Kelps - combatting climate change and a source of food

An important contributor to the climate change is that photosynthesis in the oceans has decreased. Thus, any way of increasing it would be beneficial for combatting carbon dioxide accumulation. One way of doing this would be to cultivate kelp. It could be eaten as part of sallads, as binding the rice in sushi meals etc. The growing kelp would convert carbon dioxide to oxygen. Simultaneously the kelp fields would harbour a rich invertebrate fauna. I have yet not come across any negative sides in advocating kelp farming to combat climate change.