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The drivers of the tidal and residual flows in estuaries can vary spatially and temporally due to geomorphic complexities, fortnightly tides, and climatic influences. In this paper, we explore the mechanisms that give rise to the circulation patterns in Frenchman Bay, Maine, on the Eastern Coast of the USA, under varying freshwater input conditions and fortnightly tidal phases, using idealized simulations from a high-resolution, three-dimensional numerical model. The results of the simulations at the tidal timescale reveal a tidal asymmetry in vorticity, where vorticity generated during flood tide is not spun-down during the subsequent ebb. This asymmetry prompts the investigation of the residual circulation in the bay which is characterized by large tidal residual eddies. These eddies are found to persist in the depth-averaged residual flow regardless of the freshwater input or tidal phase, leading to the conclusion that the eddies are “geomorphically-constrained” in the bay. Analysis of the horizontal momentum terms and a simulation performed without Coriolis forcing demonstrates that the tidal stress terms predominantly balance the barotropic pressure gradient to give rise to the eddy patterns, while the Coriolis force acts to strengthen their vorticity. The eddies create a laterally sheared residual flow structure with depth, however the flow is more vertically sheared during the neap tide when the baroclinic pressure gradient plays a larger role. These findings demonstrate the persistence of tidal residual eddies regardless of freshwater input or fortnightly tidal phase in a geomorphically complex deglaciated coastal bay with low freshwater input. 

Understanding local hydraulic conditions is imperative to coastal harmful algal bloom (HAB) monitoring. The research summarized herein describes how the locations and tidal phases selected for coastal hazard sampling can influence measurement results used to guide management decisions for HABs. Our study was conducted in Frenchman Bay, Maine, known for its complex deglaciated coastline, strong tidal influence, and shellfishing activities that are susceptible to problematic HABs such as those produced by some species (spp.) of the diatom genus Pseudo-nitzschia. In-situ measurements of current velocity, density, and turbulence collected over a semidiurnal tidal cycle and a companion numerical model simulation of the study area provide concurrent evidence of two adjacent counter-rotating sub-mesoscale eddies (2–4 km diameter) that persist in the depth-averaged residual circulation. The eddies are generated in the wake of several islands in an area with abrupt bathymetric gradients, both legacy conditions partly derived from deglaciation ∼15 kya. Increased concentrations of Pseudo-nitzschia spp. measured during the semidiurnal survey follow a trend of elevated turbulent dissipation rates near the water surface, indicating that surface sampling alone might not adequately indicate species abundance. Additional measurements of Pseudo-nitzschia spp. from two years of weekly sampling in the region show that algal cell abundance is highest where residual eddies form. These findings provide incentive to examine current practices of HAB monitoring and management by linking coastal geomorphology to hydraulic conditions influencing HAB sampling outcomes, coastal morphometric features to material accumulation hotspots, and millennial time scales to modern hydraulic conditions.