Reef Chemistry Question of the Day #259 The Incredible Feedback Loops of Reef Chemistry

[Randy Holmes-Farley]

Reefkeepers are extraordinarily fortunate that there are so many strong negative feedback loops in reef chemistry, and relatively few positive feedback loops.

A positive feedback loop is a situation where as a process proceeds, it alters things in a way that makes the process go faster and faster.

A negative feedback loop is a situation where as a process proceeds, it alters things in a way that makes the process slow down.

Imagine a couch on fire in a house. As the fire burns, oxygen becomes depleted. The reduced oxygen can slow down the burning process. That is a negative feedback loop.

Imagine instead if the products of the fire were able to actually feed the fire and make it burn faster. In such a positive feedback loop, the couch might burn faster and faster and maybe explode. That is actually how some nuclear weapons work, with the production of high energy neutrons triggering the fission of more fuel, releasing more neutrons, which trigger more fission...

In general, negative feedback loops enhance stability, and positive feedback loops cause instability.

All of the following examples display the properties of a negative feedback loop, except one. That one is a positive feedback loop. Think about them in a typical reef aquarium setting, not a contrived situation.

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

Each is listed as a measured value, and a process that depends on and also changes that value.

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

Good luck!
































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[FartyParty]

I would say B is the one positive feedback loop, since evaporation increases with temperature increase. We see this in desert regions where the water has so much energy it never condenses.

 

[JimWelsh]

I would say B is the one positive feedback loop, since evaporation increases with temperature increase. We see this in desert regions where the water has so much energy it never condenses.

True, evaporation increases with water temperature increase, but what effect does that evaporation have on the tank water? It cools it. In order for B to be the positive feedback loop, evaporation would have to make the tank hotter.

 

[FartyParty]

True, evaporation increases with water temperature increase, but what effect does that evaporation have on the tank water? It cools it. In order for B to be the positive feedback loop, evaporation would have to make the tank hotter.

Doh!

 

[THX1138CY]

D.

 

[Marcio Gielfi]

E
Precipitation of calcium carbonate will reduce CO2 dissolved and raise PH and raising PH will accelerate calcium carbonate preciptation generating positive loop.

 

[Chuk]

I think d. The increase in precipitation will produce more nucleation sites for precipitation to occur. You need solids to induce others to precipitate. So more solids is more precipitation.
Provided you don’t drop to a ca/ alk concentration that no longer wants to precipitate

 

[THX1138CY]

I think d. The increase in precipitation will produce more nucleation sites for precipitation to occur. You need solids to induce others to precipitate. So more solids is more precipitation.
Provided you don’t drop to a ca/ alk concentration that no longer wants to precipitate

That was my thinking, and the precipitation would only stop when the Mg to Ca ratio is high enough to prevent the precipitation.

 

[Hans-Werner]

Precipitation of calcium carbonate releases CO2 or consumption of CO2 precipitates calcium carbonate. Negative feedback loop.

I am sure that C may be a positive feedback loop but I am not sure whether this is what Randy meant. High alkalinity can kill SPS or effect SPS negatively, worsen polyp extension and growth. With a steady alkalinity supply reduced growth of corals will cause a rise in alkalinity killing further corals and so on ...

I am sure that D is the right solution: The formation of calcium carbonate acts as seed crystals causing more calcium carbonate to precipitate. So in an oversaturated calcium carbonate solution the formation of calcium carbonate precipitates may be like an avalanche until the solution reaches the state of undersaturation and precipitation stops.

 

[Hans-Werner]

And the right answer is ...

 

[JimWelsh]

..... D. Calcium carbonate surface area and abiotic precipitation of calcium carbonate.

 

[Randy Holmes-Farley]

yes...D

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.

Happy Reefing.

 

[MnFish1]

Reefkeepers are extraordinarily fortunate that there are so many strong negative feedback loops in reef chemistry, and relatively few positive feedback loops.

A positive feedback loop is a situation where as a process proceeds, it alters things in a way that makes the process go faster and faster.

A negative feedback loop is a situation where as a process proceeds, it alters things in a way that makes the process slow down.

Imagine a couch on fire in a house. As the fire burns, oxygen becomes depleted. The reduced oxygen can slow down the burning process. That is a negative feedback loop.

Imagine instead if the products of the fire were able to actually feed the fire and make it burn faster. In such a positive feedback loop, the couch might burn faster and faster and maybe explode. That is actually how some nuclear weapons work, with the production of high energy neutrons triggering the fission of more fuel, releasing more neutrons, which trigger more fission...

In general, negative feedback loops enhance stability, and positive feedback loops cause instability.

All of the following examples display the properties of a negative feedback loop, except one. That one is a positive feedback loop. Think about them in a typical reef aquarium setting, not a contrived situation.

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

Each is listed as a measured value, and a process that depends on and also changes that value.

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

Good luck!


As water tenp goes up - evaporation goes up. B





























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[MnFish1]

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.

I humbly disagree. If there is a heater in the tank this is not the case - and it is a reef tank were talking a bout

 

[JimWelsh]

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.

I humbly disagree. If there is a heater in the tank this is not the case - and it is a reef tank were talking a bout

The presence of a heater in the tank does not change the fact that the evaporation cools the water, making the water temperature / evaporation system still a negative feedback system.

The question is about a measured value, and a process that affects that measured value. Evaporation cools the water, lowering the temperature, period. Just because the heater wants to keep raising the temperature does not change the fact that the resulting evaporation removes heat from the water it is evaporating away from.

 

[MnFish1]

The presence of a heater in the tank does not change the fact that the evaporation cools the water, making the water temperature / evaporation system still a negative feedback system.

The question is about a measured value, and a process that affects that measured value. Evaporation cools the water, lowering the temperature, period. Just because the heater wants to keep raising the temperature does not change the fact that the resulting evaporation removes heat from the water it is evaporating away from.

Yes - it was a tongue in cheek comment.