Phytoplankton r Microscopic, usually single-celled, free-floating algal species that drift passively in the water r Comparison with seaweeds, phytoplanktons turnover rapidly, their blooms being only of few weeks duration and their species replacement takes place only within a month of less r phytoplanktons are responsible for more than 90% of all marine primary productivityPhytoplankton Microscopic, usually single-celled, free-floating algal species that drift passively in the water Comparison with seaweeds, phytoplanktons turnover rapidly, their blooms being only of few weeks duration and their species replacement takes place only within a month of less. phytoplanktons are responsible for more than 90% of all marine primary productivity