Is sinking Sargassum an efficient use of nutrients?
Like all lifeforms, Sargassum growth is held in check by, among other things, nutrient supplies. The ‘rogue’ Sargassum we are interested in only grows where artificially high nutrients are available, these result from human activities like agriculture and run-off from land. The Great Sargassum Belt has exploited these excess nutrients. By collecting and disposing of Sargassum in deeper waters, we are helping to maintain more natural levels, and hence supporting important local ecological conditions, while removing large quantities of carbon away from the atmosphere.
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.
The C:N:P stoichiometry of organisms and ecosystems in a changing world: A review and perspectives (Perspectives in Plant Ecology, Evolution and Systematics) ↩
The Biological Control of Chemical Factors in the Environment (American Scientist) ↩
Redfield revisited: variability of C:N:P in marine microalgae and its biochemical basis (European Journal of Phycology) ↩