Dyrynda, P.E.J. 1997 Seasonal monitoring of the Fleet Lagoon aquatic ecosystem (Dorset, UK): 1995-1996.

Mentions two references not yet in FSG archive: Ladle & Young (in press) The Fleet lagoon and Chesil beach: Proceedings of the 3rd Fleet Symposium. Dorset County Council, Dorchester, and Bamber 1996 An assessment of saline lagoons within Special Areas of Conservation. English Nature research report No. 235 by Fawley Aquatic Research Laboratories Ltd.

Aim of seasonal monitoring study has been to generate information capable of distinguishing normal seasonal and year-to-year temporal variations from unnatural adverse trends caused by human activities. Monitoring strategy aims to record natural and man-induced changes to aquatic vegetation, invertebrates and fish within one section of the aquatic ecosystem (in a transect across the lagoon from Langton Hive Point).

Divides lagoon into two ecologically and physically different areas: the lagoonal basin, with weak currents and fine sediments, and the inlet channel, with strong tides and coarse sediments - both areas being very sheltered from wave action.

Lagoonal basin also divided into three sections: Littlesea - the broadest, outermost section characterised by fine sediments with seagrass beds intersected by deep fast flowing subtidal channels; from Moonfleet to Clouds Hill is a section where the lagoonal bed is level and shallow, dominated by deep, soft organic muds mainly colonised by seagrass meadows. Zostera marina is replaced upstream by Ruppia cirrhosa and Lamprothamnium papulosum is common towards the mainland shores; the Abbotsbury embayment forms the blind head of the lagoon. Although the embayment is floored by soft organic muds, the seagrass stands are thin and patchy. The green alga Chaetomorpha linum is common, and in summer can be accompanied by tracts of sea lettuce Ulva lactuca. Two small streams discharge into the embayment. Phragmites marsh is extensive along the mainland shore. The coverage of vegetation upon the bed of the lagoon is strongly seasonal. Seagrasses grow from late spring to autumn, accompanied by swards of green algae through to mid summer. During autumn, winter and early spring much of the lagoon bed features bare mud and plant detritus. The permanently submerged central areas supported the highest densities of vegetation and invertebrates.

The seagrass and algal meadows of the lagoonal basin are in summer frequented by adult grey mullet and eels, juvenile bass and by non-economic species such as sand smelt, 3-spined sticklebacks, deep-snouted pipefish and mud gobies. A variety of waterfowl and other aquatic birds feed upon vegetation, invertebrates and fish within the lagoonal basin. The most conspicuous herbivorous bird is the mute swan - a unique herd has been farmed at Abbotsbury since the 1300=s. The herd currently stands at about 700 birds. The swans are fed, at a point within the recesses of the Abbotsbury embayment, grain and high protein pelleted feed, along with seagrass material gathered from the Fleet strandline. Winter visitors include widgeon, pochard, brent goose and coot. Plankton communities of the system are little known. The water is clear from spring to autumn, but is temporarily discoloured by intense green blooms in spring, and by short-lived but often intense red/brown dinoflagellate blooms within the Abbotsbury embayment in summer (John, E. 1995, J. Jamieson, EA pers. comm.). There is a suggestion that anthropogenic eutrophication may be exacerbating these blooms (EA, 1997). Little is known of zooplankton within the lagoon, except that mysids are very common.

An oyster purification plant was constructed on the mainland shore upstream of Smallmouth during the 1980=s. Here, and in Littlesea immediately upstream of the Narrows, oysters (Crassostrea gigas) commenced in 1987 on a large scale. Due to concern as to whether this species could viably reproduce in UK water temperatures, seawater temperatures in the Fleet were monitored and searches made for settled spat. To date, spat searches have returned a negative result. Accumulation of organic waste under the oyster racks was another concern, but an impact study (Collins & Byfield in press, Monitoring oyster farming in the Fleet. In Ladle & Young, in press) detected no evidence of an adverse effect beneath oyster racks in the Fleet.

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