Community preservation in Recent shell-gravels, English Channel

Live and dead benthic faunas have been sampled offshore from Plymouth to assess the relationship between dead shell accumulations and living benthic communities. The live fauna (354 samples) has been sampled at approximately twenty-year intervals since 1895. The dead fauna (25 samples) was collected in one survey in 1980-1981. The live and dead mollusc and echinoderm (185 taxa) abundances are compared using multivariate statistical analysis (Detrended Correspondence Analysis). Three distinct communities are defined by this analysis, inhabiting shell-gravel, sand, and mud substrates. Historic records of the shell-gravel community indicate population fluctuations, and present-day faunas are most similar to those recorded in the 1920s. The dead shell-gravel fauna is similar to the 'time-averaged' live fauna and distinct from the sand and mud communities. When bivalves alone are analysed then relative abundances are preserved in dead samples even though these show greater abundance, diversity, homogeneity, and equitability. The results give support to the interpretation of original time-averaged community structure from similar fossil shell beds. However, fluctuations in population structure are not preserved because of the slow sedimentation rates.