Hi all,
I found the answer. Let me summarise!
Seaweed farms could revolutionise the world. 2% of the world's oceans are nutrient rich enough for these farms. Nutrients come from coastal erosion or oceanic upwelling. Sometimes there is nutrient pollution which causes algal blooms and dead zones. Seaweed farming can help mop up excess nutrients and restore ocean health. A new vertical column method of farming the oceans grows both kelp and shellfish and oysters and even encourages fisheries to grow in an ocean ecosystem based approach. Watch this 15 minute TED talk about seaweed feeding the world, and even bringing some of that seaweed back up onto our farmlands to help our farmers.
Many seaweeds are a rich source of vegetarian super-food in their own right, and help form a whole variety of seaweed ice-creams, salads, sauces, and other food ingredients. Kelp farms also stimulate ocean ecosystems, and there are a variety of oysters and shellfish and even wild fish that will grow in amongst the kelp farms. We could feed the world from a small fraction of the 2% of the world's oceans that have their own nutrients. Not that we would be limited to only seaweed and seafood! Think of all the seaweed fertiliser this industry could grow.We could grow so much seaweed that we bring some onto land, get the salt out, and use it as fertiliser. Seaweed could bring our soils back to life. There is even a special seaweed that cows love and eliminates their methane burps! Methane burps are bad news, and cattle lose 15% of their growth to these energy losing burps. But a special seaweed cuts their burps by 99%, solving cattle's infamous methane climate emissions, *and* helping the cows grow faster!
https://theconversation.com/seaweed-could-hold-the-key-to-cutting-methane-emissions-from-cow-burps-66498
Now here's where it gets really bizarre, and potentially planet-saving. Some peer-reviewed work has been done imagining extending kelp farming out into the nutrient-poor open ocean. They start farming the nutrient rich waters. Then a previous season's kelp is biodigested to collect methane gas out the top, leaving the digested kelp nutrients behind. They then recycle those nutrients out in nutrient poor waters. They use slow drip feed hoses and 'tea-bags' that slowly fertilise the kelp, extending the kelp farms out into what was nutrient poor water. This means that nutrients are not a limit to where we can grow kelp!
What if we really went crazy and farmed about 9% of the world's oceans this way?
It would give:-
* a world of 10 billion people half a kilogram of seafood per person per day!
* all the biofuels and biogas we could need to backup a renewable grid (and this is coming from someone who is usually pro-nuclear because of the intermittency and unreliability of renewables!)
* remove ocean acidity
* restore our atmosphere to 350ppm by 2085
In other words, seaweed is a silver bullet to feed the world, save the oceans, and save us from climate change, all in this free PDF. "Negative carbon via Ocean Afforestation". Just register, and download it for free.
http://www.psep.ichemejournals.com/article/S0957-5820(12)00120-6/abstract