The surface and interior ocean are connected through physical processes that act in three dimensions and on subseasonal timescales. Numerical models and observations both highlight the importance of these eddying dynamics on shaping the biogeochemical and biological variability in the upper mesopelagic ocean. The resulting subsurface tracer variance outside of the photic zone has implications for ocean ventilation timescales as well as biogeochemical function and microbial community composition. Subsurface patchiness is best described using water mass frameworks that account for the dynamic processes that drive the transport of biogeochemical tracers between the photic zone and the mesopelagic. Investments in co-located and coincident measurements of physical and biological properties in the subsurface ocean will contribute to the growing understanding of mesopelagic biogeochemistry in the next decade.
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