Relative abundances of living and dead molluscs in two Californian lagoons

Abstract

Assemblages of living benthic invertebrates (predominantly bivalve mollusks) from the sand-channel habitat of 2 S California (USA) lagoons were sampled on 10 occasions over a 37 mo. period. A 1-time sampling of the corresponding assemblages of accumulating dead remains made possible a contrast of living and dead assemblages designed to assess the biasing effects of post-mortem transportation, shell dissolution and time-averaging. Species-by-species comparisons of the living and dead mollusks found together in the same samples strongly suggested that post-mortem transportation is insignificant within this high-energy habitat. A similar conclusion arose from contrasting the pattern of spatial heterogeneity of the living community with that of the dead assemblage. Species presence and absence comparisons were generally more reliable than comparisons of relative abundances. Adustments for experimentally determined rates of post-mortem shell dissolution proved significant and further decreased the correspondence in relative abundances between living and dead assemblages. Greater temporal variability of living populations at Mugu Lagoon, probably caused by a more harsh physical environment, increased the differences in composition between living and dead assemblages, which suggests that correspondence in relative abundances between living and dead assemblages generally should be expected to decrease as the life environment becomes more harsh.