Distinctive stratification regimes and their biochemical implications across Queen Charlottte Sound, a highly-productive shelf in the Northeast Pacific

Queen Charlotte Sound (QCS) is a broad shelf region off Canada's west coast that is highly biologically productive and hosts several Marine Protected Areas. However, ecosystems in QCS are becoming increasingly susceptible to climate change stressors such as marine heatwaves, ocean acidification, and deoxygenation.  In this system, stratification plays an important role in setting the physical and chemical environment, thus impacting how climate change affects the region, including its biogeochemical cycles and ecosystems. Here, one year of near-continuous underwater glider observations are used to investigate how variability in stratification and exchanges between the coast and open ocean influence the physical and biogeochemical properties in QCS. Specifically, we document how varying relative contributions of temperature and salinity to density stratification set up distinct stratification regimes: a salinity-dominated beta regime, a temperature-dominated alpha regime and a transitional regime, whose presence and spatial extent vary seasonally across the shelf. We then use this stratification regime characterisation to 1) map where and when these regimes manifest and consider the drivers of variability in regime spatial and temporal extent ; 2) quantify the stratification strength as a function of regime and understand regime impacts on the vertical structure of the upper ocean; and 3) demonstrate the usefulness of this regime characterisation to present the systematic differences in chlorophyll and oxygen concentrations and their vertical distribution in the alpha and beta regimes. In addition, we make a comparison of the upper ocean stratification from glider observations with a 1-D mixed layer model driven by meteorological data to test the sensitivity of each regime to atmospheric and lateral exchange processes. Lastly, we will discuss what these findings inform us about stratification in QCS in the future in the context of climate change with increased riverine inputs, melting glaciers, increased precipitation and warmer waters.