An Open Ocean Trial of Controlled Upwelling Using Wave Pump Technology

Abstract

AbstractIn 1976, John D. Isaacs proposed to use wave energy to invert the density structure of the ocean and pump deep, nutrient-rich water into the sunlit surface layers. The basic principle is simple: a length of tubing attached to a surface buoy at the top, and a one-way valve at the bottom can be extended below the euphotic zone to act as a conduit for deep water. The vertical motion of the ocean forces the attached valve to open on the downslope of a wave and close on the upslope, thus generating upward movement of deep water to the surface ocean. Although Isaacs’s wave-powered pump has taken many forms, from energy production to aquaculture to the more recent suggestion that artificial upwelling could be used to stimulate primary productivity and carbon sequestration, the simple engineering concept remains the same. In June 2008, the authors tested a commercially available wave pump (Atmocean) north of Oahu, Hawaii, to assess the logistics of at-sea deployment and the durability of the equipment under open ocean conditions. This test was done as part of an experiment designed to evaluate a recently published hypothesis that upwelling of water containing excess phosphate (P) relative to nitrogen (N) compared to the canonical “Redfield” molar ratio of 16N:1P would generate a two-phased phytoplankton bloom. The end result of this field experiment was rapid delivery (<2 h for a 300-m transit) of deep water to the surface ocean followed by catastrophic failure of pump materials under the dynamic stresses of the oceanic environment. Wave-driven upwelling of cold water was documented for a period of ∼17 h, with a volumetric upwelling rate of ∼45 m3 h−1 and an estimated total input of 765 m3 of nutrient-enriched deep water. The authors discuss the deployment of a 300-m wave pump, the strategy to sample a biogeochemical response, the engineering challenges faced, and the implications of these results for future experiments aimed at stimulating the growth of phytoplankton.