Impacts of reduced pH from ocean CO2 disposal: Sensitivity of zooplankton mortality to model parameters

Our group has developed a methodology to quantify mortality suffered by marine zooplankton passing through a CO2-enriched sea water plume. Here we explore model sensitivity to some of the more important biological, physical and engineering design parameters, with particular reference to CO2 injection as a buoyant droplet plume. Uncertainty in the dose-response relationship, e.g. caused by the use of data for surface organisms, will affect predicted values of total mortality by less than a factor of two for a single point discharge from 10 standard 500 MWe coal-fired electric power plants and a factor of five for a single point discharge from one standard plant. The most important design variable is the number of physically separated discharge points (diffuser ports or groups of ports) used to disperse the CO2. Predicted mortality drops to zero as the number of discharge points per standard plant exceeds two. Finally, the most important physical parameters are ambient current speed and turbulent diffusivity. As with the dose-response data, most physical oceanographic measurements have been conducted near the ocean surface. Model sensitivity suggests that a factor of 2.5 reduction in current speed or a factor of 3 reduction in ambient diffusivity, relative to our base case, would require that the number of discharge points per standard plant increase from 2 to 12 in order to avoid mortality. Thus impacts can be strongly site-specific, and additional oceanographic measurements are needed at depths appropriate for CO2 sequestration. Nonetheless, it should be easy to design an environmentally conservative multi-point discharge system that can disperse CO2 as a droplet plume without significant mortality, even under adverse environmental conditions.