Area: 2,439.2 hectares.
Soil and geology:
Acidic, sand, mud, clay, peat, sedimentary.
Geomorphology and landscape:
Lowland, island, coastal, valley, floodplain, sub-tidal sediments (including sandbank/mudbank), intertidal sediments (including sandflat/mudflat), estuary, lagoon, cliffs:
Tidal flats 55%
Freshwater marshes / pools: permanent 23%
Salt marshes 21%
Seasonally flooded agricultural land 0.5%
Peatlands (including peat bogs swamps, fens) 0.5%
Poole Harbour is a bar-built estuary covering an area of nearly 4,000 hectares. The Harbour occupies a shallow depression in the acidic, tertiary deposits towards the south-western extremity of the Hampshire Basin and has been formed over the last 5,000 years by a rise in sea level. The unusual micro-tidal regime means that a significant body of water is retained throughout the tidal cycle. The site therefore exhibits many of the characteristics of a lagoon. There are extensive intertidal mudflats supporting internationally important numbers of waterfowl in winter. These are fringed on the landward side by saltmarshes or reedbeds. The river valleys of the lower Frome and Piddle support grazing marsh which is also important for wintering waterfowl. Much of the catchment along the western and southern shores comprises the internationally important Dorset heathlands and there are unusual transitions from saltmarsh to valley mire. The Harbour is separated from Poole Bay by the internationally important Studland dunes and the site includes Littlesea, a large dune slack lake also important for wintering wildfowl.
Sub-tidal sediments are mainly fine muds and sands with hard substrate occurring locally in the vicinity of the channels which link the series of basins which make up the Harbour. Associated with the sub-tidal sands of the central Harbour are species rich communities dominated by beds of the tube-worm Sabella pavonina. Intertidal areas are again largely fine grain muds although coarser sediments occur in the north-east of the Harbour. Much of the middle and lower saltmarsh is dominated by common cordgrass Spartina anglica. Some retreat of the extent of this plant is now occurring in the Harbour. Smaller areas of more species rich upper saltmarsh also occur including areas dominated by sea-blite Suaeda vera. Both tidal, brackish reedbed and fresh water reedbed occur around the fringes of the Harbour. Brackish grazing marshes dominated by creeping bent Agrostis stolonifera occur at Keysworth and in the lower Frome and Piddle valleys there is freshwater grazing marsh. The marshes north of the River Piddle have particularly complex vegetation being influenced both by the nutrient-poor acidic water originating in the valley mire at Morden and the nutrient-rich water of the River Piddle. Low sandy cliffs and slopes occur at the edge of Poole Harbour, and the heathland beyond is included in the Dorset Heathland Ramsar site. The large mesotrophic dune slack lake called Littlesea, on the Studland peninsula, is included in both the heathland Ramsar site and Poole Harbour Ramsar site for different interests.
Ramsar criterion
1. The site is the best and largest example of a bar-built estuary with lagoonal characteristics (a natural harbour) in Britain.
2. The site supports two species of nationally rare plant and one nationally rare alga. There are at least three British Red data book invertebrate species.
3. The site includes examples of natural habitat types of community interest - Mediterranean and thermo Atlantic halophilous scrubs, in this case dominated by Suaeda vera, as well as calcareous fens with Cladium mariscus. Transitions from saltmarsh through to peatland mires are of exceptional conservation importance as few such examples remain in Britain.
The site supports nationally important populations of breeding waterfowl including common tern, Sterna hirundo and Mediterranean gull Larus melanocephalus. Over winter the site also supports a nationally important population of avocet Recurvirostra avosetta.
5. Assemblages of international importance:
Species with peak counts in winter:
24,709 waterfowl (5 year peak mean 1998/99-2002/2003).
6. species/populations occurring at levels of international importance.
Qualifying Species/populations (as identified at designation):
Species with peak counts in winter:
Common shelduck, Tadorna tadorna, NW Europe 2,120 individuals, representing an average of 2.7% of the GB population (5 year peak mean 1998/9-2002/3).
Black-tailed godwit, Limosa limosa islandica, Iceland/W Europe 1724 individuals, representing an average of 4.9% of the population (5 year peak mean 1998/9-2002/3).
Species/populations identified subsequent to designation for possible future consideration under criterion 6.
Species with peak counts in winter:
Pied avocet, Recurvirostra avosetta, 1260 individuals, representing an average of Europe/Northwest Africa 1.7% of the population (5 year peak mean 1998/9-2002/3).
Noteworthy Flora
Higher Plants.
Scorzonera humilis, Potamogeton acutifolius, Alopecurus bulbosus, Oenanthe silaifolia, Myosurus minimus, Suaeda vera, Zostera angustifolia, Zostera noltei, Isoetes echinospora and Elatine hexandra.
Birds
Species currently occurring at levels of national importance:
Species regularly supported during the breeding season:
Mediterranean gull , Larus melanocephalus, Europe 5 apparently occupied nests, representing an average of 4.6% of the GB population (Seabird 2000 Census)
Black-headed gull, Larus ridibundus, N and C Europe 1700 apparently occupied nests, representing an average of 1.3% of the GB population (Seabird 2000 Census)
Common tern, Sterna hirundo hirundo, N and E Europe 191 apparently occupied nests, representing an average of 1.8% of the GB population (Seabird 2000 Census)
Species with peak counts in spring/autumn:
Great cormorant, Phalacrocorax carbo carbo, NW Europe 402 individuals, representing an average of 1.7% of the GB population (5 year peak mean 1998/9- 2002/3)
Little egret, Egretta garzetta, West Mediterranean 91 individuals, representing an average of 5.5% of the GB population (5 year peak mean 1998/9- 2002/3)
Spotted redshank, Tringa erythropus, Europe/W Africa 5 individuals, representing an average of 3.6% of the GB population (5 year peak mean 1998/9- 2002/3)
Common greenshank, Tringa nebularia, Europe/W Africa 31 individuals, representing an average of 5.1% of the GB population (5 year peak mean 1998/9- 2002/3)
Species with peak counts in winter:
Black-necked grebe, Podiceps nigricollis nigricollis, Europe, N Africa 3 individuals, representing an average of 2.5% of the GB population (5 year peak mean 1998/9- 2002/3)
Dark-bellied brent goose, Branta bernicla bernicla, 1453 individuals, representing an average of 1.4% of the GB population (5 year peak mean 1998/9- 2002/3)
Northern pintail, Anas acuta, NW 308 individuals, representing an average of 1.1% of the GB population (5 year peak mean 1998/9- 2002/3)
Europe Red-breasted merganser, Mergus serrator, NW and C Europe 401 individuals, representing an average of 4% of the GB population (5 year peak mean 1998/9- 2002/3)
Water rail, Rallus aquaticus, Europe 12 individuals, representing an average of 2.6% of the GB population (5 year peak mean 1998/9- 2002/3)
Dunlin, Calidris alpina alpina, W Siberia/W Europe 6323 individuals, representing an average of 1.1% of the GB population (5 year peak mean 1998/9- 2002/3)
Eurasian curlew, Numenius arquata arquata, Europe 1570 individuals, representing an average of 1% of the GB population (5 year peak mean 1998/9- 2002/3)
(breeding)
Species Information
Invertebrates.
Saldula setulosa, Piesma quadratum and Limonia bezzii. The nationally rare sponge Suberites massa also occurs here.