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.

Polar marine ecosystems are particularly vulnerable to the effects of climate change. Warming temperatures, freshening seawater and disruption to sea ice formation potentially all have detrimental cascading effects on food webs. New approaches are needed to better understand spatio-temporal interactions among biogeochemical processes at the base of Southern Ocean food webs, and how these interactions vary seasonally. In marine systems, isoscapes (models of the spatial variation in the stable isotopic composition) of carbon and nitrogen identify the spatial expression of varying biogeochemical processes on nutrient utilization by phytoplankton. Isoscapes also provide a baseline for interpreting stable isotope compositions of higher trophic level animals in movement, migration and diet research. Here we produce carbon and nitrogen isoscapes across the entire Southern Ocean (>40°S) using surface particulate organic matter (POM) isotope data, collected from multiple sources over the past 50 years and throughout the annual cycle. We use Integrated Nested Laplace Approximation (INLA)-based approaches to predict mean annual isoscapes and four seasonal isoscapes using a suite of environmental data as predictor variables. Clear spatial gradients in δ13C and δ15N values were predicted across the Southern Ocean, consistent with previous statistical and mechanistic isoscape views of isotopic variability in this region. We identify strong seasonal variability in both carbon and nitrogen isoscapes, with key implications for the use of static or annual average isoscape baselines in animal studies attempting to document seasonal migratory or foraging behaviours.

The stock-specific distribution of maturing and adult salmon in the Northeast (NE) Pacific has been a persistent information gap that has prevented us from determining the ocean conditions experienced by individual stocks. This continues to impede understanding of the role of ocean conditions in stock-specific population dynamics. We assessed scale archives for 17 sockeye salmon (Oncorhynchus nerka) stocks covering the entire North Pacific, from the Columbia River to Kamchatka Peninsula, to define salmon locations during their last growing season before returning to their spawning grounds. We used the relationship between δ13C in salmon scales and sea water temperature to estimate salmon distribution based on correlation strength. Significant correlations were found for 13 of the stocks allowing us to define feeding grounds with confidence. Complementary information from δ15N, historical tagging studies, and connectivity analysis were used to further refine distribution estimates. Based on the estimated distributions of the NE Pacific stocks, we suggest a sequence of steps that could result in salmon marine distributions. This study is a first step toward determining stock-specific distributions of salmon in the NE Pacific, and provides a basis for the application of the approach to other salmon scale archives. This information will improve our ability to relate stock dynamics to ocean conditions, ultimately enabling improved stock management. For example, our estimated distributions of Bristol Bay and NE Pacific stocks demonstrated that they occupy different areas with a number of the former being distributed in the high productivity shelf waters of the Aleutian Islands and Bering Sea. This may explain while these stocks seem to have responded differently to changes in ocean conditions, and the long term trend of increased productivity of Bristol Bay sockeye.