Phytoplankton growth driven by submesoscale processes under convective turbulence: Lagrangian plankton model results

Abstract

Submesoscale turbulence influences phytoplankton growth by enhancing upper-ocean stratification, nutrient entrainment, and horizontal tracer variability. However, the specific mechanisms and their relative contributions under convective forcing remain unclear. Here, we use large-eddy simulations coupled with a Lagrangian plankton model to quantify the physical and biological impacts of submesoscale turbulence across varying cooling intensities and nutrient levels, comparing cases with and without submesoscale turbulence. Under weak cooling with low nutrients, submesoscale turbulence enhances nutrient entrainment but initially suppresses growth by subducting phytoplankton into deeper, light-limited waters. As restratification progresses, greater surface retention yields net growth enhancement alongside higher nutrient supply. Conversely, when nutrients are abundant, submesoscale turbulence enhances light exposure and promotes growth. Under strong cooling conditions, convective mixing dominates, homogenizing tracers and diminishing submesoscale influence. Nonethelss, weak restratification from submesoscale turbulence still yields small gains in light exposure and growth. Additionally, horizontal heterogeneity suppresses growth when cooling is weak and nutrients are low, but is negligible otherwise. These findings highlight the need to represent interacting physical processes in ocean models. Simplified parameterizations that omit these interactions may misrepresent biological responses, especially under weak cooling.