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Data from: Fluxes and mixing of reacting biogeochemical scalars in a stratified shear layer

Laroche, Vincent1; Kaminski, Alexis1

Published Aug 12, 2025 on Dryad. https://doi.org/10.5061/dryad.n2z34tn99

Data files

Aug 12, 2025 version files 1.47 GB

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Abstract

Biogeochemical scalars, representing many different species of phytoplankton, zooplankton, nutrients, etc., play an important role in marine ecosystems. Here we examine a simplified two-species system of generic phytoplankton and nutrient populations, modeled here as reacting scalars governed by advection-diffusion equations with added terms governing the biological reaction (nutrient uptake, phytoplankton death/remineralization). We perform direct numerical simulations of an idealized scenario in which initial layers of phytoplankton and nutrients are separated by an unstable stratified shear layer susceptible to either Kelvin-Helmholtz or Holmboe instability (canonical examples of overturning and scouring mixing events, respectively). We further vary the ratio of the reaction and flow timescales, considering cases of fast, slow, and comparable reaction rates. We find that turbulent advective fluxes show large oscillations during the evolution of the shear instabilities, associated with reversible fluid motions. To isolate the irreversible turbulent scalar fluxes, we instead calculate diascalar fluxes, that is, diffusive fluxes across isoscalar surfaces. This concept has been previously used to quantify irreversible mixing of buoyancy in stratified mixing events; here, we extend this approach to also account for the effects of biogeochemical reactions. With this analysis, we show that both instabilities accelerate phytoplankton growth, that the KH-driven mixing tends to have a larger impact on the phytoplankton growth than the Holmboe flow, and that the largest impact of the flow on the scalar evolution occurs when reaction timescales are comparable to the buoyancy period.