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This probably sounds pretty dumb, but wouldn't algae blooms produce a lot of oxygen? Although they would die out and decomposers would use up oxygen, is that more than what the algae produced?

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Algae produce $O_2$ in the upper layer of water but when they die they stop producing $O_2$. They sink to the seafloor and most get decomposed by bacteria on the seafloor. In this process, bacteria use $O_2$ contained in the bottom layer of water which decreases the dissolved $O_2$ concentration in the bottom water.

These concepts (and much more!) are well described in the excellent open-access paper by Rabalais et al. (2010) Dynamics and distribution of natural and human-caused hypoxia. It also includes case-studies of areas affected by hypoxia and eutrophication around the world. A must read if you're interested in this topic!

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    $\begingroup$ thanks for your contribution to BiologySE! Are you able to put some references to your answer? We prefer answers with references on this site, as anonymous users can claim to be an expert in anything... and having references allows other users to read more about your answer in the context of the question $\endgroup$ Jul 9, 2016 at 14:35
  • $\begingroup$ @VanceLAlbaugh that's definitely an excellent idea, thanks for pointing this to me. I edited my answer with one of my favorite paper on the topic. $\endgroup$ Jul 11, 2016 at 17:10
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As you surmise, the oxygen is consumed by decomposers. From the Wikipedia page on Eutrophication:

Phosphorus is a necessary nutrient for plants to live, and is the limiting factor for plant growth in many freshwater ecosystems. The addition of phosphorus increases algal growth, but not all phosphates actually feed algae.[2] These algae assimilate the other necessary nutrients needed for plants and animals. When algae die they sink to the bottom where they are decomposed and the nutrients contained in organic matter are converted into inorganic form by bacteria. The decomposition process uses oxygen and deprives the deeper waters of oxygen which can kill fish and other organisms.

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    $\begingroup$ Does the oxygen produced by the algae while they are alive evaporate away or what? Is it a question of producing O2 at the surface and consuming it at the bottom of the water column, where fish live? If the algae still have carbon-containing bodies there must be surplus O2 somewhere that they made while alive. As a continuous process why isn't the oxygen produced by new algae used to decompose the old algae? Does it just escape into the atmosphere? $\endgroup$
    – Resonating
    Aug 14, 2014 at 22:00
  • $\begingroup$ As the question is tagged as "marine-biology", your answer is not completely correct. Most marine ecosystems are nitrogen-limited, not phosphorus. $\endgroup$
    – RHA
    Jul 9, 2016 at 8:46
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Much like how planes fly, a proper explanation to this question is more nuanced and complex than first appears...

The total algae lifecycle is oxygen neutral

When any plant or algae grows sunlight is stored as chemical energy (oxygen + organic compounds). After the autotroph dies decomposers consume this energy and we are back to square one.

Would burying dead biomass keep the oxygen from being used up? Doing so oxygenated Earth in the first place (and oxidized soluble iron II into insoluble iron III). However, it only took a tiny fraction of fossilization to build up oxygen over geological time. Thus we can ignore burial in most ecosystems.

Water can't store enough oxygen

Water contains very little oxygen, even less if it is warm. Freezing water which is fully oxygenated (has been well-aerated with air at sea-level) will have about 14ppm of oxygen. This compares to 20% oxygen (200000 ppm) in the air. Water holds at most 1/10000 oxygen by mass and 1/10 by volume as air! This is also probably why no fish or shark is as big as a blue whale.

During the bloom oxygen rapidly exceeds the amount water can hold. It may seem strange that water with only, say, 25 ppm oxygen will give oxygen to the air. But the rule that "solutes move down their concentration gradient" assumes that the solvent they are dissolved in stays the same. When we consider multiple solvents we need to factor in the partition coefficient.

Even if air was pure oxygen, only 40ppm would dissolve in warm water. When shallow water exceeds this threshold bubbles of oxygen appear and rise to the surface.

Algae blooms are followed by a population crash once whatever nutrient caused the bloom (usually nitrogen, iron, or phosphorus) is used up. Such crashes are common with fast-growing species. Since most of the oxygen produced by the bloom has been lost the water will end up with less oxygen than it started with. Anoxic water is quite unpleasant.

Decaying biomass sinks

Algae blooms produce oxygen near the surface. The cells then die, lose buoyancy, and sink. The whole lifecycle of growth and decay has the effect of pumping oxygen out of deeper water and into the air. This can create near-permanent dead-zones in deeper water.

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