# Limiters



## beeky (26 Sep 2007)

This has kind of been prompted by a recent thread elsewhere, but I thought I'd put it here. What I wanted to ask about is what limits growth.

As far as I can see, macro and micro nutrients don't limit growth as such, otherwise we wouldn't see deficiencies - growth would just slow down, but it would still be healthy. Similarly, light isn't a limiter as plants still grow, but they are pale and spindly. So, is CO2/carbon a limiter? Do you see carbon deficiency?

What do people think?


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## Matt Holbrook-Bull (26 Sep 2007)

Any part of the cycle can limit growth.

Light  >drives> co2 take up >drives> nutrient takeup

Ideally, you want to be limiting by light only.  This will prevent algae taking advantage of an out of balance system.

If your limiting by co2, then excess nutrients will be available to algae, thus causing issues.  

If you limit by any of the main macro nutrients, NO3, PO4, MgS04 ( to a lesser extent ), KSO4( lesser extent ), then your likely to see the more major signs of plant deficiency.. these tend to stunt or stall growth almost totally, again, allowing algae to exploit the unstable system.

Micro nutrients (traces) can have a similar effect, but not necessarily as visible as macros unless its a severe case.

So, to recap, limiting in any way other than light, which drives the whole system, will cause an unstable environment, resulting in algae and unhealthy/poor plant growth.


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## beeky (26 Sep 2007)

I thought it was accepted these days that excess NO3, PO4 etc don't cause algae? I'm confused now!

I had the idea that instead of adding CO2 at 30ppm, I would keep it at, say, 20ppm which would slow down growth a bit and therefore be a bit easier to maintain. I take it from your reply, that that isn't an option?


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## ceg4048 (26 Sep 2007)

beeky,
           We'll have to get a clear idea of your semantics to address the question properly. The expression "limiting" as applied to plant growth does not have the same meaning as to "inhibit" plant growth. There are many ways to inhibit growth; herbicides, or other toxic substances, disease or predation (including algal attacks) can all inhibit growth. These are obvious so unless there is some specific substance of interest it would be laborious to attempt a list of all known _inhibiting_ substances. Probably anything under your kitchen sink if poured in the tank would be an inhibitor. As far as inhibiting substances produced within the tank's ecology, empirical data indicates that the nutrients we introduce have a far more significant effect in producing growth than the negative effects of any inhibiting compounds being produced inside the tank.

Instead, it's more useful to think about the limiting effects of nutrient shortages. If a tank is "light limited" each plant will have a limit placed on it's maximum possible growth rate. That means that no matter how much more of the other elements you feed them at some point no further increase in growth rate will ever occur until the limit of the light intensity is lifted. It's important to note that the parameter  of concern is "growth rate" not just "growth". It should be obvious that with all other things being equal a tank with highest light will exhibit highest growth rates than a tank with a low light. This second tank is therefore light limited.

Lets go back to this first tank. With all other parameters at "infinite" values there is a physical limit to photosynthesis wherein an increase in the wattage no longer increases the rate of sugar production and thus the growth rate. For some plants that may be 8 WPG, for others it may be 10 WPG. For slow growers like Anubias that might be 6 WPG.

I used the expression "with all other parameters at infinite levels..." but of course this is never really true. I started with light limiting because light is the driver of the uptake of all other things. If, instead of "infinite" value of nutrient concentration we had a "set" value for each at some point, prior to approaching the light limit, the uptake rate of a given nutrient might exceed the available concentration. It may be, for a given plant, 8WPG produces the maximum capable growth rate but this MAX growth rate may require 60 ppm CO2 concentration. If we had our set value at 50 ppm the tank would then become CO2 limited before it would become light limited. At this CO2 concentration level, even if we increased the light beyond 8WPG, growth rates would not increase due to our "throttling" of the CO2.

Light intensity drives -->CO2 uptake which then drives the macro nutrient uptake, the primary of which is Nitrogen. Lets say max possible growth rate in a tank full of plants requires 8WPG, 60ppm CO2 and 40ppm NO3. If we supplied max light and max CO2 but only supplied 30 ppm NO3 the tank would be Nitrogen limited and growth rates would be curtailed regardless of how much more light and CO2 we added. You can see that the same principles can be extended to both Phosphorous and Potassium. Finally, a tank being supplied with max light, max CO2 and max NPK then drives max uptake of the micronutrients, the primary of which is Iron.

Because plants are little more that a chemical factory production line, health and growth rate in this sense are mutually exclusive. A low tech tank is more often than not CO2 limited if no injection is offered and light limited if injection is present. Growth rates may be 1/5 that of a high tech tank yet the plants in the low tech tank can be healthier than those in the high tech tank, for example, if the high tech tank is macro or micro limited.

There is a fallacy therefore in correlating growth rates with health. Any highly lit tank with algae infestation is a testament to that. Poor CO2 limits growth but so does poor anything; poor micros, poor macros and combinations thereof. On the other hand some plants just have a tendency to grow spindly even when well lit and well fed. Frequent pruning can help to encourage bushier growth and to possibly thicken up the stems. Because so many deficiencies look similar within the same plant while appearing differently on a different species it's difficult to unilaterally diagnose and to pinpoint. Color loss on older leaves is typically a Nitrogen issue whereas color loss on new leaves can be an Iron or even Boron issue. Decay in leaf or stem can be Nitrogen and/or Potassium. Stunting/deformation can be CO2/Nitrogen/Phosphorous or Calcium/Magnesium or all. As a result of this sometimes ambiguous analysis my tendency is to increase dosages of everything, starting with a check of and adjustment of CO2 which is the most finicky. The ferts are easy because you can simply scale everything whether you are using powders or commercial - 25% more or 50% more or whatever. Deficiencies typically begin undetected as the tank grows in and when the total plant mass reaches some critical value. Uptake rates increase as a function of mass so one or more nutrients can easily bottom out (not necessarily zero but too low for comfort). The signs of stress may not show for a week or two because the plants build up a nutrient reserve and use the reserve when conditions warrant. This often causes us to draw incorrect correlations, to wit; "I did such-and-such yesterday and today I have problem X , therefore we may conclude that such-and-such causes problem X". Well, no, maybe it was something that started happening last Friday.

Chuck Gadd has a nutrient deficiency chart but it is old. It does have pretty good information though:
http://www.csd.net/~cgadd/aqua/art_plant_nutrient.htm

Hope this addresses the question.

Cheers,


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## Matt Holbrook-Bull (26 Sep 2007)

beeky said:
			
		

> I thought it was accepted these days that excess NO3, PO4 etc don't cause algae? I'm confused now!



It isnt a cause, but a lack of it will cause the plants to stop growing, and therefore algae will grow instead.



> I had the idea that instead of adding CO2 at 30ppm, I would keep it at, say, 20ppm which would slow down growth a bit and therefore be a bit easier to maintain. I take it from your reply, that that isn't an option?



limiting your system via co2.. bad idea.. limit the system by light, thats the only way youll slow it down.. light is the main driving factor, not co2, co2 takeup is a CONSEQUENCE of light, if you remove the co2, and not the light, then the plants will be driven to TRY and takeup co2, but it wont be there, so theyll be extremely unhappy, unhappy plants = algae growth.


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## James Flexton (27 Sep 2007)

nothing more to add but wholeheartedly agree to the above.

ceg4048 - another excellent post, thanks for taking the time to put all that down.


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## beeky (27 Sep 2007)

Thanks for the replies. I'm starting to understand it all a bit better now.

Any further questions and I'll let you know!


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## George Farmer (27 Sep 2007)

Nice thread.  Just a couple of things to add.

_Excess_ nutrients do cause algae if there isn't enough plant growth to utilise it.

This is why dense planting, good light and 30ppm CO2 is essential with full-EI, so the nutrient uptake is sufficient via plant growth to keep algae at bay.

There are leaner methods than EI i.e PMDD with PO4, ADA system etc. so 30ppm isn't necessary.  A stable 15ppm is fine.

Carbon deficincies are apparent in high light tanks with poor CO2, but algae will likely be the first symptom.

Poorly plants leech NH3/4 - a huge algae trigger.


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## Dave Spencer (27 Sep 2007)

Do excess nutrients cause algae directly? I assume we are talking in terms of adding nitrates and phosphates. Can it be said that the result of excess nutrients is algae, therefore they caused the algae? Recently, I took the nitrates in my 120l tank above 50ppm without noticing any change in my plants or in increased algae activity.

I feel what Matt is saying is correct, and algae can be indirectly caused by nutrients, but this is when they are deficient, not in excess.

I have accidentally caused algae in my tanks through low CO2, and carrying out a rescape that resulted in a lower plant mass and a disturbed substrate that released ammonia in to the water. I am only surmising about the release of ammonia as I never tested for it, but from what I understand, it is a recognised cause of algae.

I am of the belief that excess nutrients per se do not cause algae. Isn`t adding nutrients to excess the whole ethos of EI? Nutrient levels can cause instability, which has an adverse effect on plant growth which results in algae, but it is the failing plant growth that will trigger the algae.

What I don`t understand is how ammonia can trigger algae. Does the presence of ammonia suggest plants are not growing well because they are not processing it, or is it because the nitrogen is more readily available in ammonia than in nitrates?


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## beeky (27 Sep 2007)

From my (limited) experience in growing plants I always read that high nitrates and especially phosphates caused algae. However, my fairly low light setup with no CO2 injection and fully loaded fish stock had 50ppm nitrate and 5+ppm phosphate, yet I had no algae to speak of (a little BBA and GSA on anubias is all). This went against everything I'd read and then I came across Tom Barr saying the same thing. Does an excess of other nutrients cause algae? Iron? Potassium?


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## ceg4048 (28 Sep 2007)

From what I gather in Barr's treatises, algae cannot be induced by the addition of any nutrient alone. The algae triggers are primaily light, NH4 and CO2 fluctuations. If nutrients are added to a jar of water and is placed in a dark room or in an area of subdued lighting algae should not grow. A jar of RO water placed in full sunlight will have algae develop within a matter of days.

I surmise that In a tank NH4 can never ever be zero. NH4 production is a consequence of the metabolic processes of living systems. I believe therefore that when cycling a tank, NH4 concentration can be at it's highest therefore the risk of triggering algae is at it's highest. Once trigerred, algae will then devour whatever nutrients are in the water column and the plants will be at a disadvantage. I believe that this is the reason for the recommendation to have as many plants as possible in the beginning, NH4 concentrations can be mitigated if there is enough biomass to uptake NH4. At certain NH4 concentrations plants prefer NH4 versus NO3. Below this value NO3 is preferred so that as the tank matures NO3 uptake accellerates and the lower NH4 concentrations is more or less left to the bacteria. I believe that because the NH4 uptake is slower at the low concentrations found in mature tanks algae have an advantage if there is a spike in NH4. The spike can be at a concentration low enough to be undetectable by NH4 test kits yet high enough to trigger an algae bloom. This is another reason that circulation and filtration are critical. Poor circulation allows localized waste product buildup and therefore localized NH4 concentration buildup. Poor filtration due to clogged filters diminishes the ability of the bacteria colony to process NH4.

When the plants are healthy the nutrient flow is from the water column accross the leaf surface, however, as George says deficiencies cause breakdowns in the absorbsion mechanism. This results in a reversal of the nutrient flow across the leaf/water interface. NH4 is also leeched back into the water column across this interface. This is why the algae forms on the leaves. 

Many don't realize that although plants uptake NO3, the NO3 is first converted to NH4 before the nitrogen is stripped. It turns out that too high an NH4 concentration inside the plant is actually toxic so the plant regulates the NO3->NH4 conversion within certain "chambers". All these regulatory functions breakdown when the plant is stressed due to nutrient deficiency and the plant begins to jettison NH4 and other products across the cell walls. Is it any wonder therefore that we see our plants covered head to toe with algae?

This causes a paradox because we may be inclined to reduce the nutrient dosing so that it will not be available to the algae, yet this actually doesn't do the plants any good and they suffer more and leech more into the water column. The only way to stop the "bleeding" is to dose more. Yes, this will feed the algae but it will also stop the bleeding. The algae then has to be mechanically removed by trimming, scrubbing, water changes etc.

Therefore, I think it doesn't matter whether you dose EI and excess nutrients, or whether you use the leaner methods. Any mistakes in dosing, filtration, circulation, CO2, light will result in deficiency and subsequent dysfunction leading to one or more of the algae causal factors. I feel it's an oversimplification  to say that excess nutrients cause algae without stating the context in which the excess nutrients are found.

Cheers,


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## George Farmer (28 Sep 2007)

> I feel it's an oversimplification to say that excess nutrients cause algae without stating the context in which the excess nutrients are found.



I agree.  Some newer folk need oversimplification though I think.  I do sometimes....


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