I tried this with duckweed one year.
Mats of the stuff make fair weed barrier mulch. Works better than straw.
Collecting it: Rig your manure wagon with a removeable screen with about 1/2" mesh. Hardware cloth supported by concrete reenforcing mesh should work.
Back your wagon to the pond edge.
Use a trash pump to skim the pond, pump the water into wagon, and let the water pour back out into the pond.
May work well if you can figure out how to support the snout of the pump hose at the corner of a 90 degree float. A couple of 2x4 studs make work well.
To help it along, make another float that you can drag across the pond to heard scum to the corral.
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We have this as a problem in a local lake. (I am an occasional advisor on their board) Numbers:
It takes roughly 500 pounds dry weight of most algae to contain 1 pound of phosphorus. (the problem nutrient in ponds)
Algae runs 90 to 95 percent water.
So it takes 5000 to 10000 pounds of algae to get 1 pound P.
N usually runs several times P, but the life span of N in a lake is only about 5 years, eventually being released as N2 gas, where P keeps recycling.
What may be easier to do would be to swap water with the farmer.
Pump water from your place to his pond. Pump filtered pond water to your trees. The water will have probably a factor of 10 to 100 less nutrients in it, but it will be a lot easier to do.
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One post mentioned disease in nursery retention ponds. Not the same. In that case the ponds are part of a complete cycle, and the disease isn't usually propagating in the pond. Bacteria tend to encyst in unvarioable conditions, fungi have spores etc.
Some too depends on how fast the reuse cycle is. A pond with a one week retention time is far more likely to re-infect plants than one with a 1 year time. In this example, it's not a closed cycle. There is no runoff from the walnut grove into the pond.
The usual reason for nursery retention ponds is to save fertilizer -- you want to get that back at the trees/crops before some pond scum eats it, so retention times tend to be short. (Some ponds are present to reduce discharge into local streams and water table.)