Larval transport in an estuarine-shelf system: Interaction of circulation patterns and larval behavior

Complex circulation patterns at the interface between estuarine and shelf systems impact transport of larvae into the estuary, an important process for species that utilize estuarine habitat. Narragansett Bay is a partially-mixed estuary on the U.S. east coast, in a region of significant fisheries activity. This study combined observational data and results of a hydrodynamic model to investigate physical and behavioral controls on the ingress of fish larvae into Narragansett Bay. Data and model output demonstrated a clear effect of wind on physical exchange and rates of larval ingress into the estuary. Strong, short-lived pulses in estuarine-shelf exchange currents were observed as a local effect of wind blowing down-estuary or as rebounds after the release of wind conditions that stalled exchange. Winds observed to be favorable for strong physical exchange were also favorable for ingress of larvae in the model.;Additional factors that affected transport of larvae near the estuary-shelf interface included time of year and larval swimming behavior. Differences in circulation and larval transport patterns between the early summer and late fall were attributed to a seasonal change in the level of vertical density stratification. The addition of vertical swimming behavior modified the outcome of transport. Larvae that maintained a deep depth distribution were the most successful at entering the estuary, but cyclical vertical migration behavior increased retention time near the estuary mouth and resulted in moderate ingress success under wind conditions that were favorable or unfavorable for enhanced estuary-shelf exchange flows. Vertical migration timed to the tide cycle did not strongly promote up-estuary transport, as expected from the concept of selective tidal stream transport, due to lateral and vertical variability of currents. Model simulations predicted distinct patterns of transport that should be tested by future field studies. Specifically, water masses and larvae most consistently entered the estuary by way of a shallow reef near the estuary mouth, a potential zone of vertical mixing and into dominant inflow that was restricted to the deepest estuary channel. Questions remain about spatial and temporal details of these circulation dynamics and how larval fish respond behaviorally in a complex natural setting.