Last time, I talked about eutrophic lakes and now they can be harvested in a way that benefits both people and the lake. This also happens in the ocean, in which farm runoff, rich in fertilizers with phosphorous and iron, leech out of the farm, into rivers, and finally to the ocean. This benefits certain algae, which reproduce to an absurd degree until no other life is possible in that area. The algae stains the water red with their single-celled bodies, which is why this is called the red tide. Red tide is undesirable to us because the algae produce an anti-animal poison that kills the fish in the area and irritates the bejebus out of any humans that eat any of the sealife nearby. (And I don't just mean it's annoying. I mean brain damage from toxic chemicals in the water.)
Just like before in the eutrophic lake situation, the undesirable situation occurs because of an excess of nutrients benefiting one organism over all the others. So I propose a similar solution: Harvest the red tide by physically removing it from the ocean.
Boats would strain the red tide, physically removing the algae for later use. (This later use would almost assuredly be research into economic uses.) Kelp would be planted in its place. Kelp can be eaten by humans, but it is also a reliable source of grease. Grease being important because it can be refined into biodeisel to provide useful mechanical energy. Biodeisel would replace, to a degree, petrodeisel, saving large amounts of money annually. (Deisel fuels power most trains, trucking, and some aircraft in the United States.) To provide for the entire energy needs of the United States, an area of the ocean the size of Texas would have to be dedicated to growing algae and farmed regularly. The ocean has room for that, seeing as it is 70% of the earth's surface. Probably the entire earth's energy needs could be farmed out of the Pacific ocean alone, although the environmental effects of this would need to be demonstrated to be safe first, as more and more countries rely on a crop of fish to keep their populations fed.
To top off this cycle, the harvesting boat could also be powered by biodeisel, entirely utilizing the biological power of the red tide.