It sounds like signs of an aging pond and nutrient loading, lots of phosphorus feeding algea blooms. The Fremont state lakes had a similar issue and were treated with alum.

http://www.flatwatergroup.com/projects/f...-alum-treatment

Quote:
Why all the phosphorus in the sandpits of the Fremont Lakes?

Phosphorus is taken in by plants from the soil for growth. Nutrients enter the lake through decay of leaves and other organic matter and then accumulate over time on the lake bottom sediments. Phosphorus is released from sediments under anoxic conditions when a lake stratifies and oxygen is depleted from the lower layer. This released phosphorus results in algae blooms in the lake. Uncontrolled algae growth can clog these lakes with bluish-green scum, lead to fish kills and make then unsafe for fishing, boating, and water skiing.

This alum formulation (2 parts aluminum sulfate and 1 part sodium aluminate) is added to the lake from a boat (see project photos). When it contacts water, the formulation forms a �fluffy, cotton ball-like substance� known as colloidal aluminum hydroxide floc. This floc binds to phosphorus and then settles to the bottom of the lake. A chemical barrier is formed between the lake bottom and water. The floc binds and holds onto the phosphorus so that it can�t escape into the water above. By keeping the phosphorus on the bottom, lake algae is reduced. The treatment is supposed to remain effective for 5-10 years.