Still don't know that this problem of yours is, if its not phytoplankton, Are we using the correct terminology? I looked up dinoflagellates and it is defined as phytoplankton.
According to what I searched,
"The dinoflagellates (Greek ????? dinos "whirling" and Latin flagellum "whip, scourge") are a large group of flagellate protists that constitute the phylum Dinoflagellata. Most are marine plankton..."
https://en.wikipedia.org/wiki/Dinoflagellate
Does it form a film on your glass? that's them swimming towards the light and resting on the glass.