Healthy plants = no algae. True or false?

[Marcel G]

What keeps algae at bay in our tanks?

Very often I read here on the forum (and elsewhere) that the abundance of nutrients do not cause algae issues in densely planted aquariums. Also I have found some examples of people claiming this to be true based on their practical experiences with dosing high amounts of fertilizers without any visible algae issues – e.g. Tom Barr has writen in one thread that he was dosing 3-times a week 5 mg/l PO4 + 15 mg/l NO3 (with light of 0.53 W/l) for the time period of 2 years; ceg4048 from ukaps.org was saying that he was dosing even more fertilizers: 4-times a week 50-60 mg/l NO3 + 6-9 mg/l PO4 + severalfold more micro-elements than recomended, accompanied with weekly water changes of 50% RO water (he is also known for his obsession for purity/cleanness). In both cases there was no visible signs of algae infestation in their tanks. And I believe it (I have a similar experience). So the general conclusion made out of this is „healthy plants = no algae“. In some threads Tom also said that healthy growing plants are not a good place for algae to attach and grow. Also when there are some problems with CO2 (whether be it the stable level, amount or distribution in the tank) the algae seems to propagate. Many times Tom is also saying we should be careful in making conclusions based on our own findings without any real scientific evidence.

So I would like to know if there is such a scientific evidence to claim that „healthy plants = no algae“, or in other words: that the plants (and also good + stable CO2 level) is the real cause for algae to be kept at bay in our tanks? The only scientific study which Tom Barr is citing when speaking of the „plants-algae-nutients“ relationship (and that abundant nutrients do not cause algae) I know of, is the study „Bachmann, R. W., B. L. Jones, D. D. Fox, M Hoyer, L. A. Bull, and Daniel E. Canfield, Jr. 1996. Relations between trophic state indicators and fish in Florida (USA) lakes. Can. J. Fish. Aquat. Sci. 53:842-855.“ … but the main concern of this paper is to answer the question of whether the concentrations of phosphorus and nitrogen in the water of Florida lakes determine the abundance of aquatic plants (in similar way as the amounts of plankton algae in the water are closely related to the concentrations of plant nutrients). They found no good relationship between the amounts of plant nutrients in lake water and the abundance of aquatic plants. So this study does not relate to the „algae-nutrients“ relationship, but rather to „plants-nutrients“ relationship. The conclusion of this study is that aquatic plants in these Florida lakes do not respond to nutrients in the water in the same way that the phytoplankton (algae, cyanobacteria, protozoans) do. The end result is that there is no correlation between the nutrients in the water and the aquatic plant abundance. BUT the same does not apply to algae! In the case of algae the results are exact opposite! BTW, the scientists clearly state that in some Florida lakes the aquatic plants outcompete the phytoplankton for nutrients => which is the reason why these systems are macrophyte-dminated (and not phytoplankton-dominated).

I seriously doubt this to be true, or stated in other words: I believe it to be true only partially. I think the statement „healthy plants = no algae“, should be corrected to „healthy plants = no visible algae on these plants“. So in the densely planted aquarium, where there is much more plants than anything else, it could look like there are no algae at all … particulary if we make regular maintenance, pruning and water changes, with the help of algae grazers (like shrimps, otocinclus and SAE). But if we would leave our tanks without maintenance and without algae grazers for 2-3 weeks, these tanks would become „algae heaven“ (given we will keep the EI dosing and light intensity) … regardles of the condition of our plants or CO2 levels. I think the main reason for algae to stay at bay in our planted tanks being manyfold: 1) regular maintenance, 2) algae grazers, and 3) „not enough time“. Many kinds of algae need some time to grow (reproduce) into visible mats or filaments (in the case of green algae it could take at least one week – although there are some exceptions based on specific environmental conditions and particular type of algae). So if we make 50% water changes each week accompanied with good cleaning techniques and big help of algae grazers, than the algae can have quite a hard time to establish and expand to larger volumes … BUT I would not ascribe it to the plans only. The plants are not any mysterious organisms which could make our tanks algae free by means of some sacred proceses or willpower.

What do you think about this? Why do you believe the plants are the key to keep the algae at bay in our tanks? And what arguments for supporting your theory you can use?

Update: A good (scientific) article on "What stops growth of algae" (in Florida lakes) can be found here: http://lakewatch.ifas.ufl.edu/LWTEAMFOLDER/CanfieldPubs/LimitFact.pdf. Nutrients are at the first place. Also it is stated, that "if macrophytes (large aquatic plants) are abundant over most of the shallow lake area, the lake waters will tend to have lower amounts of plankton algae". But why? "Because the macrophytes and their attached periphyton remove nutrients from the water leaving less for the plankton." So again the limiting factor #1 = nutrients. There are also other limiting factors like light energy, temperature, time, and grazing.

 

[Tom Barr]

Well, certainly on the plants. Observation from natural systems:
We can walk down most any stream anywhere from Denmark, to Calififornia USA, Florida USA, Brazil, NT Australia, India, Cameroon and find nice healthy plants, and a few meters away, plenty of algae in streams and rivers.

There is a profound difference between natural systems and our highly groomed aquariums: light, CO2, current are the larger factors, temp is yet another.
Grooming the plants and adding high densities of algae eaters plays a huge role also. We selectively provide good conditions for plants and keep them clean.
Light is often reduced photoperiod(rarely over 10 hours these days for most) and maybe at MOST 10% of natural lighting.

Comparing natural systems is rather tempting but it's not wise.
Light and CO2 are huge differences, and as we all know all too well, light and CO2 are the main drivers in polanted aquariums, not nutrients so much.

I agree with your assessment about no visible or problematic algae.
Some tanks have much less grooming requirements also than others. Why?
Ferts can be similar, or be more/less or less/more in each tank.

=> Light and CO2 play a larger role for us than natural systems typically.
Natural systems really only have nutrient variation to control growth.
Light will be the same over similar water depths typically(say the top 1 meter).

CO2 can vary a lot. Spring fed rivers/streams also tend to have more stable nutrients and more differences in current, CO2 and shading along their course.

If you look at tropica's article on Light and CO2, look at the matrix on light vs CO2 for 9 different cases(the nutrients are non limiting thus independent).
Plants still grow well, but at very different rates. 18X difference.

So the nutrients can be non limiting over an extremely large range for aquarist.
We could add nutrients to that model and temperature also, but it would tend have 27 different growth rates and with temp added 81 different data points.
If you did replications of say 4, then 324 reps, and then repeated 2x= 648.

Yikes.

And that is for only one species.

I think shading by aquatic plants reduces phytoplankton and sedimentation/reduction of floc(which is a good source of nutrients for plankton. Light would be the largest factor in RESOURCE competition. Aquatic plant tanks also demonstrate this.
Our main pest algae are attached periphyton, not phytoplankton. Also, our main plant groups are submersed plants, not floating or emergent. Not many lakes are 1-2 meters deep and nice and clear.

I think spring fed rivers with rich CO2 are a better model to study. But there is little comparative data on those even though there are many in Florida(hundreds all with lush plant beds and clear waters).
These spring fed rivers have been stable and with plant beds for at least 500 years documented by the French explorer: Ponce De Leon.

Nutrients are low, but non limiting and continuously replenished. Dupla made mention of many such systems in the Optimum Aquarium.
They also wisely said it is not possible to add nutrients continuously in a similar manner for aquariums. We will add more, but the light and CO2 make the up for that in aquariums, where in natural systems, the inputs are very different.

 

[Marcel G]

You say „Abundance of nutrients doesn’t mean algae“ elsewhere on this forum (and also on some other planted tank‘s forums). From time to time you cite some scientific studies (articles), and relate their results to our planted aquaria. The one (study) I mentioned above you did use as argument for the statement that „abundance of nutrients does not cause algae“. Now you say we cannot compare it with the conditions in our aquaria. This seems to me as disproportion. I was trying to point out that many (maybe majority of) scientists had proven that nutrients are the main control method for nuisance algae in the natural systems (some of them being similar to our aquaria). I agree that in our aquaria the light is also a big player (probably the biggest one). BUT I don’t think we can say that nutrients have nearly no impact on algae growth. Just try to leave your tank 2-3 weeks without maintenance and algae grazers (given other factors being the same = same light, same CO2) … you’ll see the results = algae infestation. I think that the MAIN reason the nutrients SEEMS to play no role in algae growth (in our planted aquaria) is our REGULAR maintenance + FREQUENT water changes + algae grazers + not enough time for algae to grow/multiply to bigger volumes. In our eutrophic aquaria the algae would grow FOR SURE … IF we give them enough time (more than 1 week) and enough rest/holiday from our maintenance. This alone is very strong proof/evidence of nutrients being BIG player with regard to algae growth/infestation. So I would be careful with making such conclusions that „EI dosing (or abundance of nutrients) doesn’t lead to algae infestation“. Yes, it could be true … BUT there is one big „IF“ => IF we do regular maintenance, frequent water changes, and have a big croud of algae grazers.

To say "abundance of nutrients doesn't cause algae" is like saying "the sun is not shining on me" (but what you forgot to mention is: "because I am under the roof" = "because I make regular maintenance and have hosts of algae grazers").

I think there is a big confusion between aquarists on „what is the main control method for algae growth“. You say it‘s light and CO2. And I’m in agreement with you in this. But you add to this statement that „healthy plants = no algae“, and „abundance of nutrients doesn’t cause algae“. And this is confusing. It sounds great, it is revolutionary and provocative statement, it’s something new … but I don’t think this statement is correct/valid. If we speak of controling algae growth, we should use holistic approach, and mention ALL the facts/factors … not picking just some of them (e.g. light and CO2). Abundant nutrients could be dangerous in the similar manner as high light in combination with very low CO2. The only reason for nutrients not „causing algae“ in our tanks is not the harmlessness (un-dangerousness) of nutrients, but our maintenance execution (+ algae grazers). So dosing high amounts of nutrients could make our life easier (= no plant deficiencies), but it‘s not wise according to me (as there is a risk of algae infestations with abundant nutrients in the water column … if we neglect our regular maintenance). As there are more factors influencing nuisance algae growth, it is wise to pay attention to ALL of them: light, nutrients (including CO2), temperature, time, algae grazers, maintenance. I don’t think we can get along just with „healthy plants“. That’s all.

 

[thegasman]

I'm sorry, but I don't understand your point here. I have seen it said many times that when you dose the typical *EI levels of nutrients, you should be doing regular weekly maintenance including 50% or more water changes to prevent build up. I've also seen the importance of having proper CO2, lighting, temperature, and algae clean up crew mentioned numerous times. If anyone thinks otherwise, well then they should have read more than one post. I don't think that it must be mentioned in every post that has or will ever be posted.*

 

[Tom Barr]

You say „Abundance of nutrients doesn’t mean algae“ elsewhere on this forum (and also on some other planted tank‘s forums). From time to time you cite some scientific studies (articles), and relate their results to our planted aquaria. The one (study) I mentioned above you did use as argument for the statement that „abundance of nutrients does not cause algae“. Now you say we cannot compare it with the conditions in our aquaria. This seems to me as disproportion. I was trying to point out that many (maybe majority of) scientists had proven that nutrients are the main control method for nuisance algae in the natural systems (some of them being similar to our aquaria). I agree that in our aquaria the light is also a big player (probably the biggest one). BUT I don’t think we can say that nutrients have nearly no impact on algae growth. Just try to leave your tank 2-3 weeks without maintenance and algae grazers (given other factors being the same = same light, same CO2) … you’ll see the results = algae infestation. I think that the MAIN reason the nutrients SEEMS to play no role in algae growth (in our planted aquaria) is our REGULAR maintenance + FREQUENT water changes + algae grazers + not enough time for algae to grow/multiply to bigger volumes. In our eutrophic aquaria the algae would grow FOR SURE … IF we give them enough time (more than 1 week) and enough rest/holiday from our maintenance. This alone is very strong proof/evidence of nutrients being BIG player with regard to algae growth/infestation. So I would be careful with making such conclusions that „EI dosing (or abundance of nutrients) doesn’t lead to algae infestation“. Yes, it could be true … BUT there is one big „IF“ => IF we do regular maintenance, frequent water changes, and have a big croud of algae grazers.

To say "abundance of nutrients doesn't cause algae" is like saying "the sun is not shining on me" (but what you forgot to mention is: "because I am under the roof" = "because I make regular maintenance and have hosts of algae grazers").

I think there is a big confusion between aquarists on „what is the main control method for algae growth“. You say it‘s light and CO2. And I’m in agreement with you in this. But you add to this statement that „healthy plants = no algae“, and „abundance of nutrients doesn’t cause algae“. And this is confusing. It sounds great, it is revolutionary and provocative statement, it’s something new … but I don’t think this statement is correct/valid. If we speak of controling algae growth, we should use holistic approach, and mention ALL the facts/factors … not picking just some of them (e.g. light and CO2). Abundant nutrients could be dangerous in the similar manner as high light in combination with very low CO2. The only reason for nutrients not „causing algae“ in our tanks is not the harmlessness (un-dangerousness) of nutrients, but our maintenance execution (+ algae grazers). So dosing high amounts of nutrients could make our life easier (= no plant deficiencies), but it‘s not wise according to me (as there is a risk of algae infestations with abundant nutrients in the water column … if we neglect our regular maintenance). As there are more factors influencing nuisance algae growth, it is wise to pay attention to ALL of them: light, nutrients (including CO2), temperature, time, algae grazers, maintenance. I don’t think we can get along just with „healthy plants“. That’s all.

you have 2 different things here you are trying to say: you seem to suggest that I use citations inappropriately.

I was trying to point out that many (maybe majority of) scientists had proven that nutrients are the main control method for nuisance algae in the natural systems.

No, this is not correct: Not where aquatic plants are present in high densities similar to planted tanks, eg, 30-50% surface area coverage, not where the lakes are shallow and not where the temperatures are tropical to subtropical.
Northern lakes are very very different than tropical and subtropical lakes.

Many northern lake research have focus on lakes that Freeze, that tend to be very deep and unable to have a large % of submersed macrophyte coverage. You need to have shallow lakes, warmer temperatures and plants present to compare.
That's been a problem for a number of years in Limnology. There is a strong bias towards northern lake studies.

There are also many sampling errors such as was the case with Philips et al., 1978. They measured the P contained in the water but also the P in phytoplankton.
They did not include the P contained in the root macrophytes. When the P from the rooted macrophytes was included, then there was no correlation between nutrients and algae or macrophyte dominance.
The paper was cited 494 times.
http://www.sciencedirect.com/science/article/pii/0304377078900128
Several researchers I had as professors made this point in class at UF at Gainesville, Florida.

So bad information can easily get into the research. Main thing is to correct it once you see the error.
What Philips did was not unreasonable at the time. Everyone thought it made sense at the time. But we learn from the mistakes and try to rule things out.
One of the people that questioned the paper was also a Brit, Dr Fox, who is now at UF. She's pretty sharp. She made this point and she was right.

There is a lot more light in those natural systems. Maybe there's CO2, maybe not.
Let's look at that:

http://lakewatch.ifas.ufl.edu/LWTEAMFOLDER/BACHMANN/CO2_FL_lakes.pdf

They report a mean value of 3550 uatm vs 370 uatm for the air above. Roughly 9-10X higher.
Maybe that plays a large role?

I do agree with you when it comes to say Lake Geneva in Switzerland, but not in Lake Kissemme, Florida USA.
Likewise, I disagree with aquarist who claim nutrients cause algae in planted tanks("excess", we will say "non limiting" to plants).
There are few plants to define Lake Geneva, there are billions of plants that define nearly the entire lake in Florida.

We cannot find exact matches to aquariums, but we can be a lot better by using a high number of replications, shallow habitat that is suitable for submersed plant growth, tropical to subtropical habitats and that look at plant biomass and algae biomass.
If you have no plant biomass and add nutrients, then you will generally get more algae.
If you have an aquarium with no plants, high light and add nutrients, you will also get the same results.
This is not inconsistent with the research or our observations in the hobby of keeping aquarium plants.
Clearly northern lakes have far less in common with our tropical planted gardens than Florida lakes.

Let me pose a question to you: Why might one aquarium have few issues, require virtually no care with respect to algae and lack much plant biomass, whereas another tank have a lot more plant biomass and both tanks have the same non limiting nutrients?

Here's they are:

I've done virtually nothing to this tank for months, once a month water change(likely does not even need that), ferts added 2x a week, I've cleaned the glass maybe 2-3 times since the tank was set up.

Another tank that would fit more into your criteria above:

I have never once stated that natural systems and aquariums are the same thing. I have suggested that natural systems can be used to understand things in our aquariums and that rich nutrients does not imply algae and no plants in natural systems.
I have stated that there is little if any correlation between algae dominance and nutrient levels in natural subtropical systems that are shallow and possess 30-50% coverage.
I have also applied these same concepts to ponds and have client's where I have solved their algae issues, by adding 40% plant biomass and keeping it there during the growing season(beginning of April till end of Oct). So even with all the light outdoors, the plants still did their job.
Nutrients can be ALL OVER THE PLACE. But we can have bad algae or no/little algae issues.

These are merely correlations based on observations.

What we can infer is that nutrients independent of other factors, do not cause algae in these systems where the plant biomass is high and the water is warmer, shallow, and also the same can be said for planted aquariums.
This is true where plant biomass covers 30-50% of the area and we find something similar in planted tanks.
I have not stated with those studies or any other, what causes algae.

There's a distinct difference there. One shows what is NOT causing something, the other professes ignorance at what causes algae.
You and everyone else, myself included knows there are many aquariums with very limited nutrients, aquatic plants and horrible algae.
Limiting nutrients does not save anyone from algae issues either. Many get worse algae when they stop dosing nutrients.

 

[Tom Barr]

I am afraid that adding non limiting nutrients is not NEW. I mentioned these same ideas to the researchers at University of Florida and they looked at me and said "yes, of course! If you add more nutrients to these lakes, you get more weeds!"
This was not just one person, this was the entire department and some of the world's top people in aquatic weed research. I studied algae there. I focus on aquatic weeds these days.
We also see this same thing in planted tanks, my growth rates are higher than many people in the local club and the larger USA group that limit or dose leaner nutrients.

However, I have and can slow growth down, but unlike the researchers studying natural lakes, I have control over light, and CO2 and nutrients. They really do not, so the only control measure for algae and weeds is nutrient control.
You cannot adjust the sun or add CO2 to a 500 hectare lake. Your ONLY management tool is nutrient management. We have far more control.
Still, comparing agriculture(which is pretty much what we do) to natural systems is a stretch also, they are similar in some ways, but different. I do try and qualify them and point out those differences and what we can and cannot say about them.

To say "abundance of nutrients doesn't cause algae" is like saying "the sun is not shining on me" (but what you forgot to mention is: "because I am under the roof" = "because I make regular maintenance and have hosts of algae grazers").

But I did not say this like you suggest, I mention all the tools to reduce algae, not just excess nutrients do not cause algae. Light/CO2/general care are included with that.
I can say that non limiting (for plants and certainly for algae) nutrients do not induce algae. I cannot say what causes algae. All it takes are a few examples where the nutrients are non limiting and there's little if any algae to falsify your claim that higher nutrients induce algae. You seem to imply that it's trimming and care, algae eaters etc that are the only off set controlling algae in such systems, this is false.

The hairgrass tank above is clearly lacking algae eaters and I do almost nothing other than feed fish and dose 2x a week. This falsifies that claim grooming and trimming, algae eaters etc are the main ways that control algae.
I do agree that adding algae eaters, cleaning, and trimming well is a WISE and in some aquariums this seesm the only way to control algae, but this observation is hardly universal.
The question is, why are some tanks more prone to issues than others?

How does management and light/CO2 affect those tanks?
I know the answer to that question.

I can also set up tanks that achieve little to virtually no input. But I can also set up tanks than need trimming weekly to look as nice as I want.
Aquarist have many different goals, no one method will achieve every goal. A holistic understanding is wiser using all the tools we have to get to that goal.
So add lots of algae eaters, add lots of plants and do lots of water changes from the start until the plants are doing well, clean the tank as needed, consider temp, light, whether or not to use CO2 or Excel etc, chose plants that need trimming often or plants that can go months to years without trimming.

Use all the tools available, not just focus so much on one single thing. Nutrocentric thinking I've called it.

 

[Tom Barr]

BTW, I think we agree on 95% of the topics you have brought up
You conclude at the end, that the holistic view is best.

I strongly agree with that!!

 

[Marcel G]

Tom, first of all I want to thank you for all your comments. I would be really happy to understand the processes in our aquaria, and be a better horticulturalist. That’s my goal.

As I read your comments, it seems quite reasonable (convincing) that in tropical aquatic systems with high plant biomass there is little (if any) correlation between algae dominance and nutrient levels.

Also I have to admit that you were never stating what „causes“ algae … and if so, you always done this merely as suggestion. So that‘s my fault I attributed this statement to you … I apologize!

It’s true that in nature there is not possible to control light intensity and/or CO2, so nutrients are the main control method there. On the contrary, in our tanks we CAN control light, CO2 (and nutrients also). That’s really good point! … and that’s maybe the main difference between the natural vs. artificial systems.

I also understand the difference between positive and negative statements (e.g. „ IS the real cause of algae infestation“ vs. „This alone ISN’T [or could not be] the cause of algae infestation“). If we don’t do serious science, we can‘t make too much positive statements. In most cases we can do no more than falsify some unscientific speculations.

Still unanswered questions:

1) You say that non limiting nutrients (independent on other factors) DON‘T induce algae (= negative statement).

2) I said that non limiting nutrients DO cause algae infestations if we give algae time to utilize them + grow + multiply, and we will not „disturb“ them (by maintenance or algae grazers).

3) To falsify my positive statement you pictured your aquaria with non limiting nutrients + little plant biomass + no algae grazers + seldom done maintenance … with no visible algae infestation.

4) I have to say that I don’t know the exact parameters of this (your) aquaria, so it’s hard to say what could be the reasons for „algae not utilizing the nutrients“ (or „why are some tanks more prone to issues than others“). I recall the Liebig’s law of the minimum => some factor being limiting. I would guess lower light intensity … as light being the main driving force of growth. The soil substrate could play a role here also … as it’s able to absorb nutrients from water column. Maybe you’re using UV sterilizer also (that could play a big role also)? Maybe you limit (reduce) growth by not adding any CO2 from outside (wheather pressurized CO2 or liquid carbon)? Frankly, I don’t like this kind of riddle … if you give your aquaria as proof of falsification, but not mention all the parameters.

5) As I understand it right now the excess of nutrients in aquatic systems with high plant biomass doesn’t induce algae without the teamwork of some other factors (e.g. higher light, non limited CO2, neglected maintenance, no algae grazers). In this respect I have no other way than agree with the results of your falsification. So I must admit that excess nutrients (independently on other factors – mainly light and CO2) in aquatic system with high plant biomass do/could not induce algae.

6) Still, I would like to know what is the role of aquatic plants in all this? I mean, you said that in tropical aquatic systems with high plant biomass there is no correlation between algae dominance and nutrient levels. Why? What’s the reason? Does it relate to the „Liebig’s law of the minimum“ or there are some other factors?

 

[Tom Barr]

Tom, first of all I want to thank you for all your comments. I would be really happy to understand the processes in our aquaria, and be a better horticulturalist. That’s my goal.

As I read your comments, it seems quite reasonable (convincing) that in tropical aquatic systems with high plant biomass there is little (if any) correlation between algae dominance and nutrient levels.

Also I have to admit that you were never stating what „causes“ algae … and if so, you always done this merely as suggestion. So that‘s my fault I attributed this statement to you … I apologize!

It’s true that in nature there is not possible to control light intensity and/or CO2, so nutrients are the main control method there. On the contrary, in our tanks we CAN control light, CO2 (and nutrients also). That’s really good point! … and that’s maybe the main difference between the natural vs. artificial systems.

I also understand the difference between positive and negative statements (e.g. „ IS the real cause of algae infestation“ vs. „This alone ISN’T [or could not be] the cause of algae infestation“). If we don’t do serious science, we can‘t make too much positive statements. In most cases we can do no more than falsify some unscientific speculations.

Still unanswered questions:

1) You say that non limiting nutrients (independent on other factors) DON‘T induce algae (= negative statement).

2) I said that non limiting nutrients DO cause algae infestations if we give algae time to utilize them + grow + multiply, and we will not „disturb“ them (by maintenance or algae grazers).

3) To falsify my positive statement you pictured your aquaria with non limiting nutrients + little plant biomass + no algae grazers + seldom done maintenance … with no visible algae infestation.

4) I have to say that I don’t know the exact parameters of this (your) aquaria, so it’s hard to say what could be the reasons for „algae not utilizing the nutrients“ (or „why are some tanks more prone to issues than others“). I recall the Liebig’s law of the minimum => some factor being limiting. I would guess lower light intensity … as light being the main driving force of growth. The soil substrate could play a role here also … as it’s able to absorb nutrients from water column. Maybe you’re using UV sterilizer also (that could play a big role also)? Maybe you limit (reduce) growth by not adding any CO2 from outside (wheather pressurized CO2 or liquid carbon)? Frankly, I don’t like this kind of riddle … if you give your aquaria as proof of falsification, but not mention all the parameters.

5) As I understand it right now the excess of nutrients in aquatic systems with high plant biomass doesn’t induce algae without the teamwork of some other factors (e.g. higher light, non limited CO2, neglected maintenance, no algae grazers). In this respect I have no other way than agree with the results of your falsification. So I must admit that excess nutrients (independently on other factors – mainly light and CO2) in aquatic system with high plant biomass do/could not induce algae.

6) Still, I would like to know what is the role of aquatic plants in all this? I mean, you said that in tropical aquatic systems with high plant biomass there is no correlation between algae dominance and nutrient levels. Why? What’s the reason? Does it relate to the „Liebig’s law of the minimum“ or there are some other factors?

Some of the confusion might be language and terms that are used.
I do think we see all the ideas in a similar way.

Sometimes it is best to read what someone meant to say, than what they actually said
That is a better way to communicate often times.

1) You say that non limiting nutrients (independent on other factors) DON‘T induce algae (= negative statement).

This appears to be the case in aquariums with plants. It also has strong correlation in many natural systems in many tropical and subtropical regions worldwide.
Nutrients can be quite low in flowing rivers, but still non limiting.

2) I said that non limiting nutrients DO cause algae infestations if we give algae time to utilize them + grow + multiply, and we will not „disturb“ them (by maintenance or algae grazers).

So how do you reconcile the hair grass tank above? I've also grown a number of planted tanks without herbivores. Tanks appeared to do better.
Not maintaining the aquarium is not reasonable for most hobbyist, independent of algae, an overgrown tank looks bad.

In natural systems, light plays a huge role in algae biomass. This is due mostly to the day length, which might vary from 16 hours in the summer to 10 hours in the winter. In Florida springs, with high plant density, we see this pattern.
Natural systems like those have crawfish, turtles, larger herbivorous fish, manatee, trees and branches falling into these systems which eat, displace, uproot, or bury the pant growth. The next season's growth, we often find the plants sprouting up and growing back.
We also have human's floating down these rivers in inner tubes for recreation, they remove and uproot plants.
Or boaters etc.

The plants do not grow much during the winter. The temp and nutrients, flow etc are very stable and constant. So light is the main variable that changes seasonally.
In other systems, it might be due to turbidity that blocks light during sporing melt and runoff.
Or once the leaves start growing and shades the river banks etc. There are a number of light factors and good ecosystems that are nutrient and temperature stable.
They also tend to have very stable gas concentrations(O2 and CO2).

Lakes? Ponds? Not so much really. But we still see lots of plants in those systems also.
Non limiting nutrients for algae is EXTREMELY low, but not that low for plants.

The periphyton group in southern Florida studies mostly plants and algae mats:
http://algae.fiu.edu/

In order to restore these systems to their natural states, the P levels are incredibly low, under 10 ppb. If you add more P, you get weeds like Typha that move in and take over. If you maintain low P, you get Native sedges and periphyton. If you add more P, you get cattails.
Macrophytes tend to be limited at 50ppb or less. Algae, under 10 ppb, likely under 3 ppb, but measuring systems in those ranges is very tough.

Plants always have higher nutrient demands. Same with Elephants vs mice. Both are herbivores. Both live in the same area. Both have very different life histories.

 

[Tom Barr]

3) To falsify my positive statement you pictured your aquaria with non limiting nutrients + little plant biomass + no algae grazers + seldom done maintenance … with no visible algae infestation.

Well, that's the issue, we can only KNOW for certain what something is not by falsifying it. If we say that adding PO4 to 3 ppm as from KH2PO4 dosed 2x a week to a 200 liter aquarium with 100 % plant coverage yields = no algae bloom. We must reject the hypothesis that excess PO4 = algae blooms.
We have some algae still, but it's certainly very manageable. More so in fact than the aquariums with limiting PO4.

Would adding management and algae eating herbivores improve the likelihood of reduced, easy to manage algae issues? Certainly. Would those same things reduce user/hobbyists errors? Yes! Reduced light in the 30-40 umol range? Yes!!!
So all these different bits of information begin to add up and makes the ecosystem much more stable and the chance of success much much higher.

I would agree with this:

4) I have to say that I don’t know the exact parameters of this (your) aquaria, so it’s hard to say what could be the reasons for „algae not utilizing the nutrients“ (or „why are some tanks more prone to issues than others“). I recall the Liebig’s law of the minimum => some factor being limiting. I would guess lower light intensity … as light being the main driving force of growth.

I also would say my goal is to make this aquarium easy to care for also. Why use much more light than I need?
I would also say that algae like BBA need less light than most aquatic plants. So light as the controlling factor independent of plants is not always the case either.
Rather, good healthy plant growth at whatever light level we provide. When we add more light, we add more nutrients and CO2. If we add very little light, say just barely enough for the plants to grow only a bit each week, then adding CO2 is very helpful.

See Ole and Troel's article for more on that:
http://www.tropica.com/en/tropica-abc/basic-knowledge/co2-and-light.aspx

The soil substrate could play a role here also … as it’s able to absorb nutrients from water column.

Yes, but plants leach nutrients from the sediment up into the water column, decaying leaves, plants are pretty messy overall
The leaching of nutrients from the soil to the water is well documented.
And there is virtually no way to a rich soil based sediment in a planted tank that is nutrient limiting to algae.

Maybe you’re using UV sterilizer also (that could play a big role also)?

No and not really, maybe for green water, but that's about all.
Few would disagree in the hobby on this point.

Maybe you limit (reduce) growth by not adding any CO2 from outside (wheather pressurized CO2 or liquid carbon)? Frankly, I don’t like this kind of riddle … if you give your aquaria as proof of falsification, but not mention all the parameters.

Haha
CO2: gas enrichment to about 50ppm, O2 is 7-8 ppm depending on time of day.
The 70 Gallon tank has about 40umol, maybe 30 umol in the edges of light.
I have sprayed Excel on the wood a couple of times when I do a water change.
Keeps the wood clean.

Likewise, I have cleaned the glass, mostly Green spot algae, a few times, but I can count the times on 1 hand.
GSA likely appeared due to infrequent PO4 dosing, there is enough N from fish so BGA did not appear.

Water is soft, KH is about 20ppm and goes to about 35 ppm in the winter/fall.

I dose maybe 2x a week if I remember.

Soil is ADA AS.

Fish: about 100 Brass tetras.
Feeding: flake and brine 1-2x a day.

Algae will still grow at low light, but it will not grow very quickly.
Plants will grow fairly fast overall even at lower light. If you also add CO2, then this is increased 4x, maybe less for some species, perhaps more for other plant species(see Ole's matrix in the Tropica link above for support) They have more structure to capture light and have more reduced Carbon demand than algae.
Algae really do not need much to grow.

5) As I understand it right now the excess of nutrients in aquatic systems with high plant biomass doesn’t induce algae without the teamwork of some other factors (e.g. higher light, non limited CO2, neglected maintenance, no algae grazers).

To some degree, but............do you really want another example that falsifies the statement above?
???

I can do that with a non cO2 planted tanks also.
hehe

This tank blocks the light below much more, but the plants also are not light or CO2 limited, I can add as much nutrients as I want to this tank and it does even better.


But this is a better example that addresses your questions:

No CO2 or liquid carbon, no water changes, trimming is still done, but only once every 3-6 months, mostly pinching leaves off that blocked flow. Dosign is done 2-4x a month, about 1/10th EI per dose.
Growth is 15-20X slower than a high light CO2 enriched system. Algae is still controlled by the same things.

Not much work added to this tank after set up. Sand based tank, no soil etc.

In this respect I have no other way than agree with the results of your falsification. So I must admit that excess nutrients (independently on other factors – mainly light and CO2) in aquatic system with high plant biomass do/could not induce algae.

In general, I can only say based on my observations that anything that hurts the plant's growth is bad and algae will grow then.
Now plants can grow slow, as long as the adaptations to those conditions are slow and the plants are given some time to adapt, then it should be fine.
When aquarist VARY the CO2 a lot back and forth, when they dose a lot, then nothing for inconsistent care, when they add way too much light for the CO2/dosing. That is where the issues and algae becomes a problem.

There are many reasons for algae, but we see universally that good growth, in a wide range of conditions= little algae issue.
Now if you want to test things:

A better way is to take a very well run healthy planted aquarium, then see if you can INDUCE algae blooms.
Since the aquarium is well run and growing plants well already, it acts as a "control"
Hobbyist virtually never do this unless by mistake, so they lack any "control" and newbies who have the algae problems .....lack the ability to provide a "control" for the test to begin with.
This presents a real fundamental challenge to testing why algae grows or not in planted tanks.

I know I can get rid of most any algae without that much work and effort.
So I was willing to try things to see if I could induce on purpose, algae.

I made some conclusions based on those observations and plenty of mistakes and helping other hobbyists.
I can induce many species of algae, eg, drop the CO2/shut it off and after about 1 week or more, I'll start getting mostly green algae on the glass, decor etc, then BBA will start.

Now while I can say poor CO2 induces algae species X, Y and Z and can repeat this..............I cannot rule out something: the possibility that there might be multiple causes for algae blooms, there may be more than one way to induce algae.
I can never rule out that possibility.

I may get most of the probable inducing criteria for algae blooms, but never all.
By inducing algae blooms on purpose, I have a much greater chance at understanding what is going on in the planted aquarium.
I go from observation based, to actually manipulating the system to test a hypothesis.

6) Still, I would like to know what is the role of aquatic plants in all this? I mean, you said that in tropical aquatic systems with high plant biomass there is no correlation between algae dominance and nutrient levels. Why? What’s the reason? Does it relate to the „Liebig’s law of the minimum“ or there are some other factors?

Many people would like to know. The researchers at UF also would like to know, the researchers in the Everglades would like to know.
Allelopathy has been put forth, but is extremely difficult to test and measure in natural systems. In aquariums, allelopathy is actually easy to test and set up a control using activated carbon to remove the chemicals.
While a pressing question for a a very few researchers and many plant hobbyists, there is not much funding or economic interest in this issue. Most of the funding is on how to kill and control aquatic weeds.

 

[Marcel G]

Tom, now I'm a little confused again.

I would say that (besides other control methods) light is probably the #1 in our aquaria. As you described some of your low-tech aquaria, it seems to me that light plays the biggest role in the algae control there.

BUT cca 100 µmol PAR at the water surface in low-tech tank (30-40 µmol PAR at the substrate) is MUCH less then 1000-2000 µmol PAR in the nature systems.

So although light seems to play the biggest role in algae control in our aquaria, how do you explain that there are no algae issues in the natural (eutrophic) systems with high plant biomass where there is VERY strong light (much higher than in our aquaria)?

I understand how balanced light, CO2, nutrients + good filtration, good maintenance, and host of algae grazers all contribute to algae control ... BUT I'm not sure I do understand how algae can be no issue in the "30-50% plant biomass system" with extremely high light (= some natural systems like Florida lakes you mentioned)?

The only explanation that comes to my mind is, that the plant biomass itself somehow controls algae growth. How do you explain this for yourself? What is the reason/reasons why algae don't grow in these eutrophic high light systems?

 

[Tom Barr]

Yes, light is the main factor in any system.

Then you move to CO2 and then nutrients.

Whether or not you can manipulate those experimentally is another matter for natural systems.
You can look to see where in nature they have such systems that have less light(or when they do) and which systems have good stable CO2, or CO2 that moves around or low CO2 etc, and of course low, moderate and high nutrients.
Then you need to see if you have plenty of plants and good temps. That can be done, but is hard logistically.

In larger natural systems, there are many types, some are huge and oligotrophic and stable and cold: Lake Tahoe, yet on the south side, there are huge plant beds. The massive volume can still support plants even at low levels because of currents bring new water to the plants, much like a river with low nutrients, this also prevents CO2 from runnign out even if it's very low.

Light is MUCH higher in Lake Tahoe due to the elevation. So light is 2500 umol or more. Water is clear etc.

Creeks make a better study for your question, we can walk down many creeks locally here and find lush plant beds, free for the most part of algae at the right TIME of year. Later in the season, water current drops and the plants get algae on them.
But when they are growing nicely, there's still algae elsewhere a few meters away, same creek, same light, same CO2 etc.
Explain that???

Current is all I have, and deposition of aquatic weed propagules to those locations. Strong AREAS OF AQUATIC PLANT RECRUITMENT MAKES A BIG DIFFERENCE!

To your last 2 questions, I do not know really. Stable conditions and plenty of plants and propagules are regularly added.
In systems that are highly stochastic/variable, aquatic plants are absent independent of nutrients/light etc.

the same is true for Tide pools and rocks on the beach near the ocean, they have the most species where there's intermediate disturbances.
On large rocks, that never move due to waves, they have only a few weeds that colonize and out compete the other macro algae. Small fast rapid colonizing algae only will last a few days perhaps on smaller rocks that will be moved by larger waves.
The intermediate sized rocks will have the highest diversity. Joe O Connell is an Ecologist that used this to describe the "intermediate disturbance hypothesis".

Likely a similar thing with us.

 

[Marcel G]

when are growing nicely, there's still algae elsewhere a few meters away ... same creek, same light, same CO2 etc.

Tom, that's strange. I thought that you were originally saying that in eutrophic tropical systems with huge plant biomass (healthy growing vital plants), the WHOLE system is "algae free" (= no algae issues) ... not just PLANTS. I'm trying to understand the correlations between eutrophic tropical system with huge plant biomass (= our aquaria), and algae issues. You said that some natural systems similar to our aquaria (= tropical systems with non limiting nutrients + huge plant biomass) don't have any algae issue ... so the nutrients could not be blamed for causing (inducing) algae issues.

So is it true that plants in these systems "protect" the whole system (somehow) from algae issues ... or false? I think this is the crucial question I'm asking. And if the answer be positive, then what do you suggest as to "HOW they are doing this"?

 

[Marcel G]

Recently I was searching some full text databases like ScienceDirect, Scopus, ProQuest and EBSCO, and I have found quite a big number of articles on "allelopathy in aquatic macrophytes" (e.g. Chara, Elodea, Myriophyllum, Potamogeton, Lemna, Pistia etc.). All these studies were stating very clearly that the scientists identified these allelochemicals, and that these chemicals inhibited the growth of several kinds of algae (cyanobacteria, and some green algae also, at least).

So it seems to me that the main role the plants play in inhibition of algae is:
1) allelochemicals
2) light "traping" (so the algae could have less light due to shading effect)
3) nutrients consumption (so the algae has less nutrients => I know the algae need much less nutrients then plants, but in the case of severe algae infestation, the nutrient limitation may play some role in algae decline)
4) the plant roots support better nitrification which reduces organics.

Also I know that active charcoal should remove all (or most of) allelochemicals from water, but not everyone uses it in their tanks. The similar would apply to frequent water changes. We do frequent WC, but with the change of water we also remove big amount of microscopic algae, so on the one hand we can dilute the allelochemicals in the water, but on the other hand we reduce substantially the number of algae in the water body.

Update:
I like one of Tom's comments:

You can be an arm chair philosopher and speculate till the cows come home, offer research support etc, but unless you test and see if something works or not, you will not gain much new insight.

Still I would argue that as we (non scientists) are not able to make these tests properly (= to rule out all the factors in play), than we have no other way than speculate.

So if you say that "excess of nutrients doesn't induce algae", I have actually no way to test it. If I add lots of nutrients in my tank, and still have no algae, it says nothing about the true reason. The nutrients could still play a big role, but if I have very low light, then the role (or "algae inducing power") of nutrients is weakened substantially by the lack of light. But that doesn't mean the nutrients are harmless. So our own observations/tests are very poor proof or disproof.

 

[Marcel G]

Actually I would like to point out that I don’t think the plants alone are the key to „algae free tank“ (unless „healthy plants“ means all the following). I see the key more like a combination of many factors working together (and no one alone/isolated can do it by itself):
1) decent light
2) healthy plants (wheather be it allelochemicals or other mysterious „something“ the plants are producing/radiating)
3) decent nutrients (and as I said, excess nutrients need not to induce algae, but less nutrients could induce even less algae)
4) low organics (no rotting plants = good growing, healthy and vital plants, no dead critters ...)
5) frequent tank management (cleaning, vacuuming, pruning, trimming, water changes ...)
6) good number of algae grazers
... and maybe some other factors like good filtration, circulation, temperature etc.

 

[Tom Barr]

Tom, that's strange. I thought that you were originally saying that in eutrophic tropical systems with huge plant biomass (healthy growing vital plants), the WHOLE system is "algae free" (= no algae issues) ... not just PLANTS. I'm trying to understand the correlations between eutrophic tropical system with huge plant biomass (= our aquaria), and algae issues. You said that some natural systems similar to our aquaria (= tropical systems with non limiting nutrients + huge plant biomass) don't have any algae issue ... so the nutrients could not be blamed for causing (inducing) algae issues.

So is it true that plants in these systems "protect" the whole system (somehow) from algae issues ... or false? I think this is the crucial question I'm asking. And if the answer be positive, then what do you suggest as to "HOW they are doing this"?

Well flowing waters, streams etc, vs say lakes/ponds, are VERY different, but often have the same plants and less commonly, at leats from what I have seen, the same algae, algae seems different in lakes vs streams.

Walk down a creek sometime where you have plants, you will find spots/locations with a lot of algae also.
It makes for a more interesting question in a stream since the plants and algae are sometimes only 1 meter away from each other but doing nicely.

No system is truly algae free, but blooms seem to occur based on some factor or germination/inducement.
So what causes a bloom or a sudden pulse in algae growth in planted tanks? That's a more specific question.
And we can try and test that question by adding things we think might be the trigger for blooms.

Plants appear to protect some regions that are optimal for the growth of plants.
But non optimal locations seem unaffected.

Lake Tahoe is a rather large oligotrophic lake, very low nutrient, but some locations are rich in nutrients and rich sediments.
Very murky water in those regions, but 50 meter clarity in the open lake.

Plants grow very well in some very defined areas, but not in other placed.

We can walk down a stream with plants and find similar cases. Generally, the more plants that grow in the system, the less algae.
But not always. Correlation is weak there if you have enough sampling where plants are present.

Current, temperature differences,nutrient and landscaping run off, aquatic weed propagule pressure from boats coming from other areas, Destruction of a historic wetland for homes, these all contribute to the aquatic weed problem in this area of Lake Tahoe.
There are many native plants, not the entire lake etc, but pretty small patches. Very nice, but cold water plants. Vernal pools are interesting because they last about 2-5 months and algae has been studied in those also.

Current and light likely influence things as well as plant presence in the streams. If the plants are scoured but too much current or there's little sediment to root or get established, then that might be good for algae.
If the water gets too warm and shallow for the plants, then algae might take hold.

If you look at where plants grow in streams, it's typically where current is moderate and even, and ofter on the inside of bend/turn in the stream where there is good sedimentation/soil to root into.
This still does not quite answer why the algae do not grow on the plants really in those locations.

So are aquariums more like lakes or more like streams?
Or can they be both or a combination of the two?

 

[Tom Barr]

Recently I was searching some full text databases like ScienceDirect, Scopus, ProQuest and EBSCO, and I have found quite a big number of articles on "allelopathy in aquatic macrophytes" (e.g. Chara, Elodea, Myriophyllum, Potamogeton, Lemna, Pistia etc.). All these studies were stating very clearly that the scientists identified these allelochemicals, and that these chemicals inhibited the growth of several kinds of algae (cyanobacteria, and some green algae also, at least).

So it seems to me that the main role the plants play in inhibition of algae is:
1) allelochemicals
2) light "traping" (so the algae could have less light due to shading effect)
3) nutrients consumption (so the algae has less nutrients => I know the algae need much less nutrients then plants, but in the case of severe algae infestation, the nutrient limitation may play some role in algae decline)
4) the plant roots support better nitrification which reduces organics.

Also I know that active charcoal should remove all (or most of) allelochemicals from water, but not everyone uses it in their tanks. The similar would apply to frequent water changes. We do frequent WC, but with the change of water we also remove big amount of microscopic algae, so on the one hand we can dilute the allelochemicals in the water, but on the other hand we reduce substantially the number of algae in the water body.

Update:
I like one of Tom's comments:


Still I would argue that as we (non scientists) are not able to make these tests properly (= to rule out all the factors in play), than we have no other way than speculate.

So if you say that "excess of nutrients doesn't induce algae", I have actually no way to test it.

Ole Petersen refute to Diana Walstad's comments about Allelopathy:
http://www.bio-web.dk/ole_pedersen/pdf/TAG_2002_15_7.pdf

My refute is different and based on carbon usage which is a standard control method for allelopathy research.
The other observation/s is/are what are the chances that all 300+ species of aquatic plant all produce such similar effects on algae as we see in planted aquariums?
I just think the chance of that last question is so slim, I could win the lottery 5X before that would occur by chance alone.

So that's one way to "test"

Light: shading can be done with screen or they do sell wavelength specific filters if you want to spend some $ and test that.

Algae and nutrients: induction of algae bloom is the key, not nutrients after the algae is in the full growth stage, at exponential growth or inducement, that's where the test needs done.
Few people bother with that outside Phycology. Few aquatist try and see if they can induce algae independent of other factors. Those aquarist with the control to grow plants independent of excess nutrient rarely are experimental type people.

4 might have some impact, but indirectly. Sediment bacteria might play some role with algae. BOD also.
Still does not explain stream algae.

If I add lots of nutrients in my tank, and still have no algae, it says nothing about the true reason.

Yes.
It does say that it CANNOT be the reason that algae is growing INDEPENDENT of other factors, it does NOT say why algae grows.
We have falsified that claim/hypothesis and can move on to another possible reason/cause for algae.

The nutrients could still play a big role,

Sure, but then they are DEPENDENT. Big difference compared to the above and hypothesis testing.
I can strongly limit say PO4, this limitation can affect CO2 demand in plants if say the CO2 is low, say 14 ppm.

I can add PO4 and then induce algae under high light in such a tank. The PO4 causes the demand for CO2 to skyrocket.
Thus there is a CO2 dependency on the PO4 concentration. If CO2 is non limiting for low PO4 and high PO4, now CO2 is INDEPENDENT.
And we have no algae in either case.

You need to test all outcomes, not just one, and most aquarist only test one outcome, rarely more.
PMDD from years ago fell into this trap.
http://www.thekrib.com/Plants/Fertilizer/sears-conlin.html

but if I have very low light, then the role (or "algae inducing power") of nutrients is weakened substantially by the lack of light.

More likely: the role of CO2 demand would be the the larger factor than nutrients.
So we'd expect that such a tank would be more stable due more to the low CO2 demand vs the low nutrient demand.
Plants have a much higher demand for CO2 generally than nutrients.

But that doesn't mean the nutrients are harmless. So our own observations/tests are very poor proof or disproof.

I disagree with you. I can back it up with a simple falsification example, so can many other hobbyists. We are not arguing what causes algae, only what does not cause algae, independent of other factors.
Regardless of the dependencies, of which, there may be MANY, it cannot be due to excess PO4(say in the case of Paul' Sear's hypothesis in PMDD).

The hypothesis MUST show we have algae in each case if we add excess PO4, NO3 or a cocktail of nutrients in each case.
Even Albert Einstein said all it would take is one example to disprove his entire theory.

Falsification is critical to ruling out the more obvious potential causes. Sometimes we cannot rule something other easily, so it remains a nagging question.

Another similar aquarium example:

Many put a chemical into their aquarium and then a fish/shrimp dies. They blame the chemical for the death. Other aquarist have used the same chemical for 10 years without any issues on the same species, they live next door and both have similar aquariums and fish.
We cannot say that the chemical killed the fish independent of other factors. There are 1001 ways to kill fish that do not include that chemical. The person using the chemical for 10 years serves as the "control". The person whose fish died, has no "control". He cannot rule out other possible reasons why the fish died.
The aquarist using the chemical for 10 years cannot say "why" the other aquarist's fish died, but he can say "why not".

If you do not understand this concept, I would suggest learning about hypothesis testing and the principle of falsification.
Please take a good look at that.

 

[Marcel G]

I think I understand the principle of falsification. I just don’t think it’s so easy in this case to rule out all the factors … to know what is the true reason behind it.

Let’s say I have a toxin. When I add it to my aquaria with fish, all the fish would die. My neighbor has the same aquaria with the same fish, and the same toxin in his aquaria ... but no fish deaths. According to your opinion (your understanding of the principle of falsification) we can boldly rule out the toxin as the reason behind the fish deaths. Why? Because the neighbor has this toxin in his aquaria too, and the toxin (in his case) causes no harm to fish. I hope you get the point.

The problem is that the neighbor could have some "antidote" (or other stuff) inside his aquarium that neutralized this toxin from being so toxic. So there is no doubt about the high toxicity of the toxin. So unless some other factor comes into play, the toxin will kill all your fish.

I say that it could be the same with nutrients or just anything else. You can’t say that you ruled out (or falsified) the excess nutrients just by the simple fact that it doesn’t induce algae in some tanks. You would need to use sterile tanks with distilled water … and totally controled environment to be able to proove or disproove something for sure.

The thing you are trying to rule out (falsify) could be dependent on other factors, so if you are not able to identify all these relationships and their correlations, than you cannot be sure what did actually happen! You say the excess nutrients do not induce algae. Maybe you’re right. But maybe not! Maybe the nutrients could really induce algae … if you stop using other factors (like lower light, shading plants, water changes, grazers, filtration, frequent cleaning …).

This applies also to the DEPENDENT vs. INDEPENDENT problem. How do you know the CO2 or nutrients are totally independent on other factors? I’m not bold enought to say that. Maybe you are.

Please, understand I’m not against you. I'm just trying to understand how all this could work together. Probably I’m wrong in my conclusions. But I don’t agree it’s so easy to rule things out (even nutrients).

I asked the same question prof. Roger W. Bachmann from University of Florida (as well as some other limnologists). He said that „large infestations with aquatic macrophytes tend to lead to reduced concentrations of planktonic algae“ (in lakes with high plant biomass)“ … so that’s exactly what you say. But when I asked him to propose the reason (why it is so?), he stated that „the evidence seems to indicate that the macrophytes are causing the concentrations of the nutrients in the water to decline, and thus leading to a decline in the phytoplankton populations … and that this might be due to the uptake of nutrients by the plants or their attached periphyton“. He don’t believe the allelopathy plays significant role in it (same as you), but still he attributes this algae decline (in lakes with high plant biomass) mainly to nutrients decline.