Yes, that would have been my guess also, but not using the correct word like you or the exact process that you laid out...but yes. The problem is that it is Modern in a Lady in the Lake milieu...any moment she should rise with Arthurian Sword in hand...while the square, whatever its use, is very obviously modern. Traveller
My guess that is an aerator for the pond. Some induce air bubbles and some spray up in the air like a fountain. Some do both, helps keep the water healthy and alive. I’ve hooked up and serviced a few over the years. Paul