Variations in abundance and size distribution of carbohydrates in the lower Mississippi River, Pearl River and Bay of St Louis Xuri Wang, Yihua Cai 1 , Laodong Guo * Department of Marine Science, University of Southern Mississippi, Stennis Space Center, MS 39529, USA article info Article history: Received 16 September 2012 Accepted 10 April 2013 Available online 24 April 2013 Keywords: carbohydrates dissolved organic matter colloids particulate organic matter the Mississippi River Pearl River Bay of St Louis abstract Riverine export of dissolved and particulate organic matter to the sea is one of the major components in marine carbon cycles, affecting biogeochemical processes in estuarine and coastal regions. However, the detailed composition of organic material and the relative partitioning among the dissolved, colloidal, and particulate phases are poorly quantified. The abundance of carbohydrate species and their partitioning among dissolved, colloidal, and particulate phases were examined in the waters from the lower Mis- sissippi River (MR), the lower Pearl River (PR), and the Bay of St. Louis (BSL). Particulate carbohydrates (PCHO) represented a small fraction of the particulate organic carbon (POC) pool, with 4.7  3.1%, 4.5  2.4% and 1.8  0.83% in the MR, PR, and BSL, respectively. Dissolved carbohydrates (DCHO) were a major component of the bulk dissolved organic carbon (DOC) pool, comprising 23%, 35%, and 18% in the MR, PR, and BSL, respectively. Differences in the DCHO/DOC ratio between the MR, PR, and BSL were related to their distinct characteristics in drainage basins, anthropogenic impacts, and hydrological conditions, reflecting differences in sources and composition of organic matter in different aquatic en- vironments. Within the total carbohydrates (TCHO) pool, the high-molecular-weight carbohydrates (HMW-CHO, 1 kDae0.45 mm) were the dominant species, representing 52e71% of the TCHO pool, fol- lowed by the low-molecular-weight carbohydrates (LMW-CHO, <1 kDa), representing 14e44% of the TCHO. The PCHO accounted for 4e16% of the bulk TCHO. Variations in the size distribution of carbo- hydrates among the MR, PR, and BSL were closely linked to the cycling pathway of organic matter and the interactions between different size fractions of the carbohydrates. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Carbohydrates, including dissolved, colloidal, and particulate carbohydrates, are a major organic component in the biogeo- chemical cycling of carbon and trace elements in aquatic systems (Pakulski and Benner, 1992; Hung et al., 2003; Hassler et al., 2011; Xu et al., 2011). Dissolved carbohydrates are the prominent iden- tified class of dissolved organic carbon (DOC) compounds, ac- counting for 10e30% of the DOC pool in riverine, coastal, and marine waters (Pakulski and Benner, 1994; Hedges et al., 1994; Hung et al., 2005). Carbohydrates are bioactive and play a critical role in ecosystems and biogeochemical processes, serving as a major structural component for planktonic cells and as an energy transportation medium for heterotrophic metabolism (Ittekkot et al., 1981; Myklestad and Borsheim, 2007) and controlling the bioavailability of Fe in the ocean (Hassler et al., 2011). While a growing body of literature has reported the abundance, distribu- tions, and sources of carbohydrates in riverine and marine envi- ronments (e.g., Pakulski and Benner, 1994; Hedges et al., 1994; Myklestad et al., 1997; Hung et al., 2003, 2005; Guo et al., 2004; Wang et al., 2006; Gueguen et al., 2006; Khodse et al., 2007; Cai et al., 2008) and the geochemical behavior and fate of carbohy- drates in the estuarine mixing zone (Senior and Chevolot, 1991; Hung et al., 2001; Witter and Luther. 2002; Steen et al., 2008; Wang et al., 2010), few have reported the size distribution or phase par- titioning of carbohydrates and their relative abundance in the bulk organic carbon pool. Data concerning particulate carbohydrates are even scarcer, and the partitioning of carbohydrates among dis- solved, colloidal, and particulate phases and their cycling pathways in aquatic environments remains poorly understood. Nevertheless, knowledge of the abundance and partitioning of carbohydrates among dissolved, colloidal, and particulate phases is important for * Corresponding author. Current address: School of Freshwater Sciences, University of Wisconsin-Milwaukee, 600 East Greenfield Avenue, Milwaukee, WI 53204, USA. E-mail address: guol@uwm.edu (L. Guo). 1 Current address: College of Ocean and Earth Sciences, Xiamen University, Xiamen 361005, China. Contents lists available at SciVerse ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 0272-7714/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ecss.2013.04.008 Estuarine, Coastal and Shelf Science 126 (2013) 61e69