Oligotrophic systems
Most natural lakes in Britain belong to this category, and are mainly found in the base-poor upland areas of north Wales, the Lake District, Galloway and the Scottish Highlands and Islands. Most were formed by the processes of glacial erosion during the last glaciation, but many of the smaller examples are in areas of morainic deposition. In the main valleys occupied by the larger glaciers, long, narrow and generally very deep, fjord lakes were formed, and all the larger oligotrophic lakes in Britain are of this type. At higher altitudes many lakes were formed in corries and are usually roughly circular, deep and steep-sided. In more level areas, a combination of glacial erosion and deposition has often led to the formation of numerous small, shallow, irregularly shaped oligotrophic lakes. In lowland Britain oligotrophic lakes are very rare, being confined to areas of base- poor sandstones or drift where they have generally been formed as kettle-holes. South of the area of glaciation there are only a few artificial oligotrophic lakes and a few small natural rock basins.
The shorelines of these oligotrophic lakes almost invariably consist of coarse inorganic sediments, boulders, stones or gravel. With increasing depth these give way to finer material and in the deeper areas the bottom is covered by a mud deposit known as dy. This consists largely of allochthonous peaty material admixed with a gelatinous precipitate of ferric salts. Because of the low productivity of these lakes, the contribution of decaying phytoplankton and macrophytic remains to the deep water sediments is low, and the rate of decomposition in the dy is slow. Thus, this sediment and the overlying deep water of the lake (the hypolimnion) remains well oxygenated even during summer, when it becomes isolated from the surface of the lake by a stable layer of warmer water (the epilimnion) which occupies the upper 5-24 m of water. Phosphorus is only released from the sediments of lakes when these become anaerobic, so in oligotrophic lakes this element remains bound in the dy and the unavailability of phosphorus is generally the factor limiting phytoplankton production. The zooplankton and phytoplankton of these lakes are both sparse and generally lacking in diversity. The biomass of phytoplankton, as measured by the concentration of chlorophyll a, is much lower than in eutrophic lakes, with values from about 0.5 to 20 mg/m3. Maximum algal cell numbers reach concentrations of only hundreds per milli-litre, except for the minute nannoplankton which may reach a few thousand per millilitre. Since light penetration is in part a function of algal abundance, a characteristic of oligotrophic lakes is the low phytoplankton production per unit volume of water down to great depths. This contrasts with eutrophic lakes where high production per unit volume of water limited to the upper water is typical. Production rates may not be correspondingly low per unit of biomass in oligotrophic lakes because nannoplankton, which often have higher production rates than the larger algae, commonly predominate. Typical values of phytoplankton production range from 5 to 25 g organic carbon/m2 of lake surface per year.
As many algae are found in a wide variety of lakes, the major qualitative differences between the phytoplankton of oligotrophic and eutrophic lakes are in the relative abundance of the various groups. Discolouration of the water by blue-green or other abundant algae is very rare in oligotrophic lakes and few diatoms are found, the most abundant being species of Cyclotella, Tabellaria, Rhizosolenia and sometimes Melosira distans. Asterionella may be present but not in large numbers. Chrysophyta are prominent and are represented by a considerable variety of nannoplanktonic flagellates of which Dinobryon and occasionally Uroglena are characteristic genera. These two species may produce some turbidity or discolouration of the water.
Dinoflagellates such as Peridinium, Gymnodinium and Ceratium cornutum may be present especially in summer, but C. hirundinella is rare or absent. Among the Chlorophyta, desmids are not usually very numerous per unit volume of water. However, a wide variety of species is present, some of which are also benthic or come from boggy areas surrounding the lakes. Coccoid and colonial green algae are relatively common, especially in summer, including genera such as Raphidonema, Chlorella, Sphaerocystis, Gloeocystis and Botryococcus. There is a seasonal succession of plankton with diatoms and Chrysophyceae predominating from winter to midsummer, and green algae and dinoflagellates from midsummer to late autumn.
Few species of zooplankter are confined in Britain to oligotrophic waters, but the assemblage of species is generally very distinct from that of eutrophic waters. The cladocerans Holopedium gibberum and Bythotrephes longi-manus and the copepods Limnocalanus macrurus, Diaptomus (Mixodiaptomus) laciniatus and D. (Arctodiaptomus) laticeps are only found in the plankton of oligotrophic lakes in the north of Britain. Holopedium has, however, disappeared from a number of lakes in the Lake District that have received increasing loads of sewage effluent, and Limnocalanus, which was previously recorded only in Ennerdale (where it was thought to be a glacial relict) may now be extinct in Britain.
The most common assemblage of species in the open water of these lakes comprises Cyclops strenuus abyssorum, Diaptomus gracilis, Bosmina coregoni (usually var. obtusiros-tris) and small numbers of Leptodora kindti. In all but the poorest lakes, Daphnia hyalina var. lacustris is also usually a constituent of the zooplankton. Variations on this basic community are frequent, for example Cyclops strenuus may be accompanied or replaced by other Cyclops spp. such as C. agilis or C. leukharti (the latter, a tropical species at the northerly edge of its range in Britain, is not found in Scotland). In some shallow oligotrophic waters Diaptomus (Arctodiaptomus) wierzejskii may replace D. (Eudiaptomus) gracilis, and in others one or more of the common species may be absent.
In the profundal zone of these lakes there is no auto-trophic plant production and the flora consists only of bacteria and fungi whose production is limited by the relatively non-degradable nature of the peaty solids forming the organic fraction of the dy. The invertebrate fauna living in or on the dy mud consists of a few well-defined groups.
1. Filter feeders, such as sponges, lamellibranchs and some Chironominae and Orthocladiinae, which depend on fine paniculate organic matter falling from the upper parts of the lake.
2. Burrowing forms feeding directly on the mud and its microflora and microfauna, e.g. oligochaetes and nematodes.
3. Largely carnivorous forms such as leeches, Tany-podinae, Chaoborinae and Hydracarina.
The sponge Spongilla lacustris may form large palmate colonies in still deep waters but is replaced by the encrusting species Ephydatia fluviatilis in more turbulent shallow water. Nematodes are often present but little is known about the biology of this group. Oligochaetes are little noted, but species such as Peloscolex ferox, Tubifex tubifex and the naidid Arctonais lomondi are recorded from the profundal of a few oligotrophic lakes in Britain. The most typical leech species of the deep-water zone is Helobdella stagnalis. Glossiphonia complanata and Erpobdella octoculata are also found in muddy conditions, usually in shallow water. Few gastropods extend to great depths: the pulmonate Lymnaea (Radix) pereger is restricted in Loch Lomond to depths of less than 6 m, but the opisthobranch Valvata piscinalis can go much deeper (down to at least 11 m in Loch Lomond) and is usually the most successful snail on a mud substrate in oligotrophic waters. Planorbis (Gyraulus) albus, Potamo-pyrgus jenkinsi and Physa fontinalis will also occur in the mud zone, but these snails are more typical of the shallower vegetation zones. Pisidium spp. generally form a considerable portion of the biomass of the deep water benthos. Pisidium casertanum the most widespread and adaptable member of this genus, P. hibernicum, P. lilljeborgii and P. personatum, are the most typical species and may be accompanied by the larger Sphaerium corneum. Pisidium conventus which is found in the mud of a number of cold-water northern lakes, is considered to be a glacial relict species.
Several genera of Hydracarina swim above the mud surface together with cladocerans such as Latona setifera, Ilyocryptus acutifrons, I. sordidus and various harpacticoid copepods. The glacial relict Mysis relicta, which was recorded from Ennerdale, is the only macrobenthic Crustacean found in the profundal of oligotrophic lakes in Britain. This species is partially benthic and partially planktonic,
A characteristic group of the deeper water is the Chironomidae of which Orthocladiinae may figure prominently, e.g. Orthodadius spp., Cricotopus spp. and Metriocnemus spp. Chironominae are less well represented than in nutrient-rich lakes and Chironomus spp. are not found, but Cryptochironomus spp., Polypedilum spp., Pentapedilum spp., and Lauterborniella spp. may occur together with Tanytarsus spp. These genera are mainly tube dwellers, unlike the carnivorous Tanypodinae which are free-living in the mud, feeding on chironornid larvae and oligochaetes. The genera Procladius, Ablabesmyia and Anatopynia are common in oligotrophic lakes. The phantom midge Chaoborus flavicans occurs widely in deeper water, and migrates between the mud and the water mass, where it is a specialist feeder on zooplankton. Small numbers of biting midge larvae, Ceratopogonidae, are also found in the deep-water mud.
The Trichoptera are not found in the deepest water but extend down into the aphotic zone. In oligotrophic lakes Polycentropus flavomaculatus, Cyrnus flavidus, Athripsodes aterrimus, Mystacides azurea and Oxyethira spp. are characteristic. Similarly the alder fly Sialis lutaria is a common predator in the mud zone of the slightly richer lakes but does not extend down into the deeper water.
Progressing into shallow water, the transition to the sub-littoral zone is marked by the appearance of macrophytes such as Nitella opaca, Isoetes lacustris and occasionally Fontinalis antipyretica, usually growing in single-species stands. These may grow down to a depth of about 12 m, but the last two species can also occur in shallow water where competition from other plants is low. In shallower regions of this zone where the sediments are generally of fine inorganic material species such as Myriophyllum alterniflorum, Juncus bulbosus, Callitriche hermaphroditica, and occasionally Potamogeton gramineus and P. perfoliatus replace the deep-water species. In more sheltered situations floating-leaved communities of Nymphaceae, Sparganium angustifolium and Potamogeton natans may be found. In sheltered bays and along the margins of small oligotrophic lakes where peaty organic sediments accumulate and the sublittoral zone may extend up to the water's edge, emergent communities occur of species such as Carex rostrata, Equisetum fluvialile, Schoenoplectus lacustris and generally rather depauperate stands of Phragmites communis. In most cases these sparse reed- beds are not advancing with time and hydroseral progression is limited to areas around the mouths of inflow streams where reed-swamp progresses to poor-fen and wet alder-willow scrub growing on the deposited silt.
The benthic algae of the sublittoral zone of oligotrophic lakes are not well known. Desmids and diatoms, particularly certain species ofFrustulia and Pinnularia are abundant on the surface of the mud, while the higher plants may be covered with a wide range of epiphytic diatoms and filamentous algae. The latter are more abundant, however, in the littoral zone where they attach to stones.
The fauna of the sublittoral benthic zone is more diverse than that of the profundal mainly because of the diversity of habitat provided by the presence of aquatic macrophytes and benthic algae. The carnivorous triclads appear where the bottom is firm enough to support them, or on vegetation. The only abundant species in these waters are Polycelis nigra and P. tennis which feed on oligochaetes and insects. The oligochaetes in this zone are not well known but conspicuous species are Lumbriculus variegatus and Stylaria lacustris, the latter closely associated with macrophytes. Tubificidae and other Naididae are also present. The same species of leech found in deeper water may occur, and the fish leech Piscicola geometra. The firm substrates and greater variety of prey of this region favours a greater density of leeches than in the deeper mud zone. Gastropods also achieve their greatest abundance in this zone, though the variety of species is limited to those which are tolerant of low calcium concentrations. Lymnaea (Radix) pereger, Valvatapiscinalis and Planorbis (Gyraulus) albus are the most abundant species in the vegetation zone but other species, such as Planorbis (Bathyomphalus) contortus, P. (Gyraulus) laevis, P. (Anisus) leucostoma, Physa fontinalis and the limpet Acroloxus lacustris, which are more typical and abundant in richer conditions, may also occur. Sphaerium corneum and several Pisidium spp. are found in this zone, both climbing among the shoots of the vegetation, and burrowing in the sediment. Hydracarina may be plentiful and weed-dwelling Entomostraca such as Sida crystallina and Eurycercus lamellatus often occur in huge numbers amongst the submerged vegetation, but few large Crustacea are found at this depth. Gammarus (Gammar acanthus) lacustris or G. (Rivulogammarus) pulex may be found among weed, but are more characteristic of the wave-washed littoral zone. Asellus spp. are found in small numbers in a few of the richer lakes but are not characteristic members of the fauna.
Where the bottom substrate is of silt or mud the mayfly nymph Caenis horaria is found, but is replaced by C. moesta where the substrate changes to sand. The typical mayfly fauna of weed-beds in oligotrophic lakes consists of Centrop-tilum luteolum, Cloeon simile, Leptophlebia vespertina, Ephemerella ignita and Siphlonurus lacustris. Few stoneflies are found in the silted conditions of this zone, Nemoura spp. perhaps being the most common.
Dragonflies are not usually found in the larger lakes in Britain but will occur where sheltered bays produce suitable conditions for emergence and breeding and in the smaller oligotrophic lakes. They are then usually associated with vegetation and in oligotrophic lakes the most frequent species are Enallagma cyathigerum, Pyrrhosoma nymphula and Libellula quadrimaculata, which are widespread and adaptable.
The aquatic Hemiptera must visit the surface to respire and are thus mostly restricted to shallow water. Of the submerging species Glaenocorisa propinqua extends to greater depths than others and is the characteristic species of the sublittoral zone of oligotrophic lakes. Shallow-water species such as Sigara distincta and S. scotti also occur. The aquatic beetles are also restricted to relatively shallow water but a few species, such as Deronectes (Potamonectes) dcpressus and Haliplus fulvus, will occur in deeper water among vegetation. The larvae of the alderfly Sialis lutaria are found throughout this zone wherever silt, mud or peat accumulates.
The net-spinning caddis larvae Cyrnus flavidus and Poly-centropus flavomaculatus occur in vegetation, together with a variety of cased species. The latter include a number of Limnephilidae such as Limnephilus lunatus and Anabolia nervosa. The Leptoceridae are also well represented by Mystacides azurea, Athripsodes aterrimus, Triaenodes bicolor and others. Other caddises commonly found are the predatory Phryganea spp., and the small hydroptilid Oxyethira costalis which are associated with vegetation, and Molanna angustata which is typically found on sandy bottoms.
Several weed-dwelling Chironomidae occur, including tube-building forms such as Stempellina, Cricotopus and Endochironomus. In the underlying silty substrate there are several genera of Chironominae, though Chironomus itself is usually absent from oligotrophic waters. This is the typical Tanytarsus zone. Free-living Procladius and Anatopynia are found in both the weeds and mud and Tipula spp. occur in small numbers wherever organic muds accumulate.
The fauna of the sheltered shoreline, where fine deposits
are found and emergent vegetation such as Carex rostrata often occurs, differs little from that of the deeper sub-littoral. Dragonflies, corixids such as Sigara scotti and S. distincta, and some of the water beetles such as Platambus maculatus and several Hydroporus spp. are more frequent in the shallow water. In contrast to the open water the surface film is calm enough in this habitat to support surface dwelling insects such as Collembola, Gerris lacustris, G. odontogaster, Gyrinus marinus and G. aeratus.
As well as the true benthos, there is also a community of cladocerans and copepods which lives inshore swimming near the bottom or amongst macrophytes of the sublittoral zone. Even in oligotrophic lakes this community can be very varied with a large number of species. A typical constituent, especially in high-altitude, rock-shored lakes is Alonopsis elongata, a species with a northerly distribution. This community also contains several species such as Chydorus sphaericus, Alona spp., Diaphanosoma brachyurum and Cyclops (Paracyclops) fimbriatus, which are not selective in their choice of habitat and are consequently found in nearly every water-body. Where there is submerged vegetation, species such as Sida crystallina, Eurycercus lamellatus, Ceriodaphnia spp., Peracantha truncata and Cyclops (Mega-cyclops) viridis can be extremely abundant and form a major component of the fish diet. Ophryoxus gracilis and Eurycercus glacialis are Arctic species found in a few cold lakes in the north of Scotland.
In the littoral zone the nature of the flora and fauna is largely dependent upon the degree of exposure to wave action. On the relatively sheltered shores where the bottom consists of stones, gravel, or sand in depths of i m or less, the plant communities consist predominantly of rosette-leaved species such as hoetes lacustris, Littorella uniflora, Lobelia dortmanna and Subularia aquatica, with emergent Juncus bulbosus, J. articulatus and Ranunculus flammula at the water's edge. On more exposed shores where the substrate is of stable boulders or bedrock, the macrophytic flora is restricted to bryophytes such as Fontinalis anti- pyretica and Jungermannia cordifolia. In both these situations the substrate is generally covered by a film of epilithic diatoms and desmids, while filamentous algae belonging to the genera Mougeotia, Zygnema, Spirogyra, Microspora and Hormidium may also be present. In the most exposed situations where the shorelines consist of unstable storm beaches, macrophytic vegetation cannot gain a footing and even the epilithic film of algae may be removed by abrasion in storms.
The physical conditions in the stable wave-washed stony shoreline of large oligotrophic lakes approximate to those of eroding rivers with moving, well- oxygenated water, and a clean silt-free substrate. This enables a number of riverine species of invertebrate to exist here also. The invertebrates of this zone consist mainly of grazers feeding on the epilithic algae, and predators, while a few species are filter feeders and rely on the waves to bring particulate matter to them.
Stones may be encrusted on their undersides with the filter-feeding sponge Ephydatia fluviatilis and ectoprocts such as Cristatella mucedo and Plumatella repens. The triclads Polycelis tennis and P. nigra are most abundant in this zone and, in some cold-water lakes in the north, are joined by Crenobia alpina (which elsewhere is a stream species). The typical and often most abundant mollusc of the stony shore is the freshwater limpet Ancylus fluviatilis, but the other soft-water gastropod species, especially Lymnaea (Radix) pereger, may also be present. Pisidium spp. and Sphaerium corneum occur only in small numbers where there is sufficient gravel in which to burrow. Nematodes, and tubificid and naidid worms, are also present in small numbers in the gravel but the most conspicuous oligochaetes of this zone are the larger Eiseniella tetraedra and Stylodrilus heringianus. Among the leeches the horse leech Haemopis sanguisuga is confined, in oligotrophic lakes, to stones in shallow water where the more adaptable soft-water species Glossiphonia complanata, Erpobdella octoculata, Helobdella stagnalis and Piscicola geometra also occur. Some Hydra-carina are also found swimming amongst the stones. The only common malacostracan Crustacea on the stony shores are Gammarus spp. G. (Rivulogammarus) lacustris is the typical lacustrine species in the north, but south of its range in Britain the stream species G. (R.) pulex may take over this habitat. The latter is also often found in oligotrophic reservoirs which it has colonised from the inundated river. In lakes where the two species occur together G. (R.) pulex is generally confined to the areas near the mouths of streams and appears to be unable to compete against G. (R.) lacustris within the lake itself.
The most characteristic insects of the stony lake shore are the mayflies and stoneflies. Many species live in this habitat and some, such as Ecdyonuridae and most stone-flies, are adapted to living in water currents. The mayfly species are Ecdyonurus dispar, Heptagenia lateralis, Lepto-phlebia marginata, Ephemerella ignita, Cmtroptilum luteolum and Siphlonurus lacustris. In the far north of Scotland Ame-letus inopinatus, which is a high-altitude stream species, is found on stony lake shores down to sea-level. Other true stream species such as Baetis spp. are occasionally found on lake shores in small numbers.
The most abundant stoneflies are Chloroperla torrentium, Leuctra fusca, L. inermis, Nemoura avicularis and Diura bicaudata. The last species has a peculiar distribution in that it is found in lakes at all altitudes but in streams only above 300 m. The nymphs of Capnia bifrons and C. atra occur only in autumn and winter, and the former species is confined to Scotland. Isoperla grammatica and Per lodes tnicro-cephala are stream dwellers which, in Scotland, occur only on lake shores. Nemoura cambrica, N. erratica and Leuctra hippopus are also stream-dwelling species found occasionally in lakes.
Micronecta poweri often swims just above the bottom in large numbers, particularly in sandy areas, and Sigara distincta occasionally also occurs. The beetles of the stony lake shore are also found in rivers. The larvae and adults of helmids such as Elmis aenea, Limnius volckmari and Oulimnius tuberculatus are found in the gravel often several centimetres below the surface, while the adults of species such as Oreodytes rivalis and Haliplus lineolatus swim over the bottom, although their larvae are benthic.
The stony cases of Agapetus fuscipes frequently coat the stones of the littoral zone of oligotrophic lakes while other caddises such as Tinodes waeneri and Lype phaeopa live in silk tubes secreted on the surface of the stones. The adults of Tinodes often swarm in a band along the shores of large oligotrophic lakes. The Polycentropidae, of which Plectroc-nemia conspersa, P. geniculata and Polycentropus flavo-maculatus are the most frequent species occurring, live under and between the stones where they spin their nets. The mobile case-dwelling caddises of the exposed shore mostly have sturdy heavy cases of sand particles and include some Limnephilidae, Sericostoma personatum and Molanna angustata. The small Hydroptila spp. may also be found here.
Various chironomids inhabit the shoreline but little is known of the species composition. Orthocladiinae, Tany-podinae and Tanytarsini are amongst the groups present and the larvae of the tipulids Dicranota spp. andPedicia spp. burrow deeply into the sand and gravel beneath stones.
On unstable storm beaches there is little organic matter on which invertebrates can feed; erosion and the frequent movement of the substrate makes it difficult for them to remain attached to the bottom and there is the danger of the animals being crushed. Most animals living in this habitat are therefore either deep burrowers, which escape the movement of the substrate by penetrating the more stable gravel below the mobile surface, or are strong swimmers living for the most part above the level of the stones. Included in the first category are the worms Eiseniella tetraedra and Stylodrilus heringianus, dipteran larvae such as Dicranota, Pedicia and certain chironomids, and adults and larvae of the helmid beetles Oulimnius tuberculatus and Limnius volckmari. The last two are small and have a strong hard exoskeleton which resists crushing by the stones, while the worms and diptera larvae are leathery and elastic. Among the species which swim over the substrate are Gammarus, Micronecta poweri and a number of beetles such as Oreodytes rivalis. A few stoneflies, mayflies and caddises of the stable stony shore may also cling precariously to the stones of the storm beach or wander over sandy areas.
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