The impacts of ocean acidification on marine trace gases and the implications for atmospheric chemistry and climate

Abstract

Surface ocean biogeochemistry and photochemistry regulate ocean-atmosphere fluxes of trace gases critical for Earth's atmospheric chemistry and climate. The oceanic processes governing these fluxes are often sensitive to the changes in ocean pH (or pCO₂) accompanying ocean acidification (OA), with potential for future climate feedbacks. Here, we review current understanding (from observational, experimental and model studies) on the impact of OA on marine sources of key climate-active trace gases, including dimethyl sulfide (DMS), nitrous oxide (N₂O), ammonia and halocarbons. We focus on DMS, for which available information is considerably greater than for other trace gases. We highlight OA-sensitive regions such as polar oceans and upwelling systems, and discuss the combined effect of multiple climate stressors (ocean warming and deoxygenation) on trace gas fluxes. To unravel the biological mechanisms responsible for trace gas production, and to detect adaptation, we propose combining process rate measurements of trace gases with longer term experiments using both model organisms in the laboratory and natural planktonic communities in the field. Future ocean observations of trace gases should be routinely accompanied by measurements of two components of the carbonate system to improve our understanding of how in situ carbonate chemistry influences trace gas production. Together, this will lead to improvements in current process model capabilities and more reliable predictions of future global marine trace gas fluxes.

Keywords: atmospheric chemistry; climate; marine trace gases; ocean acidification.

Figure 1.

Overview of the production of marine trace gases and their roles in atmospheric and climatic processes. (Online version in colour.)

Figure 2.

Overview of the DMS response from all published OA mesocosm experiments carried out under natural environmental conditions, to date. Four experiments took place in early summer in Raunefjord, Norway (60.3°N, 5.2°E): (a) Avgoustidi et al. [77], (b) Vogt et al. [87], (c) Hopkins et al. [84], (d) Webb et al. [79]; two in the coastal waters of Jangmok, Korea (34.6°N, 128.5°E): one in winter (e) Kim et al. [61] and the other in early summer (f) Park et al. [86]; single experiments were carried out in (g) summer in the Svalbard Archipelago (78.9°N, 11.9°E) Archer et al. [82], (h) summer in the Baltic Sea off Finland (59.8°N, 23.2°E) Webb et al. [88], and (i) late-summer in the subtropical North Atlantic (27.9°N, 15.4°W) Archer et al. [83]. In order to compare results between experiments, the percentage changes in DMS concentrations between the pCO₂ treatments (approx. 350 : 750 µatm, shown as a percentage change on each panel) were calculated using time-integrated DMS concentrations over the duration of each experiment. See electronic supplementary material, table S2. For experiments A, B, C and D, the % response in DMS was calculated from two pCO₂ treatments (duplicate mesocosm for (a) and triplicate for (b–d)); for the remaining experiments, the % response was obtained from the linear fit between pCO₂ and DMS concentration (n = 8, pCO₂ treatments for e, f and h; n = 6 for g). (* note in (b) value not significant at 95% confidence interval [87]). (Online version in colour.)

Figure 3.

Outputs from the fully interactive Earth system model run from Schwinger et al. [44]. The top row of panels show DMS sea–air flux (a), sulfate aerosol burden (d) and surface temperature (g) in the reference simulation (no sensitivity of DMS fluxes to OA). The corresponding changes in a simulation assuming a decrease of DMS production with increasing pH are shown in the middle row of panels (b,e,h) and zonal mean changes are depicted in the bottom row (c,f,i). The grey shaded area in the zonal mean plots gives the range of natural variability (defined as the standard deviation of the zonal mean found in the control run). (Online version in colour.)

Figure 4.

Summary of our knowledge on multiple stressors and their anticipated direct and indirect effects on trace gas production. Coloured arrows represent known/anticipated trace gas response (red, increase; blue, decrease; green, no net change), and black arrows describe the direction of change of the related process. HABs, harmful algal blooms; TEP, transparent exopolymer particles; DON, dissolved organic nitrogen. (Online version in colour.)