Turbulent mixing is a key physical process in the ocean, setting the distributions of momentum, heat, salt, and other important tracers. The latter includes biogeochemical scalars, representing many different species of plankton and nutrients that play an important role in marine ecosystems; representing their fluxes is a major challenge in earth system models. A common approach in studying ocean mixing is to represent individual mixing events in terms of canonical stratified shear instabilities. In this talk, I will first describe these instabilities and give an overview of their mixing behaviour. I will then show the results of recent direct numerical simulations in which these instabilities are coupled to an idealized reaction-advection-diffusion model for phytoplankton-nutrient interactions. With these simulations, I will discuss both the impact of the type of instability and the reaction timescale on the resulting fluxes.