Author's personal copy Research papers Historical eutrophication in the Changjiang and Mississippi delta-front estuaries: Stable sedimentary chloropigments as biomarkers Jun Zhao a,b,1 , Thomas S. Bianchi b,n , Xinxin Li b,c , Mead A. Allison d , Peng Yao c,e , Zhigang Yu c,e,nn a Key Laboratory of Marine Environment and Ecology, Ministry of Education, Ocean University of China, Qingdao 266100, China b Department of Oceanography, Texas A&M University, College Station, TX 77843-3146, USA c Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, China d Institute for Geophysics, University of Texas, Austin, TX 78758-4445, USA e Institute of Marine Organic Geochemistry, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China article info Article history: Received 31 October 2011 Received in revised form 21 June 2012 Accepted 15 July 2012 Available online 25 July 2012 Keywords: Large-river delta-front estuary Changjiang river Mississippi river Eutrophication Sedimentary chloropigments Biomarkers abstract Eutrophication is one of the most significant ecological problems in large-river delta-front estuaries (LDEs) around the world. We used TOC, TN, d 13 C, d 15 N and three stable sedimentary chloropigments (pyropheophytin-a [pPHtin-a], sterol chlorin esters [SCEs] and carotenol chlorin esters [CCEs]) as geochemical proxies to examine historical trends of eutrophication over the last few decades in the Changjiang and Mississippi LDEs. Concentrations of sedimentary pPHtin-a, SCEs and CCEs increased from 15, 12 and 120 nmol g 1 OC in 1960s to 51, 32 and 256 nmol g 1 OC in 1990s on the inner shelf of the East China Sea, respectively, and from 57, 69 and 31 nmol g 1 OC in 1950s to 70, 90 and 44 nmol g 1 OC in 2000s in the Mississippi Canyon, respectively. Riverine loading of DIN flux to LDE increased from (261 7109)  10 6 kg yr 1 in pre-1980 to (1385 7209)  10 6 kg yr 1 in post-1990 in the Changjiang LDE, while nitrate flux increased from (322 789)  10 6 kg yr 1 in the 1950s to 1960s to (589 7123)  10 6 kg yr 1 in the 1970s, and DIN flux kept relatively stable in (963 7250)  10 6 kg yr 1 from the 1980s to 2000s in the Mississippi LDE. This work reveals that the stable sedimentary chloropigments in accumulating sediments on the inner shelf are suitable biomarkers for examining past changes in eutrophication in the Changjiang and Mississippi LDE ecosystems. The historical record of riverine nutrient inputs as related to changes in the watershed (e.g., fertilizers and manure) is well- correlated with down-core concentrations of stable pheopigment biomarkers in sediments at both LDEs. These results support the coupling between enhanced phytoplankton abundance and increasing anthropogenic nutrients input to the inner shelves of these LDEs. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Large-river delta-front estuaries (LDEs) are important flux inter- faces between terrestrial and marine environments where sediment accumulation is relatively rapid and organic matter production, burial, and transformation are occurring in globally significant quantities. Hence, these environments are natural ‘‘recorders’’ of global environmental changes across large watersheds that are heavily impacted by anthropogenic activities (Bianchi and Allison, 2009). Over the past century, with the development of agriculture and industry, LDE ecosystems have varied significantly due to land- use changes in the watershed, such as enhanced fertilizer usage, damming, and deforestation (Milliman and Farnsworth, 2011, and references therein). Consequently, estuarine and coastal primary production has risen dramatically over the past decades (Harding and Perry, 1997; Lohrenz et al., 2008; Zhou et al., 2008). Previous work indicates that this acceleration of eutrophication (Nixon, 1995; Cloern, 2001) causes a general decline in water quality, ecological shifts, loss of biodiversity, and an overall disruption of ecosystem functions (Rabalais, 2002; Diaz and Rosenberg, 2008; Conley et al., 2009). During the past ca. 40 years, our knowledge of eutrophication has expanded greatly (e.g., Rabalais and Nixon, 2002; Joye et al., 2006). However, details on differences in the ecological effects and mechanisms controlling hypoxia in different coastal systems (e.g., estuarine versus open shelf) requires further work (Bianchi et al., 2010). Anthropogenic activities have been shown to play an increas- ingly important role in the biogeochemical cycling of coastal systems (Turner and Rabalais, 1994; Zhang et al., 1999, 2010; Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/csr Continental Shelf Research 0278-4343/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.csr.2012.07.005 n Corresponding author. Tel.: þ1 979 845 5137; fax: þ1 979 845 6331. nn Corresponding author at: Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, China. Tel.: þ86 532 66781006; fax: þ86 532 66781659. E-mail addresses: jzhao@sio.org.cn (J. Zhao), tbianchi@tamu.edu (T.S. Bianchi), zhigangyu@ouc.edu.cn (Z. Yu). 1 Present address: Key Laboratory of Marine Ecosystem and Biogeochemistry, Second Institute of Oceanography, State Oceanic Administration of China, Hangzhou 310012, China. Continental Shelf Research 47 (2012) 133–144