General features

Habitat, communities and species

Conservation objectives

General features

The Fleet lagoon (see linked figure) lies between Chesil Beach, a large shingle barrier, and the impounded mainland Dorset shore, just west of Weymouth and the Isle of Portland. It is a natural lagoonal inlet with features of a percolation lagoon. The channel at Smallmouth connects the lagoon with Portland Harbour and the sea. The English Channel in this area is microtidal, with a tidal range of only approximately 1.5 m at spring tides. The Fleet has been described as the finest example of a lagoon of its type within the British Isles (Barnes, 1989), and is also one of the largest (480 ha) and best studied B hence its inclusion here as the case study. It is largely natural, with a predominantly rural catchment and adjacent shoreline, partly because the lagoon bed and the majority of the shore has been owned and managed privately by the Ilchester Estate for over 400 years.

The Fleet lagoon has evolved over the last 5000 years by impoundment of marine waters behind the shingle storm ridge of Chesil Beach. Originally, the lagoon was probably longer and wider than it is today. Its shape reflects the irregularities of the impounded mainland shore, with the broadest reaches (approx. 900 m wide) at Littlesea, and the shingle beach almost in contact with the mainland shore at the Narrows (minimum 65 m wide), where strong tidal currents erode back the shingle beach and prevent closure of the lagoon from the sea (Dyrynda 1997).

The shape and tidal regime of the Fleet result in two ecologically distinct zones within the lagoon (see linked figure):

the narrow inlet channel, with strong tidal currents, mostly fully marine salinity, good flushing characteristics and coarse sediments; and

the lagoonal basin, with much weaker currents, lower and more variable salinity, considerably reduced seawater flushing and fine sediments (Dyrynda 1997).

The inlet channel extends from Smallmouth to the head of the Narrows. Tidal rapids are accompanied by coarse sediments and boulders, with slacker currents and finer sediments within the intervening section. The coarser sections of the channel bed support unusual and diverse assemblages of algae and sedentary invertebrates, particularly within the Narrows. A variety of unusual species are common here (eg the red alga Gracilaria bursa-pastoris and the sponge Suberites massa). Two southern species present in exceptional abundance are the snakelocks anemone Anemonia viridis which occurs in vast numbers on shingle and cobbles, and the sea squirt Phallusia mamillata which is abundant on stable bedrock and man-made structures. The southern black faced blenny Tripterygion delaisi occurs both at Ferrybridge and in the Narrows alongside a profusion of other small fish and crustaceans. Tracts of fine sand and mud are uncovered at low tide between Smallmouth and the Narrows. These sediment flats are not colonised by seagrasses, but resemble low-energy estuarine flats, with an abundance of the lugworm Arenicola marina and with algal mats in summer. Rarities found within this area include the specialist lagoonal polychaete worm Armandia cirrhosa (Dyrynda 1997).

Upstream of the Narrows the lagoonal basin is divisible into three sections according to physical and biological factors (see linked figure):

1. Littlesea is the broadest and outermost section of the lagoonal basin, and is a transitional zone. Extensive platforms of seagrass-stabilised mud are dissected by relatively deep and steep-sided subtidal channels within which tidal currents are typically strong. Coarser sediments occur at the downstream end of Littlesea towards the Narrows. Towards the upstream end of Littlesea a declining tidal influence is reflected in the attenuation and ultimate phasing out of the low water channels and a transition to soft mud (Dyrynda 1997).
2. Upstream of Littlesea from Moonfleet to Clouds Hill (west of Rodden Hive) the bed of the lagoon is level and shallow and dominated by deep soft organic muds mainly colonised by subtidal seagrass meadows. The shores in this section of the lagoon are very narrow due to much reduced tidal range. Moving westwards through the middle lagoonal basin seagrass Zostera marina is increasingly replaced by tasselweed Ruppia cirrhosa. Zostera noltii and Ruppia maritima occur in many parts of the lagoonal basin, and in some areas (eg close to Langton Hive Point) all four species occur together. Zostera angustifolia has been recorded in the Fleet as a separate species in the past, but it is now generally regarded as a narrow leaved form of the species Zostera marina (Davison and Hughes 1998). The rare foxtail stonewort Lamprothamnium papulosum is common towards the mainland shores of the lagoon (Dyrynda 1997).
3. The Abbotsbury embayment forms the blind head of the lagoon, which is significantly different from the main sections of the lagoonal basin. The embayment is floored by soft organic mud, but the seagrass stands are thin and patchy. The green alga Chaetomorpha linum is common and in summer can be accompanied by tracts of the green sea lettuce Ulva lactuca. Much of the Abbotsbury embayment is permanently submerged, but the mud beds are partly exposed on rare occasions (Dyrynda 1997). Phragmites marsh is extensive along the mainland shore (where the Abbotsbury Swannery is located).

The coverage of vegetation on 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 debris (Dyrynda 1997).

A variety of lagoonal invertebrates, including some rarities, occurs within the lagoonal basin. Recent work indicates that some invertebrates are zoned in abundance both along and across the lagoon. In all areas a near-shore gradation of decreasing vegetation cover and invertebrate numbers was identified. The permanently submerged central areas supported the highest densities of vegetation and invertebrates. The fauna was found to be a mixture of both common and rare lagoonal specialists, and brackish species also found in estuaries. Well known lagoonal specialists that are widespread include the lagoonal cockle Cerastoderma glaucum, the crustacean Idotea chelipes and the gastropod molluscs Hydrobia ventrosa, Rissoa membranacea and Littorina saxatilis. A dwarf variety of the nudibranch mollusc Akera bullata (var. nana) is also common. Most of the common species were found to peak within specific areas of the lagoonal basin. Other lagoonal specialists were much more localised, e.g. the starlet anemone Nematostella vectensis and the crustacean Gammarus insensibilis, both of which were found to be centred within the upper, more brackish reaches of the basin adjacent to Clouds Hill. Generalist brackish species which are abundant in the Fleet, but which also occur within estuaries, include the polychaete worm Scoloplos armiger and the nemertine Lineus viridis (Dyrynda 1997).

The seagrass meadows 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 (Dyrynda 1997).

Plankton communities are a little understood element of the lagoon system. The water is characteristically 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. Little is known of zooplankton communities within the lagoon, except that mysids are very common and are likely to feature in the diet of many small fish such as juvenile bass and pipefish (Dyrynda 1997).

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 Swannery since the 1300s. The birds nest within the Abbotsbury embayment in spring and early summer and otherwise forage along the length of the lagoonal basin consuming seagrasses or algae according to seasonal availability. Other common herbivorous grazers include wigeon, pochard, brent geese and coot, all of which are winter visitors. All of these birds feed on both seagrasses and green algae, depending on seasonal availability. In summer little terns (from the breeding colony on Chesil Beach) can be seen diving for small fish, and other common fish eaters include cormorants and mergansers (Dyrynda 1997).

Statutory advice on the Chesil and the Fleet European marine site includes a conservation objective for the lagoon feature (English Nature 1999). The role of conservation objectives is to set out what needs to be achieved to deliver the aims of the Habitats Directive in relation to the site. As such, they are the starting point from which a management scheme and a monitoring programme for the site are to be developed as they provide the basis for determining what is likely to cause a significant effect. The conservation objective for the lagoon feature is:

Subject to natural change, maintain the lagoon in favourable condition, in particular:

Seagrass bed communities

Tide-swept communities

Subtidal coarse sediment (gravel, cobble, pebble) communities

Intertidal sediment communities

Shingle spring line communities

Details of how to recognise favourable condition are provided in an Annex.

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