Even with aerators running it's still possible for DW to have that big an effect? Wow wouldn't have guessed that...
Oxygen is transferred to the pond water by the interface between the water and the atmosphere. Some O2 is transferred by the interface between the water and the aeration bubbles, but the majority comes from the atmosphere. That's one of the reasons why finer bubbles that are found in membrane diffusers are better for summer aeration - finer bubbles bring more water to the pond surface than bigger bubbles.
If the DW was covering 90% of the pond, then there might not be that large of an area from the diffuser boils that is in contact with the atmosphere, and not enough O2 was transferred to the water.
Then you have all the surface area of the pond that is covered by DW, and the resulting shade does not allow sunlight to the phytoplankton, which actually produces a LOT of dissolved O2 during the day via photosynthesis. When there is considerable shade, but especially at night, the bacteria, phytoplankton, algae and plants in the pond consume O2, along with the fish. Cloudy, windless days in ponds that have a high phytoplankton count (i.e. bloom) will also experience "summer kills", especially in the wee early morning hours due to an O2 crash. That's why it's important to watch the water clarity, and any BOW with less than 12" visibility due to a bloom runs the risk of a "summer kill".
In addition, a pond can have an aerator that produces bubbles, but not properly designed/sized for that BOW. So a pond could be "aerated" but it still won't help a lot in times of high oxygen demand.
