Equilibrium in our Tanks

[Servillius]

Randy,

I’m not going to pretend to be an expert on this subject. Please correct what I’ve gotten wrong, but to the extent some of it is right, can you give us a crash course in how an equilibrium works.

I ask this because I see loads of discussions where it looks like someone is not accounting correctly for this mechanism. For instance, having some arbitrary value for phosphates, say 0.01, is only part of the story. Two tanks can have water with 0.01 phosphates but with very different total phosphates in the system. Tank 1 has effective export mechanisms such that even though 1000 goes in, only 0.01 is left in the water at a given time. Tank two has only 0.001 going in but much less than tank A is coming out (0.001) so it’s at 0.01. Assuming the removal is dynamic (usually true in our case), adding more to both tanks will have very different results.

This also means a tank with a lot going in and a lot coming out like A, has a lot more available to corals at least some of the time (like during the time it’s being slurped back out) so might not be a problem for the corals while tank B is leaving them perpetually starved.

Thanks in advance for the well reasoned smack down and further discussion!

 

[Brew12]

Randy,

I’m not going to pretend to be an expert on this subject. Please correct what I’ve gotten wrong, but to the extent some of it is right, can you give us a crash course in how an equilibrium works.

I ask this because I see loads of discussions where it looks like someone is not accounting correctly for this mechanism. For instance, having some arbitrary value for phosphates, say 0.01, is only part of the story. Two tanks can have water with 0.01 phosphates but with very different total phosphates in the system. Tank 1 has effective export mechanisms such that even though 1000 goes in, only 0.01 is left in the water at a given time. Tank two has only 0.001 going in but much less than tank A is coming out (0.001) so it’s at 0.01. Assuming the removal is dynamic (usually true in our case), adding more to both tanks will have very different results.

This also means a tank with a lot going in and a lot coming out like A, has a lot more available to corals at least some of the time (like during the time it’s being slurped back out) so might not be a problem for the corals while tank B is leaving them perpetually starved.

Thanks in advance for the well reasoned smack down and further discussion!

I'm curious to see Randy's answer, but I hope you don't mind my taking a shot at it.

I see this as two separate issues. If you have a large amount of PO4 going in and a large amount coming out I would consider this a high nutrient system even if water tests show 0.01ppm PO4. If you have a small amount of PO4 going in and a small amount coming out and still read 0.01ppm PO4, I would call this a low nutrient system. Everything else being equal, the total PO4 bound in the system would be nearly identical.

This is separate from phosphate equilibrium. Consider a system that runs for a year at 0.08ppm PO4 and then wants to drop to 0.02ppm by using GFO. They not only need enough GFO to account for new PO4 sources, but also to absorb the PO4 that is bound in their rock. Compare that to a tank that has been running 0.02ppm PO4 and wants to go up to 0.04ppm. They not only have to add enough new PO4 to raise phosphate levels, but they have to add extra to account for the rock absorbing it as levels increase.

 

[Randy Holmes-Farley Verified Community Expert]

On the issue of a needed element in the water, here's how I would suggest thinking about it.

To a coral, the only thing that matters in terms of its ability to take up this element is the concentration of it in the very near vicinity of its surface.

Let's say that value is 1.0 (of anything in any units). The coral is able to take up a certain amount of that element at that concentration, and because of the significant flow around it, it stays about the same concentration 24/7.

The coral doesn't know or care what is happening elsewhere in the system.

1. It could be the whole tank is at that value (1.0) and none is ever significantly added or removed elsewhere. A compound like magnesium comes close to fitting this description. Each magnesium ion might stay in solution for months or years, on average, before being taken up somehow.

2. For a different element, it could be that that there is a big ongoing source somewhere and a big sink elsewhere. Say, ammonia being released from all sorts of heterotrophic organisms and being taken up by all sorts of photosynthetic organisms (and nitrifying bacteria). Any given ammonia molecule might be around only a few minutes, on average, with a big flux from addition and removal happening all the time.

But a coral neither knows nor cares how much magnesium or ammonia is fluxing through the system. It only cares about local availability, and that local availability says nothing about the overall flux.

Does scenario 2 have more availability of the element to the coral? No, not if the concentration at the coral surface is the same.

If the concentrations are varying over time (as many will), then the question becomes a lot more complex to answer. Is 2 hours of 4 ppm nitrate and 22 hours of none providing more nitrate than 24 h of 0.5 ppm? It all depends on the gory details of the uptake mechanisms available in corals. The necessary information is almost certainly not available to answer that question, short of setting up an experiment and measuring it.

 

[Scott Campbell]

On the issue of a needed element in the water, here's how I would suggest thinking about it.

To a coral, the only thing that matters in terms of its ability to take up this element is the concentration of it in the very near vicinity of its surface.

Let's say that value is 1.0 (of anything in any units). The coral is able to take up a certain amount of that element at that concentration, and because of the significant flow around it, it stays about the same concentration 24/7.

The coral doesn't know or care what is happening elsewhere in the system.

1. It could be the whole tank is at that value (1.0) and none is ever significantly added or removed elsewhere. A compound like magnesium comes close to fitting this description. Each magnesium ion might stay in solution for months or years, on average, before being taken up somehow.

2. For a different element, it could be that that there is a big ongoing source somewhere and a big sink elsewhere. Say, ammonia being released from all sorts of heterotrophic organisms and being taken up by all sorts of photosynthetic organisms (and nitrifying bacteria). Any given ammonia molecule might be around only a few minutes, on average, with a big flux from addition and removal happening all the time.

But a coral neither knows nor cares how much magnesium or ammonia is fluxing through the system. It only cares about local availability, and that local availability says nothing about the overall flux.

Does scenario 2 have more availability of the element to the coral? No, not if the concentration at the coral surface is the same.

If the concentrations are varying over time (as many will), then the question becomes a lot more complex to answer. Is 2 hours of 4 ppm nitrate and 22 hours of none providing more nitrate than 24 h of 0.5 ppm? It all depends on the gory details of the uptake mechanisms available in corals. The necessary information is almost certainly not available to answer that question, short of setting up an experiment and measuring it.

Very cool explanation. Wouldn't large swings in overall flux increase the probability that local availability fluxes in response? But maybe that doesn't matter? In other words - would corals have the ability to take advantage of a sudden blast of ammonia or does a coral have a rather constant ability to absorb ammonia? Thanks Randy!!

 

[Randy Holmes-Farley Verified Community Expert]

Very cool explanation. Wouldn't large swings in overall flux increase the probability that local availability fluxes in response? But maybe that doesn't matter? In other words - would corals have the ability to take advantage of a sudden blast of ammonia or does a coral have a rather constant ability to absorb ammonia? Thanks Randy!!

I don't know how much "excess capacity" for short term uptake of needed nutrients and such a coral has. It might be a lot. It's not the sort of thing that usually gets studied.

 

[Servillius]

I don't know how much "excess capacity" for short term uptake of needed nutrients and such a coral has. It might be a lot. It's not the sort of thing that usually gets studied.

Thank you Randy, your responses are very helpful. In a sense the fact that it’s not well understood may actually be what I’m trying to grapple with.

We’re not really watching the availability of things with time when we test, just some discrete estimates at a specific time. As you point out, in some cases those could be misleading (I test every morning and it’s zero so it’s always zero may ignore the fact that it, whatever it is, gets added at lunch and only gets back to zero at 3am).

It also reminds me of the case of things like GHA (and maybe Zoa) that form their own micro environment by trapping detritus in a zone of slow moving water so may actually have lots of available nutrients even though the tank doesn’t show it. We’re just testing water, we’re not testing in time or in space. That would suggest there are all sorts of local phenomena we’re not yet account for, correct?

 

[Randy Holmes-Farley Verified Community Expert]

Yes, I agree. Local concentrations of some things may be quite different then the bulk water.

 

[Servillius]

On the issue of a needed element in the water, here's how I would suggest thinking about it.

To a coral, the only thing that matters in terms of its ability to take up this element is the concentration of it in the very near vicinity of its surface.

Let's say that value is 1.0 (of anything in any units). The coral is able to take up a certain amount of that element at that concentration, and because of the significant flow around it, it stays about the same concentration 24/7.

The coral doesn't know or care what is happening elsewhere in the system.

1. It could be the whole tank is at that value (1.0) and none is ever significantly added or removed elsewhere. A compound like magnesium comes close to fitting this description. Each magnesium ion might stay in solution for months or years, on average, before being taken up somehow.

2. For a different element, it could be that that there is a big ongoing source somewhere and a big sink elsewhere. Say, ammonia being released from all sorts of heterotrophic organisms and being taken up by all sorts of photosynthetic organisms (and nitrifying bacteria). Any given ammonia molecule might be around only a few minutes, on average, with a big flux from addition and removal happening all the time.

But a coral neither knows nor cares how much magnesium or ammonia is fluxing through the system. It only cares about local availability, and that local availability says nothing about the overall flux.

Does scenario 2 have more availability of the element to the coral? No, not if the concentration at the coral surface is the same.

If the concentrations are varying over time (as many will), then the question becomes a lot more complex to answer. Is 2 hours of 4 ppm nitrate and 22 hours of none providing more nitrate than 24 h of 0.5 ppm? It all depends on the gory details of the uptake mechanisms available in corals. The necessary information is almost certainly not available to answer that question, short of setting up an experiment and measuring it.

The part I left out of the story and the difference between the systems is how you solve a particular problem. Let’s leave aside the question of whether X causes Y and just stipulate it for a second.

In a system where you have too much X and you want to fix it, you may be at a particular equilibrium point because you have a reserve of it (detritus trapped in the sand bed for instance). Taking some out, by maybe doing a water change, is not going to have the anticipated effect. We see this all the time. I have 40ppm nitrates, I do a 50% water change and the next day I’m at 39ppm nitrates. You actually have to remove enough of the trapped stuff to slow its leeching in before you get an appreciable effect. This might mean a lot of removal over a long time. By comparison the tank without the trapped reserves could be pushed much lower much faster. Two tanks with the same test results, but the stability of nutrient X is wildly different so the fix for Y requires a lot more patience in one case.

 

[Randy Holmes-Farley Verified Community Expert]

The part I left out of the story and the difference between the systems is how you solve a particular problem. Let’s leave aside the question of whether X causes Y and just stipulate it for a second.

In a system where you have too much X and you want to fix it, you may be at a particular equilibrium point because you have a reserve of it (detritus trapped in the sand bed for instance). Taking some out, by maybe doing a water change, is not going to have the anticipated effect. We see this all the time. I have 40ppm nitrates, I do a 50% water change and the next day I’m at 39ppm nitrates. You actually have to remove enough of the trapped stuff to slow its leeching in before you get an appreciable effect. This might mean a lot of removal over a long time. By comparison the tank without the trapped reserves could be pushed much lower much faster. Two tanks with the same test results, but the stability of nutrient X is wildly different so the fix for Y requires a lot more patience in one case.

There are several effects like this.

There is a strong effect for phosphate because phosphate binds to rock and sand. That effect buffers the phosphate against changes, and a 100% water change doesn't eliminate it, and may hardly lower it.

I have a reef question of the day on positive vs negative feedback loops that also pertains...

Reef Chemistry Question of the Day #259 The Incredible Feedback Loops of Reef Chemistry
https://www.reef2reef.com/threads/r...ible-feedback-loops-of-reef-chemistry.380420/

Which of the following feedback loops is sometimes a positive feedback loop, where all of the others are more often negative feedback loops?

A. Silicate and diatom growth
B. Aquarium water temperature and evaporation
C. Alkalinity and SPS coral calcification
D. Calcium carbonate surface area and abiotic precipitation of calcium carbonate
E. pH and abiotic precipitation of calcium carbonate

Going through the possible answers...

A. As diatoms grow, they consume silica. Eventually they could use up the available silica/silicate, and would become growth limited by lack of silica. Thus the lowered silica can slow growth, and hence it is a negative feedback loop.

B. As water evaporates, the water temperature drops. When the temperature is reduced, the chance that more water molecules will leave the surface by evaporation (i.e., the vapor pressure of the gas above the liquid) declines. Hence it is a negative feedback loop.

C. As SPS corals grow, they consume alkalinity. As the alkalinity declines, the calcification rate declines. Thus the lowered alkalinity can slow growth, and hence it is a negative feedback loop.

D. As calcium carbonate precipitates, it creates new calcium carbonate surfaces. In many cases, the precipitation will cause an increase in surface area. Calcium carbonate is especially prone to happen on a fresh calcium carbonate surfaces that act as a seed crystal for further precipitation. Thus the cycle can continue faster and faster, and many reefers, especially those with new tanks with lots of fresh rock and sand surface area find themselves unable to maintain calcium and alkalinity with reasonable doses of supplements due to this effect. It also happens in limewater/kalkwasser overdoses where the tank turns milky white with suspended calcium carbonate. Many people find that despite the overdose, the alkalinity and pH quickly drop to normal after the positive feedback loop took all of the excess alkalinity out of the water in a massive precipitation event.

E. As calcium carbonate precipitates, it tends to lower pH. The reason is it does so is due to this equilibrium:

HCO3- <---> H+ + CO3--

By Le Chatelier's Principle, if you remove carbonate from the right hand side (by precipitation), the equilibrium shifts and more bicarbonate breaks up into H+ and CO3--. That produced H+ lowers pH.

In turn, the lowered pH tends to shift the total alkalinity more into HCO3- and less CO3--. Abiotic precipitation is driven by the presence of calcium and carbonate ions, and reduced carbonate reduces the abiotic precipitation.

Hence, abiotic precipitation lowers pH, and lowered pH tends to reduce abiotic precipitation. Thus, those processes form a negative feedback loop.

 

[Servillius]

There are several effects like this.

There is a strong effect for phosphate because phosphate binds to rock and sand. That effect buffers the phosphate against changes, and a 100% water change doesn't eliminate it, and may hardly lower it.

I have a reef question of the day on positive vs negative feedback loops that also pertains...

Reef Chemistry Question of the Day #259 The Incredible Feedback Loops of Reef Chemistry
https://www.reef2reef.com/threads/r...ible-feedback-loops-of-reef-chemistry.380420/

Which of the following feedback loops is sometimes a positive feedback loop, where all of the others are more often negative feedback loops?

A. Silicate and diatom growth
B. Aquarium water temperature and evaporation
C. Alkalinity and SPS coral calcification
D. Calcium carbonate surface area and abiotic precipitation of calcium carbonate
E. pH and abiotic precipitation of calcium carbonate

Going through the possible answers...

A. As diatoms grow, they consume silica. Eventually they could use up the available silica/silicate, and would become growth limited by lack of silica. Thus the lowered silica can slow growth, and hence it is a negative feedback loop.

B. As water evaporates, the water temperature drops. When the temperature is reduced, the chance that more water molecules will leave the surface by evaporation (i.e., the vapor pressure of the gas above the liquid) declines. Hence it is a negative feedback loop.

C. As SPS corals grow, they consume alkalinity. As the alkalinity declines, the calcification rate declines. Thus the lowered alkalinity can slow growth, and hence it is a negative feedback loop.

D. As calcium carbonate precipitates, it creates new calcium carbonate surfaces. In many cases, the precipitation will cause an increase in surface area. Calcium carbonate is especially prone to happen on a fresh calcium carbonate surfaces that act as a seed crystal for further precipitation. Thus the cycle can continue faster and faster, and many reefers, especially those with new tanks with lots of fresh rock and sand surface area find themselves unable to maintain calcium and alkalinity with reasonable doses of supplements due to this effect. It also happens in limewater/kalkwasser overdoses where the tank turns milky white with suspended calcium carbonate. Many people find that despite the overdose, the alkalinity and pH quickly drop to normal after the positive feedback loop took all of the excess alkalinity out of the water in a massive precipitation event.

E. As calcium carbonate precipitates, it tends to lower pH. The reason is it does so is due to this equilibrium:

HCO3- <---> H+ + CO3--

By Le Chatelier's Principle, if you remove carbonate from the right hand side (by precipitation), the equilibrium shifts and more bicarbonate breaks up into H+ and CO3--. That produced H+ lowers pH.

In turn, the lowered pH tends to shift the total alkalinity more into HCO3- and less CO3--. Abiotic precipitation is driven by the presence of calcium and carbonate ions, and reduced carbonate reduces the abiotic precipitation.

Hence, abiotic precipitation lowers pH, and lowered pH tends to reduce abiotic precipitation. Thus, those processes form a negative feedback loop.

Thanks Randy!

 

[taricha]

Thank you Randy, your responses are very helpful. In a sense the fact that it’s not well understood may actually be what I’m trying to grapple with.

We’re not really watching the availability of things with time when we test, just some discrete estimates at a specific time. As you point out, in some cases those could be misleading (I test every morning and it’s zero so it’s always zero may ignore the fact that it, whatever it is, gets added at lunch and only gets back to zero at 3am).

It also reminds me of the case of things like GHA (and maybe Zoa) that form their own micro environment by trapping detritus in a zone of slow moving water so may actually have lots of available nutrients even though the tank doesn’t show it. We’re just testing water, we’re not testing in time or in space. That would suggest there are all sorts of local phenomena we’re not yet account for, correct?

Here's a paper on surge uptake of N in macroalgae.

https://www.int-res.com/articles/meps/161/m161p155.pdf

Looks at fast weeds (like ulva) vs slow growers (like codium).
Found that everyone can do a little surge uptake for 1-2hr, but it only lasts you like 12hr. Found that under limitation, slow growers do much better even though they can't uptake N as well as the fast weeds.

In algae the uptake rates can be mostly guessed from the shape- thin leafy vs fat stalks.

Wonder if similar applies to corals - birdsnest, vs porites etc.