We investigate a nonlocal reaction-diffusion-advection equation which models the growth of a single phytoplankton species in a water column where the species depends solely on light for its metabolism. We study the combined effect of death rate, sinking or buoyant coefficient, water column depth and vertical turbulent diffusion rate on the persistence of a single phytoplankton species. Under a general reproductive rate which is an increasing function of light intensity, we establish the existence of a critical death rate; i.e., the phytoplankton survives if and only if its death rate is less than the critical death rate. The critical death rate is a strictly monotone decreasing function of sinking or buoyant coefficient and water column depth, and it is also a strictly monotone decreasing function of turbulent diffusion rate for buoyant species. In contrast to critical death rate, critical sinking or buoyant velocity, critical water column depth and critical turbulent diffusion ra...