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scottshilala t1_jac3tpt wrote

Reply to comment by edXel_l_l in ELI5: Why is it that when fertilizers make their way into waterways, all the oxygen disappears, killing the fish? by Psychological-Dog994

The algae bloom also limits the transfer of oxygen from the air to the body of water by creating a layer that blocks adsorption. The surface agitation is dampened dramatically by the floating mats, thus lowering the uptake of oxygen by a large percentage (that I can’t remember, so I don’t want to quote it, but it’s over 50%).

More simply, the floating mats of algae cut down the oxygen and create a blocking layer so very little oxygen gets through it.

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GeorgeCauldron7 t1_jacwrdf wrote

So does the algae itself consume oxygen (aerobic respiration?), or produce oxygen (photosynthesis?) in order to survive? Sorry, I study inorganic geochemistry but don't know much of anything about biology or botany.

Is there any difference between green algae and orange algae? I monitor water quality on a few streams, and the stream with the lowest dissolved oxygen (~30%) has both green and orange algae present, while the other streams have only green algae and have DO of 70-90%.

Also, are there any other plants that have a significant matting effect that dampens oxygen dissolution? Like a pond full of lily pads?

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RedditOR74 t1_jadidze wrote

Algae does both. During the day it contributes highly to oxygenating the water, but it consumes it at night. Since it also limits he surface area to replenish the oxygen, the water goes anaerobic during night time.

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Kingreaper t1_jadjc2x wrote

> So does the algae itself consume oxygen (aerobic respiration?), or produce oxygen (photosynthesis?) in order to survive? Sorry, I study inorganic geochemistry but don't know much of anything about biology or botany. > >

The Algae both consumes and produces oxygen - at first it's necessarily producing more than it consumes, in order to get the carbon it needs to grow in size, but when it's as big as it's going to get those numbers start balancing out.

Add in the fact that dead algae decaying take oxygen solely from the water, while living algae photosynthesising put their oxygen both into the water below and the air above, and you get a net decrease in oxygen over time (barring gas transfer, which as mentioned is blocked by the matting effect)

Another part of the problem is that the algae only produces oxygen in the daytime, and consumes it constantly, so at night the oxygen level in the water drops - and if something can't survive at night-time oxygen levels then it dies, and starts decaying, further lowering the oxygen levels.

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glampringthefoehamme t1_jadkekh wrote

Can we use the algae as fertilizer? As in, direct ask runoff to large ponds with non-harmful algae, allow it to absorb the leftover nutrients, harvest abs dry the algae, and then use it as fertilizer later?

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GeorgeCauldron7 t1_jadww4u wrote

So is it safe to say that if algae is present in a body of water, then you can expect the overall effect to be a net decrease in oxygen?

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