Using seasonality to trace water masses in a coastal ocean

The Strait of Georgia is an estuarine system of glacial fjords and basins located on the south-eastern coast of British Columbia, Canada, which can be divided into three constituent water masses: (1) a riverine surface layer; (2) a deep oceanic layer; and (3) an intermediate layer, composed of a mixture of the surface and deep waters. The intermediate water (IW) layer is the largest layer volumetrically and a primary component of the region’s subsurface estuarine circulation, yet a comprehensive understanding of the system has proved challenging. Here, we determine a seasonal cycle in IW mass characteristics and use the spatial variation in seasonal cycle phase and amplitude to age the water mass, infer circulation and mixing, and understand the transport and eventual fate of IW in the Strait of Georgia. The IW circulation is found to be more complex than previously anticipated, with a direct alteration of circulation via inputs at the north of the basin, a distinct cyclonic flow in the primary basin, and a strong inflowing boundary current on the eastern slopes. Furthermore, we find evidence that wastewater tracer concentrations downstream of major effluent outfalls are consistent with our inferences of IW pathways. Finally, we compare our observations with hydrographic and particle tracking outputs from a regional model.