It is likely something to do with the chemistry of the water changing significantly. When clay is suspended in the water, it has something to do with how ions attract and repel other particles of clay. When you add Alum for instance, the clay particles clump up, and fall to the bottom. Likely the same thing is happening with the runoff water, salts being picked up, low salinity, etc. Just something has changed enough to settle the clay and likely kill off or flush out the plankton also.
