Habitat point records from 1982 MBA Isles of Scilly littoral sediment survey

This report is the result of a brief visit to the Isles of Scilly in September 1982 when surveys were made of sediment shores between Tresco and St Martin's and of the shore at Porth Hellick, St Mary's during 4 low tides. This complements the account of the sediment shores given by Nichols and Harris, 1982 (report to NCC) by providing information on shores not included in their surveys and notes the importance of the sediment shores of Scilly in a national context. The shores of Scilly are of particular interest for a number of reasons. The position of the islands in the Atlantic well to the south-west of mainland Britain has the effect of maintaining an exceptionally mild winter climate as well as ensuring an absence of fresh water inflow from streams and rivers, with consequent absence of water-borne pollutants and of suspended solids. There are wide contrasts in wave exposure of the shores, from fully exposed on the outer coasts, grading to sheltered conditions within the archipelago. The sediments, formed of quartz derived from the granite, are coarse-grained and inhabited by species which usually occur offshore in coarse shell or gravel deposits. At sites surveyed between the more northerly isles, the shores at St Helen's and Foremans Island supported moderately-developed Spatangus-Fasciata communities (with characteristic species including a heart urchin and bivalve only occasionally found on the shore in the British Isles) as described by Bishop and Holme, 1980 (report to NCC). Dense beds of the burrowing anemone Cereus pedunculatus were also present, these populations extending into the shallow water channels between the islands. On the more sheltered beaches the stability of the sediment was indicated by the relatively heavy growth of some green and red algae. At Pentle Bay, Tresco, a Tellina community at mean low water was thought to give way to a Spatangus-Fasciata community at low water springs. The eel grass, Zostera marina, was only found in patches on the shore, although extensive beds were present in shallow water on the south and east of Tresco. Almost the only sheltered shore on St Mary's was surveyed at Porth Hellick, where some interesting muddy sand and gravel communities were present on the lower shore. Scilly is stated to be unique in the occurence of extensive flats of coarse sediment under sheltered conditions, and where tidal scour is not excessive, so that relatively rich animal communities are supported. (Coarse grained sediments are often associated with wave and/or tidal scour, and tend to be barren of life, particularly when they lie in the intertidal zone). In a national context, Spatangus-Fasciata communities are rare and localised on the shore, and the Isles of Scilly provide by far the best examples in southern Britain. Other examples of this community occur in Scotland, usually associated with Lithothamnion gravels ('maerl'), but these northern examples, although of considerable interest, are limited in extent. The report concludes that the sediment shores of the Scillies must be considered as of at least national, if not international importance. It recommends that, although these shores are not at present subjected to any severe pressures, the sediment shores and the adjacent subtidal communities are of such high conservation value that efforts should be made to ensure that conservation of the marine environment is effective.

Organization: Joint Nature Conservation Committee