Water filters and Blue Green Algae?

We did a few days on Lake Champlain and found out when we got there that there had been a Blue Green Algae (aka cyanobacteria) bloom. There were all sorts of warnings against allowing your pets and small children near the water but no real information as to what the hazard was.

So do water filters take out what ever the contaminants created by Blue Green Algae?

My guess…
I think the only reliable answer to this would come from the manufacturer of the filter in question.

“the bride” just ordered one
that supposedly will make mud puddle water drinkable.

jack L

Be careful
Water filters will only remove bacteria and in some cases viruses from the water. The toxins released by cyanobacteria are much smaller and like most small molecules will not be removed by the filters. So if all the toxin is still in the algae, you will be safe as the filter can remove the algae and the toxin load it carries. If the toxins have left the algae and are dissolved in the water, you might regret it.


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Reading this makes it sound as though only activated charcoal will take the toxins out of the water. Even then you would need to test to findout how effective that was.


A lot of illnesses aren’t bad from the pathogen itself. It is the toxins that such pathogens produce. Much like contaminated seafood making you sick even if it is cooked. You might not even be able to boil it and make it safe.

Ryan L.

You release the toxin by boiling the water.

This is a dangerous topic. Get it wrong and you or your dog could be dead.
We need some more research to know for sure.

Agree with ppine and djo. I once took a 400 level course in phycology - dealing with “algae”, which is really a catch-all classification. (Many are motile, not all photosynthisize… etc.) You’ve caused me to pull out my old textbook on this… and, actually there has been quite a bit of research done but, alas, it doesn’t yield simple “no exceptions” rules to live by.

And djo’s right, its the toxins, not the algae itself, that is toxic. And there are quite a few species of toxin-producing algae that are dangerous, even deadly, that are endemic to different parts of the world and habitats.

That said, in temperate North American fresh water the most problematic blue green (cyanobacteria) species seems to be of the genus Microcystis, though there are others as well. (Species of at least six other genera of cyanobacteria are also known to produce toxins.) And there are three classes of toxins they are known to produce - lipopolysaccharides, hepatotoxins, and neurotoxins. The commonly dangerous one around here, and I presume L. Champlain, though, is Microcystis and it produces the hepatotoxin microcystin. The complication is that not all species of Microcystis produce the toxin and even the ones that do don’t produce it all the time. Toxin production seems to increase during and after exposure to intense sunlight. Microcystis, like many other algae, most often produce blooms where higher phosphorus levels, associated with fertilizer run off, occur. Even in the absence of the algae, the toxin can persist in the water for two weeks or more before bacteria break it down.

Cities that draw their water from lakes containing microcystin, after mechanical filtering (such as might be used by paddlers), then filtered through activated carbon, ozonation, and chlorination can remove from 80 -98% of the toxin.

Symptoms of microcystin poisoning include weakness, heavy breathing, pallor, cold extremities, vomiting, diarrhea, and massive bleeding of the liver - the usual cause of death. Prolonged low level exposure is suspected to cause tumors of the liver. Ppine’s right, this is not something to mess around with.

Algae and their associated toxins are often concentrated in shallow bays where they are driven by the wind. So your best bet, if you must draw water from a lake known to contain high levels of blue-green algae (and without putting the algae under a microscope to determine the genus), is to take your water from the middle of the lake. Take it early in the morning, filter it mechanically, then run it over activated carbon. Perhaps the toxin can be broken down by boiling - I don’t know about that and haven’t heard of any research being done in that area. I hear boiled arsenic is just as poisonous as it is when iced. (There was, back when I took the course, a bacteria - Sphingomonas - that had been found that breaks down microcystin and might possibly be useful in the treatment of affected water supplies. There were also some enzymes that showed promise. I don’t know where that research now stands.) BTW, microcystin is also toxic to zooplankton, so the presence of daphnia or other copepods is a good sign that the water is safe.

But look on the bright side - cyanobacteria are what made the stromatoliths , arguably the oldest known signs of life on the planet. They are thought to be the first oxygen-producung life forms - so the very air we breathe is a gift to us, and every other air breathing living thing, from the cyanobacteria. We may think disparagingly of blue green algae, but really we do owe a lot to pond scum

Microcystis and Anabaena caused the city of Toledo OH to shut down their water system a few years back. Too bad Lake Champlain has that much fertilizer running into it, it is (was?) a beautiful lake. https://greatlakes.org/2019/08/five-years-later-lessons-from-the-toledo-water-crisis/

Thanks to PJC. I worked a lot in surface water hydrology during my career, but never ran across algae as an issue. We mostly worked in clean ecosystems far from human impacts.

Blue-green algae and its associated toxins, seems to be the common problem that shows up in lakes late in the season. Nitrates and phosphates and warm water temperatures are conducive to algae blooms. In a practical sense, keep an eye out for any algae that you see on the water. Assume it is dangerous and don’t allow your pets to make contact with it. Even with a filter, it is best to pull drinking water from side streams or bring water from home.