Glannau Porthaethwy

Also within

Gwynedd County YNYS MON/ANGLESEY Scenic Area

Climate exposure

This is about exposure to a nationwide pressure this habitat type carries — not a verdict on this site's own management.

Geology & landscape

Landscape processes: Coastal

Description

Description The site extends along 4 km of the shore of Menai Bridge/Porthaethwy in the Menai Strait from Britannia Bridge to Craig y Don, and has been selected for its marine biological features. This south-east facing shore is sheltered from wave-action and consists of a mixture of mud, bedrock, pebbles, cobbles and boulders. The shores of the islands of Ynys Gorad Goch, Ynys Welltog, Ynys Benlas, Ynys Tysilio, Half Tide Rock, Ynys Faelog, Ynys Tobig, Ynys Gaint, Ynys Castell and Ynys y Big are also included in the site. The shore is of special interest as it is the most extensive sheltered rock shore in the area between Bardsey Island and great Orme’s Head and because it supports the greatest diversity of marine plant and animal communities on this type of shore within this area. The shore is also important for the presence of five marine communities of restricted national distribution, five diverse rockpool and overhang communities, and for exhibiting the most comprehensive community zonation characteristic of sheltered rocky shores. In areas such as Ynys Welltog and beneath the Menai Bridge, rocky shore communities form visible ‘zones’ down the shore. Such patterns of zonation are the result of different species’ tolerance’s to desiccation, temperature extremes and sunlight and their differing abilities to compete with other species for space. Above mean high water, rock surfaces subject to sea-spray support a zone of yellow and grey lichens. Beneath this zone the black-tar lichen Verrucaria maura grows on bedrock, cobbles and pebbles, forming a wide band. A band of channel wrack Pelvetia canaliculata and spiral wrack Fucus spiralis occurs just below this level, followed by successive zones lower down the shore of knotted wrack Ascophyllum nodosum, bladder wrack Fucus vesiculosus, and serrated wrack Fucus serratus. Mixed kelps such as oarweed Laminaria digitata, sugar kelp Saccharina latissima and cuvie Laminaria hyperborea dominate the lower reaches of the shore in a narrow band, with a diverse range of associated seaweeds including sea-oak Halidrys siliquosa, bootlace weed Chorda filum, and the red seaweeds Cystoclonium purpureum, Cryptopleura ramosa, Phycodrys rubens and species of Ceramium. The area of the Menai Strait known as the Swellies experiences strong tidal currents of up to 8 knots during spring tides and, as a result, supports a wide range of filter-feeding animals, including four tide-swept communities of restricted national distribution. After the Menai Strait, the second largest concentration of these communities in Wales is within Milford Haven, with a few small patches of certain of these communities on the Gower Peninsula, Ramsey Island and St. Tudwal’s East. These tide-swept communities are individually characterised by knotted wrack, serrated wrack on bedrock, serrated wrack on mixed substrata, or by oarweed, as well as a diverse array of filter-feeding animals. Such species, occurring on seaweeds and rock surfaces, include the sponges Hymeniacidon perleve and Amphilectus fucorum, the bryozoans Dynamena pumila, Flustrellida hispida, Membranipora Stauroneis membranacea and Crisularia plumosa, the ascidian Botrylloides leachii and star ascidian Botryllus schlosseri, and hydroids. Near Church Island an abundant population of the daisy anemone Cereus pedunculatus is associated with the tide-swept oarweed community. Another unusual community – species of small marine worms and molluscs in sheltered muddy gravel – is found at three locations along this shore. In Wales, this community is restricted to the Menai Strait and Milford Haven. Distinct communities of plants and animals inhabiting rockpools and the surfaces under rock overhangs are particularly diverse on this shore. Sediment-floored rockpools with fucoids and kelps and hydroid-dominated pools on mixed substrata are present throughout the site. Deeper rockpools dominated by serrated wrack and oarweed are found on the shore between the two bridges, and on Ynys Gorad Goch. Overhanging surfaces on the Menai Suspension Bridge support a diverse community that includes purse sponge Grantia compressa, breadcrumb sponge Halichondria panicea, the sponge Ophilitaspongia seriata and the red seaweeds Lomentaria articulata, Membranoptera alata, and dulse Palmaria palmata. Other lower shore rock overhangs on the site support a diverse community of filter-feeding animals, such as sponges, ascidians and bryozoans. Species commonly found here include the sponges Hymeniacidon perleve, Haliclona rosea and breadcrumb sponge, the bryozoan Dynamena pumila, dahlia anemone Urticina felina, plumose anemone Metridium senile and the star ascidian. Owing to its close proximity to Traeth Lafan, an internationally important site for wading birds and wildfowl, relatively undisturbed parts of this shore provide important high tide roost areas for waders that feed at Traeth Lafan and elsewhere in the Menai Strait. For example Ynys y Bîg provides an important roost site for oystercatchers. Additionally, otters have been recorded in and around the Afon Cadnant. Remarks: Certain areas of the site below mean high water form part of Y Fenai a Bae Conwy/Menai Strait and Conwy Bay Special Area of Conservation (cSAC), under the EC Habitats Directive (Directive 92/43/EEC on the Conservation of Natural Habitats and of Wild Fauna and Flora 1992). The site contains areas of Intertidal Mudflats and Sandflats and Reefs; habitats listed on Annex I of the Directive which are important features of the cSAC.

Species mentioned