Your blue green algea clumps that have floated off the bottom look identical to the clumps I see in years when we had very good weed kills using chemicals. Basicaly when the aquatic vegetation is decomposing on the bottom of the pond that type of algea grows on the surface of the dead & matted decaying vegetation. When the water warms and clears enough this type of algea seems to produces enough oxygen/gas and starts to float up in chunks late afternoon this time of year. Usually there are some visible remnents of the old decaying clumps of aquatic vegetation on the underside of it. Did you have a big rain event that muddied the water up and then suddenly killed off a significant amount of aquatic vegetation ealier this year? You could have had a significant fish kill at that time, a good portion of dead fish never float and those that do only float for several days before sinking again.
