POTENTIAL OF MARINE MACROALGAE AS A SINK FOR CO2 - CONSTRAINTS FROM A 3-D GENERAL-CIRCULATION MODEL OF THE GLOBAL OCEAN

A 3-D global ocean model used previously to determine natural oceanic uptake of anthropogenic CO2 is used here to evaluate another proposed strategy for mitigation of rising atmospheric CO2. As a reference, this study bases itself on previous efforts with the same model to evaluate the potential of Fe fertilization as a means to enhance oceanic CO2 uptake. From that base, we test the feasibility of slowing the rise in atmospheric CO2 by enhancing growth of seaweed, a proposal resurrected from previous efforts considering it as a means to grow marine biomass as fuel for energy production. To determine its maximum potential, logistical and financial constraints are ignored. An enhanced growth of 1 GT C yr-1 is prescribed to be evenly distributed over a large ocean area such as the equatorial band from 18-degrees-S to 18-degrees-N and the northern and southern subtropics from 18-degrees to 49-degrees latitude. Results from these simulations clearly demonstrate that the CO2 invasion from the atmosphere is substantially less than C removed from the surface via enhanced growth. When enhanced growth is supported only by naturally available nutrients, the enhancement to the air to sea CO2 flux averages 0.2 GT C yr- 1 for the first 100 yr. When nutrients are supplied artificially to support the enhanced growth, the mean enhanced air to sea flux is more (for the first 100 yr it averages 0.72 GT C yr-1 when all enhanced growth is harvested but only 0.44 GT C yr-1 without harvesting); however, generating enhanced marine growth at 1 GT C yr-1 requires an unreasonably large supply of nutrient-close to the world's current rate of fertilizer production for P and substantially more than that for N. Less nutrient is needed if the enhanced algal growth is not harvested and thus respired, but respiration increases demand for oxygen so that significant anoxia develops. We conclude that growth of macroalgae is an inefficient mechanism for sequestering anthropogenic CO2 and that the use of macroalgae as an additional fuel source will actually result in a net transfer of CO2 from ocean to atmosphere; however, there would be a reduction in the atmospheric CO2 increase rate if macroalgae were used as a partial replacement for fossil fuel.