I would go with Keith on the dissolved oxygen levels. As elevation, temperature, and organic matter increase, your dissolved oxygen decreases. For the last Anaheim show I got smoked off the basketball court in Oracle, Arizona
( Tucson, Scottsdale, Vegas run ). Oxygen level difference between Florida Gulf Coast and 4500 feet elevation busted my gut.
There are dropper tests you can do for DO and adjust for temp and elevation. Breaking up thermal layers and water aeration are good ideas.
www.epi.state.nc.us/epi/oee/bluegreen.html says blue green algae are not toxic to fish
Michael Rogers
Down to Earth Water Management
Golf Industry Liaison / Wastewater Solutions