Habitat point records from 1987 OPRU HRE Portland and Weymouth Harbours survey

Portland and Weymouth harbours are situated on the south coast of Dorset to the north of the isle of Portland (or Portland Bill). Portland harbour encloses a large area of seabed and is sheltered in all weather by the Portland peninsula and Chesil beach to the south and west and a series of breakwaters to the east. The entrance to the Fleet lagoon lies within the harbour on the north-west side. The bottom of the harbour is 10 to 17m below chart datum over most of its area, but slopes gradually up to intertidal sediments on the west side. Temperatures in Portland harbour become significantly elevated during the summer and freshwater input is minimal. Muddy sediments cover most of the bottom, but the artificial breakwaters are formed of limestone block tumbles with abundant crevices and interstices. Rocky outcrops, boulders, cobbles and pebbles on the bottom of Portland harbour increase the habitat diversity. Weymouth harbour is small and narrow and lies at the mouth of the River Wey, less than 1km north of Portland Harbour, with a north-east facing entrance. It is dredged to 5 to 7m and has no intertidal areas except for vertical walls. Freshwater inputs are also small and the water is, for the most part, fully marine. The breakwater is made of concrete walls and pilings, and there is little other hard substrata. The bottom is covered in muddy sediments. Portland harbour is a navel base and a certain amount of research and other Navel operations are carried out within designated areas. Commercial operations are few, but the fish quay at Castletown pier is thriving ahd there are some small fish-farming operations. There are recent proposals to increase the scale of salmon farming in the harbour. The harbour is popular for water sports, particulary sailboarders, and there are some small boat yards and sailing clubs. It is also a popular diving site. While most marine biological studies have concentrated on the Fleet lagoon, there is a long history of specimen collecting and other studies in the harbour. The Fleet study group have surveyed stations in the harbour and have compiled records of species found there. Weymouth harbour is more commercially orientated with a cross-channel ferry port and a Ro-Ro terminal. It is also used by commercial and sports fishermen and there are a large number of moorings in the Backwater. There are a number of wharves and three boatyards. There have been no known previous marine biological studies in Weymouth harbour. Twelve diving sites were surveyed during this survey, nine from within Portland harbour, one outside the harbour and two in the entrance to Weymouth harbour. In addition, grab samples of subtidal sediments from four sites in Portland harbour, sampled by Wessex Water Authority, werte analysed and the results are included in this report. Sixteen subtidal habitats and associated communities have been described from these sites. Descriptions are given of these and lists of species found are tabulated. The findings of other workers are incorporated into the discussion and conservation assessment. The conditions of sheltered, warm, clear, fully marine water in Portland harbour are unique to Britain and have allowed a large number of warm water species to live there. These include the Black-faced blenny Tripterygion atlanticus, the Red-band fish Cepola rubescens, the burrowing anemone Scolanthus callimorphus, the nudibranch mollusc Aeolidiella alderi and the polycheate worm Sternapsis scutata. The habitats and communities in the harbour are otherwise similar to those found in other inlets, but the bottom sediments contain very rich communities of infaunal animals, there are interesting communities of sessile fauna and flora on the vertical sides of the limestone blocks of the breakwater and the subtidal communities in the entrance to the Fleet are also fairly rich. The mudstone platform habitat surveyed outside the harbour was colonised with diverse assemblages of sponges, hydroids and ascidians which are similar to those recorded by the Dorset Underwater Survey. The communities recorded in the entrance to Weymouth Harbour included some fairly rich algal communities in shallow water on the pier piles, but the muddy sediments were poorly colonised. The scientific interest and conservation importance of the area has been assessed using standard criteria and the conservation importance of habitats and communities in the area have been provisionally graded as of Local, Regional or International importance. Species of particular scientific interest have been tabulated and there conservation importance provisionally graded as of Regional or National importance. Portland harbour is considered to be of high scientific and conservation importance. Overall diversity of species is high and there are a large number of species of conservation interest. Weymouth harbour is not considered to be of any conservation importance. Records currently considered sensitive have been removed from this dataset.