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 d.ependency 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-l d-l) and on the exposed site (2,600 ind. m I d-l) coincided with peaks of bedload sediment transport (-35 kg m-l d-l). At both sites, bedload transport was positively correlated with clam transport (r = 0.33 and 0.5 1; sheltered and exposed sites, P < 0.001); on the sheltered site, clam transport was negatively correlated with clam density (Y = -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 sandllat 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.

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We combined extensive water sampling with monthly growth measurements of juvenile sea scallops held in cages 0 to 200 cm above the bottom to (1) construct predictive empirical models of shell and soft-tissue growth based on oceanographic variables, and (2) determine whether scallops on or near the bottom can derive a food supplement from resuspended sediment when seasonal phytoplankton production is low. Variation in growth was strongly dependent on depth, but this relationship was not consistent over time or tissue type. In late fall, when phytoplankton biomass was generally low (-1 pg chl I-'), the adductor muscle of scallops on the bottom lost mass (-1.5 mg dry wt d.'), but for scallops held only 20 cm higher in the water column, growth was 2.5 mg d.'. During the winter, softtissue growth on the bottom was significantly lower than that of scallops held above the sediment surface. At this time, there was no variation in shell growth with respect to depth. At the end of the study, soft-tissue weight (excluding muscle tissue) of scallops on the bottom was -40% less than that of scallops growing 250 cm above bottom. Rather than providing an energetic benefit, results suggest that high concentrations of seston near the bottom inhibit growth. Empirical regression models of scallop growth using data from water sampling every 2 wk accounted for up to 68% of growth variation, with temperature and seston quality being the most important predictor variables. Marginal improvements to the model uslng data collected hourly with In situ probes suggest that estimates of food supply should be corrected, i.e. reduced, when high flows or high seston concentrations limit filtration rates. In addition, results indicate that attention to the magnitude and variation of predictor variables without consideration of their seasonal coherence may be a primary factor limiting the ability to construct truly predictive models of bivalve growth.

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The importance of sediment disturbance and water flow to the production of the soft-shell clam, Mya arenaria, was assessed in laboratory flumes by measuring growth rates of clams exposed to a gradation of bed shear stress, free-stream velocity, and frequency and depth of sediment disturbance over a 10-m period. In the absence of sediment disturbance, growth of soft-tissue was directly proportional to both free-stream (U) and shear (U*) velocity (r2 = 0.64 and 0.72, respectively). It was suggested that increased organic seston flux linked higher water flow to higher growth. In all treatments, maximum growth rates were observed with daily disturbance of the top centimeter of sediment. No level of disturbance resulted in growth rates lower than those of undisturbed clams in low flow (U = 0.4 cm∙s−1, U* = 0.1 cm∙s−1). The stimulation of growth under maximum sediment disturbance was removed when U exceeded ~3 cm∙s−1(U* = 0.7 cm∙s−1). An energy budget for M. arenaria indicated that the amount of organic material suspended during sediment disturbance was insufficient to account for the increased growth in clams subjected to high levels of disturbance. It was suggested that the sediment disturbance associated with intense clam harvesting, and changes in local hydrography resulting from coastal development, may be responsible for some of the unexplained growth variation in commercial clam stocks.

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