Teenage Mutant Ninja ....Frog?

So for the past several weeks we have had this frog in our pond. I thought there was something wrong with him. He never came out of the water and he had a huge head. Whenever I would get anywhere near the window no matter how still I was it would see me and duck under the water. Well today I was on the phone so I was just looking out the window and talking and I saw these long claw things and I thought what the heck is that thing. Then he came a bit closer to the top of the water and what do I see? A shell....my poor mutant frog is actually a big ol' turtle. We have had this pond for several years now and NEVER had a turtle in it. I hope he didn't have our fish for dinner! This might explain why the spout to the fountain keeps getting knocked over every night.

Not a good picture (I had to take it through the window) but you can kind of see him on the far left. That is all I had seen of him until today. Just that big ol' head. Any idea what kind of turtle it might be? I know nothing about them.