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 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, O-2, particles, SCM, and turbulence. We find large O-2 excess at middepth, identified by positive saturation (Delta O-2) maxima of 15%-20% that unambiguously indicate significant production occurring in middepth waters. The Delta O-2 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 Delta 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% +/- 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 O-2 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. Plain Language Summary Coastal seas in the Arctic are experiencing large changes as the region experiences warming. Some of these changes may be occurring beneath the surface, where we have fewer observations. Here, we used a unique sampling approach that allowed us to collect more observations throughout the whole water column than are typically possible, giving us a clearer picture of distinct layers at different depths. We interpreted observations of dissolved oxygen, chlorophyll, nutrients, light, and small-scale physical motions known as turbulence in different ocean layers to indicate biological growth occurring beneath the surface represents an important fraction of the total primary productivity for the region. We also find that we can explain vertical and spatial patterns in our data based on our understanding of how light and nutrients influence phytoplankton growth, and how ocean circulation impacts the availability of light and nutrients in the water column. These results contribute to an improved understanding of Arctic primary productivity and potential sensitivities to future change, and argue for the need to use observational tools that see below the ocean's surface to track this change.

Automated summary

This study finds that middepth waters in Arctic marginal seas play a significant role in regional productivity, as indicated by the presence of subsurface chlorophyll maxima (SCM) and large oxygen excess. The SCM coincides with a compensation point for balanced growth and loss, and is associated with turbulence minima and increased light attenuation. The vertical and spatial patterns of SCM and productivity are influenced by stratification, turbulence, light availability, and seasonal water mass formation.