These were harder to find...did you find them?
These small organisms are dinoflagellates. Dinoflagellates are a type of single-celled phytoplankton that can be autotrophic or heterotrophic.
Have you ever heard of a red tide?
Dinoflagellates can be the cause of red tides, as well as another type of phytoplankton, cynobacteria. These tiny organisms release red to brownish pigments into the water, staining it red in high enough concentrations. They can also be toxic to marine organisms. The coloration decreases or eliminates light from penetrating into the water, which eventually decreases oxygen levels as these organisms decay. As a result there can be massive deaths of marine life below red tides from the lack of light and oxygen and the presence of the toxin.
Back to phytoplankton
All pictures are borrowed from Spenser-Jones, David and Max Wade. Aquatic Plants (1986). ICI Professional Products; Surrey.