Alum binds with phosphorous dropping it to the pond bottom. This robs planktonic algae of an essential nutrient required for it to grow. Without nutrients necessary for growth, it will abate. I'd google alum treatment for algae blooms and I'm sure it will be covered in greater detail.


"Recreational and aesthetic uses of ponds are often impaired by excessive algal
growth. This condition is generally manifested by low transparencies and scum
conditions leading to odor problems and, upon decomposition, anoxic conditions in deep
waters. Historically, nuisance algae has been controlled with algicides, most commonly
copper-based chemicals (i.e. copper sulfate). The mechanism of action involves
disruption of photosynthesis. While the effect of this treatment may be immediate, it is
also temporary and may, in fact, compound the problem. Algal densities are usually
limited by either nutrient (phosphorus or nitrogen) concentrations or by grazing by
zooplankton. Upon their death, algal cells release nutrients, allowing for subsequent
blooms. Since copper compounds also kill zooplankton, which rebound more slowly than
algae, decreased grazing may benefit the rebounding algae. A sudden, widespread algal
die off often results in massive reductions in dissolved oxygen, leading to fish mortality.
Many species of blue-green algae develop a tolerance to algicides, leading to a situation
where these undesirable organisms are favored unless treatment rates and frequencies are
increased. Also, copper compounds are non-selective, killing vascular plants (through
interruption of photosynthesis) and certain animals (through direct toxicity) (Anon.,
1990).
An alternative method for managing algal populations (as well as non-algal
controlled water clarity) involves the use of buffered alum (aluminum sulfate buffered
with sodium bicaronate). Buffering is necessary to maintain pH between 6-8; aluminum
may become toxic when pH drops below 4.5-5.5. When dissolved, aluminum sulfate
dissociates, releasing aluminum ions. In the appropriate pH range, these ions immediately
undergo a series of hydrolysis reactions resulting in the formation of aluminum hydroxide
(AI(OH))). This compound forms a visible floc which, over several hours, settles through
the water column. As it falls, inorganic phosphorus adsorbs to the floc; also, the water is
clarified as particulate matter (algae cells, particulate organic and inorganic material)
becomes entrapped in the coagulated aluminum hydroxide. The potential for long-term
control exists because after phosphorus is removed from the water column it remains
adsorbed to the aluminum hydroxide in the sediments, preventing intemalloading which
is often a significant phosphorus source in shallow waterbodies (Cooke et at., 1993). "[b][/b]

Many men go fishing all of their lives without knowing that it is not fish they are after. ~ Henry David Thoreau

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