If enough nutrients are in the water, like run-off from agricultural land  
loaded with nitrogen, it could very easily be an algae bloom that deprives the  
fish of their oxygen.
Back in Wisconsin many of the lakes get clogged by mid summer with blue  
green algae that blooms when the water warms up. It feeds off the nitrates and  
can get quite thick on the water surface and really stinks making the lake  
unusable. Streams that run through farm land now have setbacks to prevent cattle  
from destroying the banks and adding nitrogen to the water. Farm run-off is a  
big problem, but can be taken care of with some good old fashioned
common sense. 
 
Rummy