Spartina Salt Marshes in Southern England: IV. The Physiography of Poole Harbour, Dorset

Abstract

Poole Habour is a tidal estuary formed by recent marine submergence, with sand spits bordering an entrance maintained by tidal currents. Its shoreline configuration has been modified by wave action at high tide, the greatest erosion being on shores exposed to prevailing south-westerly winds, but inter-tidal topography is shaped partly by currents and partly by the sediment-trapping effects of salt marsh vegetation. The pattern of tidal channels has changed little since 1785, but intervening areas have been built up by accretion of sediment carried into the harbor by rivers. The rate of accretion increased after the harbor was colonized by Spartina townsendii in 1899, there being stratigraphic evidence of up to 1 meter accretion on Spartina marshland. Phragmites communis has colonized built-up Spartina marshland on areas of diminished salinity, and the two types of salt marsh now occupy 36% of the inter-tidal zone. Erosion of the margins of Spartina marshland follows die-back of this plant in the seaward limits of the harbor, possibly a physiological response to developing substrate conditions, perhaps associated with a contemporary rise in sea level.