Offshore farms of macroalgae have the potential to be important sources of food and biofuels in the future. Tropical waters offer benefits such as a year-round growing season, but in oligotrophic waters, additional nutrient input is required to support large biomass growth. As part of the ARPA-E MARINER program, the EPA-approved Environmental Fluid Dynamics Code (EFDC) was used to model a cluster of large (1 km x 2 km) macroalgae farms suspended at 10 m below the surface and supplied by artificially upwelled nitrogen (N)-rich deep seawater (DSW) west of the island of Hawai?i. The EFDC domain was nested in a local ROMS model for hydrodynamic boundary conditions, and ambient concentrations of water quality variables, including nitrate, ammonium, carbon dioxide, and three phytoplankton types, were taken from Station ALOHA, Hawai?i. Phytoplankton and the biogeochemical cycles were included as important contributors to nutrient competition and regeneration for regional-scale designs.
were run for 14 days for three farm layouts (Fig. 1) under multiple conditions, including 1 gallon per minute (gpm) of DSW provided per m2 of farm; 2 gpm of the same; no DSW provided; no DSW, but higher surface N present; 1 gpm of DSW per m2 with modified values for macroalgae growth rate and N half-saturation constant; 1 gpm of deeper (600 m) DSW per m2; and 1 gpm of DSW per m2 and no phytoplankton modeled. Default DSW was drawn from 250 m with 150 µg N L-1 of nitrate; 600 m DSW had 520 µg N L-1 nitrate. Default surface nitrate was 0.24 µg N L-1; higher surface nitrate was 5.0 µg N L-1. Default maximum daily growth rate (DGR) and N half-saturation constant were 22.5% per day and 5.0 µg N L-1, respectively, while modified values for the same were 11% per day and 1.4 µg N L-1.
Results were analyzed to provide average net DGR for the farm systems as a function of layout and design assumptions, showing the impact of these factors on macroalgae growth at regional scales (Fig. 2). In all scenarios, DGR was primarily controlled by nitrogen limitation. Sparser clusters of farms were advantageous when no DSW was provided, as they experienced less competition, while denser clusters fared better when DSW was provided on a per-farm basis, as nutrients advected away from one farm were more likely to benefit neighbors. Phytoplankton was a critical factor in determining macroalgae farm success and should not be neglected when modeling offshore macroalgae systems.