Came for the deadly algae, stayed for the giant white fish

The state of Virginia has a handy little page.

Decaying bacterial blooms will deplete the dissolved oxygen in the water and fish may die as a result. Dense bacterial blooms floating on the surface will also block out sunlight that is needed for other organisms to survive. Some bacterial bloom toxins are directly harmful to fish and can cause fish kills.

http://www.vdh.virginia.gov/epidemiology/dee/habs/cyanobacteria/index.htm

What's more interesting is the cause.. a nutrient overload. Probably dog crap in this case. It accumulates and then a major rain carries it into the pond. It's kind of funny that dogs are now at risk there like some karmic payback.

What causes blue-green bacterial blooms?

High quantities of nutrients in the water often lead to the formation of bacterial blooms. Nutrients may concentrate in natural bodies of water due to:

Inadequate water flow or exchange of water when tides change.

Nutrients such as nitrogen and phosphorus from fertilizers used on farms and in urban and suburban yards, sewage, industrial waste, etc. being washed into waterways.

Changes in rivers and their surroundings due to land clearing, farming practices, construction and housing.