Designing Ocean Acidification Experimental Systems: How to Visit the Year 2100 without a Flux Capacitor

To explore how ocean acidification may impact marine species, researchers generally expose organisms to controlled carbon chemistry conditions and measure parameters of interest, such as growth or survival. Experimental carbon chemistry conditions typically simulate global mean current and projected future pCO₂ levels. Accurately and precisely creating carbon chemistry conditions is technically challenging; results from species-exposure experiments are suspect if carbon chemistry is not adequately regulated and documented. Some of the issues confronted in developing an experimental system to study ocean acidification are 1) delivery of CO₂ and other gases, 2) removal of CO₂ to create conditions below ambient, 3) monitoring pH or other carbon system parameters, 4) controlling dynamic conditions, 5) appropriate use of validation protocols, and 6) collecting and archiving chemistry data. We will review approaches to system design and describe a system constructed at the NOAA Northwest Fisheries Science Center that incorporates several unique features to more realistically mimic natural environments. These features include: 1) independent control of CO₂, DO and temperature for factorial experiments on interactions of OA, eutrophication, and climate change, 2) dynamic control of all parameters to mimic natural patterns at tidal, diurnal and other scales (i.e., conditions are not just constant global averages), 3) simulation of pre-industrial conditions by removal of CO₂, 4) over-parameterization of carbon measurement for accurate and precise control, and 5) large volume per treatment for simultaneous experiments on multiple species. We address design issues we encountered when using the system for experiments on larval bivalves, crustaceans and fish.