Biogeosciences Discuss., 10, 4611-4635, 2013
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This discussion paper has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG.
Enhancement of photosynthetic carbon assimilation efficiency of phytoplankton assemblage in the future coastal ocean
J.-H. Kim1, K. Y. Kim1, E. J. Kang1, K. Lee2, J.-M. Kim2, K. Park2, K. Shin3, B. Hyun3, and H. J. Jeong4
1Department of Oceanography, Facility of Earth System and Environmental Science, Chonnam National University, Gwangju, 500-757, Korea
2Pohang University of Science and Technology, School of Environmental Science and Engineering, Pohang, 790-784, Republic of Korea
3Korea Ocean Research and Development Institute/South Sea Institute, Jangmok, 656-830, Republic of Korea
4School of Earth and Environmental Sciences, College of Natural Sciences, Seoul National University, Seoul, 151-747, Korea

Abstract. A mesocosm experiment was conducted to evaluate the effects of future climate conditions on photosynthesis and productivity of coastal phytoplankton. Natural phytoplankton assemblages were incubated in field mesocosms under the ambient condition (present condition: ca. 400 ppmv CO2 and ambient temp.), and two future climate conditions (acidification condition: ca. 900 ppmv CO2 and ambient temp.; greenhouse condition: ca. 900 ppmv CO2 and 3 °C warmer than ambient). Photosynthetic parameters of steady-state light responses curves (LCs; measured by PAM fluorometer) and photosynthesis–irradiance curves (P–I curves; estimated by in situ incorporation of 14C) were compared to three conditions during the experiment period. Under acidification, electron transport efficiency (αLC) and photosynthetic 14C assimilation efficiency (α) were 10% higher than those of the present condition, but maximum rates of relative electron transport (rETRm,LC) and photosynthetic 14C assimilation (PBmax) were lower than the present condition by about 19% and 7%, respectively. In addition, rETRm,LC and αLC were not significantly different between and greenhouse conditions, but PBmax and α of greenhouse conditions were higher than those of the present condition by about 9% and 30%, respectively. In particular, the greenhouse condition has drastically higher PBmax and α than the present condition more than 60% during the post-bloom period. According to these results, two future ocean conditions have major positive effects on the photosynthesis in terms of energy utilization efficiency for organic carbon fixation through the inorganic carbon assimilation. Despite phytoplankton taking an advantage on photosynthesis, primary production of phytoplankton was not stimulated by future conditions. In particular, biomass of phytoplankton was depressed under both acidification and greenhouse conditions after the the pre-bloom period, and more research is required to suggest that some factors such as grazing activity could be important for regulating phytoplankton bloom in the future ocean.

Citation: Kim, J.-H., Kim, K. Y., Kang, E. J., Lee, K., Kim, J.-M., Park, K., Shin, K., Hyun, B., and Jeong, H. J.: Enhancement of photosynthetic carbon assimilation efficiency of phytoplankton assemblage in the future coastal ocean, Biogeosciences Discuss., 10, 4611-4635, doi:10.5194/bgd-10-4611-2013, 2013.
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