Observations of  Exchange Between Eastern Casco Bay and the Western Gulf of Maine
(in press, Deep Sea Research, II - June 2005)

Exchange of water between eastern Casco Bay and the adjacent Gulf of Maine shelf is examined to assess the circulation processes that impact the distribution and occurrence of a toxic dinoflagellate, Alexandrium fundyense, in eastern Casco Bay. Over the inner shelf adjacent to the bay, tidal variance is weak, and the across-shelf current is highly coherent and in phase with the along-shelf wind stress. Although tidal current variance increases as one advances into the bay, non-tidal currents account for 30-40% of the across-shelf current variance at the bay entrance. Between the shelf and the bay interior is a transition region, where the circulation response to wind forcing changes as the wind adjusts to the changing orientation of the shoreline. Far from shore, the overall large-scale coastline orientation dominates the wind-driven response, but within a few internal Rossby radii, the local coastline clearly dominates the flow patterns as across-shelf wind becomes locally shore-parallel inside the bay. Within the bay interior, the across-shelf wind is highly coherent and in phase with the near surface subtidal across-shelf current. The Kennebec River north of the study area supplies freshwater to eastern Casco Bay in all seasons. A pool of low-density, relatively fresh water at the entrance to the bay sets up an across-shelf density gradient that is reveresed from a typical estuary, and likely contributes the mean surface on-shelf transport in this region. Surface drifter trajectories observed over the course of the study suggest that both the across-shelf wind and the across-shelf density gradient are important in forcing surface up-bay tranpsortand in the retention of surface dwelling organisms in eastern Casco Bay.
 

Map of 1998 Casco Bay study area, showing station locations of moorings (Md1, Md2, and Md3); drifter release location (Md2), wind and sea level stations,  and CTD locations along Transects B, D, E and F.  Note that our observations concentrated on the eastern portion of Casco Bay.