Papa Stour and the adjacent mainland are surrounded by both submerged bedrock and boulder reefs. They provide an excellent example of a wave exposed environment. In some areas the very exposed nature of the site is reflected in the considerable depths to which the kelp forests extend, reaching down to almost 30m. This compares with depths of only a few metres in adjacent sheltered sites. The kelp forests are made up of oarweed Laminaria hyperborea, sugar kelp Laminaria saccharina and a luxuriant sub-flora of associated species. 
The deeper reefs are dominated by invertebrates highly characteristic of the marine environment in Shetland. These include the soft coral Alcyonium digitatum, the feather star Antedon bifida and the polychaete Pomatoceros triqueter. The sub arctic starfish Hippasteria phrygiana is also present in the sublittoral communities of Papa Stour.
Amongst the reefs are wave exposed gullies where a rich, surge-tolerant community thrives. The presence of this habitat further increases the diversity found within the cSAC with species such as the jewel anemone Corynactis viridis and Tubularia indivisia covering the gully walls.
The Papa Stour cSAC contains numerous sea caves, as well as tunnels and arches, in very exposed sea conditions. They are the best examples of their type in Shetland and among the most extensive of such systems in the British Isles. The caves are subject to a variety of wave, current, scour and light conditions and support rich communities that reflect the changing environmental gradients. Shade-tolerant algae at the cave entrances give way to extensive faunal turfs on their inner walls.
Although much of the site is generally exposed or very exposed to wave action, there are some areas of shelter. The voes of Papa Stour offer rare areas of hard substrata sheltered shores. It is here that plants such as Ascophyllum nodosum and the fucoid seaweeds (commonly known as wrack) can be found, together with filamentous red and brown algae. Sheltered boulder beaches may contain pools and rich underboulder communities of encrusting animals, such as the sponge Halichondria panicea.
The Sound of Papa is subject to very strong currents. Rapid tidal flows lift material from nearby sandy areas and carry it though the Sound. Only those species specially adapted to withstand scour and abrasion by the suspended sand can survive these harsh conditions. This has lead to the formation of an unusual community composed only of scour-tolerant species. Mixed kelps and a variety of red algae may be found, with animals such as the anemone Urticina felina and the brittlestar Ophiocomina nigra present. |
The exposed reefs within the littoral zone are also of conservation interest. Along the Sandness Coast and around Papa Stour
these biotopes are considered to be both highly natural and representative of the Shetland environment. They illustrate the principle that in conditions of extreme wave exposure the height range that littoral communities span can be considerably greater than the tidal range at the site. This is particularly evident in the wide, vertical lichen and barnacle zones that can by seen extending up parts of the shore. The littoral sites further exhibit a transition from classic exposed shore communities on the
open coast to sheltered communities in the heads of voes. Another important feature is the presence of Fucus distichus, an exposed shore fucoid of limited distribution in the British Isles. It is known from both Papa Stour and the mainland at this site.
