Submesoscale turbulence influences phytoplankton growth by modulating upper-ocean stratification, nutrient entrainment, and horizontal tracer variability. However, the specific mechanisms and their relative contributions under convective forcing remain poorly understood. In this study, we employ 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. Under weak cooling and low-nutrient conditions, submesoscale turbulence enhances nutrient entrainment but initially suppresses growth by subducting phytoplankton into deeper, light-limited waters. As restratification progresses, increased surface retention eventually leads to net growth enhancement. Conversely, when nutrients are abundant, submesoscale turbulence promotes growth primarily by increasing light exposure. Under strong cooling, convective mixing dominates, homogenizing tracers and diminishing the relative influence of submesoscale dynamics. Nevertheless, weak restratification still yields marginal gains in light exposure and growth. Furthermore, horizontal heterogeneity suppresses growth under weak cooling and low nutrients but becomes negligible in other regimes. These findings underscore the necessity of representing interacting physical processes in ocean models; simplified parameterizations that neglect these interactions may misinterpret biological responses, particularly in weak-convective regimes.