Perspectives on Northern Gulf of Alaska freshwater pathways, wind, and linkages to phytoplankton biomass

Abstract

The biologically productive northern Gulf of Alaska continental shelf receives freshwater from surrounding watersheds and wind stress from atmospheric pressure systems. A better characterization of the relationship between wind and salinity spatiotemporal variability is important for understanding the fate of freshwater and its ecological importance. In Chapter 2, I analyzed 25 years of northern Gulf of Alaska hydrographic data to reveal mid-shelf and shelf­ break freshwater pathways that impact stratification, currents, and freshwater distribution. Process studies of the Copper River plume’s propagation towards the shelf-break and beyond inform ecological function on the northern Gulf of Alaska shelf and productivity in the high- nutrient, low-chlorophyll waters of the adjacent slope and basin. Horizontal density fronts are often sites of locally enhanced ecological activity and accumulated biomass that contribute to biological patchiness on continental shelves. In Chapter 3, I analyze a high-resolution hydrographic dataset spanning spring, summer, and fall to identify wind-driven symmetric instabilities in the upper water column, improving our understanding of northern Gulf of Alaska submesoscale dynamics, its seasonality, and its role in driving phytoplankton patchiness. Autonomous underwater glider observations can improve our understanding of environmental forcings influencing spring bloom timing. In Chapter 4, I analyzed four environmental parameters: the potential energy anomaly, the mixed layer depth, light, and chlorophyll-a concentrations, assessed in relation to the stabilizing and destabilizing effects of wind, surface buoyancy flux, and tides. Bloom onsets coincided with high irradiance throughout the mixed layer, reduced wind stress and, over the mid-shelf, shoaling of the mixed layer above the euphotic zone driven by stratification, while in a coastal embayment, stratification had less influence on bloom onset. Primary productivity was estimated using a bio-optical model. These results clarify how interacting physical processes shape production in the northern Gulf of Alaska, improving our ability to anticipate ecosystem responses to climate change.