DO Concentration for Fishes in Different Water Volumes

[needhelpperson]

there are 2 identical containers with 2 identical fishes, but they both contain a different amount of water; one less and one more. Which one has more DO available to the fish?

I'm guess they both contain the same amount of DO. is this correct?

 

[gerben]

No, the container that contains most water contains most dissolved oxygen. There will be the same amount of dissolved oxygen per litre of water, so if there are less litres of water there will be less oxygen. This does however not matter for the fish in it (as long as the container is not too small), because the fish is using oxygen from only a little volume of water at a time, which is independent of the container the fish is in. As the concentration of dissolved oxygen decreases (and that of dissolved carbon dioxide increases), oxygen from the air will dissolve (and carbon dioxide will escape from the water into the air) so that the oxygen concentration will stay more or less the same at all times.

EDIT: Oh yeah, I am sorry I misread your post, there will be same amount of oxygen available to the fish

 

[needhelpperson]

No, the container that contains most water contains most dissolved oxygen. There will be the same amount of dissolved oxygen per litre of water, so if there are less litres of water there will be less oxygen. This does however not matter for the fish in it (as long as the container is not too small), because the fish is using oxygen from only a little volume of water at a time, which is independent of the container the fish is in. As the concentration of dissolved oxygen decreases (and that of dissolved carbon dioxide increases), oxygen from the air will dissolve (and carbon dioxide will escape from the water into the air) so that the oxygen concentration will stay more or less the same at all times.

EDIT: Oh yeah, I am sorry I misread your post, there will be same amount of oxygen available to the fish

After reading gerben's post, was i correct or not?

 

[gerben]

After reading gerben's post, was i correct or not?

Yes you were correct, the same amount of dissolved oxygen is available to the fish. This is the case because the amount of oxygen that is available to the fish depends on the concentration of the dissolved oxygen in the water which is the same in both containers. It does not depend on the total amount of water in the container.

 

[Phobos]

Both containers start with the same DO concentration per unit volume (and that is what the fish breathes). The greater volume of water starts with a greater total mass of DO.

Although, as gerben said, atmospheric O2 will dissolve into the water to replenish the consumed DO, this process is not as efficient as the fish's respiratory system. In other words, if it's a small container (small area exposed to the atmosphere), the fish will use up DO faster than it is replaced by the atmosphere. So, the fish in the smaller volume may be in trouble a little bit sooner.

The wider the container mouth (i.e., more surface area exposed to the atmosphere) and the more agitation of the surface, the faster the DO replenishment from the atmosphere. So, the balance would depend on the size of the tank and the activity of the fish.

If the replenishment is totally/quickly efficient, then yeah, the 2 fish see the same DO concentration.