| Summary: | 碩士 === 國立臺灣海洋大學 === 海洋環境與生態研究所 === 106 === Since the beginning of industrial revolution, more than one quarter of anthropogenic CO2 emissions have been absorbed into the oceans, driving down ocean pH and aragonite saturation state (ΩAragonite), a process known as ocean acidification (OA). The coral reef is a sensitive ecosystem to OA, and previous studies show that OA has a negative impact on the calcification rate for most of coral species. These earlier studies have mainly focused on individual species and/or community-level responses to OA. However, to better predict OA impact on ecosystem level, it needs to consider the interactions among the biological communities. Moreover, recent studies also suggest that seagrass meadows could have potential in serving as chemical “refugia” for the associated coral reefs facing future OA. This potential derives from their ability to modify seawater carbon chemistry through high levels of primary productivity. Therefore, the purpose of this study is to examine if seagrass could be beneficail for coral reef calcification at present and future OA scenarios using mesocosm approach. In this study, we monitored the variations of carbon chemistry parameters (DIC, TA, and pH) in 6 tanks of mesocosm under present (stage 1) and OA (stage 2) conditions, in which 3 tanks with seagrasses (experimental group), and the other three without seagrass (controlled group). Results show that net ecosystem production (NEP) under the present condition in the controlled and experimental groups is 4.76 and 5.26 μmolC kg-1 day-1, respectively, and there is no statistically significant difference between the two groups, suggesting that NEP from algae would increase in the controlled group due to the lack of competition from seagrass. The average net ecosystem calcification (NEC) in the first and second stages is 5.23 and -1.12 μmolC kg-1 day-1, respectively, the fomer is statistically significant higher than the latter. This suggests that OA has a negative impact on the NEC of the coral reef ecosystem no matter with segrass or not. Fianlly, by compiling NEC and ΩAragonite data of the present and previous studies, we suggest that the threshold value of ΩAragonite is approximately 1.82, at which NEC will switch from positive to negative. Assuming this threshold value of ΩAragonite, the simulated result shows that coral reef in tropical will begin to decline at a atmopsheric pCO2 level of approximatively 882 μatm.
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