face72,
Clearer water in the cold months compared to warm months is common. Usually the pond water will be
clearest during the winter season.
Numerous things by themselves or in combination can cause cloudy water. As ewest says, a denser or more abundant plankton community will reduce water transparency. Plankton growth slows dramatically when the water is near 39F. Bacterial populations also dramatically decrease during cold water periods. Bacterial concentrations that are responsible for decompositional activity can become quite abundant during summer.
I have learned that the activity of frogs and tadpoles even at faaaaairly high densities have little influence on causing water cloudines in established ponds. You are correct that during cold water periods animal life becomes less active. Animals that can contribute to water cloudiness are bottom or bank burrowing animals such as muskrats or other mammals. It would take a real high number of turtles working on the bottom to casue continual water cloudiness.
Fish such as catfish-bullheads types, koi or carp, even numerous goldfish, species of large sucker or redhorse, and or very high numbers of bottom oriented minnows such as fatheads, bluntnose, mudminows, etc can contribute substancially to water cloudiness.
