Mesotrophic lakes are a relatively small
series, intermediate in all respects
between oligotrophic lakes and eutrophic lakes. In Britain they are frequently
situated on the borders between uplands and lowlands where run off from base-
poor rocks is slightly enriched by drainage from the more calcareous soils of the
lower lying areas of the catchment. Drainage from the base-poor Tertiary sands of
southern England also produces mesotrophic water.
The most productive large glacial trough
lakes in Britain are mesotrophic, as are
the least productive examples of kettle-hole lakes. In lowland Britain, a few
mesotrophic lakes have been formed by coastal processes while others are of
artificial origin. Mesotrophic lakes sometimes represent a successional stage
between oligotrophic and eutrophic conditions, but they may occasionally evolve
into more oligotrophic waters where peripheral mire development isolates them
from ground water.
Since mesotrophic lakes generally occur
in regions of somewhat softer rocks than
oligotrophic lakes, their shorelines tend to be of finer particles, often of gravel or
sand instead of stones and boulders. They are frequently shallow and rarely have
steeply shelving shorelines, so that emergent vegetation may extend well out into
the water and some hydroseral progression may take place. Phytoplankton
production is sufficiently high to cause discolouration of the water in summer and
produces a significant contribution to the bottom mud in the form of dead algal
cells. The deep-water mud is thus intermediate in character between the largely
allochthonous dy muds of oligotrophic lakes and the largely autochthonous gyttja
muds found on the bottom of eutrophic lakes. The sediment of mesotrophic lakes
has thus been termed dy-gyttja. In the deeper examples, where thermal stratification
takes place, the decay of the algal crop in the hypolimnion may cause some
depletion of oxygen during the summer months. This may cause conversion of the
ferric salts in the dy-gyttja to the ferrous state and thus allow the release of
phosphorus from the sediment.
The phytoplankton of these lakes is also
intermediate, in production and species
composition between oligotrophic and eutrophic lakes, and species of algae
characteristic of both types of lake may occur. Diatoms generally predominate in
the spring, Asterionellaformosa being a characteristic species, but the algal
blooms produced by blue-green algae in late summer in eutrophic lakes do not
generally occur.
The composition of the zooplankton community
is similar to that of oligotrophic
lakes except that species such as Bythotrephes longimanus and Holopedium
gibberum, which are confined to the most nutrient-poor lakes, are absent. The
zooplankton is, however, much denser and, in southern Britain, species
characteristic of eutrophic conditions may be present. Bosmina longirostris
replaces S. coregoni, a characteristic species of oligotrophic waters, although in
some mesotrophic waters both species may exist. This change-over has occurred
in recent years in a few oligotrophic lakes where the nutrient levels have been
increased by man's activities.
The most striking qualitative change in
the composition of the profundal benthic
fauna of mesotrophic lakes, as compared to the oligotrophic situation, is the
appearance of a number of species of invertebrate adapted to the somewhat
anaerobic conditions which prevail in the dy-gyttja muds in the summer months.
These include haemoglobin pigmented species such as the tubifid Potamothrix
hammoniensis and Chironomus spp. such as C. anthracinus, C. (Camptochirono-
mus) tentans and C. cingulatus. All these have appeared in recent years in
oligotrophic lakes which have suffered from artificial eutrophication. Another
characteristic species adapted to living in the oxygen-depleted hypolimnia of
mesotrophic lakes is the phantom midge larva Chaoborus flavicans which lives
partly in the plankton and partly in the bottom mud. Asellus meridianus and A.
aquaticus, although more frequent in the sublittoral zone, may extend down into the
profundal where they can survive very low oxygen concentrations. In quantitative
terms the profundal fauna is much more abundant and productive in mesotrophic
lakes.
Because of the restricted light penetration,
the profundal zone extends into
shallower water and submerged plants are confined to depths of 6 m or less. With
increasing alkalinity and nutrient content the number of macrophyte species in a
lake generally increases. Most of the species of oligotrophic lakes are also found
under suitable conditions in mesotrophic lakes with the exception of the
acidophilous Sphagnum spp., Eriophorum angustifolium and Utricularia minor. In
addition there are a number of species more or less restricted to mesotrophic
lakes together with those which are more characteristic of the eutrophic
environment but which will tolerate relatively low ionic concentrations. Species
characteristic of the mesotrophic condition include Isoeles echinospora (which may
replace /. lacustris), Pilularia globulifera, Elatine hexandra and E. hydropiper, the
last being far more common on sandy shores than was previously believed. Among
the species also found in the eutrophic environment which will penetrate into
mesotrophic lakes are the submerged species, Chara spp., Elodea canadensis,
Potamogeton praelongus, P. obtusifolius and P. alpinus. Emergent marginal
communities are usually more diverse in species and more vigorous in growth than
those of oligotrophic lakes. Fen communities may develop at the inflows of streams
or in sheltered bays, the hydroseral progression being very variable, and in
sheltered or ungrazed conditions the most typical emergents are plants such as
Phragmites communis, Phalaris arundinacea, Typha spp., various carices, and
Schoenoplectus lacustris. Under certain conditions successions from floating-
leaved communities through to Menyanthes trifoliata, Potentilla palustris and Juncus
spp. occur. In conditions of heavy grazing, bryophyte-dominated communities may
develop around the margins of mesotrophic lakes, with bryophytes such as
Acrocladium cuspida-tum, and low-growing herbs such as Myosotis secunda,
Hydrocotyle vulgaris and Galium palustre. In more exposed conditions the marginal
communities of mesotrophic lakes may be very similar to those of oligotrophic
waters. One of the most characteristic communities of the moderately exposed
shoreline with a substrate varying from gravel to a loam is a dense sward of
Littorella uniflora which is often exposed at periods of low water. This community
also extends into lakes belonging to the poorer end of the eutrophic spectrum.
Filamentous algae are more abundant than
in oligotrophic lakes and may coat the
submerged macrophytes, and Cladophora, the most abundant filamentous species
in eutrophic lakes, may be present.
The fauna of the sublittoral zone is more
diverse and much more abundant than in
oligotrophic waters. The increased diversity arises mostly from the addition of
species associated with richer conditions. These newcomers include the triclads
Dendrocoelum lacteum and Dugesia lugubris which are restricted to waters whose
calcium content is greater than 10 p.p.m. (This restriction is probably associated
with the distribution of their prey organisms which are respectively, Asellus and
gastropods.) The soft-water mollusc species found in oligotrophic waters become
more abundant and, as the calcium content increases, are joined by species such
as Acroloxus lacustris, Planorbis carinatus and the duck mussel, Anodonta anatina.
As with the triclads the distribution of the mollusc-feeding leech Glossiphonia
heteroclita is linked to that of its prey. This species and the fish leech Hemiclepsis
marginata, which are absent from oligotrophic waters, occur in mesotrophic lakes
together with the leech species which extend into the poorer waters. The
distribution of the Malacostraca is closely linked to the ionic content of water-
bodies, species such as Asellus aquations, A. meridianus and the introduced
American species, Crangonyx pseudogracilis, appearing in mesotrophic waters,
and Gammarus spp. becoming more abundant.
In the Ephemeroptera there is an increased
abundance of those species
associated with macrophytes. The weed-dwelling Cloeon dipterum appears, and
where there are silted conditions, Ephemera danica may be found and the numbers
of weed-dwelling dragonfly nymphs such as Ischnura elegans and Enallagma
cyathigerum increase. A number of species of water bug absent from the poorer
lakes appear in mesotrophic waters. Sigara falleni and Callicorixa praeusta are
associated with relatively open conditions amongst vegetation whilst
Hesperocorixa sahlbergi is found in the thicker parts of the marginal vegetation and
Corixa punctata is a typical species of weedy ponds. Sigara scotti and S. distincta
decline in mesotrophic waters and are not found in rich lakes.
The weed-dwelling species of caddis generally
increase in numbers and variety in
mesotrophic waters. Most of the species of oligotrophic lakes are probably
encountered. Phryganeidae, Limnephilidae, Leptoceridae and Hydrop-tilidae are
often present in variety and abundance and large numbers and many species of
Chironomidae occur, up to 100 species having been found in one small lake.
Chirononws spp., Ablabesmyia spp. (particularly A. cingulatd), Procladius spp.
(particularly P. choreus), Cricotopus spp., Psectrocladius spp., Dicrotendipes spp.,
Cryptochironomus spp. Endochirono-mus albipennis, Microtendipcs chloris,
Pentapedilum spp. and Tanytarsini typically form the major components of the
chironomid fauna.
The extent of the wave-washed littoral
zone may be limited in mesotrophic lakes by
their increased tendency to silting and by the encroachment of marginal vegetation
around the shorelines. Storm beaches are generally absent and the bryophyte
communities associated with stable exposed shores in oligotrophic lakes may be
missing. The most exposed shores are often of gravel or sand, the pebbles being
covered with growths of algae, particularly Cladophora, in which a layer of silt is
trapped. Littorella uniflora often grows between the stones on this type of shoreline.
Because of the more silted nature of the
shoreline in mesotrophic lakes,
invertebrate species requiring a clean, firm, wave-washed substrate are scarce or
absent. Species which are generally rare or absent include mayflies such as the
Ecdyonuridae and Ameletus inopinatus and most of the stoneflies, including Diura
bicaudata and Capnia atra, although Chloroperla torrentium may be plentiful even
in relatively silted conditions in rich lakes.
Many of the species found in the sublittoral
extend into the littoral, including
especially the gastropods, leeches, malacostracans, caddises and chironomids.
Polycentropid caddises are often very abundant under the stones of gravel shores,
while Molanna angustata is characteristic where the shoreline is sandy. Even
species such as Chironomus spp. and various tubificids which are more
characteristic of the profundal may find a niche in the littoral zone by burrowing in
the silt accumulated between stones.
