Sorry to hear about your fish kill.
I am NOT an expert on this topic, so I am just repeating what I have learned on Pond Boss.
If your fish kill is due to a low oxygen situation, then it is typically the largest fish that suffer the highest mortality rates. (I think it is a surface area of gills to body mass ratio issue, which gets worse as a fish gets larger.)
Lots of Water Shield usually means lots of shallow water. It also means very poor wind mixing due to the surface layer of your plants. At the start of August, you probably had very warm water in your shallow areas plus unoxygenated, unmixed water in your deepest spots. That is perfect conditions for a cool summer rain to cause your pond to turn over and put your fish into that unoxygenated water. Further, your pond water probably gets most of its oxygen from the Water Shield, rather than normal wind mixing. Several cloudy days in a row, either before or after the rain, would have shut off your oxygen generating plants. Finally, when your algal bloom died off, the decomposition process actually CONSUMES dissolved oxygen in your pond.
I think you got hit by all of the possible "bad" things at the exact same time.
Can you go out to your pond right at dawn? That will be the lowest oxygen point of the 24-hour cycle. Can you observe your bass and BG piping at the surface for oxygenated water? If so, you are still in the middle of your low oxygen event.
I do not believe that your Water Shield killed the algal bloom, I believe it was more likely just part of the bloom boom-and-bust cycle. However, I am pretty certain you do NOT want to kill your Water Shield right now. That will only make things worse with additional decomposition of plants in your pond!
Lots of speculation above on my part. Hopefully we can get some more expert advice into your thread to give you some better guidance.
I'm leaning more towards the bass being stressed by lack of forage and that the extensive water shield is providing too much cover for smaller fish. I do think the bloom was keeping the water very oxygenated and a lower o2 level may also have a impact.
I absolutely believe the watershield was competing with the bloom for nutrients - the pattern seems that the bloom kicks off every summer around late may and visibility drops from 3'-4' to maybe 12" to 2' and then clears up in late summer as the watershield is at peak.
I suspect the bass were able to catch forage better in the lower visibility conditions of the bloom using their lateral line.
i noticed that the BG were much more aggressive feeders when the bloom was on. They seem much more leery of feeding in the clearer water (3' to 4' visibility).
I've been battling water shield over last 5 years - it all tends to die off by November and creates something off a bloom then. I plan to kill it off in sections.
As far as shallow water - I think my pond may not have enough shallow water as I'm not sure my BG are breeding fast enough to support the bass. Most of the pond drops to 4-5' deep within a few feet of the shore. The water shield seems to grow in water up to about 6' deep. I wonder if it is getting too thick for the BG to spawn effectively.
