I dont have a thermometer but I'd say it's pond dependent due to the setup. Some shallower than others, one has the entire size held by concrete retaining wall so I imagine it gets colder as well. Give how temps have been in central/southern ohio I would assume none of them are over 40 degrees yet. Water in other ponds comes from rain but the smallest pond has a minor leak in it which since I have no good plan on how to figure out where it is and how to fix it, I end up filling it with hard water a few inches every couple weeks. Luckily its the smallest of my ponds and just really an overglorified koi pond (20'x30'x3.5') that I use to display fish to visitors.