Natural Swimming Pools, Biofilms, Cyanobacteria & The Science of Uncertainty

In this thought-provoking Floc It Friday episode, Rudy Stankowitz takes a step away from chemistry myths, manufacturer sound-offs, and social media debates to explore a topic that has generated considerable discussion in both the pool industry and online communities: natural swimming pools. Drawing from four peer-reviewed scientific studies provided by Professor Charles Gerba, Rudy examines what the current scientific literature actually says about biological water treatment systems, pathogen control, microbial communities, and public health.

Before diving into the science, Rudy also shares a personal message recognizing National PTSD Awareness Month, discussing the unseen challenges many industry professionals carry and reminding listeners that they are never alone in their struggles. 

In This Episode

•  Why natural swimming pools represent a fundamentally different philosophy from traditional disinfected pools 
•  The role of biological treatment systems, regeneration zones, gravel beds, and microbial communities 
•  A review of a documented 2001 German outbreak involving more than 200 illnesses associated with a public nature-like swimming pond 
•  What researchers discovered about swimmer exposure, water ingestion, and viral transmission 
•  The findings of a Canadian risk assessment examining pathogen behavior in natural swimming ponds 
•  How filtration rates, turnover times, and treatment efficiency influence health outcomes 
•  The potential role of UV disinfection and why questions remain about its interaction with biological ecosystems 
•  Research from Spain examining microbial populations and fecal contamination in natural swimming pools 
•  Wildlife as a potential source of contamination in recreational waters 
•  The importance of biofilms and the complex microbial communities that inhabit them 
•  Why cyanobacteria, algae, and aquatic microbiology continue to raise important scientific questions 
•  The challenges of identifying microbial populations without site-specific testing 
•  What a 2024 One Health review reveals about algae, cyanobacteria, recreational water quality, and public health 
•  The difference between visible water quality and the unseen biological processes occurring beneath the surface 
•  Why scientific uncertainty is not a weakness, but a critical part of the scientific process 

Key Takeaway

The current scientific literature does not conclude that natural swimming pools are inherently unsafe, nor does it suggest that all questions surrounding their operation have been answered. Instead, the research consistently points toward the need for continued study, monitoring, challenge testing, and a deeper understanding of the biological communities responsible for water treatment. As Rudy emphasizes throughout the episode, science advances not by defending positions, but by asking better questions. 

Topics Discussed

•  Natural swimming pools 
•  Biological water treatment 
•  Recreational water health risks 
•  Pathogen control 
•  Biofilms 
•  Cyanobacteria 
•  Algae ecology 
•  Public health 
•  Water quality monitoring 
•  Environmental microbiology 
•  Charles Gerba 
•  Risk assessment 
•  One Health research 

Mentioned During the Episode

•  Professor Charles Gerba 
•  Canadian Natural Swimming Pool Risk Assessment 
•  German Nature-Like Swimming Pond Outbreak Investigation 
•  Spanish Natural Swimming Pool Microbial Study 
•  2024 One Health Review on Algae and Recreational Waters 
•  National PTSD Awareness Month 

Sponsors

The 2026 Talking Pools Podcast Pool Industry Mentor Award is proudly supported by:

•  BlueRay XL 
•  LaMotte Company 
•  Revved Up Apparel 
•  Aqua Comfort Water Group 

Research on Natural Pools https://drive.google.com/file/d/1QpahWoVh3DDoNPwdw3oFsnbmUEj_umrS/view?usp=sharing

Connect With Talking Pools

🌐 Website: https://www.talkingpools.com

Transcript

SPEAKER_05 0:00

Welcome, it's Flocky Friday on the Talking Pools podcast. Weekend Eve. I posted something on on social media and I want to share it with you guys as well. Because a lot of you have known me for a long time, and I have a lot of friends in this industry. A lot. I know a lot of you are listening, and whether we met at a trade show through social media, or however, you're friends. And that means a lot to me. I know I've got haters too. There's probably a good five, six people I could name right off the top of my head that I can't stand. That's just real life. I have letters after my name. A lot of us do. C P O, C P O I, A F O F O I, C P I A E E, that's aquatic energy something. I forget what it stood for.

SPEAKER_01 0:52

Anyway, I also have letters that people can't see. G-A-D. M D D P T S D. Why share this? I'm not the only one.

SPEAKER_05 1:10

I know a lot of folks listening also have letters that no one else knows, but I do know that there's really nobody that talks about it in our industry, and someone should be fucking open and honest about it. So here I am. And if it makes you feel differently in a bad way about me, go touch grass. Because you don't know what battles I have fought, you don't know what wars I have been in. So if you're carrying letters like that, like the ones I have, or different ones, or no letters at all, it doesn't matter. You are not alone.

SPEAKER_02 1:54

If today is heavy, if the hard days are winning ground, I want you to know I am here. One VM, one text, one email away, talkingpools at gmail.com. No judgment.

SPEAKER_05 2:13

Just a friend who gets it. We don't let the hard days win. And on the days it looks like they're ahead, we fight back. Together. This is National PTSD Awareness Month. My demons know I don't quit, and yours do too. You've got this, and you've got me. If you need me. I've been getting service industry news since I first stepped into this business, and every time it landed, I did the same thing. Flip straight to the horror file. The weird installs, the absurd finds, the stuff only pool pros ever see. Then I'd go back and read the articles. Service Industry News is a twice-monthly trade publication for pool and spa service text, 24 issues a year, emailed free to over 10,000 texts and available on their app. Every issue covers nationwide industry news and real technical content you actually will use.

SPEAKER_04 3:10

Get your free subscription at serviceindustry news.net. Again, that's serviceindustrynews.net. Do it now.

SPEAKER_00 3:20

The sponsors of the 2026 Talking Pools Podcast Pool Industry Mentor Award are Blu-ray XL, Lamotte Company, Revved Up Apparel, and the Aqua Comfort Water Group. These manufacturers truly understand the importance of mentorship in the industry.

SPEAKER_05 3:36

Okay, so on a lighter note, cuz I'll tell you what, all that, I still live my life. Every injury I have, I earned it, and I'm proud of it. Today I want to do something a little bit different. No chemistry myths, no manufacturer sound-offs, no industry gossip, no social media arguments. Today I want to look at four peer-reviewed scientific papers that landed in my inbox after a conversation with Professor Charles Gerba of the University of Southern Arizona. And honestly, the papers were more interesting than the argument, because after reading these, I realized this isn't really a discussion about natural swimming pools. It's a discussion about certainty. More specifically, how much certainty we actually have. Natural swimming pools have been around for decades. Thousands have been built throughout Europe. Some are private, some are open to the public, some are absolutely beautiful. And that's important to acknowledge right from the beginning. They are gorgeous. But this is not an episode about aesthetics. These systems can be beautiful, crystal clear water, native plants, natural landscapes, no chemical smell, no traditional swimming pool appearance. I understand the appeal. I really do. What interested me wasn't how they looked. What interested me was how they worked. And even more importantly, how we know they work. Because that's where the science gets interesting. Every question I asked about the science in the process was not met with the sharing of data. Every question I asked resulted in a personal attack. Everything from Rudy wants to chlorinate the world, Rudy won't leave the house unless he is wrapped in bubble rap. Rudy probably drinks chlorine to promote gut health. And as clever and humorous as some of these were, it told me one thing loud and clear. According to the 2021 Canadian Risk Assessment, natural spoon pools are recreational waters that substitute biological treatment processes for traditional chemical disinfection. Listen to that carefully. Substitute, not supplement, substitute. Instead of relying upon chlorine residuals continuously present in the water systems, these systems depend upon biological treatment processes, plants, gravel beds, filtration, microbial communities, regeneration zones, natural nutrient cycling, biological activity. It's a completely different philosophy than conventional swimming pools. And honestly, it is an absolutely fascinating idea. Because for more than a century, public swimming pool safety has been built around a simple concept. Maintain a disinfectant residual. If contamination enters the water, chlorine attacks it. The system assumes contamination will occur. Natural swimming pools approach the problem differently. The treatment system is expected to process contamination through ecological mechanisms. And that's where the questions begin. Not because the concept is wrong, but because biology is complicated, much more complicated than chlorine. And the scientific literature repeatedly reminds us of that fact. The first paper caught my attention, wasn't theoretical, it wasn't a laboratory study, it wasn't a computer model, it was a real outbreak. In the summer of 2001, Germany experienced an outbreak of aseptic meningitis involving 215 people in and around the city of Kassel. Investigators eventually identified bathing in a public nature-like swimming pond as a significant risk factor for the illness. Stop there. 215 people. Not five, not ten, not a family cluster. The facility involved was not an abandoned pond. It was not a neglected swamp. It wasn't somebody's backyard experiment. It was a facility open to the public. Originally built as a chlorinated swimming pool, it had been converted into a nature-like swimming pond and reopened earlier that year. The pond relied upon natural treatment processes because oxidizing disinfectants such as chlorine can't be used in those systems. During summer, as many as 1,500 people a day visited that facility. Researchers conducted an extensive epidemiological investigation. What they found was remarkable. Individuals who spent more time in the water had greater risk of illness. Individuals who swallowed more water had the greatest risk of illness. Eventually, investigators identified ecovirus infections among the patients and recovered viral genetic material from pond water samples that closely matched patient isolates. Now, here's where nuance matters. The study does not conclude that all natural ponds are dangerous. It does not conclude that natural pools cannot work. It does not conclude biological treatment is ineffective. What it does conclude is that nature-like swimming ponds have the potential to cause widespread community infection with substantial public health impact. That sentence should make every pool professional pay attention. Because regardless of where you stand on natural pools, outbreaks matter. They tell us something, they reveal weaknesses, they expose assumptions, and they force us to ask difficult questions. The second paper attempted exactly that. Researchers in Canada performed a screening level risk assessment on a public natural swimming pond. Their objective was simple. Determine what happens when pathogens are introduced into a recreational water system that contains no disinfectant residual. And if you're a pool guy or a pool gal, the answer should immediately grab your attention. Because every swimmer sheds microorganisms. Every swimmer, children, adults, Olympic athletes, pool guys, pool girls, everybody. Researchers evaluated three representative pathogens norovirus, campliobacter, cryptosporidium. Not because those are the only pathogens that matter, but because they represent different categories of microbial risk. Viruses, bacteria, protozoa. The authors noted something incredibly important. In natural swimming ponds, human pathogens introduced by swimmers remain untreated until the water passes through sufficient natural barriers. Think about that. It's not criticism. That's engineering. That's literally how the system functions. In a chlorinated pool, contamination immediately encounters disinfectant residual.

SPEAKER_06 11:28

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SPEAKER_05 13:27

In natural swimming ponds, human pathogens introduced by swimmers remain untreated until the water passes through sufficient natural barriers. Think about that. It's not criticism. That's engineering. That's literally how the system functions. In a chlorinated pool, contamination immediately encounters disinfectant residual. In a natural pool, contamination encounters treatment processes later. The difference may be minutes, hours, maybe longer. Timing matters. The treatment efficiency matters. The turnover rate matters. The researchers found that risk was heavily influenced by the filtration rate and turnover characteristics of the treatment system. They also found that adding UV disinfection reduced uncertainty and improved pathogen reduction. But these systems don't use UV, do they? They did also acknowledge that the impact of UV on biological treatment ecosystem was unclear. Maybe that's why UV is not in play. Maybe it kills the plants. The authors concluded that further information is needed regarding natural barriers for pathogen removal and specifically recommended challenge studies. Challenge studies. Challenge studies, scientists asking questions, scientists admitting uncertainties, scientists identifying knowledge gaps. That is how science is supposed to work. And that's where the third paper becomes super important because it asks the question I already found myself asking repeatedly. Who exactly lives in these systems? So researchers in Spain examined microbial populations associated with natural swimming pools. Their findings were actually fascinating. Three of the four pools they were looking at exceeded recommended limits for either E. coli or enterocochi. The researchers concluded that wildlife appeared to be an important source of fecal contamination and noted that health risks may be higher than in conventional swimming pools. Again, notice what they didn't say. They didn't say every natural pool is unsafe. They didn't say the concept doesn't work. They didn't say abandon the technology. They said further research is necessary, and that phrase keeps appearing. And honestly, I think that's the most important finding in all four papers. Not certainty, but uncertainty. Because uncertainty tells us where the science still needs work. The Spanish study describes systems where water treatment occurs through biological filters, gravel beds, plant root systems, and microbial communities inhabiting substrate biofilms. Biofilms. There it is. The word that kept pulling me back into the papers. Because biofilms are not single organisms, they're communities. Complex communities, cities of microorganisms, bacteria, fungi, protozoa, algae, cyanobacteria, viruses, predators, prey, competition, cooperation, evolution happening in real time. That's what a biofilm is. And here's what fascinated me. The papers repeatedly discuss the importance of these biological communities. Yet they also repeatedly identify knowledge gaps regarding their behavior, stability, and pathogen removal capabilities. As someone who spent years studying algae, cyanobacteria, biofilms, and aquatic microbiology, that's where my attention goes. Because when somebody tells me a treatment system depends on biology, my next question is always okay, great. Which biology? Who is there? What dominates? What changes seasonally? What happens under stress? What happens when nutrient loading increases? What happens when wildlife contributes contamination? What happens during periods of high bather load? Those aren't anti-natural pool questions. Those are microbiology questions. Interestingly enough, they are the same questions researchers continue asking. Here's another factor. If there's no testing on site, that's the scary part because I can't tell you what's going to be in a biofilm. I can go collect samples around wherever, and I can analyze those samples and I can identify what is in those samples. It does not mean that what I see is going to be the same in a biofilm at your pool. The primary constituents of these biofilms, which in my experience have always been cyanobacteria, vary depending on geographic location, sometimes in distances as short as a mile. So if you have one person in one city saying, oh, well, this is what's in the biofilm, you cannot guarantee that that is what is in the biofilm on the next block unless you're actually testing. The fourth paper, a 2024 review examining algae in recreational waters through a one health perspective. This paper is probably the important of all four because it broadens the discussion. We're not talking just about pathogens, it's not just about ecosystems. The review discusses algae, cyanobacteria, biofilms, water quality, human health, animal health, environmental health, and how all of these things interact. The author notes that cyanobacteria and algae can produce biologically active toxins and emphasized that recreational exposure may occur through ingestion, aspiration, and inhalation. Now, before anybody starts hearing things, I didn't say not every cyanobacteria produces toxins. Not every algae bloom is toxic. Not every natural pool contains harmful species. The paper doesn't say that. The paper says these organisms are important enough to monitor because they can affect water quality and public health. That's a very reasonable position. The authors noted that visible algae may represent only the visible component of a much larger microbial community. Now is the part that I love, because this is exactly how I've viewed aquatic systems for years. The thing you see is often the symptom. The thing you don't see is the story. A patch of algae isn't just algae, it's evidence. Evidence of nutrients, evidence of ecology, evidence of biology, evidence of an entire community functioning beneath the surface. And that's what makes these papers so interesting. Because collectively, they paint a picture that is far more nuanced than the internet conversation. The literature does not support the claim that natural pools are automatically dangerous. Literature also does not support the claim that all questions have been answered. Instead, the literature repeatedly points toward uncertainty, research needs, data gaps, additional study, monitoring, characterization, challenge testing, further investigation. And frankly, that's where I think the conversation belongs. Not in marketing, not in ideology, not in camps, not in tribes, in science, in science, because science is comfortable saying we don't know yet. That's not weakness, that's honesty. So where do I land after reading all four papers? Natural swimming pools are fascinating. They are innovative, they are beautiful, they are biologically complex, and they represent a fundamentally different approach to recreational water treatment. What the literature demonstrates is that outbreaks have occurred, pathogen risks exist, wildlife contamination exists, microbial uncertainties exist, biofilms matter, algae matter, cyanobacteria matter, and researchers continue asking important questions about how these systems function and how consistently they protect public health. Maybe that's the most important lesson from all of it. The goal of science isn't to defend a position. The goal of science is to ask better questions. And after reading these papers, I don't think the biggest question is whether natural swimming pools can work. Clearly they can. The bigger question is whether we fully understand why they work, when they work, when they don't work, and exactly who is living in the biological communities we trust to keep swimmers safe. And that, my friends, is a question worth asking. I'm gonna attach links to those papers that Professor Gerber sent me. So you can take a look at them yourself if you want. They'll be in the show notes. That's all I have for this week. I'm Ruby Stankowitz.

SPEAKER_02 22:41

This is the Talking Pools podcast. Until next time, be good.

SPEAKER_01 22:48

Be safe.