Pond Boss Forums INTRO Questions & Observations DO crash and dieoff?

Postbeetle -Situation Details
1. pond appx one acre
2. lack of moisure the last two years it is now at 1/2 acre.
3. I feed during summer Purina pellets, 1/2 coffeecan daily.
4. stainless steel diffuser (made by engineer friend) in 6' of water powered by 1/4 hp Gast rotary vane - run 24/7
5. � ac pond with low water is at 12' depth
6. pond is extremely fertile - residence to 100's of migrating ducks and geese. Water pea green and scummy. That's what I saw that evening, the next morning the water was brown and my fish did not eat.

fish kill opinion -analysis

Here is what I think happened in your situation.
1. Diffuser was originally placed too shallow at 6 ft deep which from your description so far -I assume the diffuser is currently sitting in about 3 ft of water due to the current low water level. In smaller ponds the diffuser should be at pond's deepest point so water is moved out of the deepest bottom area (belly) and to the surface of the pond. Even though the boil is active, the water is only being pulled from 3 to 6 ft deep which in YOUR case leaves a layer of 6ft to 10 ft of water deeper than the diffuser. The resulting water circulation from putting an air diffuser shallow is similar to that of those who move their diffuser to shallow water during winter to reduce super cooling of the entire pond. In your specific situation, even with a diffuser at 6ft deep, when hot calm weather occurs, the pond can/will stratify (form a warm layer) at level (depth) close to the depth of the diffuser�s head; in your case 3-6 ft deep. A diffuser at a shallow depth in a small pond does not create strong enough water currents to cause the circulating warm, less dense water to penetrate the cooler more dense deep water zone. The circulating upper warm water will move outward and down along the bottom contour and it seeks the path of least resistance in its return circulation to the diffuser which in your case was at a depth of 6 ft or shallower. Wind mixing of the upper layer in most shallow ponds is also at about 6 ft. The placement of the diffuser enhanced this upper warm layer. This means your pond formed a deep anoxic bottom layer (devoid of dissolved oxygen and formed a build up of hydrogen sulfide). This anoxic layer could have compounded the dissolved oxygen loss after the bloomed died.

2. Pond was fertile with a heavy algae bloom in the warm layer. Feeding pellets enhanced fish biomass. Hot weather reduced amount of DO that water can absorb. Bloom crashed, water turned brown from rapid decay of dead bloom. Heavy bacterial growth from consuming the dead bloom consumed most of the available dissolved oxygen. Weather conditions or physical conditions from the bloom crash may have helped cause a partial turnover where the upwelling water containing poisonous hydrogen sulfide might have assisted in the degradation of the top layer to cause fish suffocation. One diffuser may not be able to add enough dissolved oxygen to a small pond during a massive bloom crash. Diffusers have different efficiencies of transferring dissolved oxygen to water (bubble size and depth of diffuser are important). Blooms can be at different sizes (biomass) when they crash. A pond�s natural or normal daily oxygen consumption (BOD) also enters the situation during a crash. Together, the two will exert varying amounts of oxygen loss. One time, a crash may not kill fish whereas another time fish will die. Water temperature and the ability of water to hold dissolved oxygen also can be important in determining if fish die during a crash.

3. I think if the pond was completely mixed during the crash of the bloom the overall amount of dissolved oxygen in the entire pond might have been enough that only a partial fish kill or no fish kill would have occurred. Complete mixing of the pond might have reduced the overall size of the bloom and prevented a total algae die off.

Hind sight is always better than fore sight. Lots of possibilities in cases like this.