. Stoneflies and mayflies
typical of exposed wave-washed shores in oligotrophic lakes may occur alongside
invertebrates (including a wide variety of leeches and gastropods) of the littoral
zone of eutrophic waters. Gam-marus (Riuulogammarus) lacustris or G. (R.) pulex
are frequently the most abundant invertebrates and the freshwater crayfish Astacus
pallipes is characteristic of southern marl lakes and has been introduced farther
north.
