The marine algae of the Hebridean Island of Colonsay

Colonsay is situated in the Inner Hebrides, 40 km from the mainland of the west of Scotland (Fig. 1).At low tide it is connected with the adjacent island of Oronsay by an expanse of sand known as the Strand.Both islands are composed chiefly of sedimentary rocks, such as sandstone, limestone and grit, all dipping predominantly eastwards, but around the harbour of Scalasaig there are outcrops of granite (Craig, Wright and Bailey, 1911).Although the land flora of Colonsay has been examined in detail (McNeill, 1910), the marine algae have received little attention.Loder (1935) lists 17 species of seaweeds, largely taken from McNeill (1910), andGorvett (1958), in a brief discussion of the intertidal ecology of Oronsay, mentions only eight species.Similarly, few algal records exist for the adjacent islands of the Inner Hebrides, although Muirhead (1962) included a short list in a survey of the Garvellachs, a group of islets 25 km north-east of Colonsay, and McAllister, Norton and Conway (1967) have described the distribution of the sublittoral marine algae of the adjacent mainland of Scotland.The present list was compiled from collections made during several visits to Colonsay and Oronsay in the months June-September 1959, culminating in a diving expedition in May 1967.The divers collected in three-metre zones from just below Chart Datum to a depth of 21 m.Depths were measured with a diver's depth-gauge, and the height of the tide, calculated from the Admiralty Tide Tables, was subtracted to give the depth below Chart Datum.The plants were collected in nylon-mesh bags (a separate bag being used at each depth sampled) and were subsequently identified in the laboratory.At all the sites investigated by the divers, steeply sloping rock extended down to a depth of approximately 9 m.The rock bore an extremely dense, almost impenetrable forest of Laminaria hyperborea, with Delesseria sanguinea the most conspicuous species in the rather sparse underflora of Rhodophyceae.The preponderance of Rhodophyceae beneath the canopy of L. hyperborea was also noted by Kain (1961) diving in the Gairloch, Ross-shire, and seems to be a general feature of such a forest.Colonsay is characterised underwater by the absence of any deep-lying rock.Below 9 m the rocky substrate gives way to shelving shelly sand, on which L. saccharina was found to be the dominant laminarian.In this region L. hyperborea was confined to large isolated boulders protruding from the sand, but a great variety of other algae was found attached to small stones or shells buried in the sand.It should be emphasised that all these 125