[ben_w]

Many crop fertilisers have that issue, but fertiliser is not only one thing.

One hypothesis is that algae are limited by some single resource which could be gradually dropped out the back of cargo ships as they cross oceans, seeding carbon-absorbing algal blooms as they go.

(I heard about this nearly 20 years ago, so I assume that has either been tested or banned since then…)

[analog31]

I'm thinking mainly about the Haber-Bosch process, which runs on methane, and feeds more than half of the world's animal population.

As mentioned in a sibling comment, it could run on some other heat source, but that hasn't been demonstrated yet on the scale of agriculture.

[ben_w]

Indeed, that's one of the main possibilities I guessed you were thinking of; for triggering algal blooms, one of the things suggested was iron. (And given it was immediately going into salt water, ore would presumably have been fine… if the hypothesis was correct).