| The world is facing unprecedented ocean acidification due to the increasing anthropogenic emissions of CO2.The average pH of the global ocean surface has fallen from 8.2 to 8.1 since Industrial Revolution,indicates that the concentration of hydrogen ion(H+)in the seawater has increased 30%.If the present acidification speed persists,the average seawater pH will decrease to around 7.8 to 7.9 by the end of this century.Ocean acidification affects the relative stability of marine chemical environment directly,and thus inhibits growth and development of marine organisms.Meanwhile,it will also alter the inter-species competition mechanism,biodiversity,marine food chain as well as the overall marine ecosystem.Seaweed aquaculture has great potential to remediate eutrophication and absorb heavy metal,in addition,it can also enhance the capacity of ocean carbon sequestration by absorbing inorganic carbon of seawater.Thus,seaweed aquaculture is by far the most promising solution for mitigating ocean acidification.In this paper,we have selected several representative Seaweed Cultivation areas situated in the northern and southern parts of China—Laminaria japonica aquaculture area of the Lidao Bay in Weihai City,Shandong province,Porphyra haitanensis aquaculture area of the Fodu Island in Zhoushan City,Zhejiang province and Gracilariopsis lemaneiformis aquaculture area of the Nan'ao Island,Shantou City,Guangdong province—to explore the carbon sequestration capacity and factors of different seaweed species in different sea areas and different time by in-situ monitoring,laboratory test and the later data processing.Through this work,a more accurate,reliable and realistic approach for bioremediation of ocean acidification and more choices of seaweed species could be provided.The main results are as follows:1.According to the three cultivation and control areas results,the changes of indexes in the sea area of the Lidao Bay and Fodu Island showed a significant regular pattern of the semidiurnal tide,while the Nan'ao sea area presented a typical full-day tide.During the monitoring process,the seawater pH of cultivation area in the Lidao Bay and aquaculture area of the Fodu Island were less than 8,indicating that there was a risk of ocean acidification in these areas.The average pH of seawater and dissolved oxygen in the three cultivation areas were higher than that in the control areas at three statistical ranges of time.This indicates that these three species of seaweed-Laminaria japonica,Porphyra haitanensis and Gracilariopsis lemaneiformis-can increase the pH and the concentration of dissolved oxygen in seawater.The mean values of Aragonite saturation and Calcite saturation in these three experimental areas were higher than that of the control areas,but the values were all more than 1,indicating that the saturation of calcium carbonate in these three areas was still in the supersaturated state.There were significant differences between the indexes of the cultivation and control area in the Lidao Bay.Only the turbidity of the cultivation and control area in the Fodu Island was significant different.Except for the temperature and the concentration of dissolved oxygen,there were significant differences in other indexes in two areas of the Nan,ao Island.2.The mean value of partial pressure of CO2 and sea-air fluxes of CO2 in cultivation area of the Lidao Bay were 325.78 ± 0.15 ?atm and-30.48 ±0.06 mmol·(m2·d)-1,respectively,which indicates a strong carbon sink,while those of the control area were 432.48±0.38 ?atm and 11.11 ±0.14 mmol·(m2·d)-1,respectively,which representing a weak carbon source.The mean value of pCO2 and FCO2 in the cultivation area of the Fodu Island were 447.37 ± 0.26 ?atm and 16.55±0.10 mmol·(m2.d)-1,respectively,while those of the control area were 480.84±0.16 ?atm and 29.30 ± 0.05 mmol·(m2·d)-1,respectively,indicating a weak carbon source in this area.The mean pCO2 and FCO2 of cultivation area in Nan'ao were 270.56±0.53 ?atm and-39.11 ±0.16,respectively,while those of the control area were 305.68 ± 0.48 ?atm and-28.72 ± 0.14 mmol·(m2·d)-1,respectively,indicates a strong carbon sink.The mean pCO2 and FCO2 in three aquaculture areas were lower than those of the control area,so that seaweed could absorb inorganic carbon in sea water and played a role in biological carbon fixation.3.The results of stepwise regression analysis showed that the seawater pH and temperature were the main factors of aquaculture area in the Lidao Bay affecting carbon sequestration,whereas the turbidity and dissolved oxygen concentration were secondary factors.Temperature and salinity were the main factors affecting carbon sequestration in cultivation area of the Fodu and Nan'ao Island.The turbidity and chlorophyll a were the auxiliary factors for the Fodu Island cultivation area.The seawater pH,dissolved oxygen and chlorophyll a for the Nan'ao Island cultivation area were auxiliary factors.4.The ability of large algae to regulate seawater ApH in descend order was Laminaria japonica in Lidao Bay>Gracilariopsis lemaneiformis in Nan'ao Island>Porphyra haitanensis in Fodu Island,and the ApCO2 regulation ability in descend order was Laminaria japonica in Lidao Bay>Gracilariopsis lemaneiformis in Nan'ao Island>Porphyra haitanensis in Fodu Island.The amounts of carbon sequestration was 9.70,0.05 and 0.11 tons per day in the cultivation area of Lidao Bay,Fodu Island,Nan'ao Island,respectively and total carbon sequestration was 3699.37,1.47 and 1.95 tons,carbon sequestration per kilometer is 697.99,17.40 and 650.00 tons,respectively.In conclusion,Laminaria japonica is the most suitable seaweed for bioremediation of ocean acidification,followed by Gracilariopsis lemaneiformis and Porphyra haitanensis. |
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