Sedimentary evidence for recent eutrophication in the northern basin of Lake Taihu,China: human impacts on a large shallow lake

Environmental change in Lake Taihu and its catchment since the early to middle part of the twentieth century has left a clear geochemical record in the lake sediments. The human activities in the lake and its catchment responsible for the change include agriculture, fishery, urbanisation, sewage and industrial waster disposal. Sediment cores were collected from Meilian Bay of northern Lake Taihu to investigate the record of anthropogenic impacts on the lake’s ecosystem and to assess its natural, pre-eutrophication baseline state. Two marked stratigraphic sediment units were identified on the basis of total phosphorus concentration (TP), pigments, total organic carbon (TOC)/total nitrogen (TN), δ¹³C and δ¹⁵N corresponding to stages in the lake history dominated by phytoplankton, and by aquatic macrophytes. Results show that as TP loading increased from the early 1950s the lake produced sediments with increasing amounts of organic matter derived from phytoplankton. In the early 1950s, the first evidence for eutrophication at the Meilian Bay site is recorded by an increase in C/N values and in sediment accumulation rate, but there is little change in phosphorus concentrations, pigments, δ¹³C and δ¹⁵N at this time. After 1990 a more rapid increase in trophic status took place indicated by increased levels of phosphorus, pigments, δ¹⁵N and by decreased δ¹³C and TOC/TN values in the lake sediments. The first increase in trophic status of the early 1950s results mainly from agricultural development in the catchment. In contrast, the acceleration from ca. 1990 originates from the recent development of fisheries and the urbanisation and industrialisation of the catchment.     

Late glacial environment and climate development in northeastern China derived from geochemical and isotopic investigations of the varved sediment record from Lake Sihailongwan (Jilin Province)

Total organic carbon (TOC) content, total nitrogen (TN) content, stable nitrogen isotope (δ¹⁵N) and stable organic carbon isotope (δ¹³C_org) ratios were continuously analysed on a high resolution sediment profile from Lake Sihailongwan (SHL), covering the time span between 16,500 and 9,500 years BP. Strong variations of the investigated proxy parameters are attributed to great climatic fluctuations during the investigated time period. Variations in organic carbon isotope ratios and the ratio of TOC/TN (C/N ratio) are discussed with respect to changing proportions of different organic matter (OM) sources to bulk sedimentary OM. Phases of high TOC content, high TN content, depleted δ¹³C_org values and high δ¹⁵N values are interpreted as times with increased productivity of lacustrine algae in relation to input of terrigenous organic matter. Two distinct phases of enriched nitrogen isotope ratios from 14,200 to 13,700 and 11,550 to 11,050 years BP point towards a reduced phytoplankton discrimination against ¹⁵N due to a diminished dissolved inorganic nitrogen pool. The combination of geochemical (TOC, TN, C/N ratio) and isotopic (δ¹³C_org, δ¹⁵N) proxy parameters points to a division of climate development into four stages. A cold and dry stage before 14,200 years BP, a warm optimum stage with high phytoplankton productivity from 14,200 to 12,450 BP, a colder and drier stage from 12,450 to 11,600 BP and a stage of climatic amelioration with high variability in TOC and TN contents after 11,600 BP. These results are discussed in relation to monsoon variability and Northern Hemisphere climate development of the late glacial.     

Sediment geochemical records of environmental change in Lake Wuliangsu, Yellow River Basin, north China

We present a paleolimnological record from shallow Lake Wuliangsu in the Yellow River Basin, north China, using a short (56 cm) sediment core. Our objective was to investigate environmental changes in this semi-arid region over the past ~150 years. The sediment core was dated using ¹³⁷Cs and ²¹⁰Pb. We examined stratigraphic trends in core lithology, nutrients, stable isotopes (δ¹³C and δ¹⁵N) and trace element concentrations in the Lake Wuliangsu core to discern between natural sediments and those affected by human agency. A lithologic transition from yellow, coarse-grained sediment to grey, fined-grained sediment marked the lake’s formation about 1860. Until ~1950, sediments displayed relatively low and constant heavy metal concentrations, indicating little human influence. In the 1950s, enrichment factors (EFs) increased, reflecting greater impact of human activities. Carbon and nitrogen stable isotopes in organic matter (OM), along with heavy metal concentrations, were used to infer past shifts in trophic state and identify pollutants that came from agriculture, industry and urbanization. In the late 1950s, the first evidence for environmental change is recorded by increases in total organic carbon (TOC), total organic nitrogen (TN), TOC/TN, EFs, δ¹³C and a decrease in δ¹⁵N. After about year 2000, a more rapid increase in trophic status occurred, as indicated by greater total phosphorus (TP), EFs, δ¹⁵N and lower δ¹³C values. Changes in isotope and TOC/TN values in the lake sediments may reflect a shift in lake ecology during this period. The first increase in trophic status during the late 1950s was mainly a result of agricultural development in the catchment. In contrast, the change after ca. AD 2000 was driven largely by urban and industrial development. Agreement between paleolimnologic data from Lake Wuliangsu, and both instrumental and written records, indicates that the lake sediments provide a reliable archive for investigating the formation and environmental history of the lake.     

Elemental (C,N, H and P) and stable isotope (δ<Superscript>15</Superscript>N and δ<Superscript>13</Superscript>C) signatures in sediments from Zeekoevlei,South Africa: a record of human intervention in the lake

We used elemental carbon, nitrogen, phosphorus and hydrogen ratios (C/N, N/P and H/C) with total organic carbon (TOC) and total phosphorus (TP) as well as stable carbon and nitrogen isotopes (δ¹³C and δ¹⁵N) to investigate the source and depositional conditions of organic matter in sediments from Zeekoevlei, the largest freshwater lake in South Africa. Typical C/N (10–12), H/C ratios (≥1.7) and δ¹³C_organic values (−22 to −19‰) together with the increase in TOC concentration indicate elevated primary productivity in lower middle (18–22 cm) and top (0–8 cm) sections of the sediment cores. Seepage of nutrients from a nearby waste water treatment plant, rapid urbanization and heavily fertilized farming in the catchments are responsible for the increased productivity. Consistent with this, measured δ¹⁵N_organic values (∼11‰) indicate increased raw sewage input towards the top-section of the core. Although cyanobacterial blooms are evident from the low δ¹⁵N values (∼3‰) in mid-section of the core, they did not outnumber the phytoplankton population. Low N/P ratio (∼0) and high TP (100–2,200 mg l⁻¹) support cyanobacterial growth under N limited condition, and insignificant input of macrophytes towards the organic matter pool. Dredging in 1983, caused sub-aerial exposure of the suspended and surface sediments, and affected organic matter preservation in the upper mid-section (12–14 cm) of the core.     

Evaluating sedimentary geochemical lake-level tracers in Walker Lake, Nevada, over the last 200 years

Walker Lake, a hydrologically closed, saline, alkaline lake located along the western margin of the Great Basin of western United States, has experienced a 77% reduction in volume and commitment drop in lake level as a result of anthropogenic perturbations and climatic fluctuations over the last century. The history of lake-level change in Walker Lake has been recorded instrumentally since 1860. A high-resolution multi-proxy sediment core record from Walker Lake has been generated through analysis of total inorganic carbon (TIC), total organic carbon (TOC), and oxygen and carbon isotope ratios (δ¹⁸O and δ¹³ C) of both downcore bulk TIC and ostracods over the last 200 yr. This allows us to examine how these sediment indices respond to actual changes in this lake’s hydrologic balance at interannual to decadal timescales. In Walker Lake sediments, changes in %TIC, %TOC, and δ¹³C and δ¹⁸O of TIC and ostracods are all associated to varying degrees with changes in the lake’s hydrologic balance, with δ¹⁸O of the TIC fraction (δ¹⁸O_TIC) being the most highly correlated and the most effective hydrologic indicator in this closed-basin lake. The δ¹⁸O_TIC record from Walker Lake nearly parallels the instrumental lake-level record back to 1860. However, comparison with sporadic lake-water δ¹⁸O and dissolved inorganic carbon δ¹³C (δ¹³C_DIC) results spanning the last several decades suggests that the isotopic values of downcore carbonate sediments may not be readily translated into absolute or even relative values of corresponding lake-water δ¹⁸O and δ¹³C_DIC. Changes in the lake’s hydrologic balance usually lead to changes in isotopic composition of lake waters and downcore sediments, but not all the variations in downcore isotopic composition are necessarily caused by hydrologic changes.     

Biomarker-based paleo-record of environmental change for an eutrophic,tropical freshwater lake,Lake Valencia,Venezuela

The lipids in a sediment core from Lake Valencia, a hypereutrophic freshwater lake in Venezuela, are examined to understand environmental changes over the last ∼13,000 years. From the latest Pleistocene to the earliest Holocene, total organic carbon (TOC) substantially increased from 2.2 to 10%, while total organic carbon over total nitrogen (TOC/TN) decreased from as high as 34 to as low as 10. Correspondingly, the concentration of terrestrially derived triterpenoids markedly decreased, and the dominant n-alkane shifted from C₃₁ to C₂₃ or C₂₅. During the same period, algal biomarkers such as botryococcenes, dinosterol, isoarborinol, C₂₀ HBIs and 1,15C₃₂ keto-ol markedly increased in abundance. These changes suggested a greater contribution of algal organic matter at the onset of the Holocene, which was concurrent with increasing rainfall and the formation of a permanent lake (Lake Valencia) in the Aragua Valley, Venezuela. The age profile of Paq, a n-alkane based proxy, showed large oscillations (0.20–0.81), reflecting historical variations in source strength of submerged/floating vs. terrestrial/emergent OM inputs. An abrupt increase in tetrahymanol abundance at ∼7,260 cal years BP suggests the establishment of an oxic–anoxic boundary in the lake’s water column. After reaching its maximum abundance at ∼2,100 cal year BP, botryococcenes, a biomarker of Botryococcus braunii, gradually decreased to below the detection limit in the uppermost sediments, while different algal/microbial biomarkers such as diploptene, dinosterol and isoarborinol substantially increased. These different historical profiles of algal/microbial biomarkers reflect different responses of source organisms to environmental changes throughout this period. The δ¹³C determinations presented exceptionally enriched values for botryococcene isomers (−7.7 to −15.1‰), indicating the utilization of bicarbonate as carbon sources in an extremely productive ecosystem.     

Environmental changes in northern New Zealand since the Middle Holocene inferred from stable isotope records (δ15N, δ13C) of Lake Pupuke

Maar lakes in the Auckland Volcanic Field are important high-resolution archives of Holocene environmental change in the Southern Hemisphere mid-latitudes. Stable carbon and nitrogen isotope analyses were applied on bulk organic matter and the green alga Botryococcus from a sediment core from Lake Pupuke (Auckland, North Island, New Zealand) spanning the period since 7,165?cal.?year BP. The origin of organic matter was established using total-organic?Ccarbon-to-nitrogen ratios (TOC/TN) as well as organic carbon (??¹³C_OM) and nitrogen (??¹⁵N) isotope composition of potential modern sources. This approach demonstrated that the contribution of allochthonous organic matter to the lake sediment was negligible for most of the record. The sedimentary TOC/TN ratios that are higher than Redfield ratio (i.e. >7) are attributed to N-limiting conditions throughout the record. Variations of nitrogen and carbon isotopes during the last 7,165?years are interpreted as changes in the dominant processes in the lake. While epilimnetic primary productivity controlled isotope composition before 6,600?cal.?year BP, microbial processes, especially denitrification and methane oxidation, caused overall shifts of the ??¹⁵N and ??¹³C values since the Mid-Holocene. Comparisons with climate reconstructions from the Northern Island suggest that changes in the wind-induced lake overturn and a shift to more pronounced seasonality were the most likely causes for lake-internal changes since 6,600?cal.?year BP.     

Holocene records of carbon and hydrogen isotope ratios of organic matter in annually laminated sediments of Lake Korttajärvi, central Finland

An 11.6 m long continuous succession of annually laminated sediments from Lake Korttajärvi in central Finland was investigated for the isotopic composition of carbon and hydrogen in organic matter. The sequence covers a time period of 9590 years, and the varve chronology has been thoroughly described in earlier studies. From 7100 to 4400 BC the lake was part of the Ancient Lake Päijänne, but in 4400 BC it became separated and formed the present independent lake system. Two organic fractions were investigated. One fraction obtained by HCl-treatment was analyzed for δ¹³C and another HCl-HF-digested organic fraction was analyzed for both δ¹³C and δD. The isotopic data were compared to atomic C/N ratios, carbon contents, diatom-inferred pH values and other environmental parameters. The diatom-inferred pH values and organic carbon contents provide evidence for a long-term change towards more acidic conditions and lower productivity in Lake Korttajärvi. The inferred pH values decrease from 7.0 to 6.1, followed by a slight increase during the last millennia. Variations in pH are accompanied by an increase in the δ¹³C_HCl-HF values of organic matter from ?31.6 to $-29.2\permilleAn 11.6 m long continuous succession of annually laminated sediments from Lake Korttaj?rvi in central Finland was investigated for the isotopic composition of carbon and hydrogen in organic matter. The sequence covers a time period of 9590 years, and the varve chronology has been thoroughly described in earlier studies. From 7100 to 4400 BC the lake was part of the Ancient Lake P?ij?nne, but in 4400 BC it became separated and formed the present independent lake system. Two organic fractions were investigated. One fraction obtained by HCl-treatment was analyzed for δ¹³C and another HCl-HF-digested organic fraction was analyzed for both δ¹³C and δD. The isotopic data were compared to atomic C/N ratios, carbon contents, diatom-inferred pH values and other environmental parameters. The diatom-inferred pH values and organic carbon contents provide evidence for a long-term change towards more acidic conditions and lower productivity in Lake Korttaj?rvi. The inferred pH values decrease from 7.0 to 6.1, followed by a slight increase during the last millennia. Variations in pH are accompanied by an increase in the δ¹³C_HCl-HF values of organic matter from −31.6 to , followed by a subtle decrease to . The changes in pH and δ¹³C_HCl-HF are closely related (r = − 0.91, P < 0.01), and apparently reflect changing environmental conditions in the lake and in its catchment area. δD values show a marked shift to higher values during the early Holocene, which may be partly related to a climatic amelioration leading to the Holocene Climatic Optimum in 6000–2500 BC. The Medieval Warm Period in AD 980–1250 is associated with a local maximum in δD, lending support for a significant warming during that time.     

长江中游网湖近代沉积环境演变及其对人类活动的响应

采用放射性核素²¹⁰Pb和¹³⁷Cs计年方法,建立了长江中游网湖近代沉积物的年代序列,并对网湖沉积物的粒度组成、磁性参数、重金属元素、花粉组合以及营养盐含量等指标进行了分析。根据沉积指标垂直分布的综合特征,结合湖区史料,重建了网湖100年来沉积环境的演变过程。通过对沉积指标与湖区人口变化、工农业产值等数据的比对,发现中华人民共和国成立以后沉积物中TOC和TN浓度与人口变化存在较高的相关性,相关系数分别达到0.8892和0.8928(P<0.001);沉积物中重金属元素富集序列、营养盐富集序列与孢粉含量的变化序列与湖区人类活动具有较好的耦合性。表明网湖沉积指标可以敏感地反映湖区的人类活动。     

Stable isotope (δ<Superscript>13</Superscript>C and δ<Superscript>15</Superscript>N) values of sediment organic matter in subtropical lakes of different trophic status

Lake sediments contain archives of past environmental conditions in and around water bodies and stable isotope analyses (δ¹³C and δ¹⁵N) of sediment cores have been used to infer past environmental changes in aquatic ecosystems. In this study, we analyzed organic matter (OM), carbon (C), nitrogen (N), phosphorus (P), and δ¹³C and δ¹⁵N values in sediment cores from three subtropical lakes that span a broad range of trophic state. Our principal objectives were to: (1) evaluate whether nutrient concentrations and stable isotope values in surface deposits reflect modern trophic state conditions in the lakes, and (2) assess whether stratigraphic changes in the measured variables yield information about shifts in trophic status through time, or alternatively, diagenetic changes in sediment OM. Three Florida (USA) lakes of very different trophic status were selected for this study. Results showed that both δ¹³C and δ¹⁵N values in surface sediments of the oligo-mesotrophic lake were relatively low compared to values in surface sediments of the other lakes, and were progressively lower with depth in the sediment core. Sediments of the eutrophic lake had δ¹³C values that declined upcore, whereas δ¹⁵N values increased toward the sediment surface. The eutrophic lake displayed δ¹³C values intermediate between those in the oligo-mesotrophic and hypereutrophic lakes. Sediments of the hypereutrophic lake had relatively higher δ¹³C and δ¹⁵N values. In general, we found greater δ¹³C and δ¹⁵N values with increasing lake trophic state.     

Sedimentary geochemical record of recent environmental changes around Lake Middle Marviken,Sweden

Geochemical properties of sediments deposited in Lake Middle Marviken over the last 185 years record the impacts of a succession of environmental changes that have occurred in the watershed. Clear-cutting of forests for wood and charcoal and extensive water harnessing to support the local iron mills from 1897 to 1957 is recorded by low C/N ratios, high black carbon, and low TOC and N_total accumulation rates. Larger δ¹³C and δ¹⁵N values in sediments deposited during this period imply higher productivity. Fluctuations in N_total and P_total accumulation rates show that the lake chemistry has varied between P or N-depleted systems that affected the δ¹⁵N values. Organic matter in the sediments is predominantly immature terrestrial material. Furthermore, hydrocarbon CPI, TAR, and P_aq values conform with the observed geochemical trends, variations in organic matter sources, and changes in the watershed. Accumulation rates of Cd, Pb, Zn, and S remained mostly unchanged throughout the period of mining, but an increase from 1957 to 1980 is most likely associated with air-borne industrial and fossil fuel emissions from regional urbanization. In situ microbial processes, such as iron and manganese reduction, also appear to be important in carbon cycling and in affecting the sediment and water chemistry of this lake.     

Isotopic fingerprints on lacustrine organic matter from Laguna Potrok Aike (southern Patagonia, Argentina) reflect environmental changes during the last 16,000 years

A combination of carbon-to-nitrogen ratios (TOC/TN), Rock Eval-analyses, and stable isotope values of bulk nitrogen (δ¹⁵N) and organic carbon (δ¹³C_org) was used to characterize bulk organic matter (OM) of a piston core from the Patagonian maar lake Laguna Potrok Aike (Argentina) for the purpose of palaeoenvironmental reconstruction. Sedimentary data were compared with geochemical signatures of potential OM sources from Laguna Potrok Aike and its catchment area to identify the sources of sedimentary OM. Correlation patterns between isotopic data and TOC/TN ratios allowed differentiation of five distinct phases with different OM composition. Before 8470 calibrated ¹⁴C years before present (cal. yrs BP) and after 7400 cal. yrs BP, isotopic and organo-geochemical fingerprints indicate that the sediments of Laguna Potrok Aike consist predominantly of soil and diatom OM with varying admixtures of cyanobacterial and aquatic macrophyte OM. For a short phase of the early Holocene (ca. 8470–7400 cal. yrs BP), however, extremely high input of soil OM is implied by isotopic fingerprints. Previous seismic and geochronological results indicate a severe lake-level drop of 33 m below present-day shortly before 6590 cal. yrs BP. It is suggested that this lake level drop was accompanied by increased erosion of shore banks and channel incision enhancing soil OM deposition in the lake basin. Thus, isotopic data can be linked to hydrological variations at Laguna Potrok Aike and allow a more precise dating of this extremely low lake level. An isotopic mixing model was used including four different sources (soil, cyanobacteria, diatom and aquatic macrophyte OM) to model OM variations and the model results were compared with quantitative microfossil data.     

Alternate trajectories in historic trophic change from two lakes in the same catchment, Huayang Basin, middle reach of Yangtze River, China

In order to assess how best to manage impacted lake systems, one needs to understand the trophic functioning of the lake system and the recent states through which the lake may have transitioned. Lakes in the middle and lower reaches of the Yangtze have been heavily impacted over recent decades. In order to understand recent changes in functional status, we examined sediment cores covering the last 120?years from two lakes in the same catchment with differing status: one algal-dominated (Taibai Lake) and the other macrophyte-dominated (Longgan Lake). Chironomid head capsules were identified from both sites and an expanded chironomid-total phosphorus (TP) transfer function (21 sites were added to the 30-lake model previously developed by Zhang et al. 2006) was used to assess the lakes?? response to recent anthropogenic change. Quantitative chironomid-inferred TP (CI-TP) reconstructions showed that Taibai Lake experienced clear changes in trophic status since the 1860s. Before the 1950s, the CI-TP concentration was relatively stable around 50?C80???g?L^?1, while it reached to 80?C130???g?L^?1 in the latter period. CI-TP for Longgan Lake, however, showed a relative decline from the range of 50?C75???g?L^?1 since the 1880s to 30?C40???g?L^?1 in recent years, accompanied by strong evidence from the chironomids for increased macrophyte biomass as TP levels declined. Both reconstructions agreed with diatom inferences of TP from the same lakes. The stark difference between these two sites is thought to reflect a function of macrophyte development, with Taibai Lake losing its plants through increased nutrient levels and internal recycling, whereas Longgan Lake, which is much bigger in area and hence potentially more resilient to change, was able to develop macrophyte communities over the same time period. The positive feedbacks associated with abundant macrophytes retained the clear water state of Longgan Lake, but a further increase in nutrients might lead to decrease in resilience of the relatively stable macrophyte state and loss of benthic pathways of primary production, which would push the lake towards eutrophication. Unless nutrient inputs to Longgan are controlled, Longgan Lake might lose macrophyte communities and follow a developmental pathway similar to that observed in Taibai Lake.     

A geochemical record of recent anthropogenic nutrient loading and enhanced productivity in Lake Nansihu, China

Total organic carbon (TOC), total nitrogen (TN), stable carbon and nitrogen isotopes (δ¹³C, δ¹⁵N), total phosphorus (TP) and organic phosphorus (OP) were measured in surface sediments and two short cores (DU-3 and WS-4) from Lake Nansihu, China to infer historical changes in anthropogenic nutrient inputs and corresponding shifts in lake primary productivity. Results indicate that organic matter preserved in the sediments is mainly autochthonous and that analyzed sediment variables were affected little by post-burial diagenesis. Increasing TOC, TN, OP and TP concentrations since the 1940s reflect increased P loading and elevated lake productivity. The δ¹³C values varied from ?21.5 to ?26.6‰ in the two sediment cores. Values were relatively more negative before the 1940s, but thereafter increased until the mid-1980s, reflecting elevated lake productivity. Since the mid-1980s, δ¹³C values remained relatively constant in core WS-4 and decreased in core DU-3, perhaps reflecting a change in the phytoplankton community. The δ¹⁵N values ranged from ?0.5 to 1.3‰ in core DU-3 and from 1.2 to 2.5‰ in core WS-4 before the mid-1980s, and increased to between 2.1 and 8.0‰ and 5.2 and 7.8‰, respectively, thereafter. Topmost sediments in the two cores display δ¹⁵N values similar to those recorded in the surface sediments (5.5–7.5‰). Higher δ¹⁵N values in recent deposits correspond to greater nitrogen concentration in water, and likely indicate anthropogenic nitrogen input, mainly from human and animal wastes.     

Carbon isotopes of marl and lake sediment organic matter reflect terrestrial landscape change during the late Glacial and early Holocene (16,800 to 5,540 cal yr B.P.): a multiproxy study of lacustrine sediments at Lough Inchiquin,western Ireland

A 7.6-m lake sediment core from a marl lake, Lough Inchiquin, records variation in landscape evolution from 16,800 cal yrs B.P. to 5,540 cal yrs B.P. We observe significant variations (up to 12‰) in δ ¹³C_org and δ ¹³C_calcite values that are interpreted to reflect secular changes in lake water δ ¹³C_DIC values that result from a regional landscape transition from barren limestone bedrock to a forested ecosystem. Lake water δ ¹³C_DIC values are therefore influenced by two isotopically distinct sources of carbon: terrestrial organic material (−27.1 to −31.2‰VPDB) via oxidized soil organic matter and weathered limestone bedrock (+3.4‰VPDB). Isotope excursions in lacustrine sediment records are forced not only by changes in productivity but also by changes in the terrestrial environment. This has profound implications for the interpretation of paleoclimate records derived from lacustrine sediment and suggests that selection of appropriate lakes can provide records of terrestrial change where other related records are not available.     

Water and sediment chemistry of Lake Pumayum Co,South Tibet,China: implications for interpreting sediment carbonate

A combination of water and sediment chemistry was used to investigate carbonate production and preservation in Lake Pumayum Co (altitude 5,030 m a.s.l.), south Tibet, China. We compared the chemical composition of lake water in various parts of the lake with that of input rivers and found that the loss of Ca²⁺ results from calcite sedimentation induced by evaporation and biogenic precipitation. This is supported by evaporation data from the catchment and δ¹⁸O measurements on water. Results suggest that CaCO₃ is the predominant carbonate in this lake. There is a positive correlation in the sediments among concentrations of total inorganic carbon (TIC), Ca, total organic carbon (TOC), and total nitrogen, confirming that most carbonates in sediment are endogenic. The Jiaqu River is the largest inflow to Lake Pumayum Co and has a strong influence on both lake water chemistry and sediment composition. The river and lake bathymetry influence carbonate sedimentation by affecting water flow velocity and growing conditions for macrophytes. Different carbon contents and relationships between TIC and TOC in the two long cores from different depths in the lake reveal that hypolimnetic conditions also influence carbonate precipitation and preservation.     

Biogeochemical records of paleoenvironmental changes in Nainital Lake,Kumaun Himalayas,India

Rapid urbanization and increased tourism around Nainital Lake in the Kumaun Himalayan region in north India has raised concerns about sediment and water pollution. Lead-210 dated sediment cores from the lake represent ~95 years of accumulation and yield a mean sedimentation rate of ~4.7 mm year⁻¹. Total organic carbon (TOC), percent N and S and their atomic C/N and C/S ratios, stable isotopes (δ¹³C, δ¹⁵N, and δ³⁴S), and specific biomarkers (n-alkanes and pigments) were measured in the core. Organic matter is primarily derived from in-lake algal production and TOC flux varies from 1.0 to 3.5 g m⁻² year⁻¹. Sediments are anoxic (Eh −328 to −187 mV) and have low (0.10–0.30 g m⁻² year⁻¹) N, but high (0.37–1.0 g m⁻² year⁻¹) S flux. Shifts in δ¹³C, δ¹⁵N, and δ³⁴S suggest in-lake microbial processes dominated by denitrification and sulfate reduction. The sediments are dominated by short-chain hydrocarbons with low Carbon Preference Index values. The pigments indicate a gradual shift to cyanobacterial domination of the phytoplankton community in recent years. Despite an increase in external input of nutrients, the trophic state of the lake has remained largely unchanged, and the perceived human-induced impacts are limited.     

Carbon cycling in Lake Erie during cultural eutrophication over the last century inferred from the stable carbon isotope composition of sediments

A ~106-cm sediment core from the eastern basin of Lake Erie was examined to investigate biogeochemical processes in this large lake during its cultural eutrophication over the last century. We measured stable carbon isotopes of total organic carbon and calcium carbonate (δ¹³C_TOC and d¹³ \textC_{\textCaCO 3} \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} ) as well as the concentrations of total organic carbon (TOC) and calcium carbonate (CaCO₃). δ¹³C_TOC and TOC show a strong positive correlation throughout the core and record changes in phytoplankton productivity and nutrient loading. CaCO₃ and TOC concentrations display a negative correlation throughout the core, suggesting that CaCO₃ concentrations are controlled primarily by decomposition of TOC in the hypolimnion and the sediments, although temperature and invasive mussels are also potential controlling factors. d¹³ \textC_{\textCaCO 3} \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} values show a positive correlation with δ¹³C_TOC between 1909 and 1969, indicating phytoplankton productivity was the primary control for d¹³ \textC_{\textCaCO 3} \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} values during eutrophication. However, a negative correlation between d¹³ \textC_{\textCaCO 3} \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} and δ¹³C_TOC from 1970 to 2002 suggests that these two proxies tracked different aspects of the carbon cycle in the lake in more recent times. The cause for the negative correlation is not yet known, but it is perhaps associated with temperature variations and seasonal differences in productivity.     

Organic geochemical record of environmental changes in Lake Dianchi,China

In order to investigate the natural ecosystem of Lake Dianchi and to assess its anthropogenic impacts, a stratigraphic study of bulk and molecular compositions of organic matter was conducted using a 63-cm long sediment core. The results show that two apparent environmental changes occurred during the evolution of Lake Dianchi: (1) the first change occurred in the 43–63 cm sediment depth, and was revealed by the amount and the composition of organic matter in the stage. Natural changes were possibly major factors responsible for triggering the environmental change, but the influence of human activities could not be excluded. Subsequently, the lake entered into a relatively stable and oligotrophic stage, which maintained until 20-cm sediment depth. (2) Eutrophication started in the upper 20 cm depth. Human activities became a major factor influencing environmental changes in this stage. Vertical profiles of various organic geochemical variables in the upper 20-cm sediments show evidence that primary productivity of the lake increased progressively and that the lake started eutrophic. Especially in the uppermost 10 cm, notable excursions to less negative δ¹³C_org and δ¹⁵N_total and high TOC concentrations have recorded an abrupt change in the lacustrine environment, suggesting that the lake entered a hypereutrophic stage. In addition, enhancement of αβ-hopanes reflects the contribution of fossil fuels to the lake sediments.     

Holocene trophic state history of a subtropical blackwater lake,South Georgia,USA

Organic-rich sediment from Lake Louise, a dystrophic sinkhole lake in south Georgia, displays variations in C, N, P, C/N, δ¹³C, δ¹⁵N, biogenic silica (BSi) and diatom flora that document changes in trophic state over the past ~9,500 years. The lake initially was oligotrophic and moderately productive, but by the middle Holocene a rising regional water table, driven by eustatic sea level rise, caused expansion of wetlands around the lake and a shift to humic waters. Low rates of sediment accumulation, low C contents, rising C/N, and light δ¹³C and δ¹⁵N indicate this was a time of low productivity, more anoxic bottom waters and extensive recycling of littoral organic matter. These conditions persisted until ~1800 AD when a physical disturbance to the watershed, probably the Great Hurricane of 1780, resulted in a dramatic increase in productivity that has continued to the present day. We attribute this shift, recorded by a >tenfold increase in sediment accumulation rate, higher C, P, and δ¹⁵N, and lower BSi, to establishment of an inflow stream that increased nutrient delivery to the lake, raised water level, and expanded the wetland area around the lake. Since ~1930, logging, farming, and highway construction have impacted the lake, further accelerating biological productivity as well as the delivery of terrigenous sediment. Results of this study illustrate the potential of a single, catastrophic event to permanently alter the hydrology and chemistry of a lacustrine system and confirm that dystrophic lakes can be highly productive and therefore promising targets for paleolimnological study.