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

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 & ldquo;hidden & rdquo; 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, O-2, particles, SCM, and turbulence. We find large O-2 excess at middepth, identified by positive saturation (?O-2) maxima of 15%& ndash;20% that unambiguously indicate significant production occurring in middepth waters. The ?O-2 maxima coincided with a complete depletion of dissolved inorganic nitrogen (DIN = NO3 & minus; + NO2 & minus; + NH4+). Nitracline depths aligned with SCM depths and the lowest extent of ?O-2 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 ?O-2 maxima were associated with high nutrient winter-origin water masses (14.8% & plusmn; 2.4%), under a shallower pycnocline associated with seasonal melt while lower values were associated with summer-origin water masses (7.4% & plusmn; 3.9%). Integrated O-2 excesses of 800 & ndash;1,200 mmol m(& minus;2) in regions overlying winter water are consistent with primary production rates that are 12%& ndash;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.