Atmospheric CO₂ Enrichment and Tropical Seagrass Growth

Writing as background for their study, Jiang et al. (2010) note that "seagrasses are flowering plants that thrive in shallow oceanic and estuarine waters around the world, and are ranked as one of the most ecologically and economically valuable biological systems on earth," citing the work of Beer et al. (2006). And they say that Thalassia hemprichii "is among the most widely-distributed seagrass species in an Indo-Pacific flora, dominating in many mixed meadows," citing the work of Short et al. (2007).

In conducting their analysis, Jiang et al. collected intact vegetative plants of T. hemprichii from Xincun Bay of Hainan Island, Southern China, which they transported to the laboratory and cultured in flow-through seawater aquaria bubbled with four different concentrations of CO₂ that were representative of (1) the present global ocean, with a pH of 8.10, (2) the projected ocean for 2100, with a pH of 7.75, (3) the projected ocean for 2200, with a pH of 7.50, and (4) the ocean characteristic of "an extreme beyond the current predictions" (a 100-fold increase in free CO₂, with a pH of 6.2).

With respect to the results of their analysis, the three researchers report that the "leaf growth rate of CO₂-enriched plants was significantly higher than that in the unenriched treatment," that "nonstructural carbohydrates (NSC) of T. hemprichii, especially in belowground tissues, increased strongly with elevated CO₂," and that "belowground tissues showed a similar response with NSC."

As for the implications of their findings, i.e., that "CO₂ enrichment enhances photosynthetic rate, growth rate and NSC concentrations of T. hemprichii," the Chinese scientists list several. They say that with higher atmospheric CO₂ concentrations "colonization beyond current seagrass depth limits is possible," that the extra stored NSC "can be used to meet the carbon demands of plants during periods of low photosynthetic carbon fixation caused by severe environmental disturbance such as underwater light reduction," that it can enhance "rhizome growth, flowering shoot production and vegetative proliferation," and that it "may buffer the negative effects of transplant shock by increasing rhizome reserve capacity." They also write that "the globally increasing CO₂ may enhance seagrass survival in eutrophic coastal waters, where populations have been devastated by algal proliferation and reduced column light transparency," and that "ocean acidification will stimulate seagrass biomass and productivity, leading to more favorable habitat and conditions for associated invertebrate and fish species."

Additional References
Beer, S., Mtolera, M., Lyimo, T. and Bjork, M. 2006. The photosynthetic performance of the tropical seagrass Halophila ovalis in the upper intertidal. Aquatic Botany 84: 367-371.

Short, F.T., Carruthers, T.J., Dennison, W.C. and Waycott, M. 2007. Global seagrass distribution and diversity: A bioregional model. Journal of Experimental Marine Biology and Ecology 350: 3-20.