Abstract

Tidal mixing is recognized as a key mechanism in setting water properties in coastal regions globally. Our study focuses on Canada's British Columbia coastal waters, from Queen Charlotte Strait to the Strait of Georgia. This area is bisected by a region of exceptionally strong mixing driven by some of the strongest tidal currents in the world. We examine the influence of this tidal mixing on regional differences in water properties and nutrient ratios. Our results quantify a spatially-abrupt and temporally-persistent lateral gradient in temperature, salinity, and density co-located with the region of strongest mixing. The distributions of density on either side of this front remain largely distinct throughout the spring-neap tidal cycle, year-round, and for over 70 individual years for which data are available. Additionally, nutrient molar ratios north of the front are statistically distinct from those to the south. Seasonal changes driven by the arrival of upwelled water differ in both timing and magnitude on either side of the front. Taken together, these results indicate limited exchange of water through the region of strongest tidal mixing, and suggest that Queen Charlotte Strait and the Strait of Georgia are largely isolated from each other. As such, this area provides a valuable case study for the degree to which the reduction of estuarine exchange by tidal mixing can maintain abrupt and substantial regional differences in physical and biogeochemical water properties. Further, it demonstrates the potential of tidal mixing to modify nutrient transport pathways, with implications for marine ecosystems.