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I am a dilettante when it comes to dead zones. I realize that they vary from fleeting algal blooms to continuous dead zones. For the continuous dead zones it would seem to me that adding a "bubbler" like for fish tanks might help restore the depleted oxygen content, which would increase microbial activity and thereby increase the quantity of nitrogen removed from the water (hopefully) before it reaches the open ocean where the fleeting algal blooms occur. Has this been investigated or could it at least be assessed from a scientific perspective?

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algal blooms consume all the oxygen in the water to make it anoxic. You'd have to bubble a lot of oxygen into the water. It probably depends on the nutrient mix in the water. If the algae can continue to grow and consume the oxygen as you bubble it in, then it won't help. –  shigeta Sep 26 '13 at 5:45
    
@shigeta If the algae continue to grow and consume oxygen, they they are also consuming nutrients that would otherwise contribute to algal blooms and/or dead zones. The idea is essentially to exhaust the runoff nitrogen through speeding algae growth - not necessarily to instantaneously solve the problem of dead zones. This should reduce the extent of dead zones. –  JoeHobbit Sep 26 '13 at 6:52
    
i see. it would take some work to figure this out - it might end faster but in any case i'd be surprised if all the other living things aren't dead by then. is the goal just to end up with a little nitrogen in the water? –  shigeta Sep 26 '13 at 12:32
    
@shigeta The goal is to remove nitrogen from the river water, since in the open ocean nitrogen is the limiting reagent for algal growth. –  JoeHobbit Sep 29 '13 at 20:24
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3 Answers 3

Actually, if you could get the oxygen in there, you would probably turn the dead zone into a "feast zone," because all that algae would be food. But I think it would be prohibitively expensive. Fish tanks aren't deep, so it doesn't take much pressure to force air bubbles into the bottom. And they're not large in volume, so you don't need huge amounts of air bubbling. Oceans, or even rivers or harbors, would be another matter.
Also, some of the algal blooms are poisonous "red tides."

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Going from our comment train I thought I'd put this into an answer.

This is similar to the proposal to dump iron into the sea as that's also supposed to be a limiting reagent for growth. There the goal was to sequester carbon from carbon dioxide as dead plankton which would sink into the depths of the ocean. I'm assuming they did their home work and the cost of producing iron metal, which uses a lot of oxygen is actually worth it.

Its interesting to observe here that oxygen depletion was not a concern here. Assuming they have done enough work to show that iron is the only reagent needed to induce the algal blooms, that's a different angle on the problem than you propose.

In any case the critiques are many because of the scale proposed are likely to create other unanticipated side effects, not the least of which is the massive release of the powerful greenhouse gas nitrous oxide.

The iron dumping experiment was mostly rejected because it was too simplistic. So, this proposal would require a model - there's no simple back of the envelope calculation that would give a thumbs up/down on this proposal. The question is - what's the next limiting factor after oxygen/nitrogen? I'd be surprised if that was not dependent upon location climate and local water chemistry.

A lot of red tides are caused by fertilizer and sewage dumps so you may be on to something here. If the goal is to remove nitrogen from the water, not only would it work, but would the cost be reasonable for what you're hoping to get?

Unless someone has already built the model I'd guess it would be up to you.

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The iron dumping was shown to be quickly removed from the seawater through its incorporation into some component of the algae that then sank: much like nitrogen. I think the iron might have been incorporated into a shell of some sort as part of the algae--possibly mixed in with CaCO3 or something. That was the reason Iron dumping was abandoned: it was hoped the iron would remain in the ocean ecosystem for longer. –  JoeHobbit Sep 30 '13 at 17:23
    
My point is just that your proposal is probably deserving of more thought - similar sorts of questions would have to be answered before the proposition would be considered valid. I myself couldn't find any preliminary work on this. and it does seem to be the case that they might have already considered oxygen. best of luck. –  shigeta Sep 30 '13 at 19:00
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Bubbles have actually been tried as described on Wikipedia. Usually the issue is layer stratification where the top layer has oxygen but the bottom layer where the decomposition of algae takes place lacks oxygen. Therefore mixers have been employed to bring the de-oxygenated water to the surface where it naturally re-oxygenates. It is interesting to note that one proposed and highly experimental idea is to use mixers to bring cold water from below to reduce the surface temperature to prevent or weaken hurricanes.

During heavy rain, London's sewage storm pipes overflow into the River Thames, sending dissolved oxygen levels plummeting and threatening the species it supports.[14] Two dedicated McTay Marine vessels, oxygenation barges Thames Bubbler and Thames Vitality are used to replenish oxygen levels, as part of an ongoing battle to clean up the river, which now supports 115 species of fish and hundreds more invertebrates, plants and birds.[14]

The dissolved oxygen concentration within Cardiff Bay are maintained at or above 5 mg/L. Compressed air is pumped, from five sites around the Bay, through a series of steel reinforced rubber pipelines, laid on the beds of the Bay and Rivers Taff and Ely. These are connected to approximately 800 diffusers. At times this is insufficient and the Harbour Authority is using a mobile oxygenation barge built by McTay Marine with liquid oxygen stored in a tank. Liquid oxygen is passed through an electrically heated vapouriser and the gas is injected into a stream of water which is pumped from, and returned to, the bay. The barge is capable of dissolving up to 5 tonnes of oxygen in 24 hours.[15]

Water Aeration

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