Behavioural response of juvenile bivalves to terrestrial sediment deposits: implications for post-disturbance recolonisation

In soft-sediment systems, terrestrial sediment deposits may result in a marked change in habitat and benthic community composition. We conducted a series of experiments to investigate the behavioural response of juveniles of 2 bivalve species common on New Zealand intertidal sandflats (Paphies australis and Macomona liliana) to terrestrial sediments. Both species are able to actively disperse as juveniles and do so in response to 'unfavourable' conditions. In the laboratory, we investigated the ability of these potential colonists to burrow into thin (0.5, 1.0 cm) surface and submerged terrestrial sediment layers, and their subsequent ability to burrow into natural sediments. Field experiments assessed their response to terrestrial sediments that had been 'aged' in the natural estuarine environment for different lengths of time (up to 1 mo). Exposure to terrestrial sediment treatments negatively affected both species, with fewer individuals burrowing into the terrestrial sediment treatments. In the laboratory, burrowing was slower in the 1 cm-thick surface layers than in the 0.5 cm-thick and submerged layers, and the subsequent burrowing rate in natural sand of individuals recovered from these treatments was similarly affected. This latter effect is important as it suggests that settlement by juvenile bivalves on terrestrial sediment deposits can have a lasting effect even if an individual is able to find a more suitable habitat at a later date. In the field, dispersal of M liliana away from the terrestrial sediment treatments was reduced, even after the sediment had been 'aged' on the sandflat. Depending upon the extent and frequency of disturbance, terrestrial sediments are likely to have important effects on the population and community dynamics of mobile bivalve species, and at a wider scale than that of their immediate impact.