photo of a pool showing algae growth

The idea of clearing a pool of phosphates in order to fight algae has been gathering momentum for years. The approach is simple: Since we know algae eat phosphates, we take away their food supply and starve them into submission.

That logic has spawned a number of products in the pool and spa industry that remove phosphate from pool water in order to keep it from going green. This magazine has published a number of articles on this approach to pool water care and interviewed advocates in the service sector who aver the battle plan works great.

But what does science have to say? Quite a bit, actually. In an industry that has never had a lot of money for R&D, the reproductive cycle and dietary patterns of algae are comparatively well researched.

Karen Rigsby, a biochemist by training and experience, now business support manager at BioLab, has studied the research and believes when it comes to algae and phosphates, empirical evidence from the lab should better inform treatment options for pool water.

The main challenge that comes from the people in the white smocks concerns the eating habits of the single-celled creatures. Yes, algae eat phosphate, but the real picture is much more complicated.

There are literally thousands of different species of algae, and they all differ when it comes to their phosphate diet. So a phosphate level that would starve one type of algae will provide perfect nutrition for another.

Actually the picture is even more complex than that. For each type of algae, there is a minimum (not enough), optimum (perfect amount) and maximum (too much) level of phosphate. If there’s not enough phosphate or too much phosphate available to a particular type of algae, growth is restricted. Only the optimum level of phosphate produces the bloom (if all other growth factors, such as presence of other nutrients and lack of sanitizer are in line).

But with a multitude of algae species, each with its own dietary needs, the starvation phosphate level for one is optimum for another and too much phosphate for a third.

“The research shows that minimum and maximum phosphate levels across the range of over 7,000 species of algae can range from below 1ppb to 20,000ppb.” Rigsby says. “Therefore, at any given concentration of phosphorous, one group of algae will be deficient, one will be at optimum levels and one will be inhibited.”

Which begs the question: Is there an extremely low phosphate level, which would deny sustenance for all algae?

Not really. Phosphate levels of 100ppb are often listed as “normal,” and phosphate remover applications strive to reduce phosphate levels below 20ppb (which is difficult to maintain) but Rigsby points out that several species of algae are known to have optimum ranges even at that low phosphate level. Which means that for these types of algae, a freshly treated pool would still provide a perfect banquet.

Fat Algae

Another problem with the phosphate approach is what biologists call “luxury phosphorous uptake.”

“This is the ability of algae to store phosphorous within their cells,” Rigsby says, “allowing them to grow even at deficient levels of phosphorous.”

Which means that, even if you can temporarily clear a pool of phosphate and thereby put some species of algae on starvation rations, just like chubby humans denied food, those algae can live for a while off their stored phosphate blubber.

In addition, there’s a point to be made about the immense complexity of the bio-chemical reactions in a swimming pool. A number of factors are involved when you look at the eating habits of algae, and reducing the equation to a statement like “less phosphate, less algae food” is an oversimplification.

Or as Rigsby puts it, “The level of phosphorous is only limiting with regard to other factors.  If all other factors (i.e. nitrogen, etc.) are limiting AND phosphorous is limiting, then the addition or removal of phosphorous will affect algal growth in the ways discussed above. However, if another factor is added (and there are plenty of those in pool water) then algal growth can proceed uninhibited by phosphorous.”

Demanding Chlorine

Outside the debate about algae mealtimes, a common belief is that phosphate in a pool can create chlorine demand — and all the problems that arise when you can’t generate a chlorine residual.

Rigsby firmly contests that assertion.

“Phosphate as it relates to chlorine demand is really just basic chemistry. You can look at the preferred oxidation state of phosphorous in the phosphate molecule and know that it will not react. That can be found in any general chemistry textbook.

“If it did react,” she adds, “you would be able to remove phosphate by shocking the pool.”

Proof In The Pool

The complete physical reality of the algae/phosphate interaction in a backyard pool (like any physical reality) is beyond our ability to know; we must take available research and our own observation and experience and use them as a guide.

Any number of scientific models may be at work in backyard pools, which may include both these lab results and the efficacy of phosphate treatments. For instance, some phosphate removal treatments may reduce phosphate levels to the deficient zone for the most pernicious of algae species.

And while guided by research in the question of algae and phosphate consumption, Rigsby notes that phosphate removers may be helpful tools in some cases.

“I don’t think there is anything wrong with using a phosphate remover as an add-on to a chemical regimen,” she says. “In fact, many phosphate remover products are actually great clarifiers or filter aids and can aid in clearing the water. However, the use of a phosphate remover in place of an EPA-registered algaecide is not advised.”

In the ongoing debate about pools and algae, the points raised in this article are merely ones made by biochemists studying algae in controlled lab conditions. In the final analysis, it’s the service technician whose customer account is on the line who will have the last word. If phosphate removal treatments lead to clearer pools and happy clients, phosphate removal treatments will continue to grow. Ultimately, the proof will be found in the pool.

References used to support this article: 1. Phosphorous Chemistry in Everyday Living, Arthur D.F. Troy (chapter 8) 2. Biomineralogy of Phosphates and Physiological Mineralization, Chapter 26 entitled Phosphorous Nutrition of Algae, by Joseph C. O’Kelley 3. Phosphorous and Its Compounds, Van Waze

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Scott Webb is Executive Editor of AQUA Magazine.