Terrigenous deposits in coastal marine habitats: influences on sediment geochemistry and behaviour of post-settlement bivalves

Dispersal of post-settlement juvenile macrofauna is widespread in the marine environment, yet the cues used by these organisms to assess substrate suitability are poorly known. We investigated how deposition of fine terrestrial sediments (TS, 0.5 to 1.7 mm thick) affects both the solute concentrations at the sediment-water boundary and the burrowing behaviour of juvenile tellinid bivalves Macomona liliana. To elucidate previously observed gradual reduction in the strength of macrofaunal responses with increasing age of deposits, the effect of TS weathered in the field for 0, 7 and 14 d, and the influence of surface and buried TS layers, were also investigated. Burrowing was significantly reduced in 0 and 7 d old surface TS compared with controls, but there was no effect of buried deposits on M. liliana burrowing, irrespective of sediment age. Microelectrode measurements showed that thin surface TS layers reduced the supply of O-2 to the underlying sediment, raising the position of the vertical [O-2] and pH gradients so that most of the gradient was located in the TS layer itself, rather than deeper in the sediment below it. Consequently, the likelihood of juveniles that are active in the near-surface layers being exposed to upward diffusing end products of microbial decomposition was increased. These results suggest a mechanism for the observed negative response of post-settlement M liliana (and other macrofauna) to TS deposits associated with sedimentation events, and an avenue for further research into potential geochemical cues (especially pH and associated geochemical species).