Vincent May

Until the mid-twentieth century, knowledge of the seabed depended on charts using lead-lines and soundings, often many kilometres apart. The ocean was mysterious, the realm of danger, myth and monsters. Today our picture of the underwater world is much better. Not only can we visualize the seabed, but also we can bring its beauty to the surface for all to see. As a result, we also understand better how marine ecosystems work, although our understanding is still very limited.

The information in the archives about the undersea world of Dorset is mostly restricted to charts, although there are some photographs of fishing, wrecks and marine plants. Gradually more information about this part of the Dorset coast is being held in other databases such as the Dorset Marine Database which provides current listings of marine species (http://www.derc.org.uk/marine/datasets.htm ). Other local sources include www.coastlink.org/seasearch and www.swenvo.org.uk/environment. Information about species which are given specific protection can be found at www.jncc.gov.uk/species/protect/animals.htm .

Marine Charts

Early descriptions of the underwater world depended on what was caught in fishing nets or seen from ships. Occasionally the bodies of large fish or mammals were washed ashore. Even in modern times, beaching of marine mammals has been a source of curiosity (see a stranded whale at Boscombe). The rest was mystery: the sea on maps was often embellished with drawings of sea monsters and ships.

Photograph of a whale stranded on Boscombe Beach (RCM)

As the oceans were explored, maps of the newly discovered lands were drawn, but charts were often more important because they allowed voyages to be repeated with some degree of certainty. Some of the earliest charts, known as Portolan charts, were produced by Italian, Catalan and Portuguese seamen, from about 1300 AD. Although mostly of the Mediterranean, some show Atlantic coasts. Most depended upon survey using the mariner's compass.

Mercator's publication in 1569 of a map of the world “ ad usum navigantium emendate accommodata” which translates as “suitably accurate for the use of navigators  ” was the first map which allowed compass directions to be drawn as straight lines (Steers, 1957). One of Mercator's maps of England shows the Dorset coast but is not available in the archive. Much of the information about place names and locations was provided to Mercator by his network of contacts in England, a practice he used extensively in the compilation of his atlases and maps.

Because bearings are true on maps using the Mercator projection, they remain the most common form of marine chart. However, because both maps and charts are representing the curved surface of a spheroid (the Earth) on a plane surface (the paper of the chart), they distort elements of the natural surface. Charts typically have direction correct but compromise on area. Land maps more usually concentrate on shape and area.

Because of these differences between land maps and sea charts, maps that cover both land and sea are uncommon. In addition, because land maps (topographic or cadastral maps) focus on heights above a datum, often referred to loosely as heights above sea level (in the UK, Ordnance Datum) and marine (bathymetric) charts concentrate on depths below the sea (Chart Datum) combining them is difficult. Indeed, it is only in the last few years that the Ordnance Survey and the Hydrographic Office have worked together to produce a map of the coastal zone.

The earliest charts were drawn to help navigation along the coast. Sailors needed to know where there were anchorages and reefs. The earliest charts have very few records of depths.

One of the earliest charts to show the Dorset coast in any detail was produced by Theodore de Brys and published in 1588 in Waghenaer's Marriner's Mirrour (a copy is held by the British Museum). The chart of “ THE SEA COASTES OF ENGLAND betweene the Isle of Wight & Dover with the principal havens thereof according to their situation and Appearing “ shows Poole, Studland and “Sandwich baye”. The chart shows a depth of 5 (presumably fathoms). The misspelling of the name is quite common on these charts and is often perpetuated from one chart to the next as they were copied or revised. Other errors were also often simply copied from one chart to the next until they were corrected.

The development of engraving on copper sheets from about the end of the fifteenth century meant that revisions to charts could be made comparatively easily. Although this did not prevent errors being repeated, it did allow revisions to be made without the whole map having to be redrawn.

Charts of the coast, like de Brys' chart, often include drawings of the coast as it would be seen from the sea. Two of the most important chart makers of the seventeenth century were John Seller and Captain Greenville Collins: examples of their charts are in the archive. Seller produced 66 charts in his English Pilot published in two parts in 1671 and 1672 and then revised and re-issued until 1792. Collins was appointed in 1681 to survey the British coast. His Great Britain's Coasting Pilot was first published in 1693 and re-issued regularly until 1785. Both provided the essential guide for navigators finding their way along the coastline and provide a unique view of the coastal landscape from the sea, unlike virtually every other description which is, of course, of the land.

As trade expanded, the identification of hazards became more important especially around ports and the charts focussed more and more on bathymetric (depth) information. For larger ships moving into ports that had previously been used for centuries by shallow draught vessels, depth information became essential. Depths were measured using a leadline lowered to the seabed whilst the position of the ship was fixed using a sextant or astrolabe. Today, echo sounders and satellites provide electronic depth and position information. All these methods pose questions about their precision in describing depth and location – their important common attribute is that provided the extent of potential error is accepted, they provide remarkably consistent means for navigators to know where their ship is and how much water is under its keel. Nevertheless, ships still run aground, even when submerged rocks are well known. Because traditional charts were based on spot or transect measurements, not all features are shown on charts. In shallow waters, this can mean that rock pinnacles exist which have never been mapped on to the charts – usually they are only recorded either when a boat hits them or nets are snagged by them. Modern technology using sonar allows a more comprehensive picture of the seabed to be produced, but such images are not widely available.

Charts show most detail around the ports where the risks of running aground were greatest (see for example, Mackenzie's charts of Weymouth Bay and Poole Harbour and a chart which shows more detail around Lyme Regis and Bridport). There is a continuous series of charts of Poole Harbour from the late eighteenth century, although earlier maps also show the main channels. Traditional chart-making methods hardly changed until the mid-twentieth century.

Chart of the West Dorset Coast (DCC).

Unfortunately, the charts tell us nothing about the life which is in the sea, and for many years it was often thought that the oceans were empty, like deserts (itself a misunderstanding). Even so, our exploration of the seas and oceans is still at an early stage and new information is being discovered continuously. This is partly because the technology for undersea investigation improved dramatically during the second half of the twentieth century. As has often been the case, the most rapid advances in underwater technology came because of military research and development, some of it in Dorset. However, the rapid adoption and refinement of the same technologies for civilian use have also given many more people access to the underwater world. The first, and in many respects most significant, advance came with the use of sound in the sea.

Undersea sound and its uses

Human activity in the sea makes sounds. The development of echo-sounders, sonar, aqualungs and more recently underwater cameras and remotely-operated vehicles (ROVs) have brought about a rapid and large increase in the information which is now available. Even so, interpretation depended upon a very small number of marine explorers and scientists who provided access to the submarine world mainly through the medium of film and television. As a result, our view of the ocean depths is predominantly visual. Military use of submarine sound is for communication and detection. Seabed mapping depends on sound.

SONAR (SOund NAvigation and Ranging) was developed during the Second World War as a means of detecting the presence and distance of vessels underwater. It was used passively (i.e simply as a listening device) and actively sending out signals to detect submerged objects. Much of the early research was carried out in Dorset, at Portland.

Why use sonar? More generally, why use sound? Simply because humans have very limited use of their senses under water. Light, which we use most commonly to detect features on land, provides us with very limited information over restricted ranges. We find it necessary to visualize some sounds in order to interpret them and to transfer the information from one individual to another. We can transmit sounds by copying them or we can shown them in a visual way that ensures replicability (Figure 1).

In addition to understanding the nature of sound in the sea, we can also use sound to describe the nature of the seabed. Since the Second World War, rapid advances have been made in the use of sound to map the seabed. In particular, echo sounders and side-scan sonar use sound reflected from the seabed to measure depths or to show the form of the seabed. Sound transmitted from sonar is reflected more or less strongly depending on the nature of the reflecting surface. As a result, it is possible to distinguish, for example, between sand ripples and boulder fields. Resolution can be good enough to detect the line between lobster pots. This information needs to be interpreted before it is used. Some images, such as ripples, are easily recognised because they are familiar. Others are much more difficult and require specialised training and experience.

Nature's sounds in the sea

Marine organisms have to cope with very limited light, and depend upon sound and touch. The living submarine world communicates by sound. Light transmission in water is very poor: the human visual range underwater is typically less than 30 m and visibility is often less than 5 m. In contrast, sound can travel very large distances, particularly at low frequencies. Marine mammals communicate and locate themselves sonically. Bottlenose (Tursiops truncatus) and Common Dolphins (Delphinus delphis and Harbour Porpoise (Phocoena phocoena) make clicks and whistles as they navigate and search for food.

In 1953, Cousteau described the sea as “a most silent world. “ (p.131). He went on to say that an

“undersea sound is so rare that one attaches great importance to it....…..save among the marine mammals – the sea is a silent jungle”.

Is the sea so silent? How can we describe its sounds and what do they mean?

Although marine organisms use sight as a means of close-up recognition, many respond to variations in pressure. Marine mammals communicate and locate themselves sonically, and there is now a substantial body of research in marine bioacoustics.

How do we use and interpret sound?

In common with many mammals, we use sound for communication, marking territory, location, warning, navigating, and hunting. We use sound both passively, i.e. listening and detecting sounds, and actively by making sounds to get some form of response, e.g. shouting or whistling. Although mammals generally use their bodies to make sounds, humans have developed many other ways to generate sounds, for example by using stringed or percussion instruments such as drums, or increasingly by using electronics.

Although we can both make and hear sounds in consistent ways, our interpretations vary immensely. The ways in which we interpret sounds are both technical and behavioural. For interpretation of marine sounds, we have first to be able to hear them. Many marine sounds are outside our hearing range and are difficult to detect without specialised equipment. When we have heard the sounds, we have to interpret for two audiences, the wider audience and ourselves. Typically we interpret unusual sounds by reference to sounds we already know, so that a common response is to say that the sound is like a known terrestrial sound. Sound is interpreted by attributing characteristics to them, e.g. ‘the rushing of steam escaping” or in western musical traditions by visualising them (Figure 1) because this allows us to have an agreed interpretation or repetition of the sound.

Figure 1 Visualisation of sound

However, even if we can visualize the sounds and find terrestrial analogues for them, we still have great difficulty explaining what they mean.

To take one example: for a dolphin, sound has very important functions. Dolphins use separate clicks to DETECT, i.e. asking, “Can I find anything out there?” Increasing the rate of clicks allows them to CLASSIFY and LOCATE, i.e. “What is it? Where is it?” This stimulates a response – ACT “Should I swim away? Can I eat it?” and the click rate will alter to continue the detect-locate-classify process.

This is comparatively straightforward to interpret and there are terrestrial equivalents such as bats that use sound in a similar way. However, when we detect the sounds that are often described as whale songs, usually continuous sounds that are likened to whistles or singing, we need to know a great deal about the behaviour that accompanies these sounds. Most work recognises them as the means by which groups of cetaceans communicate with each other. There are human analogues, such as the use of alpenhorns to send messages between locations in mountains which are not intervisible and whistle- or click-based languages.

Recording requires that boats are close to the sound sources because the high frequency sounds are detectable over only relatively short distances. Because the boats have to associate with the mammals, there may be some interaction reflected in the signals. Passive listening devices, such as hydrophones on the seabed, do not interfere with the mammals, but depend upon the presence of the mammals close to the listening device.

A hydrophone was sited in 12 m depth of water 400 m offshore in Durlston Bay on the Dorset coast in 1993. Underwater sounds are carried by cable to the visitor centre for future analysis and transmitted as part of an interpretative display on underwater acoustics. Marine sounds within frequencies 10 to 20 kHz are recorded but filtering can detect sources within an envelope of 200 kHz and enhance sounds so that humans can hear them. The hydrophone picks up the echolocation clicks as dolphins map out their environment, search for food and communicate with each other. Many other ambient sounds, from other marine animals, ships, seismic exploration, power boats and personal watercraft, pile driving, and even land based quarrying have also been recorded.

Two distinct sounds are associated with cetacean activity at Durlston. One is a comparatively slow click rate, usually coinciding with sightings just offshore of one or more Bottlenose Dolphins ( T. truncatus ). A second much faster click rate with high whistles is present most of the time and is probably produced by Common Dolphins ( D. delphis ) or Harbour Porpoise (P. phocoena). Typically, we also hear a continuous crackling, which may be snapping shrimps or mussels opening and closing their shells. Unidentified explosions may come from local quarries and naval sonars have been picked up from frigates operating over 20 km away. Since July 1994, high frequency fast repetition clicks have been recorded for about 80% of the time. High frequencies propagate less well in water so the frequent recording of these clicks indicates that the sources were generally within 500 m of the hydrophone. A dolphin sightings scheme, using volunteer observers, has highlighted the importance of the waters off Durlston for feeding and migrating Bottlenose Dolphins.

The geological underwater world

The fossils of the Dorset coast demonstrate very clearly that much of our present seabed and coast was formed in the sea. To quote from the Nomination document for the Dorset and East Devon Coast World Heritage Site:

“ The Dorset coast has been known since the early days of geology as providing one of the finest sections of marine Jurassic rocks anywhere in the World ”

The earliest publication describing the wealth of fossils at Lyme Regis was in 1673 by John Ray. It was, however, the marine reptile fossils, such as Ichthyosaurs and Plesiosaurs, including the first Ichthyosaur collected by Mary Anning during 1811 and 1812 and the first complete Plesiosaur ( Plesiosaurus dolichodeirus ) which she found in 1823, which captured the interest of the scientific community. The range and quality of preservation of the marine fossils found along the Dorset coast, which represent only a small fragment of the total population of the time, demonstrate the highly biodiverse underwater world which existed in the past. At the same time, the fossils drew attention to the enormous variety of species and the ecosystems in which they existed. Attempts to explain this became caught up in the nineteenth century debate about the origins of the earth and its development. The first published reconstruction of the ancient underwater world (“ Duria antiquior ” = “A more ancient Dorset”) was drawn by Sir Henry De La Beche in about 1830 and shows not only the range of species, but also links them visually in their food web. Although De La Beche did not use that term, he recognised the importance of the links between predator and prey and the production of by-products (in this case preserved as coprolites) (see Marine Ecology ).

Ammonites were common throughout the Jurassic seas. Of the 74 ammonite zones which have been recognised worldwide in the Jurassic, only three are missing from Dorset. Because of this, the Dorset coast is an international site for comparison with other ammonite–yielding locations worldwide. There are six repeated cycles of clay, sandstone and limestone which correspond to global deepenings of the contemporary sea-level and subsequent infill with sediment (World Heritage Site Nomination Document) between 199 and 146 million years ago.

The diversity of the fossils is greater than almost any other site in the world. The biodiversity of the Jurassic Dorset sea gave rise to the splendid range of marine fossils which include turtles, fish, corals and marine mammals. During the Cretaceous as well, the sea levels changed. The fossils which are found in the Purbeck Group and the Chalk provide evidence of deeper oceanic sedimentation as well as very shallow coast waters. Lagoons and estuarine environments are very well represented.

The underwater world of the Jurassic and Cretaceous was diverse and had members which were found worldwide or were only found locally. The same is true of the modern underwater world.

The beauty and diversity of the seabed

Rocky ledges, sand ripples, boulder fields, gravel banks and deep holes in the seabed reflect the way in which the sea has cut into the geological strata and transported sands, gravels and boulders across the seabed. For submarine plants, animals and fish, this is home: a complex, often rugged but sometimes almost featureless, landscape. Nothing living in the sea sees much more than its immediate surroundings, but this is a landscape in which there are complex patterns of behaviour (see Marine Ecology ) and where the beauty of the occupants' surroundings has only recently been displayed. Many of these species and habitats are rare in English waters. For example, Maerl beds off Handfast Point are formed by the rare coral algae Phymatolithion calcareum and Lithothamnion coralloides. Eelgrass also covers large areas of chalk seabed here. In Lyme Bay, the rocky ledges are home for the Pink Sea Fan (Eunicella verrucosa) , one of a number of Mediterranean-Atlantic species at their most easterly location in the English Channel. The Black-face Blenny (Trypterigion atlanticus) and Cranch's Spider Crab (Achaeus cranchii) are amongst the rare inhabitants of the underwater world at Kimmeridge.

The underwater areas off the Dorset coast fall into three main natural units, the westernmost being Lyme Bay which is separated by the Isle of Portland from Weymouth Bay. This in turn is separated by St Aldhelm's Head and the coast of south–east Purbeck from Poole and Christchurch Bays. They continue eastwards to the mouth of the western Solent at Hurst Castle and the Isle of Wight at the Needles.

The nature of the seabed in these three areas is affected first by the underlying geology and second by the sediments that cloak the bedrock. Thus, Lyme Bay is underlain mainly by rocks which continue the geological characteristics of the mainland. Weymouth Bay very largely reflects the reality that the Portland Stone outcrops both on the land from St Aldhelm's Head via Gad Cliff, Durdle Door and White Nothe whence it runs as the Ridgeway westwards and under the sea from St Aldhelm's Head to the Isle of Portland. Weymouth Bay is a great breached anticline (or dome of rocks) which has been eroded in the centre and widens westwards to Weymouth where the older rocks are exposed. Poole and Christchurch Bays are mainly floored by the Chalk and the younger rocks of the Tertiary. However, from Swanage to the Isle of Wight the older rocks of the Cretaceous, including the Greensand and the Wealden form the seabed. This area is more extensively cloaked by sand and gravel deposits than the bays to the west.

The planation of the strata west of St Aldhelm's Head cut across a wide range of structural features. This has produced a seabed which is marked by extensive areas of rock platforms and ledges and faces. This has a dramatic impact on the patterns of seabed species and communities.

The First and Second Dorset Underwater Surveys (Brachi et al 1976; Dixon et al 1979) established that in Weymouth Bay there were 10 main seabed communities. The Third Dorset Underwater Survey (1979) was carried out between Lyme Regis and Burton Bradstock in water which was less than 20 m deep (related to Chart Datum). Here six associations were identified of which four were common with Weymouth Bay, but two were added to the existing inventory. A more extensive survey of the eastern part of Lyme Bay undertaken as part of oil exploration preparatory studies (Cleator 1995) identified seven main communities subtidal benthic epifauna. The common feature of these surveys is that they show that the seabed communities are very strongly associated with the materials and mobility of the seabed.

More recently, ROXANNE surveys carried out by the Dorset Wildlife Trust provide another insight into the nature of the seabed habitats (see for example www.coastlink.org/seasearch/survey.htm ).

(based on Brachi et al 1976; Dixon et al 1979)

The later 1995 Lyme Bay survey of benthic epifauna grouped the horizontal surfaces and the vertical faces of the rock reefs together. Thus the Calliblepharis ciliata/ Phyllophiora crispa community associated with infra-littoral bedrock reefs has luxuriant and diverse red algae on the horizontal surfaces dominated by C. ciliata and P. crispa with large tubiferous polychaetes, the sponge H. simulous and an anthozoan ( A. mutabilis) with Tompot Blenny (Parablennius gattorugine) on the vertical faces. The West Bay Ledges (offshore bedrock and boulders) had some areas covered by loose sediments in around 20-25 m water with much of the ledges marked by diverse conspicuous species such as a bryozoan P. foliacea , the sponges I. ingallii and A. dissimilis and the seafan E. verrucosa. The vertical faces of the rock ledges had large colonies of P. johnstonia. A Ponifera/Tunicate/Hydroid community was associated with flat bedrock on which there were occasional boulders and gravel. Here the sponge fauna was very diverse and included two rare sponges A. fascicularis and D. pellescens .

The other communities were associated with a range of sediment types ranging from both unstable and mobile sand gravel and cobbles, as well as boulders within sand or gravel areas.

Until the last twenty years, very few people had seen this underwater world. Now more and more divers visit these areas to survey and visit the submarine seascapes. If you were to swim gradually from the shore to deeper water, you would travel through a landscape which changes as the geology and the depth change. So, near to the shore in shallow water, the Kelps, the largest plants of the sea, anchor to the stable rocks. These long-stalked brown seaweeds, which include Sugar Kelp (Laminaria saccharina) and Oarweed (Laminaria digitata), provide food for species that feed on their fronds (e.g. Blue Rayed Limpets Patina pellucida) and shelter for many fish (e.g. Rainbow Wrasse Coris julis). However, many fish (for example, Bass Dicentrarchus labrax and John Dory Zeus faber) also move between these nutrient rich shallow waters and deeper water.

Where shallow rock ledges slope gradually from the shore into deeper water until they reach about 15 m depth, you would see that they are covered by algae and filter-feeding animals such as hydroids, sea squirts and sponges. In this deeper water, colonies of simple animals such as Sponges, Soft Corals and Sea Firs (flower-like colonies of animals related to jellyfish) occupy the sea bed. Sandy sea beds provide a habitat for burrowing bivalves and camouflage for flat fish such as Turbot (Scophthalmus maximus), Plaice (Pleuronectes platesse) and Flounder (Platichtys flesus) and are home to Dragonet (Callionymus lyra). Crustaceans, including prawns, edible crabs, and lobsters, and many fish species including Ballan Wrasse (Labrus bergylta), Cuckoo Wrasse (Labrus mixtus), Whiting (Trisopterus luscus : also known as Pout or Bib), Pollack (Pollachius pollachius) and the Greater Pipefish (Syngnathus acus) live within and around the ledges. The food web and predator-prey interactions described in Marine Ecology are very obvious here. Shoals of sand eels attract predators such as Bass and Cuttlefish (Sepia officinalis), while Hermit Crabs (Eupagurus bernhardus) feed on the remnants of the predators' meals.

As you swim out to the deeper reefs in depths between 15 and 30 metres, you will see some of the fish which were on the shallower reefs (Ballan and Cuckoo Wrasse, Whiting, Pollack and Bass for example), but now they are joined by others. Fish around deeper reefs include Goldsinney Wrasse (Centrolabrus rupestris), Corkwing Wrasse (Crenilabrus melops), Dogfish (Scyliorhinus canicula) and Conger Eel (Conger conger). Beyond about 15 metres in depth, there is insufficient light for algae to grow, but the ledges are covered by carpets of filter-feeders such as hydroids, sea squirts, sponges and anemones. Large colonies of Ross Coral (Pentapora foliacea) are common on rocky ground. Dead Men's Fingers (Alcyonium digitatum) and Pink Sea Fan (Eunicella verrucosa) are "soft corals" common to these reefs. The tiny Devonshire Cup Coral (Caryophyllia smithii) is a solitary coral found throughout this coast, but the Sunset Star Coral (Leptopsammia pruvoti) is a much rarer species found only in a few other sites in the UK.

The Pink Sea Fan forms large colonies which branch profusely. Normally salmon pink, it also occurs as a white form. It attaches itself to rock ledges and boulders and is mainly found off Dorset in depths below 15 m. The largest specimens can be 300 mm tall and 400 mm across. Because it grows very slowly here, about 10 mm per year, it is seriously endangered by any collection or damage.

Please do not collect sea fans and take care not to damage them when diving.

The Pink Sea Fan ( E. verrucosa ) is protected under the Wildlife and Conservation Act 1981: Schedule 5 (Animals) 1992 which specifies that it is illegal to kill, injure or take (Section 9 (1)), possess (S.9 (2)) or sell Pink Sea Fans (S9 (5)).

For Pink Sea Fan information see also

www.coastlink.org/seasearch/Pink seafan survey.htm

Submarine conservation is much less well developed than conservation of fauna and flora on land. There are, however, a growing number of regulations which are intended to protect the rarer and more fragile species. There are three non-statutory Sensitive Marine Areas here (Table 2).