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.

本报告系1982年9月对塞文群岛进行短暂考察的结果，期间对特雷斯科与圣马丁岛之间的沉积岸滩以及圣玛丽亚岛的波特海利克岸滩在四次低潮期间进行了调查。该报告补充了尼科尔斯和哈里斯1982年（提交给NCC的报告）对沉积岸滩的描述，提供了他们调查中未包含的岸滩信息，并强调了塞文群岛沉积岸滩在国家层面的重要性。塞文群岛的岸滩因诸多原因而备受关注。这些岛屿位于大不列颠岛西南部的北大西洋，其位置使得冬季气候格外温和，同时确保了没有来自溪流和河流的淡水流入，从而避免了水生污染物和悬浮固体的存在。岸滩的波浪暴露程度差异显著，从外海岸的完全暴露逐渐过渡到群岛内部的避风条件。由花岗岩衍生出的石英沉积物粗粒度，栖息着通常在离岸粗壳或砾石沉积物中出现的物种。在较北部岛屿之间的调查地点，圣海伦岛和法尔曼岛上的岸滩支持着适度发展的Spatangus-Fasciata群落（特征物种包括仅在英吉利群岛海岸偶尔发现的马耳他海胆和双壳类动物），这一群落由比什普和B霍尔姆1980年（提交给NCC的报告）所描述。挖掘海葵Cereus pedunculatus的密集床层也存在于这些地区，这些种群延伸到岛屿之间的浅水通道中。在较为避风的沙滩上，一些绿色和红色藻类的相对繁茂生长表明沉积物的稳定性。在特雷斯科的彭特利湾，平均低潮时的Tellina群落被认为在低潮春潮时让位于Spatangus-Fasciata群落。海草Zostera marina仅在岸滩的零星区域被发现，尽管在特雷斯科岛南部和东部的浅水区域有广泛的床层。在圣玛丽亚岛上，波特海利克是几乎唯一的避风岸滩，进行了调查，那里在低潮时存在一些有趣的泥质沙和砾石群落。塞文群岛因其粗粒沉积物广泛分布的平坦区域在避风条件下且潮汐冲刷不剧烈，从而支撑了相对丰富的动物群落而闻名（粗粒沉积物通常与波浪和/或潮汐冲刷相关，且往往生命贫乏，尤其是在潮间带）。在国家层面上，Spatangus-Fasciata群落罕见且分布局限，塞文群岛提供了英国南部最好的实例。此群落的其他例子出现在苏格兰，通常与Lithothamnion砾石（又称“梅尔”）相关，但这些北部的例子尽管颇具兴趣，但在范围上有限。报告总结认为，塞文群岛的沉积岸滩至少具有国家，甚至可能是国际的重要性。它建议，尽管这些岸滩目前未受到任何严重的压力，但沉积岸滩及其相邻的潮下群落具有极高的保护价值，应努力确保海洋环境的保护工作有效进行。