Is sinking Sargassum an efficient use of nutrients?

Check out our blogs and you will see that  we have considered a lot of different avenues for dealing with the Sargassum problem and releasing its opportunities. A common question we are asked is whether we should be making fertilisers out of the seaweed biomass, a question we have previously answered.

In theory, it’s a perfect answer to a problem created by excessive terrestrial fertiliser run-off caused by intensive farming. Unfortunately, the salt from seawater complicates matters, as does the bioavailability of the nutrients, as well as the energetic costs of recovering the biomass. There’s also a big worry about high levels of heavy metals leading to contamination of crops¹².

The list of challenges goes on and on. But, it leads on to  more important and broader questions: are we robbing the ocean of vital nutrients and what will the long term implications of this be?

There are simple and there are complicated answers to this. The simple answer is that Sargassum has ‘exploded’ thanks to artificial levels of nutrients, resulting from anthropogenic activities: nutrients wouldn’t be there if it wasn’t for intensive agricultural activity.

That’s not good enough an answer for us.

Let’s delve into the science behind what we are doing. Every living organism has its own C:N:P (Carbon:Nitrogen:Phosphorous) stoichiometry (this is what the ratio of elements is called)³.

Sargassum grows incredibly well because it makes maximum use of all the nutrients available to it. Whereas most oceanic organisms have what’s called the Redfieldian Ratio, Carbon: Nitrogen: Phosphorus of 106:16:1 (i.e. C:N of about 7:1; C:P 106:1)⁴⁵, for Sargassum the numbers are very different: C:P ratios are typically 500-1000:1 and C:N are 20-40:1⁶. That sounds very scientific and confusing, but the take home message is, Sargassum is HIGHLY efficient from a carbon absorption perspective. It needs 5-10 times less Phosphorus, and 3-6 times less Nitrogen to grow  than most ocean life. This goes some way to explaining just why it is so explosively invasive.

When we remove Sargassum from the surface ocean, we are proportionally removing much, much more Carbon in relation to Nitrogen and Phosphorus. We are sequestering carbon far more efficiently than the natural marine carbon pump (also known as the biological pump) typically does⁷.

For some perspective, around 11 gigatonnes of carbon is already removed by the biological pump each year to the deep oceans, in a natural process that has been estimated to have prevented CO2 levels being 400 ppm higher than they are right now. Seaweed Generation simply aims to mimic and speed up this proven process.