Observing the other half of Dorset

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DWT’s Peter Tinsley explores the underwater terrain of Dorset’s coast, from the seabed’s physical features to mapping its elusive ecology

Diver in the trench caused by a large ship anchoring in Weymouth Bay.
© Simon Brown

The land area of Dorset is roughly equivalent to its adjoining sea area, which stretches 12 nautical miles from the coast – an area you could call Dorset’s territorial sea. But look at any map and you will see far fewer features (if any) marked in the sea than on land.
The mappable features on the seabed are all physical – depth contours, reefs, sandbanks, even tidal currents are reliably predictable. Some of these features are named on Admiralty charts – such as the Shambles Bank, Adamant Shoal, Whitehouse Grounds. Others have local names – Lobster Rock, Blackers Bump. There is also a scattering of shipwrecks – many dating back to the two world wars – and a few man-made features such as maintained channels into Poole and Portland harbours, and pipelines such as the disused radio-active waste pipeline running out from Arish Mell.

Who sees the seashore?
The ecological features are harder to map. While land is conveniently divided into tidy packages by walls, fences or uses, the seabed is mostly in the hands of one owner – the Crown – and there are no fences and very few seabed-uses that make an obvious mark.
Gathering ecological information in the marine environment, therefore, poses challenges. Dorset Wildlife Trust makes use of a mixture of digital technology and old-fashioned volunteering. The latter ranges from shoreline strollers reporting anything from jellyfish to stranded whales (this year’s signings have included several seahorses, one caught by a child ‘crabbing’ in Poole Harbour)to Seasearch divers trained in the identification of marine species and habitats and Shoresearch volunteers grabbing that short opportunity when the tide is out to search for intertidal species.
Other volunteers use snorkels and kayaks to map out shallow water features, such as beds of snakelocks anemones in Kimmeridge Bay.
Repeat surveys of the same site can detect long-term changes – in Kimmeridge Bay, volunteers have noticed an increase in the number of furrowed crab among the ledges, compared with juvenile edible crabs, a trend which appears to hold across the south west. Another Kimmeridge Bay group is monitoring the population of peacock’s tail alga – a species which is one of the features of the Purbeck Coast Marine Conservation Zone
The use of photogrammetry underwater in Dorset has provided detailed, scaled views of garden-sized patches of seabed – views that are impossible in the real world, where underwater visibility is just a few metres. We have now established three monitoring sites, one of which has been surveyed three times over three years. The same technique has been applied to monitor one of the scars left on the seabed in Poole Bay by cruise ships during the lockdown – still very visible three years later. Photogrammetry has proved to be an invaluable way of documenting habitat change, be that recovery or damage, natural or anthropogenic. It has enabled a much better appreciation and understanding of seabed biotopes and repeat surveys provide an opportunity to monitor change in a way that has not been possible before.

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