Habitat – Protected Species

The Environment Agent provided Dulverton Angling Association with a substantial grant for habitat improvement on the Stoats Beat and they notified the association as part of this engagement about a rare, protected species of lichen that inhabits our waters. Information regarding River Jelly Lichen has been provided by the Environment Agency.

River Jelly Lichen – Collema Dichotomum

Description

River Jelly Lichen (RJL) Collema Dichotomum resembles seaweed more than a lichen. It has a gelatinous appearance, is olive-green in colour when submerged in water (see image below) and dark grey when dry, and forms circular patches on submerged rocks in upland streams. The key identification feature of the lichen is the deeply lobed thallus which are usually 2.5cm across. The distinctive lobes are fan and spoon-like with a rubbery feel when submerged in water and are usually 2mm wide.

Ecology

RJL is one of the few lichens able to grow submerged in fresh water. It will grow on rocks which are usually permanently submerged, in unpolluted water, preferring a depth of around 20cm, but will grow down to 80cm below the water surface, and only exposed in very low flows.

The river morphology and nature of the channel are critical in determining the presence of this lichen. The lichen requires very stable riverbeds, usually favouring neutral to slightly basic rock. RJL will usually colonise larger boulders or extensive areas of bedrock, avoiding areas of strong current or prone to scouring from boulders or gravels. As such, RJL is generally found in the middle reaches of a river, as higher up the river conditions are usually too unstable and lower down the river is too heavily silted, however can be found outside of the middle reaches if conditions are appropriate. RJL requires relatively well-lit sites that are not heavily shaded by bankside or in stream vegetation.

Distribution

This lichen is restricted to isolated sites in Scotland, mid Wales, the north and west/south-west of England and Northern Ireland.

It is also found across northern Europe and Russia, and North America where it is considered rare. As such, the British Isles appear to be its main stronghold with the River Barle one of the main strongholds in south west of England.

Surveys since 1999 have identified fluctuating populations of RJL along the River Barle, noting a reduction in populations following spate conditions suggesting the lichen is sensitive to flood damage but able to regenerate once the river returns to more stable conditions. The surveys have also identified periods of drought are the likely trigger of the lichen colonising greater depths of the river where they may be more vulnerable to disturbance from moving gravels and pebbles.

The map below indicates the presence of River Jelly Lichen within the River Barle (red line). However, it should be noted a walkover survey conducted within a small section of the River Barle adjacent to Dulverton in 2023 observed reduced levels of RJL at this location. This is likely due to drought conditions in recent previous years.

Protection and status

• Natural Environment and Rural Communities (NERC) Act 2006, Species of Principal Importance, Section 41

• UK post-2010 biodiversity framework (UK Biodiversity Action Plan (UKBAP) species)

• Wildlife and Countryside Act 1981, Schedule 8

• Vulnerable in the UK (Nationally Scarce)

Threat and management

Due to the requirement of very stable conditions, the RJL is sensitive to the following threats:

· in channel recreation

· reduced water levels caused by water abstraction

· water pollution and water acidification

· increased silt loads

· Eutrophication

The effects of raised temperatures and increased frequency of extreme weather conditions due to global warming (elongated dry periods and higher frequency of short heavy local rainfall events) are a further threat to this species.