Design and development of the CO2 enriched Seawater Distribution System

The kinetics of the reaction that occurs when CO₂ and seawater are in contact is a complex function of temperature, alkalinity, final pH and TCO₂ which taken together determine the time required for complete equilibrium. This reaction is extremely important to the study of Ocean Acidification (OA) and is the critical technical driver in the Monterey Bay Aquarium Research Institute´s (MBARI) Free Ocean CO₂ Enrichment (FOCE) experiments. The deep water FOCE science experiments are conducted at depths beyond scuba diver reach and demand that a valid perturbation experiment operate at a stable yet naturally fluctuating lower pH condition and avoid large or rapid pH variation as well as incomplete reactions, when we expose an experimental region or sample. Therefore, the technical requirement is to create a CO₂ source in situ that is stable and well controlled. After extensive research and experimentation MBARI has developed the ability to create an in situ source of CO₂ enriched seawater (ESW) for distribution and subsequent use in an ocean acidification experiment. The system mates with FOCE, but can be used in conjunction with other CO₂ experimental applications in deep water. The ESW system is completely standalone from FOCE. While the chemical changes induced by the addition of fossil fuel CO₂ on the ocean are well known and easily predicted, the biological consequences are less clear and the subject of considerable debate. Experiments have been successfully carried out on land to investigate the effects of elevated atmospheric CO₂ levels in various areas around the globe but only limited work on CO₂ impacts to ocean environmental systems have been carried out to date. With rising concern over the long-term reduction in ocean pH, there is a need for viable in situ techniques to carry out experiments on marine biological systems. Previous investigations have used aqua- ia that compromise these studies because of reduced ecological complexity and buffering capacity. Additionally, aquaria use tightly controlled experimental conditions such as temperature, artificial light, and water quality that do not represent the natural ocean variability. In order to study the future effects of ocean acidification, scientists and engineers at MBARI have developed a technique and apparatus for an in situ perturbation experiment, the FOCE experimental platform. At the time of this writing the FOCE system and associated ESW are attached to the Monterey Accelerated Research System (MARS) cabled observatory. Engineering validation and tuning experiments using remote control and real time experimental feedback are underway. Additionally, an extensive instrumentation suite provides all of the necessary data for pH calculation and experimental control. The ESW is a separately deployed system that stores and distributes CO₂ enriched seawater. It receives power and communications via an underwater mateable electrical tether. The CO₂ enriched seawater is pumped into the FOCE sections from the ESW. This paper describes the design, development, and testing of the underwater ESW Distribution System as well as the software control algorithms as applied to FOCE. The paper covers the initial prototype, lessons learned, and the final operational version.