Acidification research has exploded in recent years, however, experiments testing effects of co-cycling hypoxia and pH on ecological and physiological processes are rare, despite the pervasiveness and potential importance of co-varying fluctuations in these parameters. Co-cycling dissolved oxygen (DO) and pH are difficult to precisely control, as gases used for manipulation influence both parameters. We successfully developed a LabVIEW (TM)-based system capable of monitoring and controlling co-varying DO and pH in raw seawater flow-through aquaria. Using feedback from Oxyguard DO probes and Honeywell ion sensitive field effect transistor Durafet pH sensors, our system controls ratios of nitrogen, oxygen, carbon dioxide, atmospheric air, and CO2-stripped air within a total gas flow rate through mass flow controllers, to achieve target co-cycling DO and pH values in five treatments. Our system performed well in two long-term experiments investigating effects of diel-cycling hypoxia and pH on eastern oyster (Crassostrea virginica) feeding, growth, fecundity, Perkinsus sp. (Dermo) infection dynamics and immune response. In our 2013 adult oyster experiment, the severe low DO treatment averaged only 0.04 mg L-1 higher than the 0.50 mg L-1 target, and the moderate hypoxia averaged only 0.05 mg L-1 higher than the 1.30 mg L-1 target over 48 d of cycles. Mean pH for the hypercapnia plateau was within 0.02 above the 7.00 target. In our 2013 spat experiment, daily minimum DO in the severe and moderate hypoxia treatments were both within 0.06 mg L-1 of the 0.50 and 1.3 mg L-1 targets, respectively; hypercapnia plateau pH values were within 0.01 of our 7.00 target.
