Historical trends of hypoxia in Changjiang River estuary: Applications of chemical biomarkers and microfossils Xinxin Li a,b , Thomas S. Bianchi a, ⁎, Zuosheng Yang c , Lisa E. Osterman d , Mead A. Allison e , Steven F. DiMarco a , Guipeng Yang b a Department of Oceanography, Texas A&M University, College Station, TX 77843, USA b Department of Marine Chemistry, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China c Key Laboratory of Seafloor Science and Exploration Technology, College of Marine Geoscience, Ocean University of China, Qingdao 266100, China d U.S. Geological Survey, St. Petersburg, FL 33701, USA e Institute for Geophysics, University of Texas, Austin, TX 78758-4445, USA abstract article info Article history: Received 11 November 2010 Received in revised form 8 January 2011 Accepted 5 February 2011 Available online 21 February 2011 Keywords: Hypoxia Foraminifera Chemical biomarkers Paleo-reconstruction Changjiang River Over the past two decades China has become the largest global consumer of fertilizers, which has enhanced river nutrient fluxes and caused eutrophication and hypoxia in the Yangtze (Changjiang) large river delta- front estuary (LDE). In this study, we utilized plant pigments, lignin-phenols, stable isotopes (δ 13 C and δ 15 N) and foraminiferal microfossils in 210 Pb dated cores to examine the history of hypoxia in the Changjiang LDE. Two sediment cores were collected onboard R/V Dong Fang Hong 2 using a stainless-steel box-corer; one at a water depth of 24.7 m on Jun. 15, 2006 and the other at 52 m on Nov. 20, 2007, both in the hypoxic region off the Changjiang LDE. There has been a significant increase in the abundance of plant pigments after 1979 that are indicators of enhanced diatom and cyanobacterial abundance, which agrees with post-1980 record of increasing nutrient loads in the Changjiang River. The increased inputs of terrestrially derived materials to the LDE are largely woody plant sources and most likely due to deforestation that began in the early 1950s. However, post-1960 lignin data did not reflect enhanced loading of woody materials despite continued deforestation possibly due to increased trapping from greater dam construction, a reduction of deforestation in the drainage basin since the last 1990s, and soil conservation practices. The lack of linkages between bulk indices (stable isotopes, % OC, molar C/N ratios) and microfossil/chemical biomarkers may reflect relative differences in the amount of carbon tracked by these different proxies. Although NO 3 - is likely responsible for most of the changes in phytoplankton production (post 1970s), historical changes in N loading from the watershed and hypoxia on the LDE shelf may not be as well linked in East China Sea (ECS) sediments due to possible denitrification/ ammonification processes; finally, increases in low-oxygen tolerant foraminiferal microfossils indicate there has been an increase in the number of hypoxic bottom water events on the Changjiang LDE over the past 60 years. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Recent work has shown that the global number of hypoxic zones in the coastal margin is doubling every decade, primarily due to land-use changes that result in enhanced nutrient loading (eutrophication) (Diaz and Rosenberg, 2008). While the mechanistic controls of hypoxia have been examined in many traditional semi-enclosed estuarine systems (e.g., Baltic, Chesapeake, see Bianchi, 2007), fewer studies have been focused on large river-delta-front estuaries (LDE), which typically have very different physical dynamics (e.g., coastal currents, large changes in water-residence times and extensive stratification from large buoyancy-driven plumes) (Bianchi et al., 2008; Rabouille et al., 2008; Bianchi and Allison, 2009, and references therein). For example, summer hypoxic (defined as oxygen concen- trations b 2 mg L -1 ) events in the northern Gulf of Mexico (GOM) on the Louisiana/Texas inner shelf have been attributed to water stratification and decay of accumulated organic matter during phytoplankton blooms (Rabalais and Turner, 2001; Turner et al., 2008, and references therein). Over the past two decades China has become the largest global consumer of fertilizers, which has enhanced river-nutrient fluxes and caused eutrophication and hypoxia in the Changjiang LDE (Gu, 1980; Zhang et al., 2007a). However, studies on hypoxia in this region are sparse when compared to the 20-plus years of monitoring in the GOM hypoxic zone. While it is clear that primary productivity in the GOM hypoxic zone is coupled with riverine–nutrient inputs (Rabalais et al., 2002) Journal of Marine Systems 86 (2011) 57–68 ⁎ Corresponding author. Tel.: + 1 979 845 5137. E-mail address: tbianchi@tamu.edu (T.S. Bianchi). 0924-7963/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2011.02.003 Contents lists available at ScienceDirect Journal of Marine Systems journal homepage: www.elsevier.com/locate/jmarsys