The Importance of Subsurface Productivity in the Pacific Arctic Gateway as Revealed by High-Resolution Biogeochemical Surveys

Abstract

Following sea-ice retreat, surface waters of Arctic marginal seas become nutrient-limited and subsurface chlorophyll maxima (SCM) develop below the pycnocline where nutrients and light conditions are favorable. However, the importance of these “hidden” features for regional productivity is not well constrained. Here, we use a unique combination of high-resolution biogeochemical and physical observations collected on the Chukchi shelf in 2017 to constrain the fine-scale structure of nutrients, O2, particles, SCM, and turbulence. We find large O2 excess at middepth, identified by positive saturation ((Formula presented.)) maxima of 15%–20% that unambiguously indicate significant production occurring in middepth waters. The (Formula presented.) maxima coincided with a complete depletion of dissolved inorganic nitrogen (DIN = NO3− + NO2− + NH4+). Nitracline depths aligned with SCM depths and the lowest extent of (Formula presented.) maxima, suggesting this horizon represents a compensation point for balanced growth and loss. Furthermore, SCM were also associated with turbulence minima and sat just above a high turbidity bottom layer where light attenuation increased significantly. Spatially, the largest (Formula presented.) maxima were associated with high nutrient winter-origin water masses (14.8% ± 2.4%), under a shallower pycnocline associated with seasonal melt while lower values were associated with summer-origin water masses (7.4% ± 3.9%). Integrated O2 excesses of 800–1,200 mmol m−2 in regions overlying winter water are consistent with primary production rates that are 12%–40% of previously reported regional primary production. These data implicate short-term and long-term control of SCM and associated productivity by stratification, turbulence, light, and seasonal water mass formation, with corresponding potential for climate-related sensitivities.