Invertebrate macrofauna as potential indicators of sandy beach instability

Faunal and physical characteristics were compared on a range of exposed and sheltered beaches on the coast of North Wales, U.K., to determine whether the instability of beach sediments could be adequately described using biological evidence alone.An index of relative instability was generated from values of average change in beach sand level, recorded over a period of twelve months. Invertebrate macrofaunas, characteristic of the different levels of sediment instability, were identified and their affinities to these areas and also particle size were tested. To ascertain the effect of localized physical variations across the width of individual beaches the index and faunal data were considered on a zonal basis with tidal intervals acting as fixed zone boundaries.Prevalent trends in the faunal data were examined using an ordination technique. A stronger correlation was found between the principal trends in the ordination and sediment instability than with particle size. The closeness of the relationships decreased up the shore. This was considered to be due to terrestrial influences and a decreasing environmental predictability.Beaches were allocated exposure ratings, using methods described in McLachlan (1980), and these were found to be complementary to those derived from the scale of instability. It was concluded that faunal evidence could be used on lower shore zones of beaches as a guide to the degree of temporal instability expected in the sedimentary environment.