Lake Surface Sediments Research Articles

Paleo-temperature inferred from brGDGTs over the past 18 cal ka BP from Lake Barrine, tropical NE Australia

Terrestrial temperature exerts a major influence on vegetation distribution, the hydrologic cycle and many biogeochemical cycles. Yet little research into terrestrial paleo-temperature reconstruction has been conducted covering the last deglaciation in northern Australia, and recent global surface temperature reconstructions include no data from this area. Here, for the first time, we reconstructed the mean annual air temperatures (MAAT) at Lake Barrine in this region from 18.3 to 1.7 cal ka BP at a relatively high resolution (averaged 400 yr). This temperature reconstruction was based on separated 5- and 6-methyl brGDGTs (branched glycerol dialkyl glycerol tetraethers) that are increasingly used to reconstruct terrestrial temperature because of their ubiquity and a preservation window of at least tens of millions of years. A 7.2 m master core from the center of the lake was extracted, along with five surface lake sediment and seven catchment soil samples. The results indicated that the fractional abundances of all the brGDGTs in the catchment soils, surface and downcore sediments in Lake Barrine were closely correlated, with dominant tetramethylated brGDGTs, especially Ia. We applied a global calibration to reconstruct the temperature using the lake sediment samples and a binary mixing model to correct temperature bias caused by soil-derived brGDGT input. The corrected temperature trend estimate compares favorably with the temperature anomaly recorded in the EPICA ice core record, except during the Younger Dryas. The corrected MAAT estimates at Lake Barrine indicated an increase across Heinrich Stadial 1 (HS 1; 18–14.9 cal ka BP) from 19.4 °C to 21.9 °C, a stable temperature during the Antarctic Cold Reversal (ACR; 14.7–13 cal ka BP) of ∼21.6 °C, followed by a slight decrease during the Younger Dryas (YD; 12.8–11.6 cal ka BP). After a further 1.6 °C increase into the early Holocene, the temperature decreased by 1.8 °C through the Holocene until a recent rise by 1.6 °C after 1.7 cal ka BP.

Distribution and Pollution Assessment of Nutrients in the Surface Sediments of Lake in Huoqiu County

To clarify the spatial distribution and pollution characteristics of nutrients in the surface sediments of Chengxi Lake and Chengdong Lake in Huoqiu County, total nitrogen (TN), total phosphorus (TP), and organic matter (OM) in 30 surface sediment samples of Chengxi Lake and Chengdong Lake were determined, and their pollution levels and sources were analyzed. The results showed that the contents of total nitrogen (TN), total phosphorus (TP), and organic matter (OM) in Chengxi Lake were significantly higher than those in Chengdong Lake, and their average contents were 746.23 mg·kg-1, 538.38 mg·kg-1, and 1.17% and 470.80 mg·kg-1, 492.08 mg·kg-1, and 0.68%, respectively. The spatial distribution in the north and northwest of Chengxi Lake was significantly higher than that in other areas and decreased from north to south. The C/N value indicated that the organic matter in Chengxi Lake and Chengdong Lake mainly came from unbundled plants and phytoplankton. Correlation analysis showed that total nitrogen (TN), total phosphorus (TP), and organic matter (OM) in Chengxi Lake and Chengdong Lake were homologous. The average values of TN and TP single index (STN and STP) of sediments in Chengxi Lake and Chengdong Lake were 0.67 and 1.18 and 0.42 and 1.08, respectively, indicating that TN in the two lakes was clean whereas TP was moderately polluted, and the average comprehensive pollution index (FF) was 1.51 and 1.31, respectively. The evaluation of organic index and organic nitrogen index showed that both Chengxi Lake and Chengdong Lake were clean.

Anaerobic oxidation of methane does not attenuate methane emissions from thermokarst lakes

AbstractThe ongoing global temperature rise enhances permafrost thaw in the Arctic, allowing Pleistocene‐aged frozen soil organic matter to become available for microbial degradation and production of greenhouse gases, particularly methane. Here, we examined the extent and mechanism of anaerobic oxidation of methane (AOM) in the sediments of four interior Alaska thermokarst lakes, which formed and continue to expand as a result of ice‐rich permafrost thaw. In cores of surface (~ 1 m) lake sediments we quantified methane production (methanogenesis) and AOM rates using anaerobic incubation experiments in low (4°C) and high (16°C) temperatures. Methanogenesis rates were measured by the accumulation of methane over ~ 90 d, whereas AOM rates were measured by adding labeled‐13CH4 and measuring the produced dissolved inorganic 13C. Our results demonstrate that while methanogenesis was vigorous in these anoxic sediments, AOM was lower by two orders of magnitude. In almost all sediment depths and temperatures, AOM rates remained less than 2% of the methanogenesis rates. Experimental evidence indicates that the AOM is strongly related to methanogens, as the addition of a methanogens' inhibitor prevented AOM. Variety of electron acceptor additions did not stimulate AOM, and methanotrophs were scarcely detected. These observations suggest that the AOM signals in the incubation experiments might be a result of enzymatic reversibility (“back‐flux”) during CH4 production, rather than thermodynamically favorable AOM. Regardless of the mechanism, the quantitative results show that near surface (< 1‐m) thermokarst sediments in interior Alaska have little to no buffer mechanisms capable of attenuating methane production in a warming climate.

Isoprenoidal GDGTs and GDDs associated with anoxic lacustrine environments

We examined membrane-spanning archaeal lipids using ultra high pressure liquid chromatography-high resolution mass spectrometry (UHPLC-HRMS) in a suite of sediment samples from both cored sequences (Messel oil shale and Lake Chala) and surface sediments (Azorean lakes) encompassing ancient and modern (Eocene to Present) lacustrine environments. Additionally we compared the lacustrine data to those of marine (Mediterranean cored sequences, Arabian Sea surface sediments and Monterey outcrop sediments) and hypersaline sediments (Vena del Gesso marls) as well as marine suspended particulate matter (SPM) from the Black Sea. Regular isoprenoidal glycerol dialkyl glycerol tetraethers (GDGTs) and glycerol dialkyl diethers (GDDs) were the most abundant membrane-spanning lipids in all investigated settings (>90 % and 84 % respectively). Interestingly, GDGTs with a cyclohexyl ring (S-GDGTs) were also present in almost all investigated lake sediments, in relative abundances of ca. 2–7 % and, for the first time, also their S-GDD counterparts were detected (2–10 %). The producers of S-GDGTs are still unknown, however our results show that it is likely that bottom water anoxia (both seasonally induced or permanent) is the driving factor for the production of these lipids, whereas previous studies suggested euxinia was required for production. Unsaturated GDGTs (uns-GDGTs, ca. 2 %) were only detected in Lake Chala sediments and surface sediments from Azorean lakes, but without accompanying uns-GDDs. GMGTs, glycerol monoalkyl glycerol tetraethers, were present in Messel oil shale and marine samples, while GMDs were only found in Messel oil shale.

EXPERIMENTAL TAPHONOMY OF FRESHWATER DIATOMS: DISCRIMINATING BETWEEN CHEMICAL AND PHYSICAL CAUSES OF FRUSTULE FRAGMENTATION

ABSTRACT Frustule fragmentation is one of the major taphonomic factors affecting diatom preservation in Pampean shallow lakes. Although this has been demonstrated by taphonomic studies conducted on extant and fossil material, little is known about the causes of frustule breakage and its environmental significance. Field studies along modern environmental gradients showed a low but significant correlation between dissolution and fragmentation. Reworking of sediments by wind action has also been suggested as a plausible explanation for the fragmentation patterns observed in the field. To evaluate the relative importance of chemical and physical causes of diatom fragmentation, three laboratory experiments were conducted to test for (1) the effect of physical agitation; (2) the effect of chemical dissolution; and (3) the joint effect of both variables on diatom fragmentation. Diatom samples were subjected to physical agitation with a reciprocating shaker at 250 rpm for 20 days. Chemical dissolution was tested by mixing diatom assemblages with solutions of different concentrations of NaCl and NaHCO3, and pH 10 for 20 days. The joint effect of both processes was tested by dissolving diatom assemblages in salt solutions and subjecting them simultaneously to agitation for 20 days at 250 rpm. In all cases, aliquots of each assemblage were sampled at initial and final experimental times and their preservation analyzed via the application of fragmentation and dissolution indices. Significant and similar differences in fragmentation indices were observed in both dissolution alone and combined experiments, but no differences were found in samples subjected to agitation alone. In all cases, dissolution and fragmentation indices were correlated, suggesting a causal relationship between them. Overall, our results suggest that the debilitation of the diatom frustule by chemical dissolution may be the main cause of the fragmentation patterns observed in surface sediments of Pampean shallow lakes.

Spatial Distribution and Sources of Organic Matter in the Surface Sediments of Fuxian Lake, SW China

Sedimentary organic matter is an important component of the metabolism of a lake’s ecosystem, and it is generally derived from both the watershed and the primary productivity of a lake. Understanding the sources of organic matter in lakes and lake trophic status is important when evaluating the quality of lake ecosystems. We summarize the spatial distribution of total nitrogen (TN), total organic carbon (TOC), TOC/TN (C/N) molar ratios, and organic carbon isotope (δ13Corg) of the surface sediments of Fuxian Lake, Yunnan–Guizhou Plateau, Southwest China, which is the second deepest freshwater oligotrophic lake in China. The results show that the distributions of TN, TOC, C/N, and δ13Corg of the surface sediments are spatially heterogeneous, which is also the case for the trophic conditions of the lake. Compared with the adjacent eutrophic lakes and typical lakes in other areas with strong human activities, the content of organic matter is at a low level. Meanwhile, the autochthonous organic carbon in the surface sediments was characterized by lower δ13Corg (−25.3~−28.5) and C/N (8.7~12.9), suggesting that the biological carbon pump effect plays a significant part in the stability of carbon sinks by coupling with carbonate weathering. Our results emphasize the importance of the carbon sink of coupled carbonate weathering and aquatic photosynthesis in the evolution of the carbon cycle in lakes. Although modern monitoring shows that Fuxian Lake is an oligotrophic lake, there are potential risks of organic nitrogen pollution with respect to surface sediments, especially in northern and southern shallow-water areas. The organic pollution of lakes can be reduced by controlling the discharge of wastewater and reducing the nutrient loading of agricultural runoff.

Health and ecological risks assessment of heavy metals and metalloids in surface sediments of Urmia Salt Lake, Northwest of Iran.

Urmia Lake, in the northwest of Iran, is the largest body of saline water in the Middle East, which has been desiccated in recent decades. To investigate the pollution status and ecological-health risks of heavy metals and metalloids in the surface sediments of this lake, 26 sediment samples were collected along the salt marshes of the lake and were analyzed for heavy metals and metalloid concentrations. The potential ecological risk assessment was carried out using enrichment factor (EF), geo-accumulation index (Igeo), contamination factor (CF), and potential ecological risk (Eri) standard indices. The average concentrations (mg kg-1) of heavy metals and metalloids were as follows: Fe (11,714) > Sr (320.8) > Mn (274.3) > V (28.5) > Cu (24.7) > Zn (21.2) > As (17.3) > Ni (14.8) > Cr (12.6) > Pb (11) > Co (4.0) > U (1.7), Hg (0.6) > Mo (0.36). The concentrations of As, Hg, and Sr in lake sediments were higher than geochemical background values. The non-carcinogenic risks caused by heavy metals and metalloids were insignificant regarding health risks. Levels of carcinogenic risk for metal(loid)s were in the acceptable ranges (10-6-10-4). The ecological risk was low, except for As, Sr, and Hg which showed moderate to significant EF, Igeo, and CF values. Arsenic and Sr were enriched in the surface sediments in desiccated parts of the lake due to complete lake water evaporation. It seems that further drying of the lake increases the potential ecological risk of heavy metals and metalloids in the surface sediments of Urmia Lake.

Correction to: Trace element distribution and pollution status of surface sediments in lakes impacted by volcanic activity

Correction to: Trace element distribution and pollution status of surface sediments in lakes impacted by volcanic activity

Trace element distribution and pollution status of surface sediments in lakes impacted by volcanic activity

Trace element distribution and pollution status of surface sediments in lakes impacted by volcanic activity

In-situ activities of anaerobic bacteria and methanogenic archaea in shallow lakes over the Anthropocene: A case study of Lake Wuliangsu

Understanding microbial activities in shallow lake system under anthropogenic interferences is very important for evaluation on organic carbon (OC) source-sink behaviors. Theoretically, glycerol dialkyl glycerol tetraethers (GDGTs) could signal in-situ anaerobic microbial activities in shallow lakes. Here we test this theory by analyzing GDGTs of sediment samples collected in Lake Wuliangsu, a typical large and shallow lake located in the Yellow River Reaches. Considering that lake surface sediments show different occurrences and distribution patterns with lake shore and riverine sediments, brGDGTs and GDGT-0 should be mainly produced by heterotrophic anaerobic bacteria (Acidobacteria) and methanogenic archaea (Euryarchaeota) within the lake, while crenarchaeol from soil OC transported through the Yellow River system. With the assistant of a two end-member model, in-situ and soil-derived brGDGTs can be quantitatively evaluated. Following these discoveries, we further utilized GDGT-based records from a well-dated sedimentary core to disentangle the past ∼180-year history of anaerobic microbial activities in Lake Wuliangsu. The in-situ anaerobic microbial activities started to increase at CE 1926 and showed stronger predominance over soil-derived tetraether lipids after CE 1954, largely due to a combined effect from hydrological change and in-situ OC production resulting in OC-rich and anaerobic conditions for heterotrophic anaerobic microorganisms to prosperous. However, the relative contribution of in-situ anaerobic microbial GDGTs to overall sedimentary OC shows complex relations with both OC production and recycling behaviors, tentatively suggesting that anaerobic microbial activities only conduct a minor impact on OC burial in the shallow and eutrophic Lake Wuliangsu over the Anthropocene.

Spatial distribution of anemophilous pollen and its correlation with the Asian summer monsoon on the southeastern Tibetan Plateau

Previous studies have discovered arboreal pollen in non-forested areas on the Tibetan Plateau, which are mostly anemophilous type and regarded as a result of the Asian summer monsoon. However, the relationship between the spatial distribution of wind-pollinated pollen and monsoon has seldom been illustrated quantitatively. Here, we used 676 surface samples (647 compiled, 29 new samples) to examine the characteristics of the spatial distribution patterns of 13 anemophilous pollen taxa on the southeastern Tibetan Plateau, by employing the spatial interpolation technique. Then, we quantified the correlation between the distributions of these taxa and monsoon wind via Mantel test. The results show that the anemophilous pollen represents at least half percentage of the total pollen assemblages in most areas of the study region. Anemophilous tree pollen mainly distributes in the south and southeastern part, while anemophilous non-tree pollen does in the opposite and perpendicular direction. The distributional pattern of non-anemophilous pollen reflects that their corresponding plants most likely constitute the understory components of different forests or the minorities of non-forests communities. Mantel tests indicate that the anemophilous tree pollen has a highest correlation with the Asian summer monsoon. Anemophilous pollen, overall pollen assemblages and anemophilous pollen with air sacs have higher correlations with summer monsoon. Contrarily, non-anemophilous pollen has relatively weak correlation. Furthermore, we found that the relationships between various pollen groups or separate taxa and the Asian summer monsoon are dependent on sample types. The lake sediment surface samples had the strongest signal of the Asian summer monsoon among the different sample types. Our findings provide the strong evidence of the Asian summer monsoon influencing the pollen distribution patterns. More importantly, the quantification of this correlation between anemophilous tree pollen and the Asian summer monsoon offers the fundamentally theoretical basis of reconstructing paleo-monsoon by the proxy of windborne pollen taxa.

Source-oriented ecological risk assessment of heavy metals in sediments of West Taihu Lake, China

The geographical location of West Taihu Lake determines that it is the entrance of the whole Taihu Lake, and the intensive industries around it pose a great threat to the ecology of Taihu Lake. We innovatively combined Pb isotope ratio analysis with ecological risk assessment index to quantify the source-oriented ecological risk of heavy metals (HMs) in the sediments of West Taihu Lake. In this study, the representative HMs Zn, Pb, Cr, and Cd in the surface (0-2cm) sediments of West Tai Lake were determined, and the ecological risk assessment of HM sources was carried out based on the Pb isotope ratio and ecological risk index. The results showed that HMs were significantly enriched in the south and thewest of the study area. The average geo-accumulation index (Igeo) of Pb was unpolluted, Cr and Zn were between unpolluted and moderately polluted, and Cd was moderately polluted. The average ecological risk index (Ei) of Pb, Cr, and Zn was low, and only Cd reached a considerable risk (ECd = 120.7), which accounted for 89.8% of the comprehensive ecological risk index (RI). However, the RI in the whole study area (RI = 134.4) still indicated low risk. There was a significant correlation between Pb and other HMs (P < 0.05). The IsoSource analysis showed that the order of contribution rate was fossil fuels (48.0%) > industrial sources (35.8%) > natural sources (14.9%) > agricultural sources (1.3%). The HM pollution caused by fossil fuel combustion and industrial activities reaches a moderate ecological risk, whereas natural sources and agricultural sources pose a low risk. Overall, the main sources of HM pollution are anthropogenic, which pose moderate ecological risk to the study area and should be paid more attention to.

Ecological risk assessment of glyphosate and its possible effect on bacterial community in surface sediments of a typical shallow Lake, northern China

Glyphosate is a widely used herbicide worldwide and its prevalent presence in aquatic ecosystems poses a threat to living organisms. This study evaluated potential ecological risk of glyphosate to sediment-dwelling organisms and assessed the probable effect of glyphosate on structure and predicated function of sediment-attached bacterial communities from a large shallow lake in northern China based on 16S rRNA high-throughput sequencing. Results suggested that glyphosate showed a medium to high concentration (up to 8.63 mg/kg) and chronic risk to sediment-dwelling organisms (10% samples exhibiting medium to high risk quotient), especially in sites nearby farmland and residential areas in August. Bacterial community identification based on 16S rRNA sequence indicated some species of dominant phylum Proteobacteria and Campilobacterota (e.g., Steroidobacteraceae, Thiobacillus, Gallionellaceae, Sulfurimonadaceae) were stimulated while some species of dominant phylum Actinobacteriota, Acidobacteriota and Firmicutes (e.g., Nocardioidaceae, Microtrichales, Vicinamibacteraceae, Paenisporosarcina) were inhibited by glyphosate accumulation. The stimulating species were related to sulfur-oxidizing, sulfate-, iron-, or nitrate-reducing bacteria; The inhibiting species were related to plant bacterial endophytes, polyphosphate-accumulating organisms (PAOs) and denitrifers. Correspondingly, promoted bacterial metabolic functions of “sulfite respiration”, “nitrogen respiration”, “aromatic compound degradation” and “nitrification” but suppressed “cellulolysis”, “manganese oxidation”, “anoxygenic photoautotrophy S oxidizing” and “nitrate denitrification” were predicated on functional annotation of prokaryotic taxa. Although these results could only partly suggest the impacts of glyphosate on the bacterial communities due to the lack of actual results from control experiments, the identified Steroidobacteraceae could be thought as a bioindicator in the future mechanism study for the ecological effect and bioremediation of glyphosate. This work intends to arise the concern about the depletion of biodiversity and bacterial metabolic functions with contribution of glyphosate in part in eutrophic lakes.

Leaf wax biomarkers of a high-mountain lake area in western iberia—Implications for environmental reconstructions

Leaf wax n-alkane biomarkers are widely used to infer past vegetation dynamics and hydroclimate changes. The use of these compounds strongly relies on the characterization of modern plants. However, few studies have explored leaf waxes of modern plants and their application to reconstructing climate and environmental changes in the Iberian Peninsula, a region known for its high vulnerability to climate change. In this study, we characterize the distributions and compound-specific isotopic compositions of the leaf waxes of dominant plants in the vegetation cover, soil, and surface sediment of the Lake Peixão area, a high-mountain glacial lake in Serra da Estrela (central Portugal). Our results show that the modern oro-Mediterranean (subalpine) vegetation of the study area is dominated by C3 grasses/herbs and shrubs that preferentially produce long-chain leaf waxes (≥C27). The C31n-alkane display the overall highest concentration, produced by some grasses and shrubs, but especially Erica sp (heather), which is highlighted as a major source for the total n-alkane pool in the lake sediments. C29 is the second-most abundant and the most equally produced n-alkane of the vegetation cover; C25 and C27 homologs are mainly associated with aquatic-related grasses/herbs, while C33 and C35 are particularly linked to cold-drought tolerant Juniperus sp. shrubs. Shrubs show higher but proportional values than grasses/herbs in the isotopic space, suggesting a directly proportional physiological adaptation of the two ecological forms to the prevailing climatic and environmental factors of the study area. C29 is pointed as the most representative (or less plant-biased) leaf wax n-alkane in the lake sediments. Thus, δD of C29n-alkane is interpreted as a robust terrestrial hydrological indicator (δDterr), which signal is believed to be strongly influenced by the mean air temperature and/or precipitation amount. Despite the sparse vegetation and small catchment area, the apparent hydrogen fractionation factor, determined from δDterr of the lake surface sediment, is in line with the modeled global mean values for the latitude of the study area. The different molecular and compound-specific signatures of the studied oro-Mediterranean species have the potential to support future interpretations of leaf wax biomarkers in the Iberian Peninsula.

Influence of salinity on the diversity and composition of carbohydrate metabolism, nitrogen and sulfur cycling genes in lake surface sediments.

Exploring functional gene composition is essential for understanding the biogeochemical functions of lakes. However, little is known about the diversity and composition of biogeochemical cycling genes and their influencing factors in saline lakes. In this study, metagenomic analysis was employed to characterize the diversity and composition of microbial functions predicted from genes involved in carbohydrate metabolisms, nitrogen, and sulfur cycles in 17 surface sediments of Qinghai-Tibetan lakes with salinity ranging from 0.7 to 31.5 g L-1. The results showed that relative abundances of carbohydrate-active enzyme (CAZy), nitrogen, and sulfur cycling genes were 92.7-116.5, 15.1-18.7, 50.8-63.9 per 1,000 amino acid coding reads, respectively. The Shannon diversity indices of CAZy and sulfur cycling genes decreased with increasing salinity, whereas nitrogen cycling gene diversity showed an opposite trend. Relative abundances of many CAZy (i.e., carbohydrate-binding module and carbohydrate esterase), nitrogen (i.e., anammox and organic degradation and synthesis) and sulfur (i.e., dissimilatory sulfur reduction and oxidation, link between inorganic and organic sulfur transformation, sulfur disproportionation and reduction) cycling gene categories decreased with increasing salinity, whereas some CAZy (i.e., auxiliary activity), nitrogen (i.e., denitrification) and sulfur (i.e., assimilatory sulfate reduction and sulfur oxidation) gene categories showed an increasing trend. The compositions of CAZy, nitrogen, and sulfur cycling genes in the studied lake sediments were significantly (p < 0.05) affected by environmental factors such as salinity, total organic carbon, total nitrogen, and total phosphorus, with salinity having the greatest influence. Together, our results suggest that salinity may regulate the biogeochemical functions of carbohydrate metabolisms, nitrogen, and sulfur cycles in lakes through changing the diversity and composition of microbial functional genes. This finding has great implications for understanding the impact of environmental change on microbial functions in lacustrine ecosystems.

Composition, distribution, and source of organic carbon in surface sediments of Erhai Lake, China

Lake sediment is an important organic carbon (OC) sink. Nevertheless, few studies have been conducted on sediment organic carbon (SOC) in lakes, and the effects of environmental variables on SOC pools remain poorly understood. We combined physicochemical and spectroscopic analyses to investigate the composition, distribution, and source of OC in surface sediments of Erhai Lake, southwest China, and explored the relationships between environmental variables and its SOC pool. The SOC pool consists of relatively high proportions of labile organic carbon fractions, mainly from algal production, which are rapidly decomposed and exhibit high turnover rates. The relative content of humus carbon ranges from 13.5 % to 20.5 %, with fulvic acid carbon predominating (average 52.95 %), indicating weak humification and a relatively active humus carbon pool. The dissolved organic matter in water column and sediments of Erhai Lake is largely influenced by endogenous production, with a great contribution from phytoplankton. Surface sediments contained more protein-like components than overlying waters (80.0 % vs. 63.0 %), attributed mainly to abundant algal deposition and intense bacterial metabolism. Among environmental variables, sediment chlorophyll a showed the strongest relationship with the SOC pool, and was associated with rapid decomposition and promotion of the humification process, which supported the conclusion that algae had an important influence on the SOC pool. The SOC pool in the southern region of the lake is mainly contributed by algae, other microorganisms, and sewage, exhibiting a greater potential to release organic matters into the water column. The center and northern SOC pools show relatively stable characteristics and stronger OC sink capacity, mainly because of the input of terrestrial refractory organic matters from runoff. Our data shed light on the OC storage mechanisms in the surface sediments of Erhai Lake and provide theoretical bases for enhancing the OC sink of sediments in the lake.

Amino acids as indicators of seasonal variations in organic matter degradation in surface sediments from a shallow lake

Degradation of organic matter (OM) in sediments is a key link in nutrient cycling and sedimentation processes in lakes. The aim of this study was to explore the degradation of OM in surface sediments of a shallow lake (Baiyangdian Lake, China) under seasonal temperature variations. For this, we used the amino acid-based degradation index (DI) and the spatiotemporal distribution characteristics and sources of OM. Sediment OM in the lake mainly originated from freshwater aquatic plants and terrestrial C4 plants. The sediment at some sampling sites was affected by surrounding crops. The organic carbon and total nitrogen contents, and the total hydrolyzed amino acid concentrations in the sediments were highest in summer and lowest in winter. The lowest DI occurred in spring, which indicated that the OM in the surface sediment at this time was highly degraded and relatively stable, and the highest DI occurred in winter, which showed that the sediment was fresh. The water temperature was positively correlated with the organic carbon content (p < 0.01) and total hydrolyzed amino acids concentration (p < 0.05). Seasonal variations in the overlying water temperature had a large effect on OM degradation in the lake sediments. Our results will facilitate the management and restoration of lake sediments that suffer from endogenous release of OM in a warming climate.

Spatial distribution and environmental significance of modern organic carbon and nitrogen stable isotopes in the source area of the Yellow River: A case study of Gyaring Lake and Ngoring Lake

Organic carbon and total nitrogen isotopes (δ13Corg and δ15N) are useful for identifying sources of organic matter and for reconstructing environmental conditions in paleoecology and paleoclimatology. Gyaring Lake and Ngoring Lake are the two largest freshwater lakes in the source area of the Yellow River (SAYR), which lies in the northeastern the Qinghai–Tibet Plateau. These bodies of water were chosen as our research objects, and we analyzed the carbon and nitrogen isotopes in the surface sediments of the two lakes and in soil samples from around the lakes. The results indicate the consistency and endogenous characteristics of the organic matter sources of the distribution of δ13Corg and δ15N in the surface sediments of Gyaring Lake and Ngoring Lake. The δ13Corg values of the two lakes were significantly positive in areas near the inflow and outflow of the Yellow River. The distribution of δ15N in Gyaring Lake was similar to that of total organic carbon (TOC) and total nitrogen (TN), and its values were relatively higher than those affected by the joint influence of the Yellow River and some small tributaries, while δ15N was more positive in the deep-water area of Ngoring Lake. This study provides reliable and modern data on δ13Corg and δ15N for the long-term paleoenvironment and paleoecological reconstruction of the SAYR in the future.

Deterministic processes drive national‐scale patterns in lake surface sediment bacteria and eukaryotic assemblage composition

AbstractBiological communities within lake surface sediments play a vital role in biogeochemical cycling and ecosystem services. Knowledge on abundance‐occupancy patterns and assembly processes across large spatial scales and over multiple environmental gradients is limited, yet essential to aid in protection and restoration. In the present study, surface sediment samples were collected from 296 lakes across a wide spatial scale and covering multiple interacting environmental gradients including size, depth, altitude, and trophic state. A suite of physicochemical parameters were used to characterize the environmental conditions and bacterial and eukaryotic assemblages were determined using 16S and 18S rRNA metabarcoding. The majority (~ 55%) of amplicon sequence variants were only found in a single lake with eukaryotes having a more restricted distribution than bacteria. Deterministic processes were inferred to be dominant for both bacteria (78%) and eukaryotes (51%), with variable selection being especially important for bacteria (49%). Variation partitioning indicated that land use in the catchment, which is strongly related to trophic state, was the most important environmental factor in explaining the assemblage composition. This nationwide study across broad gradients provides new insights into the ecology of bacteria and eukaryotes in lake surface sediments and a platform to better understand the effects of multiple environmental stressors on lake sediment assemblages.

Concentrations and source identification of priority polycyclic aromatic hydrocarbons in sediment cores from south and northeast Thailand

In this study, the environmental fate of carcinogenic polycyclic aromatic hydrocarbons (PAHs) in tropical lake sediments and their potential sources have been discussed. 15 PAHs (i.e. ΣPAH) have been investigated in two lakes, namely Songkhla Lake (SKL) and Nong Han Kumphawapi Lake (NHL), which are located at the southern and north-eastern parts of Thailand, respectively. Since these two lakes are registered as important wetlands under the Ramsar convention (United Nations Educational, Scientific and Cultural Organization: UNESCO), the quantitative identification of potential contributors of PAHs is an inevitable analytical tool for launching an evidence-based policy. The ΣPAH concentrations observed in SKL and NHL sediments (n = 135) were in the range of 19.4–1,218 ng g−1 and 94.5–1,112 ng g−1, respectively. While the exponential decline of ΣPAH contents were detected in SKL sediments, NHL showed a trend of enhancing PAH contents with depth. The averaged benzo [a]pyrene (B [a]P) contents of surface sediments in both lakes were much below the value stipulated by the United States Environmental Protection Agency (US-EPA) guidelines for carcinogen risk assessment. Based on numerous multivariate statistical techniques coupled with source apportionment analysis, “biomass burning” and “anthropogenic activities” are two potential contributors of the PAHs detected in the study areas. To achieve the long-term conservation of nature with related ecosystem services and cultural values, it is therefore important to promote decision-making based on ecotoxicological studies of carcinogenic substances.