Alleviating hypoxia through induced downwelling

Hypoxia, a condition of low dissolved oxygen concentration, is a widespread problem in marine and freshwater ecosystems. To date, prevention and mitigation of hypoxia has largely centered on nutrient reduction to prevent eutrophication. However, nutrient reduction is an often slow and sometimes insufficient to remedy the hypoxia. We investigate the utility of an alternative strategy of pumping oxygenated surface water to depth, termed induced downwelling, as a technique to remedy hypoxia in the sub-surface of marine and freshwater ecosystems. We introduce simple energy-based models and apply them to depth profiles in hypoxic estuaries, lakes, and freshwater reservoirs. Our models indicate that induced downwelling is ~10-100 times more efficient than bubbling air, and ~10,000-1,000,000 more efficient than fountain aerators, at oxygenating hypoxic bottom waters. A proof-of-concept downwelling field experiment highlighted the potential for this technique to provide a simple, low-cost solution for localized hypoxia relief and provided information on steps needed to scale up implementation. We estimate that regional-scale downwelling for continual hypoxia avoidance would require 0.4-4 megawatts per cubic kilometer of water (depending on local conditions), or ~50-500 US dollars per hour per cubic kilometer of water (assuming 125 USD MWh^-1 of electricity). Until such time that high nutrient fluxes to the northern Gulf of Mexico are sufficiently reduced, the hypoxic “dead zone” could be alleviated at an annual operating cost of 26.3 to 263 million US dollars, less than 10% of the estimated cost of hypoxia avoidance through reducing nutrient loading from farm runoff in the Mississippi-Atchafalaya River Basin.