The control of soft‐shell clam (Mya arenaria) recruitment on intertidal sandflats by bedload sediment transport

Abstract

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.

Effects of sediment source and flow regime on clam and sediment transport

“…Although Mya arenaria and Gemma gemma have been found in the water column in some studies (M. arenaria, Möller 1986;G. gemma, Sellmer 1967), our data and other studies (Emerson & Grant 1991, Commito et al 1995a, Hunt & Mullineaux 2002 suggest that if sediment transport at a site can be predicted, the likelihood of transport of juvenile M. arenaria, and other bivalves whose dispersal is generally linked to sediment transport, also can be predicted. It is likely that some dispersal of juvenile M. arenaria and G. gemma does occur at low flow speeds when individuals do not burrow into the sediment.…”

“…Less information was available for G. gemma than for M. arenaria, and no relationship was detected between the weight of sediment and the proportion of G. gemma transported at Sites 1 and 3, possibly due to the smaller range of sediment erosion. Previous field studies on M. arenaria (Emerson & Grant 1991) and G. gemma (Commito et al 1995a) found positive relationships between the number of clams and the amount of sediment collected in bedload traps. At the shear velocities employed in this study, clams and sand grains rolled along the bottom as bedload and were collected in the trap.…”

“…Regardless of whether bivalves actively promote or resist the initiation of transport, their subsequent dispersal will be under the control of tidal or wind-driven currents and waves. On intertidal flats, the significance of storms and wind-driven cur-rents has been demonstrated by positive relationships between wind conditions and flux of sediment and bivalves in sediment traps (Emerson & Grant 1991, Commito et al 1995b, Armonies 1996, Turner et al 1997) and abundance of individuals drifting in the water column (Armonies 1992). Intertidal (Hunt & Mullineaux 2002) and subtidal (Olivier & Retière 1998) studies have found that bivalve transport also varies with tidal conditions and may be influenced by local topography (Rankin et al 1994).…”

“…gemma, Commito et al 1995a). This dispersal appears to occur primarily through bedload transport (M. arenaria, Emerson & Grant 1991;G. gemma, Rankin et al 1994), although juveniles of both species can produce drifting threads (M. arenaria, Sigurdsson et al 1976) and have been found in the water column (M. arenaria, Möller 1986;G.…”

Disturbance and recruitment: a test of solute and substrate specificity using Mercenaria mercenaria and Capitella sp. 1

“…With attainment of a larger size, and the ability to burrow more competently and more deeply (Ross et al 2003), sediment-dwelling organisms become less vulnerable. Moreover, older juveniles and many infaunal adults are capable of secondary dispersal (Emerson & Grant 1991, Hiddink & Wolff 2002, Huxham & Richards 2003, and can move to more favorable locations when size and circumstances (e.g. the flow environment, Butman et al 1988) permit.…”