|IPA Factsheet - Mainland Orkney|
Site Description Loch of Harray and Loch of Stenness The two largest lochs in Orkney form an important natural heritage site. Harray Loch drains a shallow basin in the centre of the West Mainland. Its waters flow into the Stenness Loch, the largest brackish lagoon in the UK, through a narrow channel at Brodgar, which in turn is open to the sea at the Brig O’ Waithe. The waters of the lochs range from marine at the seaward entrance of Stenness to freshwater in Harray with variability between marine and freshwater within Stenness itself. The associated flora and fauna is diverse comprising predominantly brackish and marine species in Stenness and freshwater species in Harray, with a transition zone in the vicinity of the Bridge of Brodgar. Both lochs provide an important wintering ground for a wide variety of wildfowl, including Pochard (Atteal), Tufted Duck, Scaup and Goldeneye (Gowdie Duck). A very rare caddis fly Triaenodes reuteri, previously recorded only in Kent, Essex and Yorkshire is found in the loch. It is also the only Scottish home for the snail Theodoxus fluviatilis commonly found in the calacareous rivers of England. Loch of Isbister and the Loons Loch of Isbister and the Loons are located on the west mainland, between Marwick and Twatt, in the parish of Birsay. The site is a wetland site of national importance for its botanical and ornithological interest. The Loch of Isbister and the Loons has developed into Orkney’s best basin-mire complex as peripheral vegetation has encroached into the open water of Loch of Isbister and is internationally important as a high quality example of a nutrient rich water body. The rich habitat provides a nationally important breeding ground for a range of birds including: wigeon Anas Penelope, mallard Anas platyrhynchos, teal Anas crecca, tufted duck Aythya fuligula, red breasted merganser Mergus serrator, shoveler Anas clypeata and shelduck Tadorna tadorna. The site is also notified for its nationally important breeding population of pintail, supporting on average 6% of the British population. The site is used by an internationally important number of otters. Stromness Heaths and Coasts This part of the west coast of Orkney is important for coastal geomorphology, providing the best examples of the distinctive sandstone and flagstone cliffs and associated features of Orkney. At Yesnaby and Gaulton good examples of cliff sections of the Lower Old Red Sandstone Yesnaby Group and the Middle Old Red Sandstone Flagstones are exposed, separated by an intervening unconformity. This is important evidence of the regional relationship between these two sets of Devonian sediments typical of the Orcadian basin. The south Stromness coast is the best section through the lower part of the Middle Old Red Sandstone (Stromness Flagstone Group), showing very diverse lake and playa basin sediments.
Botanical Significance Noted for stoneworts, habitat and vascular plants interest. Loch of Harray and Loch of Stenness Harray Loch is important for its large number of Potamogeton (pond weed) species (9), three of which, P. filiformis, P. praelongus and P.friessi are nationally scarce. This assemblage is unusual, representing a broad spectrum of nutrient requirements. Loch of Harray and Loch of Stenness are sites of European importance for stoneworts (Stewart, 2004). Loch of Stenness is connected to the sea through a narrow channel and is brackish: stoneworts were confined to the parts of the loch most remote from the sea connection, where the salinity is lower. Three rare brackish species have been recorded; Chara baltica, Chara canescens, and Tollypella nidifica. Only T. nidifica has been confirmed in recent years, present as a small population in the Bay of Voy in 1994, here in one of its only three British sites, but not re-found subsequently. The salinity of the site has increased since the 1920s when the other species were recorded, possibly die to higher tides resulting from the building of the Churchill Barriers across the eastern entrances of Scapa Flow. It may now be too brackish for stoneworts (Stewart, 2004). Loch of Harray, which is separated from loch of Stenness only by a valved culvert, is freshwater and contains five stonewort species (Chara aspera, C. curta, C. contraria, C. virgata and Tolypella glomerata), although two others (Chara baltica and C. globularis) have not been confirmed recently. Extensive beds of slender stonewort species, particularly Chara contraria are found in the deeper water and strong populations of nationally scarce Chara curta in the shallows at the southern end (Stewart 2004). Loch of Isbister and the Loons This site is Orkney’s best example of a basin fen and sits in a shallow natural basin, fed by burns and springs. Over thousands of years, vegetation has gradually encroached on the Loch of Isbister and formed a range of habitats, including peatland. Relatively small differences in water levels and soil fertility across the site favour different plant species and give rise to the complex mosaic of vegetation that characterises the site today. The site in particular contains excellent examples of vegetation that forms on the margins of lochs. The mosaic of habitats supports a very rich variety of wetland plants, including autumnal water-starwort Callitriche hermaphroditica, slender-leaved pondweed Potamogeton filiformis, the regionally scarce fragrant orchid Gymnadenia conopsea and abundant stoneworts (Chara aspera, C. contraria, C. curta, C. globularis,C. virgata and C. vulgaris). Stromness Heaths and Coasts Along the clifftops, extreme exposure to wind and salt spray has produced some of the best and most extensive areas of maritime grassland and maritime heath anywhere in the UK. Red fescue Festuca rubra, thrift Armeria maritima, spring squill Scilla verna and sea plantain Plantago maritima are just some of the plants that carpet the maritime grassland. The site is also a Special Area of Conservation because of its internationally important coastal vegetation. In places where cliff-top communities are strongly influenced by the sea, species such as sea plantain and thrift are co-dominant. These cliff-top communities grade into mosaics of maritime heath and grassland. The heathland is the largest area of of the distinctive northern lichen-rich maritime European dry heaths in the UK. The coastal heath is often rich in species and is dominated by dwarf shrubs such as common heather Calluna vulgaris, bell heather Erica cinerea, an unusual maritime form of crowberry Empetrum nigrum. and creeping willow Salix repens, along with lichens, a variety of grasses, sedges and wildflowers such as mountain everlasting Antennaria dioica and wild thyme Thymus serpyllum. The seaward transition to maritime grassland includes herb-rich sedge dominated vegetation. The nationally scarce Scottish primrose Primula scotica thrives locally in this maritime heathland and grassland mosaic. The largest Primula scotica colonies occur near Yesnaby. There is a natural landward transition from the more maritime plant communities into areas of wind-pruned acidic heath. Heathland away from the zone of maritime influence is naturally less species-rich. Throughout the heathland there are base-rich flush and mire communities which support a wide range of grasses, sedges and flowering plants. Loch of Banks The area is rich in nutrients and supports a wide variety of wetland plants, including a large stand of reeds, willow scrub, creeping willow Salix repens, sedges, bog-rush Juncus effusus, yellow flag iris Iris pseudacorus, branched bur-reed Sparganium erectum and meadow sweet Filipendula ulmaria.
General Habitat Description Orkney is a landscape of distinctive geology, topography, archaeology and land use. The geology is horizontally bedded and relatively uniform. The topography consists of coasts both shallow and steep, extensive lowlands in the basins of the Lochs of Harray and Stenness, and the extensive uplands of Hoy. The surface layer contains archaeological sites, modern farmland and unimproved moorland. The West Mainland lochs of Orkney are extremely rich for stoneworts. Of particular note are the two largest lochs, Loch of Stenness and Loch of Harray. The Loch of Isbister and the Loons has developed into Orkney’s best basin-mire complex and is internationally important as a high quality example of a nutrient rich water body. The Loch of Banks is a is a mosaic of reedbed, open water and wetland habitats supporting a wide range of plant species. It is a particularly good example of this habitat in Orkney. The Stromness Heaths and Coast cover a long stretch of coast from the southern end of the Bay of Skaill to the golf course in Stromness. This stretch of land is situated on the high exposed cliffs of the west coast. Along the clifftops, extreme exposure to wind and salt spray has produced some of the best and most extensive areas of maritime grassland and maritime heath anywhere in the UK. Almost in the centre of the coast line is a large area of inshore land which is a habitat noted for its subalpine dry heath.
Conservation Issues Considered of European importance for stoneworts in Stewart (2004) Harray and Stenness Loch Algal blooms in the late 1960s were blamed for the death of large numbers of fish in Harray Loch. There is still some debate over whether this was part of a natural cycle or if it was made worse by nutrient enrichment of the lochs. Sluice gates installed between the two lochs in 1968, designed to stop sea water entering Harray Loch and encouraging further algal blooms, are likely to have had an impact on the wildlife of the site. The Control of Pollution Act and farming codes of practice restrict fertiliser and slurry spreading around the lochs and are generally This enrichment reduces oxygen levels and has caused problemsin the lochs. In the 1980’s, particularly in Harray Loch, there was a huge increase in the non-native Canadian pondweed and subsequent dieback, a cycle which may recur. It is thought that this cycle was related to mute swan population fluctuations and an unusually high mortality event of the swans. Carpets of blue-green algae and bacteria with a rotten eggs smell have been a recurring problem at the north and south ends of the Stenness Loch. The Water Framework Directive aims to prevent further deterioration of water environments, bringing together, and where necessary developing, existing control mechanisms. In the future it will be an important context for management of the lochs and their catchments. Loch of Isbister and the Loons Several pools within the basin fen habitat are directly connected to the Loch of Isbister and any significant changes in water levels in the loch could adversely affect the basin fen. In particular, any reduction in the Loch of Isbister water level, even on a seasonal basis, could be damaging. Although the Loch of Isbister has a naturally high nutrient level any increase could have an adverse effect on the site. Agricultural run-off and pollution from point sources can cause increases in nutrient levels. The Orkney and Shetland Area Advisory Group to the Scotland River Basin Management Plan has identified Loch of Isbister as one of a number of high quality small water bodies within Orkney potentially at risk from diffuse pollution. The current grazing regime on the site, including that supported by SRDP within the RSPB’s holdings, is not thought to cause any adverse effects. However, changes in the grazing regime could adversely affect the notified features, with the basin mire areas being identified as particularly sensitive. Stromness Heaths and Coasts The inappropriate use of recreational vehicles on the site at Yesnaby has resulted in some damage to the maritime grassland in the area. Loch of Banks The basin fen feature is considered to be in an unfavourable condition. During the monitoring visit in 2008 a change in the form of the habitats was noted, including a lack of brown mosses and the presence of negative indicator species. The increasing encroachment of the common reed which is dominating the north end of the site is being tackled through grazing management. The input of nutrients from off site soak-aways to the north of the site could lead to nutrient enrichment of the soil and subsequent expansion of the reedbed.