My neighbors pond also has much more algea active this winter. We can see very large clusters of green growing under the surface. We do not fertilize. This is a response from my Fisheries agent:

"Algae problems always stem from nutrient loading. All ponds go though a process called eutrophication in which the pond gets ever shallower and more nutrient rich. If unnatural sources of nutrients (fertilizers, leaky septic tanks, cow manure, etc.) are present this process will be accelerated, otherwise the rate of eutrophication depends upon the natural fertility of the soil and accumulation of other organic debris (ie leaves). If your pond is nutrient rich, something is going to use those nutrients. Usually that something is algae. It is very opportunistic.

You can kill the algae with copper sulfate, but as the nutrients are still there, the algae will return. How quickly it will return depends on how heavy the nutrient load is. The other option is using Aluminum Sulfate (Alum). Alum, when applied to your pond, will bind to the phosphorus (a major nutrient) in the water and make it unusable for the algae, basically starving the algae to death. This should have a much longer lasting effect. Alum sometimes causes a lowering of pH that can kill fish. We have not had this problem in any Missouri ponds, but if you are worried about it you can buy buffered alum. Also, Alum or Copper sulfate should only be applied when water temperatures are cool, 60 - 80 degrees. Killing any vegetation in a pond can cause a dip in dissolved oxygen levels that may cause a fish kill."

I may try the Alum.

Jeff Gaines