High partial pressure of CO 2 and its maintaining mechanism in a subtropical estuary: the Pearl River estuary, China Weidong Zhai a , Minhan Dai a,b, * , Wei-Jun Cai c , Yongchen Wang c , Zhaohui Wang c a Key Laboratory of Marine Environmental Science (Xiamen University), Ministry of Education, Xiamen 361005, China b Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA c Department of Marine Sciences, the University of Georgia, Athens, GA 30602-3636, USA Received 5 January 2004; received in revised form 18 June 2004; accepted 23 July 2004 Available online 3 October 2004 Abstract We investigated distributions of surface water CO 2 partial pressure ( pCO 2 ), dissolved oxygen (DO) and associated carbonate parameters in the Pearl River estuary, a large subtropical estuary under increasingly anthropogenic pressure in China, in the summer of 2000 and late spring of 2001. p CO 2 levels, measured underway using a continuous measurement system, were high during both seasons, with levels of N4000 Aatm at salinity b0.5. p CO 2 distribution overall mirrored DO across the salinity gradient. Using the linear relationship between excess CO 2 and apparent oxygen utilization (AOU) in surface water, we conclude that aerobic respiration is the most important process in maintaining such high p CO 2 measured upstream. The material being respired is likely in a close association with the organic pollutants discharged into the system. Based on the measured excess CO 2 vs. AOU plots, we estimate that the upper limit of pCO 2 should be ~7000 Aatm in the Pearl River estuary assuming that CO 2 was produced solely by aerobic respiration. D 2004 Elsevier B.V. All rights reserved. Keywords: Carbon dioxide; Carbon cycle; Respiration; Outgassing; Estuary; China; Pearl River estuary 1. Introduction Recent observations have shown that river–estu- ary systems may release a significant amount of CO 2 into the atmosphere in addition to the commonly recognized fluvial export of inorganic/organic matter. Very high partial pressure of CO 2 ( p CO 2 , 2000– 8000 Aatm in the mainstream, N10 000 Aatm in some tributaries) and significant outgassing of CO 2 have been reported for the Amazon River (Devol et al., 1987; Richey et al., 2002). In some U.S. estuaries, high p CO 2 values of 1000–6000 Aatm were also reported (Cai and Wang, 1998; Raymond et al., 2000 and references therein). In Asia, aqueous p CO 2 of 800–2000 Aatm was observed in mixing zones of the 0304-4203/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2004.07.003 * Corresponding author. Tel.: +86 592 2182 132; fax: +86 592 2180 655. E-mail addresses: mdai@xmu.edu.cn, mdai@whoi.edu (M. Dai). Marine Chemistry 93 (2005) 21 – 32 www.elsevier.com/locate/marchem