Most primary productivity in the surface water of lakes and oceans is due to

Most primary productivity in the surface water of lakes and oceans is due to Phytoplankton.
Phytoplanktons are the autotrophic components of the plankton community and a key part of ocean and freshwater ecosystems.
Phytoplankton is photosynthesizing microscopic protists and bacteria that inhabit the upper sunlit layer of marine and freshwater bodies of water on Earth. Paralleling plants on land, phytoplankton undertakes primary production in water, creating organic compounds from carbon dioxide dissolved in the water.