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The rapid development and exploitation of the Yangtze River basin in order to ensure human food security and increase living space in recent decades has resulted in significant potential for degradation of water quality in the river and in hundreds of lakes. Understanding how lake environments have evolved to their present state under a variety of external influences is crucial for evaluating their current status and anticipating future scenarios of environmental changes. However, the lakes along the middle reaches of the Yangtze River (MRY) are as yet little studied. Here, we described the long-term anthropogenic environmental transformations of a small lake (Lake Taibai) in the MRY area, based on a detailed quantitative geochemical analysis of the aliphatic hydrocarbons, nutrients (N and P), biogenic silica (BSi), and major and trace elements present in a dated sediment core retrieved from the lake. Our data revealed that levels of short-chain n-alkanes, αβ-hopanes and the trace elements arsenic (As) and cadmium (Cd) were all low for the entire record in sediments prior to ca. 1970, reflecting unpolluted natural state of the lake. Pronounced anthropogenic effects began to appear in sediments deposited in the subsequent years ca. 1970-1990, during which the levels of all these components were elevated, most likely driven by input of nitrogen (N) and phosphorus (P) containing chemical fertilizers, pesticides and diesel oil respectively. Since ca. 1990, changes of short-chain n-alkane levels in the sediment suggested the lake had undergone dramatic eutrophication in which existing anthropogenic stressors were exacerbated by technological advances that extended the use of chemical fertilizer into aquaculture. This pattern contrasted with an otherwise comparable lake in the lower Yangtze River basin, Lake Changdang, in which trace element and petroleum pollution were much more prominent due to dramatic urbanization and industrialization of the catchment., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The exposure of humic substances to solar radiation can alter their concentration and composition and subsequently influences their bioavailability in aquatic food webs. With eutrophication increasingly prominent in lakes, nutrients, such as inorganic N and P, are a prerequisite for heterotrophic bacteria that use organic matter. Here photodegradation of terrestrial humic acids and nutrient addition were performed to investigate the response of bacterial abundance and community structure to photodegraded humic acids and increased nutrient concentrations in a eutrophic lake. Results showed that the decreasing level of absorption coefficient at 460 nm in the treatment irradiated with 40 W UV lamps was more remarkable than that of the treatment irradiated with 20 W UV lamps and the control. This reduced coefficient corresponds to the greatest decrease in humic acid concentration in the 40 W group. Bacteria showed high abundance after incubation with humic acids which underwent strong irradiation intensity. An increased nutrient concentration significantly affected bacterial abundance. The dominant bacteria were Aquabacterium for the irradiated group, Aquabacterium and Limnobacter for the 20 W group and Flavobacterium and Limnobacter for the 40 W group. Armatimonadetes-gp4 and Sediminibacterium showed evident response to high nutrient concentration. Our results showed that the exposure of terrestrial humic acids to UV light and the increasing concentration of nutrients have obviously changed bacterial community. [ABSTRACT FROM AUTHOR]

During the past hundred years, the Lake Taihu basin has been greatly impacted by human interventions. The undesirable changes in water quality of lakes, presumably caused by the human activities, remain relatively undescribed in this area. In order to investigate these anthropogenic effects, a 210 Pb dated sediment core from a relatively small lake in the upper reaches of Lake Taihu known as Lake Changdang was subject to a detailed lipid biomarker study and other geochemical analyses including quantification of biogenic silica (BSi), nutrients and heavy metals. Based on the results, the recent environmental history of Lake Changdang can be divided into three periods. The first period from approximately 1906 to 1950, represents a natural state, with minimal anthropogenic impact on the lake. Human induced environmental change is recorded in the following stage, ca. 1950–1982, during which the trophic status of the lake increased slightly in response to inputs of agricultural waste and nitrogen (N) and phosphorus (P) based fertilizers. In particular, the signs of eutrophication during this phase accelerated from ca. 1973, perhaps owing to large-scale using phosphate based chemical fertilizer around the lake at the time. A second phase of nutrient input in the most recent stage, ca. from 1982 to 2016, initiated by wastewater discharge from rapid urbanization and industrialization of the catchment, greatly enhanced the nutrient level in the lake. However, the central zone of the lake has yet to reach a phytoplankton-dominated stable state, with both algae and aquatic macrophytes tracking an increased trend in productivity driven by intermediate nutrient levels in the water of this zone. [ABSTRACT FROM AUTHOR]

The reliability of Δ7/Δ5 sterol ratios as an indicator of trophic status in Lake Fuxian was tested with in situ nutrient enrichment incubation experiment and a comparison of Δ7/Δ5 values in the strata of a sediment core with the known chronology of nutrient enrichment in the lake. The results indicated that the Δ7/Δ5 sterol ratios (including sum of Δ7/sum of Δ5 sterol ratios and C27-C29 Δ7/C27-C29 Δ5 sterol ratios) in the incubation water increased consistently with total phosphorus (TP) and total nitrogen (TN) concentration across the full experimental range from 0.012 to 0.130 mg/L and from 0.068 to 1.351 mg/L, respectively. The covariation between Δ7/Δ5 sterol ratios and TP and TN concentration could be explained by enhanced productivity of Δ7 sterol-producing green algae (e.g., Scenedesmus sp.) under the prevailing nutrient conditions. A sediment core from Lake Fuxian showed a two-stage increase in C27-C29 Δ7/C27-C29 Δ5 sterol ratios over time, including a small rise from 1951 to 1986, and a much greater increase since 1986. These results coincide with the two-stage eutrophication previously documented in Lake Fuxian using geochemical evidence and water quality monitoring data. This study therefore demonstrates Δ7/Δ5 sterol ratio as a credible descriptor of trophic status in Lake Fuxian. [ABSTRACT FROM AUTHOR]

Organic geochemical analysis of a sediment core from Lake Fuxian was carried out to reconstruct the progression of eutrophication in this lake from ca. 1857 to 2012. The results indicate that two episodes can be identified during the eutrophication of the lake water. Both bulk geochemical proxies and sediment lipid biomarkers recorded an enhanced eutrophication in this lake since 1986. Total organic carbon (TOC), total nitrogen (TN) and phytoplankton biomarkers were most abundant in this period, indicating high phytoplankton productivity. Phytoplankton composition showed an elevated proportion of dinoflagellate and green algae relative to diatoms, in response to the anthropogenic nutrient inputs (e.g., sewage, fertilizers) since the 1980s. On the other hand, only the sediment lipid biomarker detected a minor eutrophication of the lake water between 1951 and 1986. During this period, phytoplankton biomarkers, regardless of their labile or refractory nature, exhibited a little enhanced concentration in the sediment, indicating a minor increase of phytoplankton productivity. Phytoplankton composition was characterized by a slight increase of diatoms relative to dinoflagellates and green algae. Variations of algal productivity and community structure in this period could be attributed to the nutrients (e.g., soil Si) input from catchment erosion. A causal relationship might be present between the eutrophication process of Lake Fuxian and the anthropogenic activities within the catchment. [ABSTRACT FROM AUTHOR]

• FAs in surface sediments of Lake Taihu showed a spatial heterogeneity. • The abundance of phytoplankton FAs is markedly affected by degradation. • Terrestrial FAs were abundant in regions with adjoining inflowing rivers. • All FAs were low in the region subject to greatest sediment resuspension. Fatty acids (FAs) present in the surface sediments of Lake Taihu were analyzed for spatial heterogeneity in abundance and composition in order to test the suitability of these lipids to indicate variation in sources and preservation status of organic matter (OM) in sediments. The results indicate that terrestrial plant-derived FAs were relatively enriched in Meiliang and Zhushan Bays, the most eutrophic regions of Lake Taihu, while FAs of phytoplankton origin were relatively low. This unexpected result might reflect a preferential degradation of phytoplankton OM and phytoplankton-derived FAs. Furthermore, massive inputs of refractory terrestrial OM could significantly dilute concentrations of phytoplankton FAs. In sediments from East Bay, phytoplankton-derived FAs were more abundant than in Zhushan Bay, although this region had the lowest levels of nutrients and the lowest phytoplankton productivity. This result was attributed to a greatly enhanced preservation of labile FAs, probably caused by reduced sediment resuspension in this region with dense growth of submerged macrophytes. Phytoplankton productivity and terrestrial OM inputs were moderate in Central Region, but abundances of all FAs and ratios of C16:1/ n -C16, C18:1/ n -C18 and C18:1ω7/C18:1ω9 both reached their lowest in sediments from this region, possibly as a result of significant degradation of FAs driven by strong sediment resuspension. Sediments from Gonghu Bay and Southern Region were relatively enriched in phytoplankton FAs, and this was ascribed to low inputs of terrestrial OM, which limited the dilution effect. In contrast to FAs, short-chain n -alkane abundances provided an accurate indication of variation in phytoplankton input. [ABSTRACT FROM AUTHOR]

• We found that zooplankton were significant contributors of sediment FAs. • C18:1 ω9 and short-chain n -alkanoic acids might be largely derived from zooplankton. • Zooplankton might be significant contributors of middle-chain n -alkanoic acids in some lakes. • Input of zooplankton FAs was greatly reduced due to the development of aquaculture. • Contribution of algal and bacterial FAs increased with lake eutrophication and aquaculture. During the past few decades, the majority of shallow lakes in the Yangtze River basin (Eastern China) have been impacted by heavy discharges of sewage and the intensification of aquaculture. These activities have significant potential to alter aquatic ecosystems and these changes may be reflected in the composition of lipid biomarkers in sediments at the time. In order to evaluate this effect, the abundances and distribution of fatty acids (FAs) were measured in 210Pb dated sediment cores from two anthropogenically impacted shallow lakes in the Yangtze River basin, Changdang and Taibai. Results from Lake Changdang revealed an extreme enrichment of zooplankton-derived C22:1 ω11 in sediments, with C22:1 ω11/ n -C16 ratios reaching ~1.6 on average. This unusual distribution may have been caused by zooplankton blooms in the lake in the years before aquaculture was developed. Elevated abundances of C18:1 ω9 and short-chain n -alkanoic acids observed in pre-1977 sediments might be also derived from zooplankton. After 1977, FAs of zooplankton origin in Lake Changdang greatly declined in abundance, coincident with the introduction of planktivorous fish. Meanwhile, inputs of FAs from submerged/floating macrophytes, bacteria and algae increased as a result of eutrophication and reduced grazing pressure from zooplankton. In Lake Taibai, abundances of C22:1 ω11 were greatly reduced in all sediments and accounted for 38% of n -C16 on average, reflecting comparatively smaller inputs of zooplankton FAs owing to the earlier establishment of aquaculture in the lake. Relative enrichment of polyunsaturated FAs (PUFAs) and branched FAs (Br-FAs) in the sediments was attributed to high inputs of algal and bacterial organic matter (OM) in response to reduced predation and grazing by zooplankton. Rapid increases in PUFAs and Br-FAs in post-1970 sediments were driven by eutrophication in this period. [ABSTRACT FROM AUTHOR]

• Profiles of n -alkanes in sediments were dependent on water column structure, nutrient inputs and catchment disturbance. • In deep lakes, non-photosynthetic bacterial and aquatic macrophytes were major contributors of n -alkanes. • Terrestrial plant sourced n -alkanes were exceptionally abundant in alpine lakes. • Eutrophication driven enrichment of short-chain odd carbon n -alkanes was recorded in shallow lakes. In paleolimnology studies, the trajectories of n -alkane abundances and distributions in dated sediments are widely used indicators of changing environmental and ecological status in the overlying water over time. However, the factors contributing to the variable n -alkane profiles of lake systems are not yet fully understood. Here, a comparative study was conducted to examine natural and anthropogenic influences on the sediment n -alkane profiles of five representative lakes in China. Core samples from the deep oligo- mesotrophic lakes Fuxian and Lugu (FX-1 and LG-1 cores), the shallow eutrophic lakes Taibai and Changdang (TB-1 and CD-1 cores) and the alpine Lake Heihai (HH-1 core) were used. While there were striking differences in n -alkane abundances and distributions between the cores, the results suggested that the n -alkanes in all five studied lakes were derived mainly from biogenic sources, rather than from petroleum contamination. The short-chain (C 21 n -alkane profiles of different cores varied significantly. Abundances of long-chain n -alkanes (C 29 , C 31 and C 33) in HH-1 were approximately 3–7 times greater than in other cores. Coupled with the lowest Paq (average 0.29) values recorded in the study, this indicates a predominantly terrestrial plant origin for the >C 21 n -alkanes, most likely the dense alpine meadow which surrounds the lake. In other cores, however, aquatic macrophytes appear to be more important contributors of the n -alkanes, a result that corresponds with the relatively low density of terrestrial plants in those catchments. [ABSTRACT FROM AUTHOR]

• We evaluated the reliability of n -alkane proxies for recording nutrient changes in lakes. • Common proxies such as short-chain n -alkane abundances were useful in relatively eutrophic lakes. • Most proxies failed in relatively oligotrophic lake due to influence from submerged macrophytes. • CPI 15–20 and n -C 17 / n -C 16 alkane can record nutrient changes in all kinds of lakes. • CPI 15–20 and n -C 17 / n -C 16 alkane had a potential to keep constant during OM diagenesis. Profiles of n -alkane proxies in sediment cores have been frequently used to detect historical trends in the nutrient status of lakes. However, the results obtained from these proxies are occasionally confusing because the situations in which a given n -alkane proxy might make a valid indication have tended thus far to be elusive. The current study compared geochemical characteristics of n -alkanes in sediments from the oligotrophic (before 1950) and relatively eutrophic phases (after 1980) of five representative lakes in order to assess their suitability and reliability as proxies for lake trophic state. The results from Lakes Liangzi, Changdang, Taibai and Erhai demonstrated that short-chain odd carbon n -alkane abundances were significantly higher (P < 0.05) in sediments deposited after 1980 than in those dating to before 1950, whereas TAR (terrigenous: aquatic ratios) and ACL 15–33 (average chain length of C 15 –C 33 n -alkanes) were significantly lower (P < 0.05) in the more recent sediments. These results are compatible with generally accepted variation seen in lakes that have undergone eutrophication. In Lake Fuxian, however, short-chain odd carbon n -alkane abundances, TAR and ACL 15–33 were all higher in sediments after 1980 than in sediments before 1950, although the variation was only significant (P < 0.05) for ACL 15–33. These results, especially the unexpected trends in TAR and ACL 15–33 , mean these cannot be relied upon as proxies for eutrophication in this lake. The anomaly is likely to be related to the predominantly low phytoplankton productivity and OM input in Lake Fuxian, coupled with the availability of OM from submerged macrophytes in the relatively eutrophic phase. Interestingly and importantly, two new proxies CPI 15–20 ((n -C 15 + n -C 17 + n -C 19)/(n -C 16 + n -C 18 + n -C 20) alkanes) and n -C 17 / n -C 16 alkane, which are indicative of OM input from phytoplankton over non-photosynthetic bacteria, were significantly higher (P < 0.05) in post-1980 sediments than in those dating to before 1950 in all five studied lakes. These two are thus shown to be widely applicable for recording lake eutrophication. Their presence in Lake Fuxian can be explicitly attributed to the absence of influence from submerged macrophytes. Moreover, as ratio results of n -alkanes with similar carbon number, these two proxies are less influenced by processes of OM diagenesis due to comparable degradation of n -alkanes in sediments. The combined results suggest that these previously overlooked proxies may be of considerable value to studies of lake trophic history. [ABSTRACT FROM AUTHOR]

• Short-chain n -alkanol profiles in lake sediments were influenced by eutrophication and aquaculture. • Sources of n -alkanols (>C 20) in lakes were dependent on transparency and water residence time. • (C 26 + C 28)/(C 22 + C 24) n -alkanols showed a marked variation between shallow and deep lakes. • (C 26 + C 28)/(C 22 + C 24) n -alkanol ratios in shallow lakes were influenced by eutrophication and aquaculture. In order to examine how natural backgrounds and anthropogenic activities influenced the n -alkanol profiles of lake sediments, abundances and compositions of n -alkanols were determined in sediment cores taken from four lakes with notable differences in depth, nutrient level, transparency, water residence time and human impact. The results indicate the short-chain n -alkanols (C 14 , C 16 and C 18) are generally more abundant in sediments from eutrophic lakes than in oligotrophic bodies, due to greater inputs from algae and zooplankton. In individual lakes, however, high levels of short-chain n -alkanols were not always recorded in sediments of relatively eutrophic phases because contributions from zooplankton can be significant even if the lake remains relatively oligotrophic. The >C 20 n -alkanol profiles varied markedly between shallow and deep lakes. A prevalence of long-chain compounds (C 26 , C 28 , C 30 and C 32) was observed in shallow lakes and is attributable to terrestrial organic matter (OM) carried by inflowing rivers, whose relatively huge annual input is evidenced by the short water residence time in these lakes. Sediments from deep lakes were dominated by middle-chain n -alkanols (C 20 , C 22 and C 24) as a result of significant input from submersed macrophytes, which achieve high productivity in the clear, sunlit water. In shallow lakes, (C 26 + C 28)/(C 22 + C 24) n -alkanol ratios in sediments were greatly dependent on anthropogenic eutrophication and aquaculture via their influence on the productivity of submersed macrophytes. [ABSTRACT FROM AUTHOR]