[birdzoom (OP)]

I was thinking:
wouldn't it be great to maximize algae reproduction is the oceans so that there was more photosynthesis.
Thin layers of transparent plastic at the surface could be used to make them densify and reproduce more.
It would be quite modular thing and governments could finance such endeavour to depolute the air.

Any thoughts?

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[CliffordK]

For it to work, you would also have to have a method to capture the algae, and also prevent the "plastic" from breaking up.

One of the problems that occurs near the coastlines, is that fertilizer run-off (Nitrogen + Phosphates) causes algae blooms.  As the algae dies, it sinks to the bottom of the ocean where it undergoes bacterial decomposition.  While you would think that algae would be good for oxygenation of the water, the decomposition process creates large oxygen poor regions, what has been termed as "dead zones".

It might be interesting to intentionally create algae blooms in the middle of the ocean, far away from the continental shelves where the impact to commercial fishing would be less. 

An issue, of course, would be getting the desired nutrients to the algae.  Currently the majority of the reduced nitrogen (ammonia and related) is derived from chemical processes involving fossil fuels.  Potentially one could use the algae and plankton to power the process, and certainly cells use far more carbon than they use nitrogen and phosphorous, but the fertilizer could potentially be expensive.

[evan_au]

In deep ocean, iron is a nutrient in short supply - it must blow off the land in storms. There have been experiments spreading iron in the ocean.

Note that plastic is an expensive material to make in terms of consuming a lot of fossil fuel. Placing plastic on the surface of the ocean would restrict oxygen takeup by the ocean (impacting fish), and would kill whales & dolphins...