Limited transport and recolonization potential in shallow tidal estuaries

Evidence of limited larval exchange and high levels of self-recruitment in marine populations continues to accumulate, implying that marine systems are not as open as we once believed. In estuaries, some organisms with long larval periods, such as crab and bivalve species, are capable of long-distance transport, while others, including many polychaete and gastropod species with crawling or brooded larvae, lack long-distance larval dispersal mechanisms and may disperse as juveniles and adults. To predict transport rates and colonization potentials of macrofaunal life stages transported in the water column, we use a combined hydrodynamic and particle tracking model. We vary particle release location to investigate how different habitats (i.e., main channel, lower sandflat, high sandflat, tidal creek) will contribute to the supply of colonists in different parts of the estuary. Most particles traveled short distances and settled within their release habitat. Slightly negatively buoyant particles (corresponding to small larvae) traveled much farther than more negatively buoyant particles (corresponding to larger, heavier larvae and juveniles). Particles released from high intertidal locations had lower rates of transport and transfer among habitats compared to lower intertidal sites. Other influences on organism transport include timing of release (with respect to tide), release location within the water column, tidal range, and magnitude of freshwater flood events. In general, estuarine communities dominated by organisms with limited dispersal periods are likely to be recruitment limited and limited in their ability to reach mature community states when subject to increasing frequencies or spatial extent of disturbance.