Last night, the Gowanus Canal was full of jellyfish. I saw at least a dozen, donut- to plate-sized, flowing outward under the Union Street Bridge with the tide. These were the same species as the type I saw dead on the beach at Dead Horse Bay at the end of June. Alive (in the greasy green Gowanus!), they were quite beautiful, like slow flowers, blooming.
UPDATE: These are lion's mane jellyfish, and we're experiencing a population explosion at the moment. Everything connects: Holmes was on it in "The Adventure of the Lion's Mane."