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Aquarium With Fish And Water Plants - (Sep/29/2008 )

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today with biology they asked the following question.

In a Aquarium(Fish Tank) There are water plants and some fish.
The aqaurium is in a light, they check the oxygen level at regular intervals.
After 3 hours they change something. after that they keep measuring the oxygen level.
The results are in the following graph image(Tried to recreate it as accuratly as possible).


Now the options were:
A) They put the Aquarium in the dark
B) Some Water Plants Were removed
C) They added some fish.

The correct answer was A(We looked it up in the answer book), but i can't find out why.

So why does the oxygen level rise when it's put in darkness ?

Thanks for your time, Xeross


I see where you're going.

But I guess they put back the acquarium in the light after 3hres. Otherwise..


the book said after 3 hours they changed something, and the answer was they put it in the dark.

A teacher came with this weird theory that because the plant sucks up oxygen from the air the air is in the aquarium for a very short period but that period counts too.

wich i know doesnt make sense at all.

Is this just a human error while making the questions ? the stupid part is that they dont explain it.


When plants are put into the dark, they switch from photosynthesis (O2 production) to respiration(O2 absorption). You would need more points on the graph to see the actual change from light to dark and the graph would actually look somewhat different given the 3 hour change - the line shouldn't start to drop for a little time after 3 hours as the plant would be using up the O2 in the tank fairly slowly. Returning the plant to the light should cause the O2 levels to rise again.

What I suspect they have actually shown in the graph supplied is that the initial measurement of O2 levels was taken immediately after putting the plant in the dark at time 0, then the plant was returned to the light at 3 hours.

Actually, plants respire all the time, just the levels of O2 produced by photosynthesis are so high that the respiration is almost impossible to detect - the CO2 produced is scavenged directly by the photosynthetic system.


Think of it this way: the plants are oxygen producers, while the fish are oxygen consumers. Putting the aquarium in the dark will not change the fishes need for oxygen, so consumption continues. But, putting the aquarium in the dark interferes with the plants ability to produce oxygen. So, with consumption unchanged, but production slowed, the amount of oxygen goes down.

Unexplained is why the oxygen levels rose again, unless they put the tank back into the light.


Ok, so it's just a fault from the people that made it, good to know,


is it possibly a thermal response?

light warms the water which makes the gasses less soluble. in the dark, the water will cool and the gasses will be more soluble.


I though of that, too, but then convinced myself that gasses are actually less soluable at higher temperatures. This is why soda goes flat as it warms -- there's more CO2 dissolved at lower temperature: as the temperature increases the CO2 escapes, the amount dissolved decreases, and the soda goes flat...

Besides, if that were what was going on, wouldn't the graph be the opposite of what we see? No light = lower temp = higher O2...?


QUOTE (HomeBrew @ Sep 30 2008, 03:30 PM)
Besides, if that were what was going on, wouldn't the graph be the opposite of what we see? No light = lower temp = higher O2...?

yes. that is what the graph appeared to show and that is what i meant.

from the question it would seem that the light is on for the first 3 hours then it is turned off. O2 drops then recovers.


Oh -- I see what you mean. The light is on for the interval from 0 to 3, and then switched off from 3 to 6. It makes sense, if the light causes an increase in the temperature of the water -- except that the plants would not produce oxygen in the absence of light, so where is the oxygen coming from that causes the graph to rise? Dissolving in from the atmosphere, maybe?


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