Far-field Evaluation of a Lagrangian Artificial Upwelling Concept
Gerard C. Nihous
The evolution of releases of deep nutrient-rich seawater near the ocean surface is examined in the absence of ambient currents once buoyancy forces vanish (far field).The proposed model extends plume calculations (near field) into a spatial and temporal framework where slow diffusive and biological processes become important. As anticipated, large releases with Q* ª 100m 3 /s fail to produce biological enhancement because of excessive depths. For moderate releases, phytoplankton and zooplankton growth would be promoted within a narrow vertical layer extending up to a few kilometers radially. Smaller plumes with Q* ª 1m 3 /s and shallower settling depths correspond to a complete far-field conversion of excess nutrients into organic matter.With moderate plumes, i.e. Q* ª 10 m 3 /s and deeper neutral-buoyancy depths, photosynthesis is less efficient in processing the larger amount of excess nutrients; grazing pressure also becomes relatively stronger. Changes in the concentrations of microorganisms are shown to take place between one and two weeks before a steady-state is reached. Also, several models of nitrogen uptake from phytoplankton are tested; those most consistent with oligotrophic Gross Primary Production data result in lower far-field phytoplankton and zooplankton concentration maxima. Generally speaking, the magnitude and distribution of the far-field concentrations of marine microorganisms predicted in this study suggest that the proposed nutrient-enhancement scenarios do not resemble natural upwellings.