Lipid biomarker evidence for determining the origin and distribution of organic matter in surface sediments of Lake Taihu, Eastern China
Introduction
Lake sediments have been considered to play an important role in the terrestrial carbon cycle because they accumulated a significant fraction of OM produced by the biosphere (Anderson et al., 2013, Wang et al., 2015). The lake sediment is also a significant site for OM breakdown and nutrient regeneration (Xu et al., 2015). OM preserved in lake sediments could origin from production within the aquatic system, including phytoplankton and macrophytes, terrestrial OM input from the catchment and bacteria production within the sediment (Prartono and Wolff, 1998, Meyers, 2003). The abundance and chemical nature of OM in lake sediment varies considerably with local conditions (Woszczyk et al., 2011), and in turn exerts a different impact on the environment and the biological communities in the sediments and the overlying water column (Dunn et al., 2008). High levels of OM stimulate microbial metabolism and increase sediment oxygen demand and nutrient regeneration within the surface sediment (Dunn et al., 2008). Previous studies suggest that phytoplankton-based sediment OM is more effectively biodegraded than that derived from terrestrial plants (Muri et al., 2013, Xu et al., 2015) probably because increased availability of nitrogen (N) and phosphorus (P) in the former enhances the efficiency of nutrient recycling (Meyers and Ishiwatari, 1993, Muri et al., 2013). Ecological restoration might be needed to reduce N and P release from sediment to overlying water when the sediment was dominated by phytoplankton OM (Smol, 2008). Thus, assessing the composition, origin and distribution of lake sediment OM is necessary to understand the local biogeochemical cycles for carbon and nutrients and design environmental management strategies for lake systems (Prartono and Wolff, 1998, Kumar Das et al., 2009, Carreira et al., 2011, Woszczyk et al., 2011).
Lipid biomarkers are widely used as tools for assessing the origin and distribution of OM in sediments of aquatic ecosystem because they are source specific and more resistant to bacterial degradation than many other biological compounds (Pinturier-Geiss et al., 2002, Hu et al., 2009, Xu and Jaffé, 2009, Xing et al., 2011, Lammers et al., 2013). Compared to geochemical proxies such as the relative abundance of total organic carbon to total nitrogen (TOC/TN), lipid biomarkers can provide detailed information about the origins of sediment OM (Hu et al., 2009, Xing et al., 2011). It is widely accepted that the short-chain lipids (C15–C20 n-alkanes and C16–C20 n-alkanols) in sediments indicate inputs from algae (and/or photosynthetic bacteria), while long-chain lipids (C27–C33 n-alkanes and C24–C32 n-alkanols) are derived from higher plants (Meyers, 1997). Middle-chain n-alkanes (C21–C25 n-alkanes) can indicate origins of OM from submerged and floating aquatic macrophytes (Meyers and Ishiwatari, 1993, Ficken et al., 2000). Moreover, some lipids in sediment can indicate input from more specific phytoplankton taxa. For example, dinosterol and brassicasterol are associated with dinoflagellate and diatom inputs from the overlying water column (Volkman et al., 1998, Schubert et al., 1998, Zimmerman and Canuel, 2002). Highly branched isoprenoids (HBI) compounds, especially C25HBI, can indicate input of OM from diatoms (Volkman et al., 1998).
Lake Taihu is the third largest freshwater lake in China. The lake is important for aquaculture, tourism and recreation, transportation, and provides drinking water for cities including Shanghai, Suzhou, Wuxi and Huzhou (Qin et al., 2007). The lake is surrounded by the most industrialized and urbanized area of China and exhibits great heterogeneity in water quality (Qin et al., 2007). Northern parts of the lake have undergone serious deterioration in recent decades and in the summer of 2007 cyanobacterial blooms (Microcystis spp.) lead to a water supply crisis in the region (Dong et al., 2014). The southeastern reaches of the lake are generally less polluted and are dominated by clear water habit (Qin et al., 2007, Qin, 2008, Dong et al., 2014). This spatial heterogeneity in water quality and habit might be reflected in the origin and distribution of OM in surface sediment. For example, in eutrophic areas, sediments tend to receive a large fraction of OM from phytoplankton, whereas the OM in relatively oligotrophic areas derives mainly from aquatic macrophytes (Qin et al., 2007, Dong et al., 2014). Fatty acid biomarkers have previously been used to assess the sources of sediment OM in the western part of Lake Taihu (Xu et al., 2015), but as yet there has been no attempt to map the type and distribution of OM in sediments across the entire lake. In this study, the abundance and composition of lipid biomarkers (including n-alkanes, HBIs, n-alkanols and sterols) in the surface sediments of Lake Taihu were analyzed in order to identify the origins of sediment OM in different sites. The mechanisms controlling OM distribution are discussed, based on hydrology and observed variations in physical and chemical parameters within and around the lake.
Section snippets
Study area and sampling
Lake Taihu is located in the delta of Yangtze River (30°55′40″–31°32′58″ N and 119° 52′ 32″–120°36′10″ E) (Fig. 1). The lake has an area of about 2338 km2, a catchment area of about 36, 500 km2, a mean depth of 1.9 m and water retention time of approximately 5 months. Local hydrology of the lake is complex, with 117 connecting rivers and tributaries, largely flowing into the lake from the west and draining it to the east (Qin et al., 2007). Heavy pollution of some inflowing rivers has lead to strong
TOC and clay content in the surface sediment
TOC in the sediment of Lake Taihu varied from 0.44% to 1.31% (Fig. 3a). The East Bays (sites 12, 24, 25, 26, 27 and 30) and Zhushan Bay (sites 10, 16 and 17) were richest in TOC, averaging 0.99% (range 0.60–1.31%) and 0.97% (range 0.59–1.24%), respectively. TOC abundance was slightly reduced in Meiliang Bay (sites 0, 1, 3, 4, 5, 6, 9, 15 and 32), averaging 0.91% (range 0.61–1.27%). Lowest TOC levels were recorded in Gonghu Bay, Central and Western Region (sites 7, 8, 11, 13, 14, 18, 19, 20, 21,
Lipid biomarker evidence of autochthonous OM input in the sediment
The lipid biomarkers constitute less than 1% of the total sedimentary OM and may not be representative of the total material (Meyers, 1997). However, the lipid composition can provide important details about changes in origins of the OM (Hu et al., 2009, Xing et al., 2011). Short-chain n-alkanes are commonly used as indicators of OM input from algae and aquatic photosynthetic bacteria (Cranwell et al., 1987) and in particular, n-alkanes exhibiting a Cmax at C17 or C19 are thought to be
Conclusion
Lipid biomarker evidence demonstrates that the origin and distribution of OM in sediments of Lake Taihu are significantly influenced by environmental variables in and around the lake. The distribution of allochthonous OM was dependent on hydrological conditions in the watershed, because terrestrial plant OM is transported to the lake mostly by rivers. By contrast, the autochthonous OM distribution is closely related to the nutrient status in the lake. Cyanobacterial blooms promoted by the
Acknowledgments
The suggestions of two anonymous reviewers and the associate editor improved the manuscript greatly. Special thanks go to Prof. Kuanyi Li for discussion of this manuscript. The study was supported by the National Natural Science Foundation of China (Grant Nos. 41673046, 41303036 and U1033602).
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