Tob Valasay

Also within

Western Isles County South Lewis, Harris and North Uist Scenic Area

Condition (NatureScot)

Climate exposure

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

Habitats

Saline lagoon

Geology & landscape

Landscape processes: Coastal, Fluvial, Volcanic / Igneous

Description

Tob Valasay Tob Valasay is a long narrow inlet on the west side of the island of Bernera in south west Lewis. It drains into Loch Roag via Kyles Valasay after passing over a rocky sill, which is just above the mid tide level. There is a pedestrian bridge over the narrows just north of the sill. The hinterland is rocky rough grazing at the south end, becoming better quality in-bye grassland around the township of Tobson in the north. This ob is an excellent example of a rock silled saline lagoon; a habitat which is rare in Europe outside the Western Isles and Shetland. Tob Valasay displays many of the special features which distinguish rock silled obs from other marine habitats. The saline lagoonal species tasselweed Ruppia sp. is found in the muddier northern parts of the site. In the southern narrows there is a fine example of the species rich tidal rapids which are a feature of rock-silled saline lagoons. There is a marked gradient in salinities from brackish water in the upper basins to almost fully marine salt water at the narrows. This range of salinities is sometimes accompanied by the occurrence of a halocline, the presence in the water column of an upper layer of brackish or freshwater and a lower layer of saltwater. The intertidal communities within the ob are restricted to a much narrower zone than would be found on the shores of the open coast because the tidal range is greatly reduced. The tidal rapids in Tob Valasay stretch along the narrows for over a kilometre; this is one of the most extensive examples of this habitat associated with a rock-silled ob in Scotland. In terms of species richness and representativeness this is one of the two best sites in the Western Isles. This habitat is of special interest because of its high diversity of invertebrates; the underside of boulders and the seaweed fronds are densely colonised by filter feeding animals such as sea squirts, bryozoans and sponges associated with fast water flow. Many species are epiphytic on the fronds of kelp and sea oak. The tidal rapids also contain an unusual biotope which is characterised by the presence of the calcareous red alga called maerl, which forms thick pink crusts on stones. Other biotopes of particular note in Tob Valasay include those dominated by tasselweed and eelgrass in the sheltered brackish basins. The site is also of interest because of its great variety of substrates which is reflected in a wide range of different communities of seaweed and animals. It is particularly rich in the rocky substrate biotopes which are a feature of Scottish saline lagoons. These include littoral assemblages dominated by, in descending order down the shore; channel wrack, spiral wrack, egg wrack, bladder wrack. Sublittoral boulders and bedrock are largely covered by kelp, often accompanied by the bootlace weed Chorda filum. Biotopes on other substrates range from eelgrass and tasselweed beds on mud to sandier areas at the head of the ob colonised by lugworms and sand gapers.

Species mentioned

Plant communities