I do not know the source of phosphorous. There is a small feeder stream, but it originates in woods and is mostly groundwater runoff. No ag or livestock or any other point-source possibility. We don't fertilize around the pond, either.

The biologist said the high P level could be a result of the algae cycle itself. Something about once the algae gets started, it continues to feed off itself and increase the P levels.

The pond is around 40 years old and has trees around 3/4 of the bank. The leaf litter may be contributing to the excess nutrients, perhaps?

Thanks for the quick reply.