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. 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.