Assessing the influence of ocean alkalinity enhancement on a coastal phytoplankton community

[摘要] Ocean alkalinity enhancement (OAE) is a proposed method tocounteract climate change by increasing the alkalinity of the surface oceanand thus the chemical storage capacity of seawater for atmospheric CO 2 .The impact of OAE on marine ecosystems, including phytoplankton communitieswhich make up the base of the marine food web, is largely unknown. Toinvestigate the influence of OAE on phytoplankton communities, we enclosed anatural plankton community from coastal Tasmania for 22 d in nine microcosms during a spring bloom. Microcosms were split into three groups,(1) the unperturbed control, (2) the unequilibrated treatment wherealkalinity was increased ( + 495  ±  5.2  µ mol kg −1 ) but seawaterCO 2 was not in equilibrium with atmospheric CO 2 , and (3) theequilibrated treatment where alkalinity was increased ( + 500  ±  3.2  µ mol kg −1 ) and seawater CO 2 was in equilibrium with atmosphericCO 2 . Both treatments have the capacity to increase the inorganic carbonsink of seawater by 21 %. We found that simulated OAE had significant butgenerally moderate effects on various groups in the phytoplankton communityand on heterotrophic bacteria. More pronounced effects were observed for thediatom community where silicic acid drawdown and biogenic silica build-up were reduced at increased alkalinity. Observed changes in phytoplanktoncommunities affected the temporal trends of key biogeochemical parameterssuch as the organic matter carbon-to-nitrogen ratio. Interestingly, theunequilibrated treatment did not have a noticeably larger impact on thephytoplankton (and heterotrophic bacteria) community than the equilibratedtreatment, even though the changes in carbonate chemistry conditions weremuch more severe. This was particularly evident from the occurrence and peakof the phytoplankton spring bloom during the experiment, which was notnoticeably different from the control. Altogether, the inadvertent effectsof increased alkalinity on the coastal phytoplankton communities appear tobe rather limited relative to the enormous climatic benefit of increasingthe inorganic carbon sink of seawater by 21 %. We note, however, that moredetailed and widespread investigations of plankton community responses toOAE are required to confirm or dismiss this first impression.

[学科分类] 大气科学