Natural brackish standing waters in Britain
are confined to coastal areas where
fresh water mixes with sea water carried alternatively as sea spray, or ground
water, or direct inflow via a tidal channel. Some artificial lakes in the Midlands are
affected by discharge from salt mines and are distinctly brackish, while, in various
parts of the country, there are a few, small, natural brackish springs in inland
locations. As our climate has a large excess of rainfall over evaporation for most of
the year none of our larger lakes evaporates sufficiently to become saline.
Because of the marine origin of all of
our brackish lakes, the relative proportions of
the major ions dissolved in this water are approximately the same as those for sea
water, the only variations being those caused by differences in the chemical
composition of the freshwater element of the water mix. Thus in areas of base-poor
rocks the ratio of calcium to sodium is low, as generally is the concentration of
dissolved plant nutrients, so that these might be termed oligotrophic brackish lakes.
In areas of more soluble calcareous rocks the lakes are more alkaline and richer in
nutrients and could be termed eutrophic brackish lakes. The differences between
these two categories are only appreciable at the more dilute end of the salinity
gradient; as the salinity approaches that of sea water the chemical composition,
physical properties and flora and fauna become more equivalent.
Although they comprise only a small group
of lakes in Britain in many ways they are
the most variable category of lake. Not only is there great variability in nutrient
status and salinity but their morphology and origins are also diverse. In the north
and west of Britain some relatively deep oligotrophic glacial troughs with only
narrow openings to the sea are brackish at their inland ends. Other glacial lakes in
this region are shallower and lie either on base-rich rocks, as do the brackish
eutrophic lakes of Lochs Stenness and Harray on Orkney, or else lie on base-poor
rocks as do the oligotrophic brackish lakes of the Outer Hebrides. All these
northern lakes are stony shored but, in the south of Britain, shallow mesotrophic and
eutrophic brackish waters occur which have muddy reed-fringed shorelines. These
have mainly been formed by coastal deposition so that they have become semi-
isolated from the sea by bars but some, such as the Norfolk Broads, are artificial in
origin.
The phytoplankton of brackish lakes has
been little studied in this country and it is
impossible to give an account of the composition of this flora. Prymnesium parvum,
belonging to the Chrysophyta, occurs fairly regularly in eutrophic waters where it
can cause severe fish kills by the excretion of a powerful toxin. Phytoplankton
production in brackish waters, even the base-rich examples, appears to be low and
the water is generally fairly clear but the mechanism limiting production in these
waters is not known.
As one progresses from fresh water into
slightly brackish conditions many of the
characteristic freshwater zoo-plankters disappear and are replaced by brackish
and marine genera. With the exception of species with a wide range of tolerance,
such as the ubiquitous Chydorus sphaericus, which may just extend to slightly saline
conditions, Cladocera are absent from brackish waters. Several of the more
versatile species of freshwater copepods, such as Cyclops strenuus, C.
(Megacyclops) viridis, C. (Tropocyclops) prasinus and C. (Paracyclops) fimbriatus,
extend into brackish waters, but the most characteristic zooplanktonic copepods of
brackish water are members of the genera Acartia, Eurytemora, Centropages,
Cyclopina and Halicyclops which also occur in the marine environment. Several
genera of harpacticoids such as Mesochra, Tachidius, Nitochra, Horsiella,
Laophonte and Stenhelia live in, and on, mud and vegetation in brackish conditions.
As salinity is the dominant habitat factor
affecting organisms in this environment it
is more convenient to discuss the range of variation of the macrobenthos and
macro-phytes in terms of this variable than in terms of zonation with depth.
As the salinity of a water-body increases
with increasing marine influence there is a
gradual transition from a freshwater macrophytic flora to a flora characteristic of the
marine littoral. In salinities close to that of sea water, the characteristic flora
consists of various seaweeds such as Fucus serratus and Ascophyllum nodosum.
In somewhat less saline conditions these are replaced by other algal species
such as F. ceranoides and Enteromorpha
intestinalis. Of the angiosperms the most
resistant to salinity are Ruppia spiralis, R. maritirna and Potamogeton pectinatus,
the last species occurring in waters of sodium content of at least 6000 p.p.m. Other
submerged species relatively tolerant of high salinities, e.g. Zannichellia palustris,
Lemna trisulca and Ceratophyllum demersum, while Najas flexilis and TV. marina
are generally only found in Britain in waters where the ionic content is raised slightly
by infiltration of sea water. Where the brackish conditions grade into oligotrophic
conditions the brackish water species are succeeded by species such as
Myriophyllum alterniflorum, Sparganium angustifolium, Lobelia dortmanna,
Eleogiton fluitans and Littorella uniflora, whereas, when they grade into eutrophic
conditions, Ranunculus baudotii, Myriophyllum spicatum, M. verticilla-tum, Hippuris
vulgaris and Chara spp. (particularly C. baltica) are among the first of the typical
freshwater eutrophic plants to appear. In slightly brackish conditions it is not unusual
to find seaweeds and freshwater angiosperms growing side by side.
The typical emergent species is Scirpus
maritimus. Other species associated with
slightly brackish conditions are Schoenoplectus tabernaemontani and, in the north-
west of Britain, Cladium mariscus. Phragmites communis will also tolerate slightly
brackish conditions and often grows to great heights in sheltered tidal waters.
As the salinity of a water-body increases
sensitive freshwater invertebiate species
are eliminated but, in areas of intermediate salinity, brackish and freshwater
organisms are found together. The freshwater component of the fauna may be that
of a typical oligotrophic or eutrophic water depending on the initial alkalinity of the
water, but in the most marine areas of brackish lakes the brackish water
component predominates and the fauna can be similar in both eutrophic and
oligotrophic waters.
Among the Coelenterata, Hydra spp. are
fairly resistant to brackish conditions but
the colonial Cordylophora lacus-tris is the characteristic species in south-east
England. The freshwater species of triclad are intolerant of saline conditions, but
Procerodes littoralis, which is characteristic of streams on the sea shore, is also
found on the stony shores of brackish lakes. A number of freshwater gastropods
and lamellibranchs extend into brackish conditions, the number of species being
greater in eutrophic brackish lakes than in oligotrophic lakes. The most abundant
gastropod is Pota-mopyrgus jenkinsi which is often present in enormous numbers.
This species was formerly a marine organism which is now found in fresh waters of
all types and is still in the process of extending its range. In more saline conditions
marine species of Hydrobia, Littorina, Mytilus and Cardium replace the freshwater
molluscs.
A number of naidid and tubificid worms
are confined to brackish waters and
several of the freshwater species, such as Stylaria lacustris, are also found. Some
of the more adaptable polychaetes, e.g. Nereis diversicolor, will extend into almost
fresh water. Unlike worms, freshwater leeches are not tolerant of saline conditions
and there are no brackish water species. The widespread and adaptable species
Helobdella stagnalis is probably the most tolerant and is found in very slightly
brackish lakes. Insects are largely replaced by Crustacea as one progresses into
more saline water. Where there is the slightest trace of sea water, Gammarus
(Rivulogammarus) pulex and G. (R.) lacustris are replaced by G. (R.) duebeni,
which is a freshwater species in Ireland, the Scottish islands and on the extreme
west coast of mainland Scotland where the other species are absent. In more
saline conditions G. (R.) duebeni may itself be replaced by G. zaddachi and G.
tigrinus, the former especially where there is some water current and, in almost
marine conditions, Marinogammarus spp. predominate. The shrimp Neomysis
integer first appears at about the same salinity as G. (R.) duebeni, and extends into
sea water. Other common brackish crustaceans are the amphipod Corophium
curvi-spinum and isopods Sphaeroma hookeri, S. rugicauda and Jaera nordmanni
and the true shrimp Palaemonetes varians, which is tolerant of fresh water when
adult. Plecoptera and most of the Ephemeroptera are intolerant of brackish
conditions. A number of species of widespread and highly adaptable dragonfly will
breed in brackish water in Britain. They comprise Enallagma cyathigerum,
Pyrrhosoma nym-phula, Ischnura elegans, Lestes sponsa, Sympetrum danae and
Libellula quadrimaculata. Orthetrum cancellatum, Libellula depressa and
Coenagrion pulchellum, which are confined to the south of Britain, can also breed in
these conditions. The water boatmen, Sigara selecta and S. stagnalis, are confined
to brackish water. S. lateralis occurs here and in ponds, such as farm ponds, with
organic pollution and Notonecta viridis occurs both in brackish and alkaline fresh
waters, often far inland. On the other hand S. concinna is found in fresh water but
almost always close to the sea. Other species of water boatman typical of fresh
water may extend into slightly brackish conditions alongside the above mentioned
species. A number of species of water beetle occur, and Haliplus apicalis,
Laccophilus variegatus, Hygrotus parallelo-grammus, Agabus conspersus (only
found in the extreme south east of England), Dytiscus circumflexus, Gyrinus
caspius, Helophorus alternans, some Enochrus spp. and some Ochthebius spp.
are confined to such waters. A few caddis species extend into brackish water,
especially limnephilids, including Grammotaulius atomarius. Triaenodes reuteri
breeds in pools on salt marshes.
The Diptera are the most tolerant of the
insect groups to marine conditions.
Chironomids of the subfamily Clu-nioninae breed in the inter-tidal zone of the open
sea. Several species of Tanypodinae such as Anatopynia varia and Ablabesmyia
barbitarsis, Orthocladiinae such as Coryno-neura scutellata, Cricotopus sylvestris
and Hydrobaenus rubicundus, and Chironominae such as Chironomus dorsalis, C.
longistylus and Polypedilum nubeculosus are able to breed in rock pools along the
sea shore in near marine conditions as well as occurring in fresh water. Certain
species of mosquito, e.g. Aedes detritus and A. dorsalis, and the larvae of some
Ephydridae, e.g. Ephydra riparia, generally breed in brackish pools and ditches.
