THE CONTROL OF SOFT-SHELL CLAM (MYA-ARENARIA) RECRUITMENT ON INTERTIDAL SANDFLATS BY BEDLOAD SEDIMENT TRANSPORT

Bedload sediment transport, clam transport across the sediment surface, clam population density, and spat settlement were measured daily for 10 months to determine the magnitude and frequency of clam transport and its dependency on bedload transport and to evaluate the relative importance of this phenomenon to population growth of Mya arenaria. From July to April, the transport of juvenile clams was observed frequently on a sheltered and an exposed intertidal sandflat. The maximum rate of clam transport on the sheltered sandflat (790 ind. m-1 d-1) and on the exposed site (2,600 ind. m-1 d-1) coincided with peaks of bedload sediment transport (approximately 35 kg m-1 d-1). At both sites, bedload transport was positively correlated with clam transport (r = 0.33 and 0.51; sheltered and exposed sites, P < 0.001); on the sheltered site, clam transport was negatively correlated with clam density (r = -0.47, P < 0.001). Cross-spectral analysis showed that bedload and clam transport time series were significantly coherent with zero lag at periods of < 10 d. Clam transport on the high-energy sandflat accounted for an order-of-magnitude increase in clam density in early September, a precipitous decline 2 months later, and the complete removal of recently settled spat. A net population increase on this sandflat was most likely a result of clam import during bedload events.