Algal blooms are common in the heat of summer. Comfortable temperatures and abundant nutrients allow microbial species to flourish in many aquatic environments. In June 2015 I took a sample of an algal bloom from the lake in Prospect Park, Brooklyn. In late August the lake in Central park was also experiencing a bloom. I took a rowboat out on the lake and procured a sample of that too. In September I visited the Harlem Meer, up at the top of Central Park. The bloom there was starting to die off, but I still found some great cells anyway.
The thing to remember about algal blooms is that they are not usually caused by algae. All the New York City blooms were caused by an overgrowth of several species of cyanobacteria. Cyanobacteria are bacteria, bacteria are not algae, technically. But they're green and their common name is blue-green algae, hence the confusion.
Cyanobacteria photosynthesize in the same way plants and algae do. They were the first photosynthetic organisms on earth and it's thought that the oxygen they produced helped to oxygenate the atmosphere of early earth. This allowed organisms that depend on oxygen, like us, to evolve. Importantly, the chloroplasts present in all plants and algae today were once free-living cyanobacteria, but that's anther story.
In these images you can see individual cyanobacterial colonies. Each colony consists of multiple, genetically identical cells. In the three images from Prospect Park, there are several different species present. Each species has a different colony morphology. The three images from Central Park are higher magnification. No. 1 and No. 2 show single colonies of a single species. No. 3 shows both species together in one image. The last image is a single colony from the Harlem Meer.