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Discussion Starter · #1 · (Edited)
In another thread in this forum allelopathy was mentioned (again). It seems that this is a subject that comes up periodically, but never gets resolved in any way. Today I got curious enough to do some internet research to see if any progress has been made on this subject.

It is a fact that allelopathy exists between some species of plants. But, as far as I have seen so far, there is still no data that supports the existence of allelopathy between aquatic plants and algae, which is the "holy grail" of allelopathy, as far as we are concerned.

Here is a paper by Dr.Ole Pedersen, from several years ago, that discusses this: http://www.bio-web.dk/ole_pedersen/pdf/tag_2002_15_7.pdf

From 2011: http://www.skepticalaquarist.com/algae-allelopathy and not a scientific paper, but interesting.

If I can find any other data or articles of interest, I will post them here. Or, if others find any, please also post them here.

From our own forum: http://www.plantedtank.net/forums/33-plants/118986-allelopathy-aquarium-plants.html

From 2007: An interesting paper about the difficulties involved in demonstrating that allelopathy is occurring between species growing in water: https://kops.uni-konstanz.de/bitstr...d_macrophytes_on_phytoplankton.pdf?sequence=1
 

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I will be following this one... This concept is fascinating to me. Also this could very well turn into another hot topic / science battle lol
 

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Yes, I know. Ole Pedersen's paper was written later, and states the case against Ms Walstad's conclusion about allelopathy.
Dr. Pedersen's does not explain the whole story either. I do not think it makes Walstad's work invalid. All the available sources that exist should be mentionned, let the users make their own opinions.

Michel.
 

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Discussion Starter · #6 ·
Dr. Pedersen's does not explain the whole story either. I do not think it makes Walstad's work invalid. All the available sources that exist should be mentionned, let the users make their own opinions.

Michel.
It isn't possible to post all available sources here. There has been very little testing done on aquarium plants vs. algae, more on aquatic weeds in the wild, against each other, and a lot more on terrestrial plants. I posted just a few of what I found. The really technical papers are all on non-aquarium plant settings. Of course proving a negative is almost impossible, and that would be a "negative" to prove that aquarium plants don't use allelopathy to compete with algae. It is possible, but extremely difficult to prove that one species of aquarium plants shows strong evidence of using allelopathy against one species of algae. I didn't find any such proof.

In the previous Planted Tank thread I listed, Plantbrain (Tom) made what may be the most significant comment: it is extremely expensive to find what specific chemical compound has a specific effect we are seeking, and even more expensive to purify it and manufacture it. Our very small market just doesn't generate that kind of money.
 

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I could see people spending oodles of money to get rid of the hard to get rid of by natural means algaes.. Black beard and cyano come to mind

From an anecdotal point of view, the lakes in used to swimming in would qualify as being heavily planted around the perimeter and have excellent water clarity (20+ feet) and for the most part all I see are the lower tiers of algae... Diatoms and dust... Rarely any hair/brush/slime... Not direct evidence for sure, but possibly something

Also myriophyllium spicatum (invasive milfoil from Europe) grows in basically a single species environment. With almost no plant diversity, some of this in sure is due to the height the plant will grow and sharing out of litter plants, but this doesn't happen with the other two potamogeton species in the lake that also grow tall. There is a very distinct boundary of mixed plants to no diversity and it's usually within 5-6 feet off the dense myrio locations. Again nothing definitive, but possible. Also as a bonus this milfoil had been studied somewhat extensively already.
 

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I will raise this old topic. I am not trying to say what is the right or wrong answer here.

I just can't figure out what is the logic in assumptions that there is no "chemical warfare" between algae and plants.

Why do smart people say that "having healthy plants help fighting algae"? What is the proposed mechanism then? How do the plants even relate to algae issues if allelopathy is denied?


Some of these reasonings below might seem quite silly but I just try to show the mechanism and cover all aspects here:

- it has nothing to do with light, since having real vs. similar fake plants does not affect light levels.

- it has nothing to do with taking up the space, since even in highly planted tank, the volume of plants is maybe max. 5% of the aquarium. And if that would be the case, just putting more rocks to the aquarium would do the same thing.

- it has nothing to do with healthy plants sucking all nutrients before algae gets them, since people are doing fine with EI. And EI by definition has EXCESSIVE amounts of all nutrients. And algae needs very little nutrients anyway.

- it has nothing to do with CO2, since CO2 levels are adjusted based on desired ppm concentration and in high tech tank there is plenty of CO2 for algae that needs very little of it anyway. And if algae would simply suffer from high CO2 levels, CO2 would be used in non planted tanks to prevent algae.

So what difference do the plants do for algae? Since if in theory one adjusts ferts & light to maintain same exact water and light parameters and replaces the plants with fake plants, almost every aquarium keeper would place their money for the aquarium turning into an algae disaster.

And the answer is not that healthy plants are able to prevent algae growth on their leaves so there is less surface area for algae in the aquarium. If that would be the case, just having less or no plants would be a better solution.


Edit: The only somehow logical answer to this equation I could think (without allelopathy) is that algae uses ammonia and plants help keeping ammonia levels down.
 

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You also have to remember that there are lots of types of algae. Some only thrive in abnormal water conditions (cyano/diatoms), and many of the nastier types seem almost parasitic and grow mostly on the leaves of unhealthy plants. Green dust is one of the only algaes that will grow on clean glass/rock, and it tends to hang around even in healthy tanks.

I also believe that dissolved organics contribute to algae growth (or at least weaken plants to the point where algae can take over).

And the answer is not that healthy plants are able to prevent algae growth on their leaves so there is less surface area for algae in the aquarium. If that would be the case, just having less or no plants would be a better solution.
I don't think your refutation is sound, because a tank with few or no plants would:
a) have very little algae if the bioload was compatible.
b) be much more likely to produce green water/dust algae instead of nastier hair types. See goldfish tanks or ponds for an example.
 

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If I were to guess, I would wager healthy plants impede algae through two major mechanisms.

First is competition. Planted tanks are malthusian ecosystems, especially with leaner dosing regimes (vs EI). If there are many healthy, established plants, using photosynthesis resources (nutrients, CO2), then algae will be less able to do so. Some observations in support of this are that a new tank (where the new plants are not yet established and photosynthesizing vigorously) is very prone to algae, and lightly planted tanks tend to have more algae issues than heavily planted ones.

A previous poster mentions EI as a line of evidence against this, but I don't think this is the case. First of all, EI tanks, even successful ones, are prone to quick algae outbreaks if something disturbs the tank's equilibrium. Second, try dosing EI levels in a non-planted tank and see what happens. Likely in EI tanks something besides the dosed nutrients is the limiting factor, but the method is still contingent on a lot of healthy plants in the tank to stave off algae.

Second is organics. From experience it is my belief (and many others') that high levels of organic compounds in the water (not just the standard ammonia and nitrate, but other less well-defined things like carbohydrates, amino acids, etc.) are correlated with certain algaes like BBA and BGA. Such algaes seem to thrive from feeding on such compounds. Most don't realize that plants are potentially a significant source of organic wastes and biologic load in a tank. Unhealthy plants are more likely to 'leak' out organic compounds, as well as contributing through loose leaves, etc., that decompose and further raise organics.

I notice whenever my plants aren't doing well for one reason or another, that BBA (and occasionally BGA) begins to pop up, likely for the second reason above. However the algae persists and may even continue to grow after the weak plants have died off or been removed, supporting the first reason above.
 
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- it has nothing to do with light, since having real vs. similar fake plants does not affect light levels.
agree, and I have done experiment with this. i only cover half top of the tank with duck weed. but the rest of the tank is algae minimal. so its not that the duckweed is blocking light.


- it has nothing to do with taking up the space, since even in highly planted tank, the volume of plants is maybe max. 5% of the aquarium. And if that would be the case, just putting more rocks to the aquarium would do the same thing.
agree

- it has nothing to do with healthy plants sucking all nutrients before algae gets them, since people are doing fine with EI. And EI by definition has EXCESSIVE amounts of all nutrients. And algae needs very little nutrients anyway.
agree. but remember, EI only dose N, P, K, and micro.

- it has nothing to do with CO2, since CO2 levels are adjusted based on desired ppm concentration and in high tech tank there is plenty of CO2 for algae that needs very little of it anyway. And if algae would simply suffer from high CO2 levels, CO2 would be used in non planted tanks to prevent algae.
CO2 only come into effect that it helps plants grow

So what difference do the plants do for algae? Since if in theory one adjusts ferts & light to maintain same exact water and light parameters and replaces the plants with fake plants, almost every aquarium keeper would place their money for the aquarium turning into an algae disaster.

Edit: The only somehow logical answer to this equation I could think (without allelopathy) is that algae uses ammonia and plants help keeping ammonia levels down.
its not ammonia, because commercial fertilizer has ammonia in it.
i think it's not Macro and micro nutrient either that plants competing, but organic compound that is not specificly mentioned. And this organic acts similar like a catalyst. it is being consumed (while catalyst is not consumed) by algae. when this organic present, it makes algae thrive.

i have hight tech but low growth tank with EI dosing, it has no algae at all. why? because I don't keep fish in that tank. no fish = no organic waste = no organic nutrient.

also, Amano suggest that for the first 3 months, to put activated carbon in the filter. why? to absorb organic material.

allelopathy is real, but not all plants do it. for general discussion, i say it's more about nutrient competition. but the nutrient is not macros and micros, but organic.
 

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Intersting article. Thank you for sharing. I remember reading Walstad's chapter on allelopathy with a lot of excitement, though somewhat skeptical about some of her claims. It is interesting to read another side to the story.

Still, I'm not on board with Pedersen here. He cites several compelling studies on aquatic allelopathy, then hastily concludes that their findings must be irrelevant for aquarists solely because "One would have to advocate lowering the water exchange" (AKA infrequent water changes). That seems a little conclusory.

First of all, the Walstad method (introduced in a book Pederson describes as "the best") does indeed advocate for infrequent water changes, in part for exactly this reason: allelopathic accumulation. In fact, reduction of allelopathic chemicals (to spare sensitive plants) is one of the few justifications for water changes that Walstad accepts.

Second, how can Pederson be so certain that a typical water change regime would disarm allelopathy? I understand that there is a lot of uncertainty in all of this, but the author here writes "Can allelopathy be used to control algae growth in the aquarium? The answer would be NO!" Well how the hell does he know that? I don't see any citation supporting his claim that water changes would nullify allelopathy. He does not provide any information on what effect a weekly, bi-weekly, tri-weekly water change would allelopathic chemical buildup; on what basis, then, can he form such a confident conclusion? What about aquatic ecosystems that naturally have high water exchange, like rivers? What about aquariums that have low water exchange, like large shrimp tanks? The author's entire argument is based on unsubstantiated speculation that fails to account for the great diversity in both aquaria and freshwater ecosystems.

Pedersen also assumes, without reason, that allelopathy must take place in the water column. If frequent water changes would nullify allelopathy, and if there are freshwater systems which experience frequent water changes, then it stands to reason that aquatic plants would develop allopathic strategies which are resistant to dilution. Perhaps some allelopathic chemicals can adhere to surfaces? Perhaps these chemicals are excreted directly onto algae that is growing on the plant?

There is much uncertainty in this issue, and it goes both ways.

I absolutely agree that nutrient competition is probably the dominant cause of reduced algae growth in a heavily planted tank/body of water. But the studies cited in Walstad's book and in this article have some really compelling results, and I don't think this article offers any compelling argument for ignoring the applicability of these results to the keeping of aquaria.



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