Last night I got an idea. Red tides are a nuisance caused by an excess of nutrients in water, often coastal because of farm runoff. The color is not the problem -- the problem is the neurotoxins and out-of-control growth. But what if a plankton were engineered to deal with it?
We would engineer a plankton to eat harmful algal blooms, and breed them to have enzymes to neutralize the various poisons contained within. Then, when an algal bloom gets out of control, we drop a vial of the plankton in, and the algae disappears in a massive "Om nom nom nom nom nom." The engineered plankton would likely die off when the algae was all consumed, or more likely, a homeostasis would develop with a certain level of algae and plankton.
This would result in a carbon sink too, and we all know we could use more of those.