Oxygenation?

[IonaMcCluskey]

i've read a few post on here about air pumps and whether they are needed or not. the most common answer i come across is that surface agitation is what causes oxygenation.
 
i was taught and therefore belived differently. i am interested in finding out which veiw is more accurate (please don't let this thread turn to arguments? peaceful intelectual debate or scientific proof is what i'm interested in)
 
according to the veiw of surface agitation causing oxygenation that would mean that in a flat body of water there would be no oxygenation ie gasious exchange?
 
the other veiw oxygenation/gas exchange happens at any given surface between water and air.
 
the idea behind using air pumps was that every bubble produced had a water/air surface where gas exchange could occur thereby greatly increasing rate of gas exchange, surface agitation would also have a similar effect in that even a small ripple gives a larger surface area.
 
so i ask if movement is the cause of gas exchange or does gas exchange occur without movement?
 

 

[eaglesaquarium]

Oxygen/air exchange always occurs between any surface interface.  Period. There is no alternate view to that.
 
Increasing the surface area, increases the exchange rate.  Disturbing the surface (ripples) of the water increases the surface area, therefore increases the surface area.  Ripples allows about double the surface area of the top of the tank.  The airstone will also disturb the surface more, but the truth is that the bubbles themselves are rather large, so they have a small surface:volume ratio.  Secondly, I'd add that they don't stay in the water for very long, so that doesn't do much more to create a larger surface area, compared to the expense.  Having a 'splashing' effect of the water from the filter can create a similar result - water is in contact with the air while 'en route' (for the brief period that it lasts) and brings oxygenated water into the tank.  It also creates 'splash' which also increases the surface area of the water/air interface.
 
 
The issue with a non-agitated surface of water is not that the exchange doesn't happen, the issue is that there's also very little water movement.  So, if the water/air surface isn't moving, the water near the surface becomes saturated with oxygen and won't take more oxygen into it.  The diffusion rate of oxygen through the water is very slow, compared to the possibility of moving that oxygenated water mechanically.  So, creating 'surface agitation' serves two purposes.  1 - increased surface area.  2 - increased water movement allowing the water to have a more uniform distribution of oxygen throughout the tank.

 

[Fishmanic]

so how do fish live in deep water in calm rivers or large lakes?

 

[eaglesaquarium]

By using less oxygen...
 
I never said that oxygen doesn't diffuse through the water on its own, I merely stated that the rate is far slower with less water movement.  Moving the water allows the oxygen levels to be more consistent.

 

[IonaMcCluskey]

so it is surface area that is the important factor?
the movement is only to distribute and help prevent saturation at the top?
you mention the size of bubbles, would surface tention differ in different sizes of bubbles and would that affect the rate of gas exchange?
but as there is a constant flow of bubbles from an air pump (outlet usually at the bottom of the tank) wouldn't the water at the bottom still have a certain rate of gas exchange going on and the saturation would work the opposite way?

 

[DevotedToDiscus]

so how do fish live in deep water in calm rivers or large lakes?

Also, I would doubt that there are any lakes in the world that don't see wind and rain. The wind not only causes a lot of surface agitation, but also circulates the water by pushing the surface water against the bank and creating an undertow.

 

[eaglesaquarium]

so it is surface area that is the important factor?
the movement is only to distribute and help prevent saturation at the top?
you mention the size of bubbles, would surface tention differ in different sizes of bubbles and would that affect the rate of gas exchange?
but as there is a constant flow of bubbles from an air pump (outlet usually at the bottom of the tank) wouldn't the water at the bottom still have a certain rate of gas exchange going on and the saturation would work the opposite way?

 
 
Yes, surface area is the primary factor.
 
The movement is to keep levels consistent and prevents saturation...
 
Surface tension, as far as I know, is not dependent on size... but the larger the bubble, it will rise up, and the rate at which it diffuses is slower than a smaller bubble.  Surface area of a smaller bubble has a better ratio to its volume, which is why we use airstones rather than just let the hose bubble away.
 
The movement of the bubbles upwards actually creates a bit of a water current in itself, which is why it can be used to power sponge filters.

 

[CrimsonBoli]

Also the surface area of a lake/river is probably measured in square KM, the surface area of our tanks is measured in square cm/inches etc...
 
Edit: + Wind in the open world

 

[DerpPH]

 

so how do fish live in deep water in calm rivers or large lakes?

Also, I would doubt that there are any lakes in the world that don't see wind and rain. The wind not only causes a lot of surface agitation, but also circulates the water by pushing the surface water against the bank and creating an undertow.
 

 
 
so does coriolis effect which is pretty much like the rising of warm water and sinking of cold water.
 
 
 

so it is surface area that is the important factor?
the movement is only to distribute and help prevent saturation at the top?
you mention the size of bubbles, would surface tention differ in different sizes of bubbles and would that affect the rate of gas exchange?
but as there is a constant flow of bubbles from an air pump (outlet usually at the bottom of the tank) wouldn't the water at the bottom still have a certain rate of gas exchange going on and the saturation would work the opposite way?

 
 
Smaller and more frequent bubbles are more or less better than slow bug bubbles.
 
The more bubbles the more agitated the water

 

[ech0o]

An airstone releasing bubbles begins oxygenating the water before the bubbles ever hit the surface.  What I am hearing here I believe to be mostly right, although with a finer airstone, you should be getting FAR MORE oxygen to water surface area, then what is occuring at the top of your tank.   Each bubble contains the same oxygen as what is in the air, so it really is about creating as much surface area of oxygen as possible.   The smaller the bubbles, the more surface area that occurs, as 10 bubbles with the same air volume as 1 bubble, will have more surface area than the 1 bubble, simple math.  Also, the smaller bubbles have less bouyancy as eagles states, so they take much longer to travel to the surface, and therefor diffuse into the water more.
 
The water movement caused by the airstone also helps stir the water and create more gas to water contact at the surface as well.   
 
Do you HAVE to have an airstone to successfully keep a tank?  Negative, I have been running my tank without one since I set it up, but I have good water agitation to make up for it.  On the exact flip side, you can have a tank with low surface agitation, and supplement with an airstone to get the proper oxygenation.

Also think about in nature, how would oxygen come out from the bottom of a lake or body of water?  Oxygen is not one of the gasses regularly released from the earth.  The majority of a body of waters oxygen should have to come from the surface of the water.  A small fraction of it comes from plants and other organisms within the water, although many of those organisms need something else to produce it, such as co2.  This should inevitably lead to there being something required in the air above the water.

 

[TwoTankAmin]

Surface tension blocks the exchange of gasses between water and the atmosphere. In still water this means there is little exchange. The roiling/agitation of the surface permits the gasses to move between the water and the air freely. This is why one must have surface movement in most tanks. I say most because in heavily planted tanks where co2 is injected, the plants will make the O and take in the co2.
 
With airstones or sponge filters there is some gas exchange cause by the bubbles themselves, but the greatest effect comes from the bubbles breaking the surface tension as they rise and "pop."
 
In nature wind and rain cause surface aggitation in what otherwise might be still waters such as in a pond.
 
if you have ever had a hang on filter quit and you did not catch it for a day or so and you work to restart it, the first thing you might notice is the nasty smell of the water in the filter. This results from the lack of gas exchange and the resultant reactions that occur in the face of this lack.
 
This is an interesting subject for me in relation to the zebra plecos I keep and am a bit of a nut about. They live in only one section of one river n Brazil. the water is very warm and the fish need high oxygen levels. This oxygen gets into the water from all the rapids in their territory. but the reason these fish are only found in the Big bend of the Rio Xingu is due to the fact that either end of their territory is bounded by a substantial stretch of fairly calm, slow flowing waters. these waters are much lower in oxygen content and the result is they act as a barrier to the fish being able to migrate to other areas of the river system. It is a perfect example of surface tension and gas exchange and the potential consequences.
 
The amount of gas exchange that can occur in a tank is limited by the surface area. For this reason it is important, in most instances, to insure surface agitation in a tank is sufficient to promote the required gas exchange.

 

[IonaMcCluskey]

i'm probably going to negate and answer one of my own questions here, but, is surface tension not relative to surface area? ie why does one capilary tube work better than another. therefore larger surface area has less surface tension allowing more gas exchange?
 
whats the difference between injecting CO2 and O?

 

[TwoTankAmin]

Surface tension has to do with the fact that air and water meet. There is a difference between how the molecules of water inside a container work in relation to each other and those at the boundary of the water and the air.
 

 
The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. The molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. This forms a surface "film" which makes it more difficult to move an object through the surface than to move it when it is completely submersed.

from http/hyperphysics.phy-astr.gsu.edu/hbase/surten.html

 

[eaglesaquarium]

The means that are gone to- to ensure complete diffusion of the CO2.
 
 
When injecting CO2, careful attention is paid to the size of the bubble of the CO2 and to ensure that by the time it reaches the surface, the bubble has completely diffused into the water.  There are a variety of ways to do that, but care is always taken and an airstone is almost never the choice for diffusing the CO2, primarily because the bubbles from an airstone are far too large to diffuse into the water fast enough.  Generally, CO2 diffusion is done by way of a ceramic diffuser, or some other way to ensure that you get 100% diffusion.
 
 
That NEVER occurs when using an airstone in an attempt to increase O2 in the tank.  First of all, the air you are adding is free - just air from the room, not specially obtained CO2 from either a canister or yeast reaction.  So you can just add a continuous stream of air bubbles without a bump in cost, unlike adding CO2.  CO2 is 'cheap', but certainly not free.  Secondly, as pointed out, increasing surface agitation can increase the diffusion of O2 to the tank, and having bubbles from the tank rising to the top actually does that on its own. (On a side note: That is another reason you don't want CO2 to rise to the surface, you don't want it to have access to the surface and to leave the tank, but you also don't want an increased amount of surface agitation.  Increase surface agitation will cause the gas in the water to reach an equilibrium with the air and consequently, the gas diffused in the water will more closely match the concentration in the air which is far lower than the ~30ppm that aquarists want for their plants.)

 

[IonaMcCluskey]

so a larger surface area has less cohesive force therefore less resistance to gas exchange?
the rapids may still be increasing gas exchange by increasing surface area rather than driving the gas exchange?
 
if the same care and attention was taken in the injection of O as is in the injection of CO2 would there be the same result?