Phytoplankton are small, single-celled plants. Individual cells in all organisms regardless of size are small for these reasons. Some others increase buoyancy with a drop of oil. Viscosity is internal resistance to flow of a liquid. Maple syrup and honey are viscous liquids.
Organisms float more easily in a more viscous liquid.
Both increased salinity and decreased temperature increase the viscosity. Flotation is easier in cold waters for these reasons. This is an important component of the much greater stability of the ocean compared to the land. Tropical environments tend to have more species, but colder environments have larger planktonic organisms and more planktonic biomass. Higher temperatures increase growth rates and other biological process by a factor of about 2 for each 10 degree C temperature increase.