Faunal gradients in surface and subsurface shelly accumulations from a recent clastic tidal flat, Bahia la Choya, northern Gulf of California, Mexico

Abstract

Do the shelly remains already accumulated at depth reflect the faunal composition and environmental gradients evident in surface deposits, or do many years of selective destruction and post-mortem transportation limit the spatial resolution possible in studies of nearshore marine faunas? We sampled shelly molluscan remains at the surface and at depth along two 2 km transects across the intertidal zone of Bahia la Choya, Sonora, Mexico. At each site we collected surface samples from the upper 5 cm and depth samples from a ubiquitous shelly layer approximately 25 cm below the surface. Radiocarbon dates indicate that the samples differ by approximately 800 years in age. We used the 25 most common species in subsequent analyses. Faunal composition and distribution patterns vary systematically among the four sets of samples. Cluster analysis and multidimensional scaling (MDS) show that subsurface transects reflect species distributions of their surface counterparts more closely than species distributions in surface (or depth) transects resemble each other. MDS of samples also highlights a distinctive gradient in species distributions: Dimension 1 consistently expresses the depth gradient across the tidal flats. Dimension 2 appears to represent subtle changes in substrate mobility. These results suggest that, despite the potential for shell transport and extensive time-averaging, surface heterogeneity is accurately recorded in the subsurface shelly bed. Environmental gradients may have remained constant during the ∼500–1300 years since deposition of the subsurface bed, reworking may have completely mixed the sedimentary interval, or both. Spatial resolution of approximately 100 m may be possible in intertidal flat habitats in the stratigraphic record.