There's a few key ideas here: The limiting factor in plankton growth is iron. So we can hit it with other things, the plankton needs more iron to incorporate into chloroplasts to grow. Algal/plankton blooms can be toxic from the chemicals produced, but they are usually toxic because they deplete oxygen in the environment. I suspect that this would be manageable in the ocean, since there's constant mixing and the ocean is _big_. http://oceanservice.noaa.gov/facts/habharm.html
Iron seeding of the ocean already happens, in the form of dust storms blowing onto the ocean. We've been able to see the effects of it, and it does as it says-- it increases plankton life in the ocean.
I think that a lot of posts here are underestimating the size of the ocean or over-estimating the capability of humans. We would have to dump so much iron into the ocean to make the whole thing anoxic. If we did this in a controlled manner (and in small paths, so we wouldn't end up with big un-mixed anoxic patches) then I think this is very reasonable.
All of you should be careful not do have the knee-jerk "nah that sounds crazy!" reaction. This is HN, and you ought to know that the disbelievers don't have a good track record.