Why do some oceans have so few nutrients on the surface?

The open ocean often forms layers when warm, light surface water sits above colder, denser deep water. This stratification acts like a lid and reduces vertical mixing, so nutrients such as nitrate and phosphate remain trapped below the sunlit zone. In these areas, phytoplankton growth can be limited even though the water looks beautiful and blue. Regions with upwelling, storms, or seasonal mixing can bring nutrients back to the surface and trigger blooms. This is why some parts of the ocean are highly productive while others are called ocean deserts.