Littoral sediments are found across the intertidal zone and range from boulders, through gravel and sand, to silt and clay (mud). The type of sediment is critical to structuring biological community composition. Sand shores usually support a range of oligochaetes, polychaetes and crustaceans, and muddy shores also favour bivalves. The invertebrate communities present in littoral sediments can support large populations of predatory birds. Littoral sediment environments are extremely dynamic and can change markedly over time. The aim of this project was to undertake a field survey of the intertidal sediments of the SSSIs located at Brading Marshes to St. Helenâs Ledges, Kingâs Quay Shore, and the Yar Estuary, Isle of Wight in order to enable a condition assessment of the littoral sediment feature. The survey determined: the extent of the sediment feature; the biological communities present, the sediment character, and the distribution of the biotopes, at each site.
The methods followed the standard guidelines for monitoring intertidal environments (Davies et al., 2001). Background information on the study sites was collated in July-August 2008 and the field survey was conducted 17 August-20 September 2008. Maps and aerial photographs were used to compare the extent of littoral sediment in the field, and the distribution of biotopes was mapped using a walkover Phase 1 survey, including photographs. Each site was then sub-sampled representatively (15-25 cells per site) using relatively rapid techniques to record sediment type and macrofauna. More detailed sampling of sediment character and biotope composition was undertaken at five transects per site, each with a mid and lower shore sampling station, where photographs were taken and sediment cores were collected for particle size analysis and identification of infauna in the laboratory. All field data were spatially referenced and integrated within a GIS. Data analysis consisted of summary statistics and multivariate statistical techniques using PRIMER (Clarke and Warwick 2001). There were few methodological constraints, although access to the western shore at the Yar Estuary was restricted.
The total extent of littoral habitat mapped at Brading Marshes to St. Helenâs Ledges was 130ha. The most extensive biotope type was âPolychaetes on littoral fine sandâ (LS.LSa.FiSa.Po), which covered 40% of the site and formed large flats outside Bembridge harbour. Inside the harbour, âPolychate/bivalve dominated mid estuarine mud shoresâ (LS.LMu.MEst) dominated. This pattern reflected a marked contrast between the more exposed littoral sediments outside the harbour, where fine/medium sand and gravel were typical, and the poorly sorted silt with clay inside the harbour. Sediment infauna was generally not abundant and showed variable species richness and density, with a high community dissimilarity between samples. Polychaetes such as Lanice conchilega, Hediste diversicolor and Nephtys spp. were most characteristic but the snail Hydrobia ulvae dominated the community within the harbour. A patch of eelgrass (Zostera) was mapped in the intertidal zone.
Kingâs Quay Shore comprised almost 50ha of littoral sediment. A variety of sandy biotope types characterised the site, with âBarren or amphipod dominated mobile sand shoresâ (LS.LSaMoSa) and âPolychates on littoral fine sandâ (LS.LSa.FiSa.Po) each covering a third of its area. Sediments were dominated by brown, very fine/medium sand of variable thickness, overlying a basal sub-unit of stiff brown/grey clay, often with a veneer of mixed gravel at or near the surface. The sediment supported moderate faunal diversity and numbers, and there was considerable community dissimilarity between samples. The polychaete Heteroclymene robusta was most abundant, supported by polychaetes Lanice conchilega and Arenicola marina, and the periwinkle Littorina littorea. Oysters (Ostrea edulis) and eelgrass (Zostera) were both recorded at the site during the survey.
A total of approximately 42ha of littoral sediment was mapped at the Yar Estuary, with 86% of this comprising the âPolychaete/bivalve dominated mid estuarine mud shoresâ (LS.LMu.MEst) biotope type. Throughout the estuary, sediments were dominated by very poorly and poorly sorted grey/dark grey/black silt and clay (mud), which were often highly anoxic at shallow depths. Sediment samples showed that there were few faunal species present but they were in some abundance. The snail Hydrobia ulvae was by far the most numerous, contributing substantially to community similarity across the site, although the vast majority of shells were empty. The bivalve Abra tenuis and the polychaete Arenicola marina were locally quite abundant. There was some evidence of saltmarsh erosion at the site, possibly resulting in increased littoral sediment habitat.
Multivariate comparisons of all three sites highlight the dissimilarity between the Yar sediment, which has quite a uniformly small grain size, and the mixed or larger grain sizes that characterise the other two sites. The faunal composition showed high dissimilarity between all sites and a clear distinction between communities from silt substrates and those where sand predominates, suggesting that community composition is influenced both by sediment type and site characteristics. Brading Marshes to St. Helenâs Ledges and Kingâs Quay Shore both contain a good range of biotopes and could be considered to represent diverse sedimentary landscapes. In general, the sediment tended to be rather species-poor, especially at the Yar Estuary, and faunal abundance was limited, except at the Yar where snails were numerous. It is possible, however, that the sampling intensity was insufficient to adequately represent the faunal population.
It appears that the condition of the littoral sediment feature is favourable at Brading Marshes to St. Helenâs Ledges and Kingâs Quay Shore SSSIs, although at both sites invertebrate numbers seem to be quite low and there is a limited amount of comparable information so monitoring is advisable. At the Yar Estuary, a comparison with previous information indicates that the quality of the sediment infauna may have declined and the extent of littoral sediment increased over the past 30 years, although natural dynamics make establishing a baseline difficult. Thus, the condition of the littoral sediment feature at the Yar Estuary SSSI appears favourable for sediment attributes but may be declining for biotope composition.