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.

From 2014 to at least 2018, ecosystem health in the eastern boundary upwelling system along the west coast of North America was significantly impacted by a combination of a marine heatwave known as The Blob and an El Niño event, as well as by ongoing climate change. At the northern limit of this upwelling system, in Queen Charlotte Sound on the highly productive central coast of British Columbia, we have demonstrated that changing conditions on the continental shelf and in coastal waters may be skillfully predicted based on observed open-ocean and large-scale atmospheric conditions on seasonal to interannual timescales. In this work, we build on our understanding of this predictability by presenting a statistical model that relates physical and biogeochemical ocean properties in this region to conditions at and beyond the shelf break and large-scale forcing metrics. The model is based on statistical relationships developed using a multi-decadal archive of hydrographic and biogeochemical data in combination with high-temporal-resolution mooring records collected in Queen Charlotte Sound, and is supported by a conceptual understanding of the upwelling and downwelling regimes in this region. We next use the model to examine specifically how the arrival of The Blob and the subsequent El Niño modified ocean conditions on the continental shelf during both upwelling and downwelling, including impacts on nutrient concentrations, dissolved oxygen levels, stratification, and warming. Our results suggest it may be possible to predict changes in this upwelling system caused by future anomalous events and climate change using readily available large-scale data products such as the Argo dataset and NOAA Upwelling Index.