Habitat point records from 1985 OPRU HRE Exe Estuary survey

The Exe estuary is situated in southeast Devon and opens into the western side of Lyme Bay in the English Channel. The estuary is approximately 10km in length and between 1 and 2 km in width at the high water mark. Most of this area dries at low water leaving a channel of less than 500m at its widest point. The estuary is sheltered from prevailing winds and wave action to a large extent by Dawlish Warren, an unusual double spit which has been formed across the mouth of the estuary. This feature, together with an area of shoaling sand called Pole Sand, restricts the sea opening of the estuary to a long narrow channel. Currents are rapid as a consequence of river and tidal flow being restricted to a narrow channel for much of the tidal cycle. In addition, salinity is reduced over most of the estuary, particularly at low water after significant rainfall. On these occasions, the water at Topsham may almost be fresh. Benthic habitats are sedimentary in nature with extensive mud and sand flats intertidally. Patches of saltmarsh and swards of Zostera spp. occur in the muddier reaches. There are no rocky shores within the estuary and what little alternative hard substrata there is consist of stones and small boulders concentrated around the high water mark. Stone-faced railway embankments down either side of the estuary occasionally fall within the upper intertidal zone. Subtidally, habitats are also predominantly sedimentary, ranging from sand, stones and boulders at the mouth to mud at the head. Outcrops of flat bedrock occur in the narrow channel entrance. Outside the main estuary basin, rocky shores occur east of Exmouth at Maer Rocks, and at Orcombe point and beyond. There is little commercial fishing within the estuary, apart from netting for salmon, trout and eels. Shellfish, mainly oysters but also including some mussels and manilla clams, are also grown-on at sites within the estuary. The area is used intensively for recreation, including most watersports, and the sandy beaches on the open coast are popular with tourist. In addition many visitors are attracted specifically by the overwintering populations of waders and wildfowl for which the Exe is of international importance. The area has long been of scientific interest, both biologically and physiographically, and is currently used for both educational and research purposes.The geological, ornithological and marine biological importance of the estuary is well recognised and is detailed in the notification of both Dawlish Warren, and the rest of the estuary, as SSSI,s. In addition, Dawlish Warren is a Local Nature Reserve and the western half of the estuary from Dawlish Warren up to Turf has been designated a wild bird sanctuary. The present survey, carried out in August 1985, aimed to collect information on the variety of intertidal and subtidal habitats and communities present. The last descriptive marine biological survey covering the whole estuary took place in 1901. In order to determine what changes, if any, had taken place since that time, many of the old sample sites were re-visited. A total of 47 sites were sampled, of which 34 were intertidal and 13 were subtidal. At each intertidal site the abundance of epiflora, epifauna and conspicuous infaunal species was recorded in situ. Sediment cores were taken and sieved over a 0.1mm mesh to obtain samples of the macrofauna. Subtidal areas were sampled using a combination of trawling, pipe-dredging and diving. During the survey, a total of five intertidal and four subtidal habitats were distinguished. Epifaunal and algal diversity was generally low due to the scarcity of stable hard substrata. However the infauna was richer and was typically estuarine in nature with dense populations of species such as Nereis diversicolor, Scrobicularia plana and Lanice conchilega to be found locally. Diversity tended to be higher in the heterogenous muddy sand areas of the middle and lower reaches of the estuary. Further north, sediments were of uniform mud and were influenced by reduced or fluctuating salinities. Around the estuary mouth, faunal populations in the tidally swept clean sands were often comparitively sparse, but notable for including the polychaete Ophelia bicornis which has attracted much scientific study since the 1940's. The main characteristics of the communities found do not appear to have changed significantly since the turn of the century. However, in contrast to previous records, neither Nucella lapilus or Sabellaria alveolata were found during the present survey. The former species appears to have declined dramatically in southwest Britain recently, and organotin compounds in antifouling paint are suspected as a possible cause. The latter species has been shown to fluctuate widely in abundance at any one site, and it is possible that trampling pressure plays a part in such variation. Alien species are also present including Sargassum muticum on rocky shores just outside the main estuary. The scientific interest and nature conservation importance of the area has been assessed using standard criteria and the conservation importance of the habitat and communities in the area have been provisionally graded as of Local, Regional, National and International importance. Species of particular scientific interest identified during survey work have also been noted and their conservation importance provisionally graded as of Regional, National or International importance. The Exe estuary is considered to be of high scientific interest for its populations of Ophelia bicornis and because it supports infaunal communities that are representative of their type in southwest Britain. This is in addition to the well recognised importance of the area for estuarine birds. Furthermore, such scientific study has been, and still is, carried out here alongside its continuing use for educational purposes. Potential threats to these interest are all associated with the recreational usage of the estuary which is already high. These include the proposals to build marina facilities, the effects of organotin antifouling compounds and bait digging. Records currently considered sensitive have been removed from this dataset.

Organization: Joint Nature Conservation Committee