I’ve seen numerous nature-type programs (I am a Discovery Channel addict) in which pollution or other factors have lead to lakes and areas of ocean being low in oxygen. I’ve also had at least one aquarium at all times for most of my life, and never really questioned the need to aerate the water and change it out regularly.
I’m sure my logic is goofy here, but if H[sub]2[/sub]O is made of two parts hydrogen and one part oxygen, how is it possible that water can still be water with less oxygen? Why doesn’t the absorbtion of oxygen by, say, fish change the water into hydrogen?
It’s pretty obvious that I’m no chemist, but I suddenly feel the need to know.
We’re talking about gaseous oxygen dissolved in the water, not the oxygen chemically bound to hydrogen which makes up water.
Aeration is what dissolves the oxygen in the water… cold water absorbs more oxygen than hot water,BTW.
Oxygen in two forms. As part of the water molecule and as a solution in the water. Fish don’t break down the water to its constituent atoms. They take the oxygen out of the solution.
The confusion come from the term “low in oxygen” which should be “low in dissolved oxygen”.
Marine creatures can access disssolved oxygen, but not that bound within the molecule.
It seems that the question has been answered but i’ll put my 2 cents worth in anyway. The dissolved oxygen that is used by fish, animal life in the water is separate from the H2O. It is in there kind of like kool aid mix in the water. It gets in the water by at least 3 ways, (1)the plant life giving it off through photosynthesis,(2) the water being splashed(areated) by currents, waterfalls,wind, anything that stirs it up, or (3)it accepts it from the atmosphere through pressure.
The fish breathe only the dissolved oxygen. In ponds most of the oxygen is produced by photosynthesis. When the sun is shining the photosynthesis is working. At night it stops, there has to be enough oxygen built up during the day to last the fish until morning. The dissolved oxygen level drops steadily during the night. As was said above when the water is cold it will hold the oxygen better than when it is hot. On cloudy days, not much photosynthesis occurs so the night after a cloudy day is a likely time for low oxygen.
Pollution can kill all of the plant life (algaes) in the water. This also stops the photosynthesis from happening.
Also when pollution kills aquatic plants (or anything else that ends up in the water), these organisms start decomposing. The bacteria that do this use oxygen as well. Sewage pollution can be full of basically dead stuff that is well on it’s way to being decomposed, bringing with it all sorts of bacteria and little disgusting life forms that all breath oxygen, and the more they use, the less that’s left for anything else like fish. Once the oxygen is all or mostly gone, usually there are anaerobic bacteria (breath gases other than oxygen) that take over “eating” everyone who has just suffocated. These guys can really release some nasty smelling aromas, and can make the water chemistry and clarity difficult for new plants to grow for a while. All this depends on the amount/nature of pollution, currents that remove or dilute it, and time.