Source , composition , and environmental implication of neutral carbohydrates in sediment cores of subtropical reservoirs , South China

Neutral monosaccharides, algal organic matter (AOM), and carbon stable isotope ratios in three sediment cores of various trophic reservoirs in South China were determined by high-performance anion-exchange chromatography, Rock-Eval pyrolysis, and Finnigan Delta Plus XL mass spectrometry, respectively. The carbon isotopic compositions were corrected for the Suess effect. The concentrations of total neutral carbohydrates (TCHO) range from 0.51 to 6.4 mgg−1 at mesotrophic reservoirs, and from 0.83 to 2.56 mgg−1 at an oligotrophic reservoir. Monosaccharide compositions and diagnostic parameters indicate a predominant contribution of phytoplankton in each of the three cores, which is consistent with the results inferred from the corrected carbon isotopic data and C/N ratios. The sedimentary neutral carbohydrates are likely to be structural polysaccharides and/or preserved in sediment minerals, which are resistant to degradation in the sediments. Moreover, the monosaccharide contents are highly related to the carbon isotopic data, algal productivity estimated from the hydrogen index, and increasing mean air temperature during the past 60 years. The nutrient input, however, is not a key factor affecting the primary productivity in the three reservoirs. The above evidence demonstrates that some of the resistant monosaccharides have been significantly elevated by climate change, even in low-latitude regions.


Biogeosciences
Manuscript under review for journal Biogeosciences Published: 29 November 2016 c Author(s) 2016. CC-BY 3.0 License.

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The five-year mov ing average temperature (T 5 ) was calculated fro m the reported database (Duan et al., 2015). The 174 mean air temperature in the Guangzhou area has increased by about 1.5 °C since 1960, and the mean air temperature in the 175 Heyuan area has increased by about 1.52 °C between 1957 and 2004. Therefore, the above data suggests a significant trend 176 in climate warming in the investigated areas during the last six decades (Duan et al., 2015). The annual hours of daylight in

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The content and composition of sedimentary OM derived fro m the Rock-Eval analysis could provide the source and 183 early diagenetic information of OM in the reservoir cores. The S1, S2, S3, TOC, and HI show significant decreasing trends 184 with increasing profile depths in the ZT and LA cores, suggesting that the sedimentary OM has either been affected by 185 autochthonous inputs or by extensive degradation (Duan et al., 2015). Fo r the XFJ core, the TOC as well as the other 186 pyrolytic parameters (except HI pro xy) show increasing trends with depth, suggesting the degradation and oxidation of OM 187 and/or terrestrial inputs of the OM are the primary factors affecting the variat ion of OM .

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Carbon isotope analyses offer an important tool for identifying the sources of OM in lacustrine sediments. Different 189 primary producers have distinctive carbon isotope compositions. The average δ 13 C values of C3 p lants is around 27‰ to 190 26‰, whereas the C4 plants have average δ 13 C values of 14‰ to 13‰. Although the δ 13 C values in phytoplankton are in 191 a broad range of 17‰ to 45‰ (Boschker et al., 1995), it can be identified by the combination of other proxies (e.g.,

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elemental C/ N rat ios). All the corrected δ 13 C values of sedimentary OM in the three reservoirs vary fro m 25.5‰ to 19.9‰ 193 ( Fig. 1), wh ich are in the range of phytoplankton and C3 plants. However, their corresponding C/N ratios are relatively low 194 than those for higher plants (> 12) ( Fig. 1), suggesting the predominant contribution of phytoplankton in the OM of 195 reservoirs. The δ 13 C values in ZT sediments are more enriched (average: 20.8‰) than those in LA (average: 24.6‰) or 196 XFJ (average: 24.1‰) sediments, which may be attributed to high phytoplankton productivity (chlorophyll a = 90.7 μg/L),

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anaerobic sediments with high rates of methanogenesis, and lack of terrestrial carbon inputs in shallow water bodies (Gu and 198 Schelske, 2004). High phytoplankton can enhance isotopic fract ionation and result in enrich ment of 13 C in dissolved 199 inorganic carbon. The removal o f 12 CH 4 by intensive methanogenesis also leads to the accumulation of 13 C-depleted OM in 200 sediments (Gu and Schelske, 2004).

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After correct ing for the Suess effect, the OM in sediments becomes more enriched in 13 C fro m the bottom to the top of 202 the ZT core ( Fig. 1), reflecting a progressive increase in historical productivity, wh ich is consistent with the vertical 203 variations of TOC, C/N, and HI. Similar observations were also found in the LA core from a depth of 16 cm to the surface 204 layer. Therefore, both ZT and LA reservoirs undergo significant increases in primary productivity during the recent years. As

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Lake Eric (Schelske and Hodell 1995), and deep Lake Tahoe (Chandra et al., 2005), suggesting the importance, in terms of 208 productivity, of the correction for the Suess effect in the recent δ 13 C values for lacustrine sediment cores.

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For the XFJ reservoir, the corrected δ 13 C values increase from depths of 22 to 10 cm and then decrease abruptly to the contribution of algal origin in the sediments (Fig. 1). Therefore, the decrease of corrected δ 13 C at XFJ is mainly due to the not vary significantly with the sediment core, and there is no decline in wt% glucose with a corresponding increase of %wt 296 (Fucose + Rhamnose) b in each of the ZT, LA, and XFJ sediment cores (Fig. 3), suggesting that the process of degradation 297 occurs main ly during the settling period rather than after deposition. Further ev idence in support of this conclusion can be 298 obtained from the rat io of deo xy sugars (e.g., rhamnose and fucose) to C5 (e.g., arabinose and xylose) (deo xy S/ C5). The 299 deoxy S/ C5 ratios also remain almost unchanged throughout the sediment cores of ZT, LA , and XFJ (Fig. 3). Therefore, 300 although sinking organic matter-rich particles and their carbohydrates in these reservoirs suffer fro m intensive oxidation and 301 degradation in the water column during their transit to bottom sediments, some fract ions are selectively preserved in the 302 sediment cores and remain almost unchanged during post-deposition, as observed before (Cowie and Hedges, 1984;Moers et 303 al., 1990;Hicks et al., 1994).

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Carbohydrates are derived not only fro m storage polymer but also fro m the cell memb rane of phytoplankton . The

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In support of the above conclusion, the k values of deoxy S/ C5 were calculated using a "multi-G" model (Wang et al.,

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1998) to evaluate neutral sugar degradation. The k value is 0.0025 yr -1 for ZT, 0.0021 yr -1 for LA, and 0.0025 yr -1 fo r XFJ. It 316 is found that the decomposition of 95% neutral sugar in sediments will take thousands of years, which is similar to the results 317 of TCHO in the ocean sediments (Wang et al., 1998)

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The increasing downcore trends of glucose and OM in the XFJ core are different fro m those in the ZT and LA cores,  settling. However, large amounts of resistant structural carbohydrates (e.g., cell walls) containing source information can be 327 preserved in the sinking particles and sediments. Moreover, HI has been widely utilized as a useful indicator of primary 328 productivity during recent years (Gasse et al., 2001;Stein et al., 2006;Bechtel and Schubert, 2009). As shown in Fig. 4 and Table S4 in the supporting data, HI values in the ZT and LA cores are positively co rrelated with monosaccharides, especially 330 the algae-dominated, galactose, mannose, fucose, and arabinose (Ittekkot and Arain, 1986;Hamilton and Hedges, 1988;

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galactose, mannose, fucose, and arabinose) and HI. As for XFJ reservoir, the corrected δ 13 C values abruptly decrease with very 380 low C/ N rat ios, which may indicate d ifferent sources and biodegradation in the underlying sediments. Moreover, strong 381 positive correlat ion between TCHO and HI is found both in the mesotrophic reservoirs (ZT and LA) and in the oligotrophic 382 reservoir (XFJ) in this investigation, suggesting that TCHO is related to primary productivity in the studied subtropical 383 reservoirs. Furthermo re, increasing levels of carbohydrates in the three reservoir cores show significant relationships with T 5 384 during the last 60 years. Elevated temperatures lead to increasing levels of carbohydrates in the sediment profiles during last 385 six decades. Therefore, this investigation provides important evidence for the effect of climate change on the aquatic 386 ecosystems in the low lat itude region. To further develop the productivity indicator of carbohydrates, mo re work is needed to 387 organic matter in lake sediment cores fro m a subarctic lake in Yu kon, Canada: further evidence toward the 538 algal-mercury scavenging hypothesis , Environ. Sci. Technol., 43, 7684-7690, 2009. Figure 5. Temporal profiles of five-year moving temperature (T 5 ) and single neutral sugars in sediment cores from the three 645 reservoirs.