Hyrpoxia, Eutrophication, Carbon and Power

So we live in a world with all sorts of problems.  To me these problems are temporary until we can figure out how to solve them (and probably create different problems for ourselves in the process.)

So here is a problem chain:

1. We want cheap, easy, reliable food source.
2. Therefore, we use fertilizers to make food production more efficient.
3. These fertilizers eventually end up in rivers.
4. The fertilizer causes Eutrophication, lots of algae, phytoplankton or cyanobacteria growth which hamper other plant/animal growth
5. The algae, phytoplankton or cyanobacteria die and are then eaten by bacteria which use up all the oxygen.
6. This creates hypoxia, or a dead spot in which most marine life cannot live.

So, here is what I think could be a solution.

• Gather nitrogen rich/algal rich waters and confine them.
• Allow  more algae to grow.
• Harvest the algae
• Bacteria eat the algae, producing natural gas.
• Clean water returns back to the river.

Ideally the elevation head on the water would power most of the plant, and the natural gas production could pay for it.