Upwelling (natural or artificial) brings typically-cool, well-nutriated seawater to the surface, displacing or mixing with typically-stratified, warm and nutrient deficient (oligotrophic) surface water. The combination of additional nutrients and coolness will usually increase marine biomass in surface waters, chiefly by photosynthesis. Alternatively, some of our proposed methods involve periodically sinking seaweed (typically at night) so that it can absorb the nutrients that it will metabolise nearer the surface in daytime. Where the deeper water has a level of CO2 concentration that might diffuse from surface water into the atmosphere, this adverse effect might be mitigated by ensuring that the upwelled water does not quite reach the surface.