OR/14/043 Site description - Llyn Y Fan Fach Sites 1 and 2
| Farr, G, Graham, J, and Stratford, C. 2014. Survey, characterisation and condition assessment of Palustriella dominated springs 'H7220 Petrifying springs with tufa formation (Cratoneurion)' in Wales. British Geological Survey Internal Report, OR/14/043. |
Introduction
Llyn y Fan Fach is located in the western area of the BBNP in the Mynydd Du (Black Mountain) south east from Llanddeusant and south from the Usk Reservoir (SN8057123770). Llyn y Fan Fach is the name given to a glacial lake or cirque, currently dammed it forms the headwarters of the Wennalt Fawr. Several other tributaries including the Afon Sychlwch, Nant Coch, Nant Melyn and Sychnant also flow into the Wennalt Fawr in the area. The survey concentrated on two locations along the Nant Melyn which flows from the base of Fan Foel to join the Wennalt Fawr near the small bridge close to the car parking area. During the survey two sites were assessed being Site 1 (SN8057123770 at 330 maOD) and Site 2 (SN8011623781 at 280 maOD). Tufa desposition was not identified at either site, other than on a few small branches of a single bryophyte (Palustriella commutata) at Site 1.
Geological and hydrogeological data
The catchment of the springs and also the Nant Melyn comprise rocks from the Lower Devonian Brownstones Formation underlain by the Lower Devonian Senni Formation. The Devonian bedrock is overlain by glacial till and the area is important for a range of glacial and periglacial features from the Devensian late Glacial period (Campbell & Bowen, 1989)[1]. The Nant Melyn cuts down into the bedrock but also through the superficial deposits. The site is interdispersed with boulders sourced from the Devonian bedrock.
Site 1 located slightly further upstream along the Nant Melyn comprises a series of diffuse seepages that combine into a series of runnels, flowing off the break in slope and across a small flat area of river alluvium before reaching the Nant Melyn. The flow in several runnels was estimated and ranged from <0.1 l/s to 0.2 l/s and at the main outflow there was about 0.2 l/s entering the Nant Melyn. The seepages are strongly associated with the break in slope along the bank of the river.
Two water quality samples were obtained, one from the main outflow runnel at Site 1 (LyF1.1) at SN8057123770) and one from the base of Site 2 (LyF2.4 at SN8011823779), pH values within Site 1 ranged from 7.09 to 7.32, and Site 2 from pH 7.0–7.36. Nutrient levels are close to or below the level of detection in both sites, and calcium and bicarbonate are the dominantions.
Land use and pressures
Llyn y Fan Fach is located within the Brecon Beacons National Park (BBNP). Although Sites 1 and 2 are close to the car parking area, they are off the main footpath it is thought that footfall in these areas is low. The land is sheep grazed and there are sheep tracks through the site, following the break in slope at Site 1 but this is not considered to be evidence of substantial poaching. There is no evidence of burning or obvious signs of nutrient enrichment at any part of the site. It is unlikely that there are any groundwater abstractions in the immediate area.
Vegetation (spring) of Lyn y Fan Fach Site 1
Palustriella falcata dominated short vegetation between 1–3 cm height in patches around a number of small springs in a small area that ultimately discharges into the Nant Melyn. In places the flushed vegetation is species rich with abundant Campylium stellatum, Fissidens adianthoides, Scorpidium cossonii, Philonotis calcarea, Philonotis fontana, Bryum pseudotriquetrum and locally marginal stands of Breutelia chrysoscoma. Several runnels have dominant patches of Scorpidium cossonii and Leicolea bantriensis, Riccardia mutifida, Wahlenbergia hederacea occur locally.
| Agrostis stolonifera | R (<1%) |
| Anagallis tenella | R |
| Aneura pinguis | F |
| Breutelia chrysoscoma | R (<1%) |
| Briza media | R (<1%) |
| Bryum pseudotriquetrum | O |
| Calliergonella cuspidata | R |
| Campylium stellatum | F |
| Carex demissa | O |
| Carex flacca | O |
| Cerastium fontanum | R (<1%) |
| Cirsium palustre | R |
| Cratoneuron filicinum | R (<1%) |
| Ctenidium molluscum | R (<1%) |
| Cynosurus cristatus | R (<1%) |
| filamentous algae (Chlorophyta) | R |
| Fissidens adianthoides | O |
| Galium palustre ssp. palustre | R |
| Holcus lanatus | R (<1%) |
| Juncus acutiflorus | O |
| Leicolea bantriensis | O |
| Molinia caerulea | R (<1%) |
| Palustriella commutata | F |
| Palustriella falcata | D |
| Pellia endiviifolia | R (<1%) |
| Philonotis calcarea | F |
| Philonotis fontana | R |
| Plantago lanceolata | R (<1%) |
| Preissia quadrata | R |
| Prunella vulgaris | R |
| Rhytidiadelphus squarrosus | R (<1%) |
| Riccardia multifida | R |
| Scorpidium cossonii | F |
| Thuidium tamariscinum | R (<1%) |
| Trifolium repens | R (<1%) |
| Wahlendbergia hederacea | R (<1%) |
Acidic rocks within the area of flush/spring but above level of seepage water supporting occasional acrocarpous calcifuge species.
| Racomitrium aciculare | O |
| Ptychomitrium polyphyllum | R |
Vegetation adjoining spring
Acidic Juncus effusus dominated rush pasture (to 30 m height) occurring to the right of the main Palustriella springs with frequent patches of Aulocomnium palustre, Polytrichum commune, Sphagnum species within.
| Aulocomnium palustre | O |
| Calliergonella cuspidata | R |
| Juncus effusus | A |
| Polytrichum commune | O |
| Sphagnum inundatum | R |
| Sphagnum subnitens | O |
Acid sheep grazed grassland (to 10 cm height) dominated by Nardus stricta, Galium saxatile with scatterd patches of Molinia and Sphagnum.
| Sphagnum inundatum | R |
| Hylocomium splendens | R |
| Juncus effusus | O |
| Juncus squarrosus | O |
| Molinia caerulea | R (<1%) |
| Nardus stricta | F |
| Potentilla erecta | O |
| Scleropodium purum | R |
| Sphagnum subnitens | R |
Vegetation (spring) of Lyn y Fan Fach Site 2
A series of springs fall onto an area of slipped soil allowing development of a mosaic of Palustriella dominated vegetation and rush pasture. Palustriella dominated vegetation is best developed around rocky runnels where Fissidens adianthoides, Scorpidium cossonii, Philonotis calcarea, Philonotis fontana and locally Scorpidium cossonii occur. Much of the flushed vegetation is more “neutral” in character with abundant Juncus effusus, Cratanueron filicinum, Philonotis fontana, Calliergonella cuspidata and occasional Chrysosplenium oppositifolium. Emergent flowering plants are well developed including Nasturtium officinale s.l., Galium palustre ssp. palustre, Hydrocotyle vulgaris, Mentha aquatic and Epilobium sp. The small leafy liverwort Jungermania atrovirens occurs locally on rocks at the immediate spring heads.
| Agrostis stolonifera | R (<1%) |
| Anagallis tenella | F |
| Aneura pinguis | O |
| Bellis perennis | R |
| Briza media | R |
| Calliergonella cuspidata | O |
| Cardamine pratensis | O |
| Carex demissa | R |
| Carex flacca | O |
| Chrysosplenium oppositifolium | O |
| Cirsium palustre | O |
| Cratoneuron filicinum | F |
| Ctenidium molluscum | R (<1%) |
| Epilobium sp. | R |
| Equisetum plaustre | R |
| Festuca rubra | R |
| filamentous algae (Chlorophyta) | R |
| Fissidens adianthoides | R |
| Galium palustre ssp. palustre | R |
| Hydrocotyle vulgaris | R |
| Juncus acutiflorus | O |
| Juncus effusus | F |
| Jungermannia atrovirens (per.) | R |
| Mentha aquatica | O |
| Nasturtium officinale s.l. | R |
| Palustriella commutata | O |
| Palustriella falcata | A |
| Pellia endiviifolia | R |
| Philonotis calcarea | O |
| Philonotis fontana | F |
| Plantago lanceolata | R |
| Prunella vulgaris | O |
| Ranunculus acris | R |
| Rumex acetosa | O |
| Scorpidium cossonii | O |
| Scorzoneroides autumnalis | O |
| Stellaria media | R (<1%) |
| Taraxacum sp. | R |
| Trifolium repens | R (<1%) |
Rocks within the area of flush/spring but above level of seepage water supporting occasional acrocarpous calcareous and calcifuge species.
| Tortella tortuosa | R |
| Ptychomitrium polyphyllum | O |
Vegetation adjoining spring
Heavily sheep grazed calcareous grassland (to 10 cm height) dominated Cynosurus cristatus, Bellis, Trifolium repens and Rhytidiadelphus squarrosus.
| Agrostis capillaris | R |
| Bellis perennis | O |
| Cynosurus cristatus | O |
| Juncus squarrosus | R (<1%) |
| Nardus stricta | R (<1%) |
| Plantago lanceolata | O |
| Rhytidiadelphus squarrosus | O |
| Rumex acetosa | R |
| Scorzoneroides autumnalis | R |
| Taraxacum sp. | R |
| Trifolium repens | O |
Transect vegetation and chemistry data
No transect was undertaken at Llyn y Fan Fach Site 2.
Condition assessment
The overall assessment is that Llyn-yr-Fan fach (Sites 1 and 2) should be classified as being in favourable ecological and hydrogeological condition.
References
- ↑ CAMPBELL, S, and BOWEN, D Q. 1989. Geological Conservation Review Quaternary of Wales. Nature Conservation Conucil. p.103–105.