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#473136 - 06/01/17 12:21 PM Azolla problem solved
T. Gray Shaw Offline

Registered: 09/20/10
Posts: 13
Loc: Humboldt Co., California
I used a three-part solution:
- aeration
- skimming
- bacterial pellets

One month ago, I used a 100' floating boom to skim the Azolla covering our 100' x 40' pond in Southern Humboldt County, CA. The Azolla has not come back, although a tiny population persists on the extreme edge of the pond.

I began aerating the pond last year. The aerator eliminated the rotten egg smell, but the Azolla did not go away. I collected some with a 4' boom, but it grew right back.

So next I bought the 100' boom and skimmed 99.9% of the Azolla, which took half a day with one helper. I expected the Azolla to grow back, but no. I attribute this to changed chemistry in the water from the aeration over six months, because the muck was still there.

Now that the Azolla is gone, the pond can heat from the sun, so that the bacteria can work on the muck. It's possible that the Azolla will grow back unless I eliminate this nutrient source.

We are off-grid, so I bought a 500W aerator and powered it from our 1kW solar system. It's switched on and off by our charge controller, which has a 12V circuit that can be programmed to open and close according to the battery levels. I had a custom DPDT relay built to tie the controller to AC from the inverter.

Some aerator vendors tell you your pond must be aerated 24/7. My experience aligns with vendors who say that daytime operation is sufficient.

This has been a long ordeal. Some of you will have read my earlier posts about it. I offer to provide the names of the folks who sold me what I actually needed, so that others can get relief.

The frogs are croaking wildly, fish are popping the surface, and the water lilies are growing back. We filled four compost bins mostly with the Azolla. My wife is also very pleased that the pond reflects the trees again.

I'd like to upload images. "Enter an image" only offers HTML options. Any help?
T. Gray Shaw
ISA Certified Arborist # WE-1037
Redway and Berkeley, CA

#474519 - 06/20/17 11:32 AM Re: Azolla problem solved [Re: T. Gray Shaw]
T. Gray Shaw Offline

Registered: 09/20/10
Posts: 13
Loc: Humboldt Co., California
More info about the muck: The instructions on the muck-eating bacterial pellets I bought say that the water has to be at least 60°F. Last summer, under the Azolla, it never cracked 60°. The Azolla insulated the pond, and now that it's gone, the pond can heat up enough for the bacteria to thrive.

Before aeration, the pond stratified because the muck was consuming oxygen for decomposition (BOD) at the bottom. Aeration broke up the stratification and increased the dissolved oxygen, but the muck decomposition was still being slowed by the low water temperature. Not only was the Azolla keeping the muck from decomposing faster, it was contributing to it by constantly dying and sinking to the bottom.

Reducing the muck takes more oxygen temporarily. Once it's done, the oxygen level should be higher on its own. There will always be muck, it's just not good to have a thick pile of it.

I theorize that Azolla colonized the pond when the BOD dropped to a certain point. The muck itself may have contributed to the colonization, chemically. I think I have proven that Azolla will not colonize a pond in the presence of adequate oxygen, regardless of a high muck level. I think I have also proven that Azolla will not always go away by itself by just restoring the dissolved oxygen - you probably have to skim it.
T. Gray Shaw
ISA Certified Arborist # WE-1037
Redway and Berkeley, CA

#474522 - 06/20/17 12:05 PM Re: Azolla problem solved [Re: T. Gray Shaw]
Bill Cody Offline
Field Correspondent


Registered: 04/18/02
Posts: 12688
Loc: Northwest Ohio - Malinta OH
Gary - This is good information. It is good to have creative thoughtful input.
Keep This Forum Viable, Read Pond Boss Magazine -
America's Journal of Pond Management

#497058 - 10/01/18 12:32 PM Re: Azolla problem solved [Re: T. Gray Shaw]
T. Gray Shaw Offline

Registered: 09/20/10
Posts: 13
Loc: Humboldt Co., California
The Azolla has stayed gone. The aerator was only operating during the day while the sun was strong, and it had to be turned off between Sept. and March due to limited solar power (shared with the cabin), but this did not cause the Azolla to return. There was a small sulfur odor for a few minutes when I turned it back on in April, but nothing like July 2016 when I first plugged it in.

Furthermore, my aquatic plants have recovered and spread—water lilies have self-propagated, and cape pondweed grew back from near-annihilation.

The upshot is that aeration will prevent Azolla, and aeration in combination with thorough skimming will eliminate it.

I made compost with the Azolla that I skimmed, and it turned out well, but I had to greatly boost the N with another material to get up to 150°F for proper decomposition. Azolla is reputed to be high in N because the cyanobacteria within it fix N from the air. However, experience indicates that it will not serve to bring down the C:N ratio to the required 30:1 in combination with straw, as manure will. Hence I do not recommend maintaining Azolla in your pond as a high-N substrate for making compost.

I don't know how important the bacterial pellets are for reducing the muck. I applied them only once this summer, because the pond was no longer a squeaky wheel at our homestead. Time will tell whether aeration sufficiently controls the buildup of muck without a more diligent pellet application regime—more on that later perhaps.

Edited by T. Gray Shaw (10/01/18 12:40 PM)
T. Gray Shaw
ISA Certified Arborist # WE-1037
Redway and Berkeley, CA


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