Cabled instrument technologies for ocean acidification research — FOCE (free ocean CO2 enrichment)

With rising concern over the impacts of ocean acidification on marine life there is a need for greatly improved techniques for carrying out in situ experiments. These must be able to create a ΔpH of 0.3 to 0.5 by addition of CO
₂
for studies of natural ecosystems such as coral reefs, cold water corals, and other sensitive benthic habitats. Thus controlled CO
₂
perturbation experiments in the field rather than in aquaria are quickly becoming an essential ocean science tool. Free Air CO
₂
Enrichment (FACE) experiments have long been carried out on land to investigate the effects of elevated atmospheric CO
₂
levels on vegetation. However, only limited work on CO
₂
enrichment using quasi-open systems has yet been carried out in the ocean. Seawater CO
₂
has complex chemistry with significantly slow reaction kinetics, unlike land-air experiments where simple mixing is the major concern. Ocean experimental designs must to take these reaction rates into account. The net result of adding a small quantity of CO
₂
to seawater is to reduce the concentration of dissolved carbonate ion, and increase bicarbonate ion through the reaction: CO
₂
+ H
₂
O + CO
₃
^2-
→ 2HCO
₃
^-
The reaction between CO
₂
and H
₂
O is slow and is a complex function of temperature, pH, and TCO
₂
. The reaction proceeds more rapidly at lower pH and higher temperatures. Marine animals in the natural ocean will typically experience only small and temporary shifts from environmental CO
₂
equilibrium. Valid perturbation experiments must try to expose an experimental region to a near stable lower pH condition, and avoid large and rapid pH variability to the extent possible. This paper describes the design, development and testing of an in situ pH perturbation experiment deployed on a subsea cable for control. The paper addresses the differences between the deep-sea and shallow water versions of the experiments and addresses the pH sensor developments that enable long deployments.