A mesocosm experiment was conducted in Wuyuan Bay (Xiamen), China, to investigate the effects of elevated pCO.sub.2 on the phytoplankton species Phaeodactylum tricornutum (P. tricornutum), Thalassiosira weissflogii (T. weissflogii) and Emiliania huxleyi (E. huxleyi) and their production ability of dimethylsulfide (DMS), dimethylsulfoniopropionate (DMSP), as well as four halocarbon compounds, bromodichloromethane (CHBrCl.sub.2 ), methyl bromide (CH.sub.3 Br), dibromomethane (CH.sub.2 Br.sub.2) and iodomethane (CH.sub.3 I). Over a period of 5 weeks, P. tricornuntum outcompeted T. weissflogii and E. huxleyi, comprising more than 99 % of the final biomass. During the logarithmic growth phase (phase I), mean DMS concentration in high pCO.sub.2 mesocosms (1000 Âµatm) was 28 % lower than that in low pCO.sub.2 mesocosms (400 Âµatm). Elevated pCO.sub.2 led to a delay in DMSP-consuming bacteria concentrations attached to T. weissflogii and P. tricornutum and finally resulted in the delay of DMS concentration in the high pCO.sub.2 treatment. Unlike DMS, the elevated pCO.sub.2 did not affect DMSP production ability of T. weissflogii or P. tricornuntum throughout the 5-week culture. A positive relationship was detected between CH.sub.3 I and T. weissflogii and P. tricornuntum during the experiment, and there was a 40 % reduction in mean CH.sub.3 I concentration in the high pCO.sub.2 mesocosms. CHBrCl.sub.2, CH.sub.3 Br, and CH.sub.2 Br.sub.2 concentrations did not increase with elevated chlorophyll a (Chl a) concentrations compared with DMS(P) and CH.sub.3 I, and there were no major peaks both in the high pCO.sub.2 or low pCO.sub.2 mesocosms. In addition, no effect of elevated pCO.sub.2 was identified for any of the three bromocarbons.