Abstract
Using an integrated physical and biological approach, we examined across-shelf advection and exchange and the associated transport of bivalve larvae in the presence of a strong coastal current separated from the coast by a stratified inshore environment. We tested the hypothesis that the interface of the coastal current and inshore waters can act as an ecological barrier to across-shelf transport of larvae but can be overcome by wind- or tidally-induced transport. Our study region in the Gulf of Maine encompasses a coastal current that diverges from the coast as it moves downshelf. The region inshore of this current is home to several species that exhibit limited recruitment in spite of extensive upshelf larval sources. Analysis of surface water temperatures and wind velocities revealed episodic decreases in temperature along the coast correlated with alongshelf (but not upwelling) winds, indicating wind-forced onshore movement of the cold coastal current. Such wind-driven onshore migrations are more common along the northern portion of the study region where the coastal current is near the coast, tidal currents are strong, and wind directions are more conducive to onshore migration, but rarer further south where the interface between inshore waters and the coastal current is further offshore and suitable wind events are less common. The distribution of bivalve larvae was consistent with the physical measurements. There was little across-shelf variation in larval abundance where the current abuts the coast, indicating strong across-shelf exchange of larvae, but strong across-shelf variation in larval density where the stratified inshore waters separate the current from the coast, indicating weak across-shelf transport of larvae. Our results suggest that the interface between the coastal current and inshore waters may constitute a major ecological barrier to larval dispersal in the southern part of the region that may only be overcome by rare, strong wind-forced events.
Highlights
Answering questions about larval transport and population connectivity generally requires integrated physical and biological studies [1]
The coastal circulation patterns along the western boundary of the Gulf of Maine are dominated by the Gulf of Maine Coastal Current, which consists of two branches with intermittent interaction [27,28,29]: the Western Maine Coastal Current (WMCC) and the Eastern Maine Coastal Current (EMCC)
The vertical stratification extended to the most offshore station of transects 2 and 3, indicating that temperature stations reached only the inshore edge of the EMCC. These vertical transects are consistent with the sea surface temperature climatologies for June-August (Figure 1) that show that the cold surface waters of the EMCC tend to be bounded inshore by the 75 m isobath which moves offshore as the continental shelf expands southeastward
Summary
Answering questions about larval transport and population connectivity generally requires integrated physical and biological studies [1]. Such approaches have yielded considerable insight into larval transport pathways. Work to date on coastal marine organisms has made great advances in understanding the physical mechanisms responsible for alongshelf transport [2,3], the causes of across-shelf transport are still under debate [4]. To some extent, this focus reflects the general understanding of the physical flow field. The last decade has seen significant progress on these across-shelf transport phases (mainly the onshore portion), with examinations of the roles of upwelling [5], downwelling [3], tides [6,7] and internal waves [8,9] on larval movement
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