The Isles of Scilly Biodiversity Audit 2008 - Cornwall Wildlife Trust

Martin’s; east of Samson; between Bryher and Tresco(Tresco flats) and adjacent to Old Grimsby on Tresco.The largest of these is St Martin’s flats, a SSSI south ofSt Martin’s.The extent of the sandflats can be seen on nauticalcharts of the area. Much of the soft sediment is coveredby seagrass beds and these areas have been mapped.Conservation ImportanceBoth the intertidal sandflats and the subtidalsediments are of European and National significancebecause of their extent and the richness and diversityof their associated communities (Natural England,Natural Area Profile). They support species whichare normally restricted to the subtidal, possiblyowing to unusual coarse quartz sediments foundhere in the intertidal (Brown et al, 1997). Variationsin wave exposure and tides in different areas haveled to increased species diversity within the sand andgravel communities; rocky outcrops and reefs withinsandy areas also enhance overall diversity. There aresubstantial differences between the communities ofeach of the major sedimentary flats resulting fromdifferences in physical and biotic factors (Nichols andHarris, 1982).Sublittoral sands and gravels are a national BAPhabitat and a Habitat Action Plan has been produced.The shallow sublittoral and interlittoral sedimentssupport some of the most extensive and welldeveloped areas of seagrass beds in the UK. Seagrassbeds are a BAP habitat themselves and are consideredseparately within this document [Section 2.2.2.15].The area of intertidal sand flats adjacent to the islandof St Martin’s has been protected as a Site of SpecialScientific Interest (SSSI number 2000168) since 1996.These and other sand flats throughout the archipelagosupport aggregations of Ensis spp (razorshells) andEchinocardium cordatum and Spatangus purpureus (bothheart urchins), the latter is never usually found in theintertidal elsewhere in the UK, tending to live onlyin sublittoral areas (Natural England, Natural AreaProfile). In some areas such as Old Grimsby Harbour,Tresco, the density of Echinocardium cordatum is upto 25 per square metre (Nichols and Harris, 1982).The razorshell Ensis siliqua biotope is nationallyuncommon but is widespread in Scilly. Densepopulations of burrowing anemones occur on some ofthe sand flats, especially in scoured channels which donot fully drain at low tide (Nichols and Harris, 1982).In areas where the sand is mixed with gravel, pebblesand cobbles there are faunal communities livingon the stones in addition to the subsurface infauna.These areas generally occur between the islandswhere exposure and tidal influence are greater. Thecommunities include nationally rare species such asthe red seaweeds Cruoria cruoriaeformis (A UK BAPspecies), Gelidiella calciocola and Schmitzia hiscockiana.These stony areas may also support large populationsof the gastropod Gibbula magus, at population densitiesof up to 40 per square metre (Nichols and Harris,1982).Some southern (Mediterranean – Atlantic) species,rarely recorded in UK waters are present in the sandysediment areas of Scilly due to the mild climate andexposure to oceanic currents which transport larvae.Examples of these in sandy substrates in Scilly arethe spiny cockle Acanthocardia aculeata, a hermit crabCestopagurus timidus and the trumpet anemone Aiptasiamutabilis (English Nature, 2000).Trends and IssuesResearchThe SSSI at St Martin’s flats is monitored by NaturalEngland as part of their commitment to monitoringthe Special Area of Conservation features. This iscarried out at least once every six years. The methodinvolves monitoring the infauna of the sediment ofthree biotopes on the sand flats and analysis by Multi-Dimensional Scaling ordination to look at similaritiesand differences between biotopes and changes overtime (Warwick, 2004).Aggregate ExtractionIn the past, extraction of marine sands hadsignificantly affected the coastline at Bar Point onSt Mary’s and to a lesser extent at Crab’s Ledge onTresco. Since 1999 a Coastal Protection Order has beenin place to prevent large-scale sand and aggregateextraction from the seashore within the Isles of Scillydown to the 10-metre depth contour line (EnglishHeritage, 2004).Tourism / RecreationAt low tide large areas of sandflats are exposed andmay be used for recreation. There has been no study ofthe effects of trampling on the sand flats here but it islikely to be negligible due to the large area over whichthe trampling is spread.PollutionOil pollution could be very damaging to this habitat.Other pollution is also a risk, particularly as pollutantscan become bound to sediments and persist inthe long term. Heavy metals and other toxins canaccumulate in sediment. In some limited areas theremay be persistent pollution due to past refuse disposalactivities at dumping grounds.Isles of Scilly biodiversity audit Page 66