Observations of exchange between eastern Casco Bay and the western Gulf of Maine

• Published in: Deep-sea research. Part II, Topical studies in oceanography Vol. 52; no. 19; pp. 2411 - 2429
• Main Authors: Janzen, C.D., Churchill, J.H., Pettigrew, N.R.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.01.2005
• Subjects: Alexandrium fundyense; Bay/shelf exchange; Brackish; Casco; Casco Bay; Estuarine circulation; Gulf of Maine; Maine; Marine; Near-shore flows; United States; ANW, USA, Maine Gulf; ANW, USA, Maine, Casco Bay; USA, Maine, Kennebec R; Bay/shelf exchange; United States; Maine; Estuarine circulation; Casco Bay; Gulf of Maine; Near-shore flows
• ISSN: 0967-0645; 1879-0100
• DOI: 10.1016/j.dsr2.2005.06.032

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 and 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 reversed from a typical estuary, and likely contributes to 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 driving surface up-bay transport and in the retention of surface-dwelling organisms in eastern Casco Bay.