Aquifer-fed naturally fluctuating water bodies

 

This habitat is associated with very large fluctuations in water-level. This includes a period when sites are completely, or almost completely, dry. There is no inflow or outflow stream at the surface, except at times of very high water level, when temporary outflows may occur. Instead, these water bodies are directly filled by an underlying aquifer (through the groundwater system), which is periodically emptied and recharged. All have hard water because the underlying rock is calcareous.

 

Freshwater habitatsAquifer-fed water bodies are a very rare habitat type, both in the UK and internationally. They occur in two forms:

  • Turloughs – these are found over Carboniferous limestone in Northern Ireland and Wales; and are distinguished by winter flooding and a more-or-less dry floor in summer. Three intact turloughs have so far been found in county Fermanagh, Northern Ireland, together with a single example (Pant-y-llyn) in South Wales.
  • Fluctuating meres – six fluctuating meres have been identified over chalk bedrock in the Norfolk Breckland; these have a complex pattern of emptying and refilling, sometimes with a stretch of several years during which the mere may remain dry, followed by a prolonged period when water is constantly present.

 

Freshwater habitatThe vegetation of this habitat usually has a distinct zonation determined by water depth and frequency and duration of filling. When in their dry phase, their basins are normally partly or completely occupied by grassland, often with silverweed Potentilla anserina abundant. Turloughs in Northern Ireland retain some permanent swampy pools. A common element is the prevalence of aquatic and semi-aquatic mosses, such as Fontinalis antipyretica and Cinclidotus fontinaloides, which are more resistant to desiccation.

 

Fish are generally absent, but a range of amphibians can be found, including the great crested newt Triturus cristatus in the Breckland. Invertebrates include many insect species, such as dragonflies, water boatmen and diving beetles. Typically there is also a rich assemblage of micro-crustaceans, such as water fleas, which have resting stages that can remain viable in the soil during dry phases.