This is Ostreopsis, correct?

[formallydehyde]

Microscope images under a cover slip, kinda squashed:

Microscope images without cover slip (they were moving around so they're all in different orientations):

Gif attempt to show motion (Couldn't figure out how to take a proper video so I pieced it together from multiple stills, didn't turn out good):

I didn't really notice them spinning in a horizontal circle but they did twirl around the axis of their pointy ends. It reminded me of how red blood cells twirl around in videos.

Macroscopically, I typically see them as brown threads suspended in the water column or as crimson mats if their population density is high, I actually thought it might be a mix of different organisms because of this but I sampled from multiple locations and they all look identical microscopically so I think it might just be a trick of the light based on how thick their density is.

I am unsure if they enter the water column at night, but I have noticed very long threads of them suddenly appear as the lights go down even in areas I siphoned clean earlier, I assume that might be them entering the water column?

I am like 95% sure they are Ostreopsis (or a related genus) based on the images I've seen but this is my first time trying to ID dinos so I wanted secondary confirmation.

 

[rtparty]

Ostreopsis…

Sesame seed shape. They kind of circle/twirl around. Long strands of brown mucus as they clump together

 

[thedon986]

Looks like it. More likely they are stuck together than dividing maybe? Not sure but Ostreopsis likes to form the long slime strands.

 

[taricha]

...and @formallydehyde you are correct that you caught some cells dividing.