Looking at the photo, I would venture to guess that the leaves going into the pond every Fall are the biggest culprit. Ditto everything that Bill said, but yoiu can also use PhosLoc to lock up P.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).