# Does surface agitation decrease oxygen?



## RisingSun (18 May 2017)

I know this is the CO2 forum but I have a question about oxygen.  If my plants are pearling, does surface agitation decrease or increase oxygen in the water?  One theory I've read is that pearling means the water is fully saturated with oxygen.  Surface agitation at that point will lower the oxygen in the water back to equilibrium.  Thoughts?


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## dw1305 (18 May 2017)

Hi all, 





RisingSun said:


> One theory I've read is that pearling means the water is fully saturated with oxygen.


Yes, that is it. 

Plants pearl because the water is fully saturated with oxygen. The oxygen is one of the <"products of photosynthesis"> (6CO2 + 6H2O ~ C6H12O6 + *6O2*). When light energy exceeds the light compensation point a plant it become a net oxygen producer. 





RisingSun said:


> Surface agitation at that point will lower the oxygen in the water back to equilibrium.


It will, but the only way to "preserve" that oxygen would be to seal the tank, so that  there is no gas exchange surface and that is fraught with danger. I only keep planted tanks and I like a large gas exchange surface, mainly because it gives you extra capacity at night if you have a pollution event (dead fish etc.). Oxygen is the prime metric in <"biological filtration">.

The great advantage of planted tanks is that during photosynthesis all of the plants internal air spaces will have become saturated with oxygen, and it is this internal oxygen that the plant overwhelmingly uses for respiration during the night. 

So although when light levels fall below light compensation point the plant goes back to being a net oxygen consumer, that oxygen is overwhelmingly drawn from this internal source, rather than the surrounding water. 

cheers Darrel


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## Cor (18 May 2017)

dw1305 said:


> Plants pearl because the water is fully saturated with oxygen


So if have 1 plant in 1000L of water with a low amount of light, and enough CO2, and you probably will not see much.
If the plant is exposed to direct sunlight or other stong lights, you will see the bubbles appear. Do you want to say that the plant has 1000L water saturated?
Can you explain this to a noob like me?


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## zozo (18 May 2017)

It is also temperatur dependend at 25°C water is satureted at about 8 mg/l oxygen..

Mg O2/liter
4° C - 13.13
10° C - 11.33
15° C - 10.15
20° C - 9.17
22° C - 8.83
24° C - 8.53
26° C - 8.22
30° C - 7.63

Mainly it is surface exchange that mixes the oxygene from the air with the water. Surface levels contain more oxygene than lower levels, so the water circulation provides the deeper levels with more oxygene. So actualy surface agitation caused by flow in a tank encreases oxygene in the lower levels of the tank.

It is hard to calculate the precise natural equilibrium since it is depended on atmospheric pressure (altitude) and air saturation, surface area of the water, temperature, surface movement and even salinity plays a role.. 

Here is a simple approximate calculator
http://www.hbuehrer.ch/Rechner/O2satur.html

It says in my case i live 135 metre above sea level and it's 25°C and an air humidity of 90%
o2 equilibrium is 7.32 mg/l so the plants only need to add 0.7 to reach saturation.. Aint that much..

Not calculated is the demand bacteriological process in the substrate which uses oxygene as well.


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## dw1305 (18 May 2017)

Hi all,





Cor said:


> So if have 1 plant in 1000L of water with a low amount of light, and enough CO2, and you probably will not see much. If the plant is exposed to direct sunlight or other stong lights, you will see the bubbles appear. Do you want to say that the plant has 1000L water saturated?
> Can you explain this to a noob like me?


The entire volume of water doesn't have to be fully saturated with dissolved oxygen, just the local area surrounding the plant leaves. 

Have a look at the last post (by ceg4048) in <"Plants pearling">.

Gases will diffuse along their concentration gradient, so if you have limited flow the area around the plant may be fully saturated with oxygen, even in quite polluted water (water with a high Biochemical Oxygen Demand (BOD) ). Oxygen isn't a very soluble gas (it forms 21% of the atmosphere, but you only get a maximum of ~15mg/l dissolved oxygen at atmospheric pressure).

If you turn the filter off when the lights are on you will get localised pearling in nearly all tanks. It won't last long however, because the area around the plant leaf will become depleted of CO2.

Carbon dioxide (CO2) is much more soluble than oxygen, but it only forms 400ppm of the atmosphere.




cheers Darrel


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## zozo (18 May 2017)

Here you see a nice example from a minute ago.. An outdoor aquarium, still strugling a bit with algae synthesizing like mad in the daylight. Grows faster than the plants at the moment.

Cloudy and rainy day in the daylight, 15°C, natural equilibrium..  bubbling all over the place and see where the most are.. Close to the surface where the water is most saturated..


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## Cor (18 May 2017)

Thank you so much for your time and effort for this explanation and useful information


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## xim (18 May 2017)

dw1305 said:


> If you turn the filter off when the lights are on you will get localised pearling in nearly all tanks. It won't last long however, because the area around the plant leaf will become depleted of CO2.



Ah, that's why people claim they see more pearling when they reduce the (water) flow.


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## RisingSun (18 May 2017)

So let's assume I have an iwagumi style full carpet of pearling plants.  If I want to maintain high oxygen levels near the substrate, do I want...

A. high surface agitation - to circulate the oxygen rich surface waters down to the bottom
B. low surface agitation - to prevent the oxygen from escaping the tank quicker and keep the deep levels saturated with plant oxygen

Or maybe somewhere in the middle? For simplicity, you can assume:
* flow near the substrate is the same for case A and B
* CO2 concentration is the same for case A and B (higher injection rate for A)

Here's the post by Ceg I read:


ceg4048 said:


> When plants produce Oxygen it causes a higher concentration of Oxygen in the water column. Agitating the surface too much actually accelerates the departure of this Oxygen as well as the departure of CO2, which is a double whammy. In un-planted tanks the situation is opposite because there are no Oxygen producing entities, only Oxygen consuming entities, therefore the Oxygen content of the water tends to be lower. In that case, strong agitation of the surface and other standard procedures such as air bubbles accelerate the replacement of the consumed Oxygen so that Oxygen diffuses from atmosphere to water more easily. So the people giving advice about surface agitation have this in mind but they don't realize that plants saturate the water with Oxygen so there is rarely an Oxygen deficit when you have a tankful of healthy plants.


https://www.ukaps.org/forum/threads/help-algae-outbreak.1798/#post-18917

So far I've been going by the rule of "as much agitation as possible without splashing or breaking the surface", which seems to be the most common advice found online.  However, I might consider going with a "slow gentle ripple" after reading more of Ceg's posts.

As to why I'm thinking about oxygen? I'm a bba sufferer and there has been lots of talk about redox/organics/microbial growth.  Oxygen seems like a common factor here that may help.


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## dw1305 (18 May 2017)

Hi all,





RisingSun said:


> A. high surface agitation - to circulate the oxygen rich surface waters down to the bottom.
> 
> B. low surface agitation - to prevent the oxygen from escaping the tank quicker and keep the deep levels saturated with plant oxygen


High surface agitation, but it isn't really about the surface movement, it is about water circulation and the area of the gas exchange surface. 

Linear flow is effective in maintaining high dissolved oxygen levels, and so are wet and dry trickle filters.  

Have a look at <"Aeration and dissolved oxygen..">, it was written specifically for the keepers of rheophilic Plecs, and it covers this area in some detail.

cheers Darrel


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## RisingSun (18 May 2017)

Darrel, thanks for the excellent link.  It seems to me, though, that the article is talking about low tech tanks? It says:

"we should definitely not add CO2 to tanks containing plecs*"*

What I got from the ceg snippet above is that high tech tanks differ than low tech tanks.  High tech tanks have a high rate of photosynthesis and O2 production.  This could perhaps make lots of surface agitation counterproductive to maintaining high O2 levels?


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## Zeus. (18 May 2017)

RisingSun said:


> his could perhaps make lots of surface agitation counterproductive to maintaining high O2 levels?



Yes. Thats why some rise the the return at night for better surface agitation at night so the O2 can diffuse better into water, then reduce height during CO2 phase to keep CO2 from escaping. Another way is to add a simple airstone on at night  so increased surface agitation due mainly to bubbles breaking surface and slight increase in flow, off in day. Or change the flow surface agitation some other way day/night time


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## dw1305 (19 May 2017)

Hi all,





RisingSun said:


> that the article is talking about low tech tanks?


It is, I'm not a CO2 user. I know that plants are the most important factor in maintaining water quality, and I want plec keepers etc to keep planted tanks, because it gives them more chance of success. 





RisingSun said:


> "we should definitely not add CO2 to tanks containing plecs*"*


It is largely to do with fish physiology and the <"Bohr-root effect">. Rheophilic fish don't have any physiological adaptations to high CO2 levels, and can be asphyxiated even when dissolved oxygen levels are high.

The problem is that the haemoglobin in the blood transports both oxygen and CO2. If you have high CO2 levels in the tank water the gradient between blood CO2 and the water is reduced and CO2 diffuses out of the gill surface more slowly. If a CO2 molecule is bound to the haemoglobin molecule, it can't accept an oxygen molecule.





RisingSun said:


> High tech tanks have a high rate of photosynthesis and O2 production. This could perhaps make lots of surface agitation counterproductive to maintaining high O2 levels?


No, it is like @Zeus says, but it will out-gas *added CO2* more quickly. Have a look at the "Bouncy Castle" analogy (in <"pH drop & high.....">) for an explanation.

Once the water is fully saturated with oxygen it can't hold any more and it out-gases (this is "pearling"). It is different with CO2, you can dissolve a huge amount of CO2 in water (think of a carbonated drink).

Because dissolved gases are difficult to measure we often use pH as a proxy for changes in the ratio of CO2 and dissolved O2. Oxygen is a base (the hydroxyl ion is really O-H, rather than OH-) and CO2 an acid (or more correctly a small proportion of dissolved CO2 becomes <"carbonic acid (H2CO3)">), and pH is the ratio of proton acceptors and donors.

Have a look at BigTom's posts in <"Maxing CO2 in low tech....">, the whole thread is worth a read.

cheers Darrel


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