And as these zooplankton grow, the organisms that eat them begin to grow and the organisms that eat the organisms that eat the zooplankton begin to grow and so on and so forth. Everything in the food web flourishes, as energy from the sun and matter from dissolved inorganic nutrients are turned into living matter by the phytoplankton. But the good times don't last forever. As the phytoplankton photosynthesize, they absorb dissolved inorganic nutrients and remove them from the surface waters. Because the water column is stratified and because rates of diffusion are low, the surface waters become depleted.
Nutrients run out.
This is the situation at the start of summer. In summer, intense surface heating stratifies the water column even more. Layers upon layers of successively warmer water are created at the surface of the ocean. Vertical mixing practically stops. Phytoplankton are confined to the layer in which they live unless they sink to a lower layer.