River jelly lichen  (Collema dichotomum)

This curious species resembles seaweed more than the lichens that most of us are familiar with. It has a gelatinous appearance, is olive-green in colour, and forms circular patches on submerged rocks in upland streams.

This lichen is restricted to sites in Scotland, mid Wales, the north and west of England and Northern Ireland. It is also found across northern Europe and Russia.

The river jelly lichen has a preference for usually silica-rich submerged rocks such as sandstone, that form slabs in partial shade. It is typically found in swift-flowing rivers, but very occasionally it also grows on submerged rocks in upland tarns.

Although this lichen is usually associated with upland or mountain streams, it does not seem to grow in areas of very fast flowing water such as rapids. It appears to tolerate some silting, but is probably sensitive to increased pollution and eutrophication - an increase in the level of nutrients in water.

As well as increasing nutrient levels, this species is threatened by heavy silting of the stream-beds and acidification. It may also be at risk from new hydroelectric schemes, especially the small-scale operations being planned for some upland areas.

The river jelly lichen is listed in the UK Biodiversity Action Plans and included in English Nature's Species Recovery Programme. Some of the problems associated with this species are common to many other river-living plants and animals, especially the threats from pollution and increased nutrient levels through agricultural run-off. Some, however, will have a direct impact on the lichen. The increase in the number of small-scale hydroelectric schemes risks altering the ecology of many upland streams or rivers. This can have several detrimental effects, including altering the silting characteristics of the river and affecting the natural seasonal flow-rates. This lichen appears sensitive to changes in the flow of water. Riverbank management can also affect the amount of light reaching the water surface. River jelly lichen requires some shading, so clearance of too much scrub from the banks may prove detrimental. The UK BAP Species Action Plan for this lichen recommends re-introducing this species to some of its former sites, if this proves practical. In the meantime, it is important that river water quality is monitored and improved throughout the upland regions of Britain.