Transport of molluscan larvae through a shallow estuary

Dispersal of invertebrate larvae is determined by larval swimming behavior, the length of planktonic development and the hydrodynamic regime. Larvae of estuarine invertebrates must refrain from export or invade an estuary after development in the ocean. This study investigates retention patterns of estuarine molluscs by measuring time series of larval abundance in relation to hydrodynamic processes. Previous investigations of larval dynamics have generally focused on larger estuarine systems that are often stratified and have relatively long hydraulic residence times. The estuary studied in this investigation supports dense populations of infaunal clams yet has a water depth to tidal amplitude ratio near unity. To access processes affecting larval retention, the circulation patterns of the estuary were measured with time series of salinity, temperature, pressure and horizontal velocity. Transport rates of larvae between ocean and estuary, and within the estuary proper, were calculated from velocity and larval concentration time series. The daily residence time of the estuary was determined for the summer spawning period. The results demonstrate that molluscan larvae were routinely transported between the estuary and nearshore zone in tidal flows. Based on the magnitude of the horizontal current velocities, passive transport of larvae predominates during most of the tidal cycle in the estuary. Residence time calculations suggest that the ability of larvae to remain in the estuary through larval development is unlikely, and there was no evidence of selective retention of mature bivalve larvae in the estuary. Rather, larvae are exported rapidly from the estuary and undergo development in the coastal ocean. Mesoscale physical processes in the coastal ocean probably control variation in the delivery of larvae back to estuarine systems. Recruitment to this and similar estuaries must therefore be dependent on invasion.