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Near-field Evaluation of Artificial Upwelling Concepts for Open-ocean Oligotrophic Conditions
Gerard C. Nihous

The near-field behavior of artificially upwelled deep nutrient-rich water is characterized for open-ocean oligotrophic conditions. Two basic concepts are considered: the near-surface release of negatively buoyant plumes, or the direct mixing of deep and surface waters in a prescribed ratio. As expected, direct mixing would be more effective in vertically stabilizing an given water mixture. Because of a deep quasipermanent thermocline, however, direct mixing would require considerably greater dilution in tropical open-ocean waters than was necessary during a recent summertime test in Sagami Bay, Japan: the concept might be limited to very small deep water flow rates. A brief consideration of Gross Primary Production in oligotrophic waters confirms that phytoplankton is well adapted to nanomolar nitrate concentrations. Hence, a modest dilution of artificially upwelled nutrients may not be detrimental in boosting primary productivity. This is shown when considering plumes released in a small crossflow (instead of a ‘stagnant’ water column): the relative benefit of shallower vertical stabilization appears to exceed the relative drawback of greater dilution. In those cases, it is suggested that deep seawater flow rates higher than 10 to 20 m3/s may not provide much additional enhancement.

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