Calcareous flushes on the Northmoor Trust Estate
What are calcareous flushes?
Flush is term used to describe a small, often weakly flowing, spring. Calcareous flushes generally occur where porous calcareous bed rock is underlain by impermeable strata forcing ground water, rich in dissolved lime, to emerge as a spring.
Due to its high lime content, any twigs, fallen leaves or other debris in contact with the up welling water soon become covered in a chalky deposit known as tufa.
Calcareous flushes are therefore characterised by the deposition of tufa around the spring heads. Since they occur as a chance consequence of landform and geology they are a rare habitat in Britain and throughout Europe.
In Oxfordshire, calcareous flushes are particularly well developed along the Midvale Ridge Natural Area, including several prime sites, such as Cothill Fen NNR. Much is known about the invertebrate communities associated these sites.
The conservation value of calcareous flushes
Flushes provide very shallow water that supports a diverse semi-aquatic invertebrate community. These utilise either the water film, the oxygenated surface layers of silt or water-logged mosses. Where tufa encrusted substrates also occur additional specialised invertebrates (such as soldier flies) can also be found. Most of these specialist species are rare, due to the scarcity of their habitat. Many are included in the British Red Data Book (which documents species that are rare or threatened) and/or the UK BAP (which documents species of global importance). Calcareous flushes therefore provide an important and contrasting 'aquatic' habitat to lakes, rivers, ponds and streams.
