Lake habitats

 

Eutrophic lakes

Freshwater habitatsEutrophic lakes are highly productive because plant nutrients are plentiful. They may be naturally eutrophic or result from artificial enrichment, e.g. Lough Neagh has been enriched moving it from a mesotrophic to a eutrophic state. This habitat is most typical of hard water areas in the lowlands of southern and eastern Britain, but they also occur in the north and west of the UK, especially near the coast.

These water bodies are characterised by having dense, long-term populations of algae in mid-summer, often making the water green. Their beds are covered by dark anaerobic mud, rich in organic matter.  They are most typical of hard water areas of the lowlands of southern and eastern Britain, but also occur in the north and west, especially near the coast.

In their natural state, such waters have high biodiversity. Planktonic algae and zooplankton are abundant in the water column.  Submerged vegetation is diverse.  And numerous species of invertebrate and fish are present. Plant assemblages vary, but fennel-leaved pondweed Potamogeton pectinatus and spiked water-milfoil Myriophyllum spicatum are characteristic throughout the UK. Common floating-leaved plants include yellow water lily Nuphar lutea, and there is often a marginal fringe of reedswamp.

Bottom-dwelling invertebrates, such as snails, dragonflies and water beetles are abundant. Calcareous sites may support large populations of the native freshwater crayfish Austropotamobius pallipes. Coarse fish, such as roach Rutilus rutilus, tench Tinca tinca and pike Esox lucius, are typical. And salmonids also occur naturally in some places. Amphibians, including great crested newt Triturus cristatus, are often present. The abundance of food can also support internationally important bird populations.

 

Mesotrophic lakes

Freshwater habitatsMesotrophic lakes have a moderate nutrient status. They are characterised by a narrow range of nutrients, which are virtually all locked-up in algae during the growing season. Such lakes are relatively infrequent and largely confined to the margins of upland areas in the north and west.

Mesotrophic lakes have the highest potential macrophyte diversity of any lake type. Furthermore, they contain a higher proportion of nationally scarce and rare aquatic plants. Macro-invertebrates are well represented, particularly dragonflies, water beetles, stoneflies and mayflies. Rare fish, of which only three species are found in UK lakes, are well represented. The vendace, Coregonus albula is only found in two sites in Britain, one of which is Bassenthwaite Water in Cumbria. Another whitefish, Coregonus lavaretus, known as the schelly (or gwyniad, or powan), is found in a mesotrophic tarn in Cumbria. The schelly is also found in oligotrophic lakes in Cumbria, Wales and Scotland and there is uncertainty as to whether it is abnormally stressed in a mesotrophic environment. In general, fish communities in mesotrophic lakes are a mix of coarse and salmonid species, but today there are few truly natural assemblages due to introduced species.

 

Oligotrophic and dystrophic lakes

Freshwater habitatsOligotrophic and dystrophic lakes are characterised by low nutrient levels and low productivity. They occur on hard, acid rocks, most often in the uplands of the north and west, and vary greatly in size and depth. They include some of the deepest water bodies and least disturbed aquatic assemblages in the UK. While oligotrophic lakes usually have very clear water, dystrophic examples have dark, peat-stained waters.

Characteristic plankton, zoobenthos, macrophyte and fish communities occur. Fish communities, generally dominated by salmonids, may include charr Salvelinus alpinus and Coregonus spp. A number of benthic and planktonic invertebrates, found only in oligotrophic lakes, are possibly glacial relicts. Macrophytes are typically sparse, with species such as shoreweed Littorella uniflora and quillwort Isoetes spp. Shores are typically stony, and emergent vegetation is generally restricted to sheltered bays, where species such as bottle sedge Carex rostrata and bulrush Scirpus lacustris may be found.