Reef Chemistry Question of the Day #262 Oxygen in a Shipping Bag

[MnFish1]

Answer A is supposed to mean 100% water and no air (0%) above the water.

AHHHH... hmmm well... it depends on a couple things. 1. The size of the fish as compared to the size of the bag. 2. The total volume of water in the bag. Because the amount of respiration of the fish as compared to the amount of water in the bag is an issue....

But - I misunderstood the options a bit.

But - over the 2 hour drive home I say D and A.

 

[Randy Holmes-Farley]

And the answer is...

Part 1.
D. The bag 25% filled with water and 75% air from the compressor.

Part 2.
D. The bag 25% filled with water and 75% air from the compressor.

Part 1.
As Jim and Sleepy noted, there is far more oxygen in air than there is in water. So the more air in the bag, the more O2 is present.

Prrt 2.
This part is more complicated and open to interpretation. Even after being breathed in and exhaled, there is still a lot more O2 in the exhaled air than in water. All of the bags with some exhaled air in them will initially lose O2 from the water to the air (about a third of the total present in the water). That is a substantial negative, and the huge amount of extra CO2 is also a negative. But overall, there is more total O2 available to to the fish over time in a bag with exhaled air compared to no air, assuming it consumes some substantial amount of the O2.

It is less clear whether the O2 in a bag with no air ever drops below the point where the bag with exhaled air starts. That all depends on the size of the fish relative to the bag, how active it is, etc. In this context, the bag with the highest O2 in the water at the end of the trip will probably either be bag D, or bag A.