Carbon dioxide is the most important greenhouse gas and the major factor leading to the global climate change problem. In previous studies, the ocean is considered to be the major storage of anthropogenic carbon dioxide. However, evaluation of the global CO2 flux seldom includes the estuarine and coastal regions. It should be noted that the current estimate is based on a very limited data set. In particular, data from subtropical and tropical river estuaries are scarce. Many researches point out that the estuary is a CO2 source to the atmosphere, but the data is insufficient so one couldn¡¦t obtain the total CO2 flux accurately. In this study, our team sampled 25 estuaries based on field surveys covering four seasons in Taiwan, aiming to better quantify the estimation of CO2 flux in the coastal regions.
The dissociation constants of carbonic acid are unavailable to calculate the fCO2 in the low salinity (S<1). Therefore, the difference (%) between measured and calculated is very large but will be reduced with increasing salinity. Furthermore, in the process of measuring total alkalinity and pH, accuracy may reduce because of humic acid and variations of ionic strength.
No matter in the western or eastern estuaries, most of fCO2 values is higher than the atmosphere. And they decrease downstream with increasing salinity. The fCO2 is higher in the west than in the east, because of human activities. Neither group of estuaries shows obvious seasonal variability.
The fCO2 in the estuaries has a relationship with salinity, because of mixing with sea water. Because fCO2 is controlled by biological activity, it also has a relationship with AOU, pH and nutrients (NO3- and PO43-). In the east, the fCO2 has no correlation with many parameters. It is probably that the slope is steeper and the river length is shorter in the east than in the west resulting in short resident time. So that many reactions are not complete before the water exports to the sea.
The average water-to-air CO2 flux is 24.6¡Ó19.2 (mol C m-2 y-1), which is 3.5 times the value of Pearl River (6.9 mol C m-2 y-1) but similar to the world average (23.7¡Ó33.1 mol C m-2 y-1). The CO2 flux is the highest in spring (81.7¡Ó15.8 mmol C m-2 d-1) and the lowest in winter (54.1¡Ó132 mmol C m-2 d-1).
Upper/mid/lower estuaries are operationally defined as those areas of estuaries with salinities below 2, between 2 and 25, and above 25, respectively. The trends of fCO2 have good relationships with AOU and PO43- in the upper estuaries. The reason is probably caused by human activities and biological respiration. The phenomenon is more complex in the mid than in the upper estuaries. Consequently, the fCO2 has a good correlation with pH and DIC in the mid estuaries as a result of organic matter decomposition. However, in the lower estuaries, the variation of fCO2 is subjected to biological respiration and mixing with sea water.
The fCO2 is the highest in the upper estuaries (2228¡Ó92.0 uatm)¡Athe average water-air CO2 flux is 42.3¡Ó1.54 (mol C m-2 y-1). Measured fCO2 in the mid estuaries is 1302¡Ó353 (uatm) and the average CO2 flux is 25.8¡Ó1.26 (mol C m-2 y-1). The lowest fCO2 (559¡Ó14.9 uatm) is found in the lower estuaries and the CO2 flux is 7.38¡Ó7.45 (mol C m-2 y-1).
The 106 estuaries of the globe are divided into three parts by salinity. The fCO2 is 3033¡Ó1078, 2277¡Ó626 and 692¡Ó178 uatm in the upper, mid and lower estuaries, respectively. The average CO2 flux is 68.5¡Ó25.6¡B37.4¡Ó16.5 and 9.92¡Ó15.2 mol C m-2 y-1, respectively. Geographically estuaries in all three latitude bands (¡Õ23.5o, 23.5-50o and ¡Ö50o) are generally sources of CO2. Interestingly, water-to-air fluxes do not significantly, and all fall around 24 mol C m-2 y-1 although the flux is slightly lower at high latitude. The water in estuaries release CO2 in all seasons although the flux seems to be highest in autumn (73.2¡Ó 93.4 mmol C m-2 d-1) and lowest (53.4¡Ó65.1 mmol C m-2 d-1) in winter. The average CO2 flux is 23.9¡Ó33.1 mol C m-2 y-1, and the total CO2 flux is 0.26 Pg C y-1.
Next, we estimate the tropical rivers¡¦ carbon fluxes using carbon parameters concerning 175 rivers globally between 30oN and 30oS. The specific DIC yield (flux/area) are 0.63, 3.33, 9.79 and 3.38 g C m-2 y-1 in tropical Africa, the Americas, Asia and Oceania, respectively. The DIC flux in Asia is the highest among the four regions, mainly because the percentage of carbonate rock is highest there and the second highest water discharge there. The PIC fluxes are 7.40¡Ñ1012 g C y-1 in Africa, 2.82¡Ñ1013 g C y-1 in the Americas, 1.53¡Ñ1013 g C y-1 in Asia and 2.49¡Ñ1011 g C y-1 in Oceania. The DOC fluxes are 2.80¡Ñ1013, 5.82¡Ñ1013, 4.50¡Ñ1013 and 4.48¡Ñ1012 g C y-1 in tropical Africa, the Americas, Asia and Oceania, respectively, for a total DOC flux of 0.136 Pg C y-1. Tropical rivers provide 0.53 Pg C y-1 of carbon to the oceans, of which 39.8¢H is DIC, 25.7¢H is DOC, 9.7¢H is PIC and 24.8¢H is POC.
| Identifer | oai:union.ndltd.org:NSYSU/oai:NSYSU:etd-0628112-152031 |
| Date | 28 June 2012 |
| Creators | Fu, Yu-Han |
| Contributors | Ben-Jei Tsuang, Chin-Chang Hung, Jia-Jang Hung, Chen-Tung Arthur Chen, Shu-Lun Wang |
| Publisher | NSYSU |
| Source Sets | NSYSU Electronic Thesis and Dissertation Archive |
| Language | Cholon |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.lib.nsysu.edu.tw/ETD-db/ETD-search/view_etd?URN=etd-0628112-152031 |
| Rights | user_define, Copyright information available at source archive |
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