Table 1.

Overview of experimental methods employed in studies of the effects of OA on marine trace gases.

					CO2 and pH treatments
Experimental technique	Vol. (l)	Number of replicates	Duration (days)	Key studies	CO2 (µatm)	pH	Method of acidification	What can it tell us?	Strengths	Weaknesses
single-species algal cultures	<1	2–3	7–40	[76–79]	385/1000 370/760 ambient/790 395/900	8.1/7.7 no data 8.3/8.0 8.1/7.7	aeration with CO2-enriched air, pH-stat to maintain constant DIC and pH	Batch cultures: acclimated/physiological response to OA Semi-continuous culture: multiple generations allow insight into adaptive plasticity to OA Level of sensitivity to OA/high CO2	Useful tool for establishing baseline concepts Reduced complexity compared with natural populations Determines direct response on trace gas production by phytoplankton isolates (if axenic) High duplication/ reproducibility	Do not simulate complex natural systems Elimination of extracellular (bacterial) processes that may be key control on trace gas production
shipboard microcosm experiments	5–10	up to 12	4–10	[80] [91] [81]	Av. of 18 expts: 320.2 ± 38.3 533.4 ± 40.0 673.8 ± 82.2 841.5 ± 128.2 1484.0 ± 104.0 5 treatments and control over range: 509–3296	8.1 ± 0.1 7.9 ± 0.03 7.8 ± 0.1 7.8 ± 0.1 7.5 7.9–7.2	addition of strong acid/base, e.g. HCl/NaHCO3-	Physiological response and extent of the variability in response/plasticity between communities Level of sensitivity to OA/high CO2	Extensive spatial coverage Natural gradients in carbonate chemistry, temperature, nutrients Multiple short-term identical experiments on complex natural communities Results in large, highly replicated, statistically robust data sets	Short-term physiological response: representative? Bottle effects Rapid acidification
mesocosm experiments	2400–75 000	1–3	25–35	[82] [83] [77] [84] [85] [61] [86] [87] [79] [88]	175–1085 400–1252 ambient versus 700 300 versus 780 175–1085 400 versus 900 160–830 350 versus 700 280–3000 330–1166	8.3–7.6 8.1–7.6 8.2 versus 7.8 8.1 versus 7.8 8.3–7.6 no data no data 8.1 versus 7.9 8.1–7.3 7.9–7.5	aeration with CO2-enriched air, or addition of CO2-saturated seawater	Whole community response during bloom conditions Acclimation (>30 days) Net production by whole community and associated biogeochemistry	Close to natural conditions (light and temperature) + large volume Longer timescale = improved realism of representation of surface ocean Towards a whole community, adaptive response	Limited by number of experimental replicates Difficult to test multiple drivers Logistically challenging (physically and financially) Minimal geographical coverage