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. The productivity associated with these “hidden” features has traditionally not been 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 mid-depth, identified by positive saturation (∆O2) maxima of 15-20% that unambiguously indicate significant subsurface production. The ∆O2 maxima were situated immediately beneath the pycnocline and coincided with a complete depletion of inorganic nitrogen ([NO3-] + [NH4+]). The complete nutrient drawdown and O2 excess from this horizon is consistent with subsurface production that amounts to 1/3 to 1/2 the total regional primary production. Nitracline depths aligned with both the base of the mid-depth O2 maxima and with SCM depths, 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 due to high particle loads. Spatially, the largest ∆O2 maxima were associated with high nutrient winter-origin water masses, under a shallower pycnocline associated with seasonal melt. 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.
