The Fleet, on the western coast of Dorset, is the largest lagoon in the British Isles. 14 km in length, it communicates with marine waters only through a single, artificially narrowed entrance. During August of 1983, the subtidal bed of the Fleet was surveyed in order to determine the range of substrates and biological communities present. The area covered was the down-stream-most section of the lagoon, which experiences moderate to strong tidal currents and flushing, but very weak wave exposure. Coarse sediments and exposures of bedrock were identified, and were found, under these favourable hydrological conditions, to be colonized by biological communities with a high content of species (Dyrynda, 1984). The survey preceded major construction work, undertaken during 1984/85 and necessitated by the deterioration of the Weymouth-Portland road bridge spanning the lagoon entrance. The programme aimed to maintain the road link and also tidal communication at all times, and avoid any long term effects on the hydrography of the lagoon. The work entailed the construction of a new channel and bridge within what was a solid embankment, and the simultaneous closure and infilling of the old channel, 200 m to the north. This report provides a brief update on the 1983 subtidal survey in the light of the development work. Three sampling stations within the locality of the lagoon entrance have been re-evaluated according to visual assessments made prior to, at the time of, and 18 months after the entrance was transferred. The locations are, i) the former channel, now part infilled and in part a small embayment, ii) the new entrance channel, and, iii) a station within the lagoon, upstream of the two entrances. Comparison of the results obtained with original baseline information allows for the identification of changes in substrates and benthos. Some local changes were identified. The significance of these is discussed, with the conclusion that whereas a proportion are inevitable and expected, others cannot at present be gauged in importance since they may be transitory, or even, in part, the result of other processes, e.g. the recent large scale build up of Sargassum within the outer Fleet. Whether or not longer term changes have occurred or are developing further afield within the lagoon or Portland Harbour, can only be answered by studies undertaken on an adequately wide scale and after a sufficient time interval.
