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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 O₂excess at middepth, identified by positive saturation () maxima of 15%–20% that unambiguously indicate significant production occurring in middepth waters. Themaxima coincided with a complete depletion of dissolved inorganic nitrogen (DIN = NO₃⁻ + NO₂⁻ + NH₄⁺). Nitracline depths aligned with SCM depths and the lowest extent ofmaxima, 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 largestmaxima 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 O₂excesses of 800–1,200 mmol m⁻²in 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. 

Abstract Petermann Gletscher drains ~4% of the Greenland ice sheet (GrIS) area, with ~80% of its mass loss occurring by basal melting of its ice shelf. We use a high-resolution coupled ocean and sea-ice model with a thermodynamic glacial ice shelf to diagnose ocean-controlled seasonality in basal melting of the Petermann ice shelf. Basal melt rates increase by ~20% in summer due to a seasonal shift in ocean circulation within Nares Strait that is associated with the transition from landfast sea ice to mobile sea ice. Under landfast ice, cold near-surface waters are maintained on the eastern side of the strait and within Petermann Fjord, reducing basal melt and insulating the ice shelf. Under mobile sea ice, warm waters are upwelled on the eastern side of the strait and, mediated by local instabilities and eddies, enter Petermann Fjord, enhancing basal melt down to depths of 200 m. The transition between these states occurs rapidly, and seasonal changes within Nares Strait are conveyed into the fjord within the same season. These results suggest that long-term changes in the length of the landfast sea-ice season will substantially alter the structure of Petermann ice shelf and its contribution to GrIS mass loss.