I was looking for an image made over four years ago, and while doing so came across a few that I had passed over at the time, but found that I liked.
This is one of them. I had a bird bath full of Haematococcus, and at the time was set up for phase contrast. As the water evaporated on the slide, they came together in a dense group.
At this stage of this slide it was pretty much a "mono-layer", but there were were bits and pieces also floating around in the mix. (Phase contrast can be "brutal" with out-of-focus bits)