Where are phytoplankton MOST productive?

Oh, dude, phytoplankton are most productive in areas with high levels of nutrients and sunlight, like the surface ocean where they can photosynthesize and thrive. They're like the overachievers of the ocean, soaking up all that sunlight and nutrients to grow and reproduce like there's no tomorrow. So, yeah, you'll find those little guys partying it up near the surface where all the action is.