General site character

Key habitats found in Europe and requiring protection have been identified and protected by this designation.

The river Axe Special Area of Conservation (abbreviated to SAC) is a strictly protected site and the designation is concerned with protecting valuable habitats.

The specific habitat description for this part of the River Axe is:

Water courses of plain to montane levels with the water-crowfoot, species and hybrids and water-starworts, in latin parlance: the Ranunculion fluitantis and Callitricho-Batrachion vegetation.

In this case the SAC is comprised of 90% river, 5% improved grassland and 5% other.

This habitat type is characterised by the abundance of water-crowfoots which are a sub-genus Batrachium of Ranunculus spp., (Ranunculus fluitans, R. penicillatus ssp. penicillatus, R. penicillatus ssp. pseudofluitans, and R. peltatus and its hybrids). Floating mats of these white-flowered species are characteristic of river channels in early to mid-summer. They may modify the water flow, promote fine sediment deposition, and provide shelter and food for fish and invertebrate animals.

There are several variants of this habitat in the UK, depending on geology and river type. In each, Ranunculus species are associated with a different assemblage of other aquatic plants, such as water-cress Rorippa nasturtium-aquaticum, water-starworts Callitriche spp., water-parsnips Sium latifolium and Berula erecta, water-milfoils Myriophyllum spp. and water forget-me-not Myosotis scorpioides. In some rivers, the cover of these species may exceed that of the Ranunculus species. Three main sub-types are defined by substrate and the dominant species within the Ranunculus community.

The Axe is a south-western example of a variant of this habitat (sub-type two) that is found on other substrates, ranging from lime-rich substrates such as oolite, through soft sandstone and clay to more mesotrophic and oligotrophic rocks. There is considerable geographic and ecological variation in this sub-type. Faster-flowing western rivers on harder rocks, for example in Wales and southwest England, support stream water-crowfoot Ranunculus penicillatus ssp. penicillatus, while western and northern rivers on sandstone or alluvial substrates often support both stream water-crowfoot R. penicillatus ssp. penicillatus and river water-crowfoot R. fluitans. Sub-type two rivers elsewhere in the UK contain a mixture of species, and often hybrids, but rarely support stream water-crowfoot R. penicillatus ssp. penicillatus or river water-crowfoot R. fluitans. Associated species which may be present include lesser water-parsnip Berula erecta, blunt-fruited water-starwort Callitriche obtusangula, and, in more polluted rivers, curled pondweed Potamogeton crispus, fennel pondweed P. pectinatus and horned pondweed Zannichellia palustris. Flowering-rush Butomus umbellatus is an occasional bank-side associate.

Only the lower reaches of the main river have been designated, where the mixed catchment geology of sandstones and limestones gives rise to calcareous waters where stream water-crowfoot R. penicillatus ssp. pseudofluitans dominates, giving way to river water-crowfoot R. fluitans further downstream. Short-leaved water-starwort Callitriche truncata is an unusual addition to the Ranunculus community and gives additional interest.

Species listed in the Habitats Directive, that are present as a qualifying feature, but not a primary reason for site selection are:

the sea lamprey  Petromyzon marinus, the brook lamprey  Lampetra planeri and the bullhead  Cottus gobio.

 

Threats to this type of habitat are several fold. In particular:

  • invasive plants;
  • pollution;
  • inappropriate farming practices within the river catchment area; and
  • inappropriate river management.

Some introduced plants from other areas are well adapted to the prevailing conditions and through colonisation and growth can smother native plants and can lead to physical changes to the river system resulting in, for example, flooding.

In this habitat the main invasive species are monkey flower Mimulus spp., giant hogweed Heracleum mantegazzianum, Japanese knotweed Fallopia japonica and Himalayan balsam Impatiens glandulifera, can be found along the riverbanks. Canadian pondweed Elodea spp., can be problematic within the river itself.

This can also be seen where native species have been introduced to ‘new’ rivers and have subsequently proved to be dominant. This has been seen where Ranunculus fluitans has been introduced to several rivers in northeast Britain and one in Northern Ireland and is now considered to be a problem species.

The introduction of pollutants into the river is potentially always a problem from both diffuse or point sources. An example of diffuse pollution is run-off of excess fertiliser from farmland providing nitrogen and phosphorous in an available form as nutrients to plants. An example of a point source of pollution is for example where raw slurry has escaped into a water course. This may be directly toxic or can result in excessive plant growth and oxygen depletion when this vegetation dies and rots.

Farming practices that result in soil being washed into the water course are also problematic through erosion of the field concerned and silting and sedimentation of the river affected by it. To a large extent this is avoidable through the adoption of appropriate practice in the river catchment but not universally practiced.

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