Could be. So over the mentioned four weeks, the algae is reproducing more cells in sunlight, and emitting light at night, while gradually wearing out in some way and "retaining 75% of their brightness". Then at the end of the month you have a bucket of tired algae, and that's the stored carbon. I don't know what you do with it. You probably shouldn't chuck it in a river. Its likely fate is methane, wherever you put it.