Circumstantial at the moment, but a very powerful correlation.  The average temperature of bodies of water are increasing, warming earlier than in years past.  This provides a longer season for the cyanobacteria to thrive.  Warmer water equals more blooms, longer blooms.  A fraction of a degree of warming can make a big difference.

We need to be at least concerned and cautious about this.  There is a significant, and growing body of evidence that exposure to low-level cyanobacteria toxins are implicated in motor-neuron disease (ALS, Lou Gehrig's Disease).  A lot of communities draw their water from lakes and streams that could be vulnerable to these changes.  I recall Dunes City on the coast having a big problem from a bloom in Siltcoos Lake.  Data suggests that this could be more frequent in the years to come.