Ducks stopped visiting the pond...?

So all winter (and early spring) long my ducks (3 pekin, 2 blue swedes and a buff) would make several trips down to the pond which is about 200 or so feet from their coop. They loved it: swam around, hung out on the edge, they would even spend the night in it on occasion. I wasn't happy about that but I couldn't really get them out because they would just keep swimming away from where ever I came to the edge and tried to chase them out. Anyway about a month ago (early May) the just stopped going to the pond at all. First I thought it was my imagination but it's really obvious that they lost all interest in it. Which is strange because they seek out anything remotely resembling a puddle and act like they found an oasis in the desert.

Anyone know why this would be? I was really counting on them keeping the weeds and algae down in the summer. Last year the pond was covered by August in azole (sp?) and parrot feather. I'm thinking maybe a snapping turtle took up residence? If so, does anyone know of ways to confirm it's presence and then discourage it's desire to remain?

Snapping turtles in a pond will stop ducks every time from going into a pond.
The only way I know to trap them is with a large drum. You want the top of the drum sticking about 4” above the water. You do not want a lot of water in the drum and to weight it down with rocks. Use metal bars across the top. Chicken legs or necks makes great bait hanging above the drum.
You want the turtles reach up to get the chicken. When they pull back you want them to get off balance so when they are done fighting the chicken they will fall in the drum so you can make turtle soup.

Decomposing barley straw in a pond can stop or slow new algae growth.
Ohio State University did a study on this a few years ago. Something you need to read up on before trying it.

Thanks Bob. Thanks my biggest fear - snapping turtle! I'll have to see if I can set up your trap idea. I'll report back on my progress.

By the way, I'm aware of the barley straw idea but apparently you need to get it started early in spring and for my pond size (~100,000 gallons) I'd need quite a bit of those little bales that sell for $3-5 each. That's why I was counting on the ducks. Oh well. I'm not giving up yet...