The combination of oceanic influences (extreme wave exposure, deep oceanic swell, high water clarity) and local geology around the St Kilda archipelago has created a remarkable marine environment. The seabed communities in the area surrounding the islands are outstanding in terms of biodiversity and composition, including 'northern' and 'southern' species at the extremes of their range. The site offers important research opportunities for monitoring the impact of climate change in the marine environment.

Jewel Anemones

The St Kilda archipelago and adjacent sea bed supports a variety of remarkable marine communities that reflect the geomorphological history of the area over the last 50-60 million years and the highly unusual conditions that prevail. The intertidal and shallow subtidal areas are subject to extreme wave exposure dictating the composition of the seabed communities, whilst the influence of the oceanic swell is still felt at exceptional depths of 60-70m and is reflected in the composition of the animal communities present. Despite the relatively small spring tide range (3.0m), the extent of the upper intertidal communities is greatly extended as a result of wave splash with some species extending up to 100m above sea level. The remarkable clarity of these oceanic waters also has a significant bearing on the extent and distribution of the various sublittoral communities of animals and plants. The sublittoral photic zone is greatly extended with large kelp plants occurring down todepths of 45m and coralline red algae to in excess of 60m. The waters around St Kilda represent some of the most exciting and challenging scuba diving in the world attracting a small but increasing number of dive parties each year.

The towering sea cliffs and stacks evoke powerful emotions. The rugged rocky underwater topography is as spectacular as that above the waves and is a further legacy of St Kilda’s Tertiary Period volcanic origins and more recently the sea level and climate changes that have occurred during the Quaternary Period. It is unusual for such extensive areas of relatively shallow bedrock to occur in such oceanic conditions. The clarity of the water enables the true splendour of this underwater landscape to be better appreciated than it might be in more coastal waters where underwater visibility is much reduced. The plunging vertical underwater rock faces are festooned with marine life – a kaleidoscope of colour and form kept in constant motion by the Atlantic swell.

St Kilda is affected by the warmer water of the North-east Atlantic Drift, and particularly in winter, water temperatures remain much warmer than those in the enclosed North Sea. This has resulted in a number of northern species reaching the southern extreme of their range and vice versa. This not only enhances the overall marine biodiversity of the archipelago but also represents an opportunity for future monitoring of the status of these species as indicators of the impact of climate change on the marine environment.