THE INTERTIDAL INVERTEBRATE FAUNA OF THE ORWELL ESTUARY C . H . BEARDALL, S . M . GOOCH a n d R . PILCHER In 1973 the Institute of Terrestrial Ecology ( I T E ) surveyed the intertidal macro-invertebrate fauna of the south east coast of England as part of a wider study to assess the environmental impact of the proposed development of London's third airport at Maplin Sands ( I T E , 1974; Kay & Knights, 1975; B o o r m a n & Ranwell. 1977). The study embraced fourteen sites ranging from the estuarine areas surrounding the Isle of Sheppey in the south, to the Orwell estuary in the north. The present study was undertaken by the Suffolk Wildlife Trust in September and October of 1986, to assess the distribution and abundance of the intertidal macro-invertebrate fauna of the Orwell estuary and identify any changes in these communities since 1973. DĂźring the last Century the Orwell estuary has come under increasing pressure from both industrial and recreational activities, many ofwhich have the potential to substantially alter the physical and chemical characteristics of the estuary. The impact such changes have had on the invertebrate fauna is unknown. The ornithological interest of the Orwell estuary has been well documented over a n u m b e r o f y e a r s (Davidson & E v a n s , 1985; Ravenscroft, 1987; Beardall, Dryden and Holzer, 1988) and the national and international importance of this area recognised by its designation as a Site of Special Scientific Interest in 1985. T h e conservation of the ornithological importance of this site will depend upon careful management and monitoring of all aspects of the ecosystem and especially those communities at the bottom of the food chain upon which the system relies. This study forms part of a wider project researching all aspects of estuarine ecology in Suffolk undertaken by the Suffolk Wildlife Trust Estuaries Project. Site Description The tidal Channel of the Orwell estuary extends some 18km from Constantine Weir in Ipswich to Felixstowe. The intertidal zone of the upper reaches have been extensively developed to create Ipswich D o c k . The total intertidal mudflat Covers 713ha with the largest expanses found in the mid-reaches on the north shore. Whilst the catchment area of the river Gipping flowing into the estuary is 400km 2 , the freshwater input into the estuary is small, accounting on average for just under 0 . 5 % of the tidal volume (see Beardall et al., 1988). This allows conditions of high salinity to penetrate as far up the estuary as the Orwell bridge (14km upstream). The intertidal flats consist of a wide diversity of substrates but are dominated by fine silt (mean particle diameter + 5 . 5 - + 6 . 5 0 * ) which is replaced in certain regions on the north * A modification of the Wentworth scale is used to define classes of Sediment size. 0 is the - log2 of the particle diameter in millimetres.
Trans. Suffolk
Nat. Soc. 26 (1990)