If one buries them afterwards. If not, they will eventually die, rot and release the carbon back to the atmosphere. But eutrophication has other issues: it is

If one buries them afterwards. If not, they will eventually die, rot and release the carbon back to the atmosphere. But eutrophication has other issues: it is harmful for animal life and it unbalances the ecosystem, degrading biodiversity and overall making natural carbon sequestration poorer. Maybe in a controlled setting, one could use a dragnet to harvest algae and use it for fertilization. That would certainly improve the efficiency of nitrogen usage.