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1.
We used statistical analyses to determine which subset of 36 environmental variables best explained variations in surface sediment δ13C and δ15N from 50 lakes in western Ireland that span a human-impact gradient. The factors controlling lake sediment δ13C and δ15N depended on whether organics in the lake sediment were mostly derived from the lake catchment (allochthonous) or from productivity within the lake (autochthonous). Lake sediments with a dominantly allochthonous organic source (high C:N ratio sediments) produced δ13C and δ15N measurements similar to values from catchment vegetation. δ13C and δ15N measurements from lake sediments with a dominantly autochthonous organic source (low C:N ratio sediments) were influenced by fractionation in the lake and catchment leading up to assimilation of carbon and nitrogen by lacustrine biota. δ13C values from lake sediment samples in agricultural catchments were more negative than δ13C values from lake sediment samples in non-impacted, bogland catchments. Hypolimnetic oxygen concentrations and methane production had a greater influence on δ13C values than fractionation due to algal productivity. δ15N from lake sediment samples in agricultural catchments were more positive than δ15N in non-impacted bogland catchments. Lower δ15N values from non-impacted lake catchments reflected δ15N values of catchment vegetation, while higher δ15N values in agricultural catchments reflected the high δ15N values of cattle manure and inorganic fertilisers. The influence of changing nitrogen sources and lake/catchment fractionation processes were more important than early diagenesis for lake sediment δ15N values in this dataset. The results from this study suggest a possible influence of bound inorganic nitrogen on the bulk sediment δ15N values. We recommend using a suitable method to control for bound inorganic nitrogen in lake sediments, especially when working with clay-rich sediments. This study confirms the usefulness of δ13C and δ15N from bulk lake sediments, as long as we are mindful of the multiple factors that can influence these values. This study also highlights how stable isotope datasets from lake surface sediments can complement site-specific isotope source/process studies and help identify key processes controlling lake sediment δ13C and δ15N in a study area.  相似文献   

2.
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.  相似文献   

3.
Few studies have assessed the relative importance of landscape, land use history, climate, and regional heterogeneity on lake ecosystem processes, despite the likelihood that interactions among these factors must be important for controlling lake dynamics. We used 14 sediment measures from 20 lake cores in a climatically sensitive region of the prairie-forest border in southern Minnesota to (1) assess relationships between modern lake productivity (Carlson’s Trophic State Index [TSI]), modern land-use, catchment, and lake morphometry, and (2) contrast regional responses to climatic transitions from the ‘Medieval Climatic Anomaly’(1000–1350) to the ‘Little Ice Age’ (1350–1800) to ‘Modern’(~1980–1996 AD). TSI was significantly positively correlated with modern sedimentation rate, and accumulation rates of organic matter (OM), biogenic silica (BSi), and total phosphorus (TP). TSI was not significantly correlated with “modern” land cover, catchment, or lake morphometry characteristics, but total organic N(N) was negatively correlated with percent cultivation in the catchment area and negatively correlated with δ15N of bulk organic matter in sediment. Regional, among-lake heterogeneity was high over the past 1,000 years, but Little Ice Age (LIA) cooling appeared to result in an approximately 20% decline in OM, BSi and TP accumulation, while warming and cultural eutrophication of the past 150 years corresponded to a 200–400% increase in accumulation rates as well as an 80% increase in carbonates and a small but significant 10% drop in C/N ratios consistent with greater in-lake productivity. Our results indicate that climate does have regional effects on lake ecosystems but that among-lake variability is high, reflecting the importance of local factors and suggesting a need for (1) more explicit definition of what ‘regional’ means, (2) a focus on degree as well as direction of change, and (3) estimating accumulation rates based on multiple lakes and multiple cores within lakes.  相似文献   

4.
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.  相似文献   

5.
I addressed the effects of taphonomic and early diagenetic processes on the isotope composition of cladoceran remains, using both experimental and field approaches. An experiment was designed to mimic the conditions encountered by cladoceran remains when they settle through the water column and are buried in the sediment. Cladoceran exoskeletons were incubated for 4 months in oxic or anoxic water, and in sediment. Changes in their carbon (C) and nitrogen (N) content and isotope compositions were measured. Most changes in isotope composition of exoskeletons took place when they settled through the water column. Once buried in the sediment, however, the δ13C and δ15N values of cladoceran exoskeletons did not undergo further change. Taphonomic processes resulted in an increase in δ13C and δ15N of the cladoceran remains and this was related to microbial degradation, which selectively removed isotopically light C and N compounds from the remains. For δ13C, changes were minimal (<1‰) and occured within the first 3 months. Taphonomic effects on δ15N were larger, from +2 to +5‰, and occurred within the first 2–3 weeks. These effects depended on incubation conditions and were greater in anoxic waters than under oxic conditions. Monthly changes in the isotope composition of sedimenting cladoceran exoskeletons were also recorded in the field using sediment traps, and were compared to the isotope composition of the living cladoceran community. The isotope composition of sedimenting remains displayed values consistent with those that might be expected, considering the effects of taphonomic processes observed in the experiment. Because C and N in cladoceran exoskeletons might involve a different isotope routing, the δ15N value of the remains provides an annual record of the value in the parent community, with a 1-month delay, while δ13C of remains essentially reflects that of the parent community during the period of lake thermal stratification. These findings provide insights into paleolimnological interpretation of isotopic changes in cladoceran remains from sediment cores.  相似文献   

6.
This study presents detailed lithostratigraphy and stable carbon and nitrogen isotopic variations in a 520-cm-long sediment core from a cirque basin in the Labsky důl Valley, Krkonoše Mountains, Czech Republic. Detailed study of the core reveals five major periods of sedimentation during the last 7600 years: silt and sand deposition during ~7.6–5.1 ka cal BP, Sphagnum peat accumulation during ~5.1–4.0 ka cal BP, sandy silt and sand during ~4.0–2.8 ka cal BP, raised peat bog during ~2.8–2.0 ka cal BP (Sphagnum peat), and sedimentation of sandy silt since ~2.0 ka cal BP. The δ13C values of the organic matter in the core vary in the range typical for C3 plants, from −24.35 to −27.68‰, whereas the δ15N values vary from −2.65 to +4.35‰. Core sections having ash contents ≥70% have δ15N > 1‰ and δ13C < −26‰, whereas those having ≤70% ash content have δ15N < 1‰ and δ13C > −26‰. Strong linear correlations are observed between δ13C and δ15N values as well as between C:N ratios and δ15N values in the horizons with ash content >10%, primarily for sand and silt horizons. On the other hand, poor correlations between δ13C and C:N ratio, as well as δ15N and C:N ratio, were observed in Sphagnum peat layers (45–125 and 185–265 cm). We conclude that the primary stable isotope variations are not preserved in the layers where significant correlation between δ15N and C:N ratio is observed. The relatively small δ13C variation in the uppermost Sphagnum peat layer suggests stable temperature during ~2.8–2.0 ka cal BP.  相似文献   

7.
Total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) were determined in combination with stable isotope ratios of carbon and nitrogen (δ13COrg, δ15N) in a 63 cm sediment core from Longgan Lake, located in the middle reaches of the Yangtze River, China. These geochemical and isotopic records provide a continuous history of lake productivity and trophic state of Longgan Lake since 1890. Variations of δ13COrg, TOC, TN and TP indicate that primary productivity of Longgan Lake increased continuously during the last century and that the trophic state of the lake shifted from oligotrophic to mestrotrophic conditions accordingly. Anthropogenic sources of organic carbon (OC), nitrogen (N) and phosphorus (P) were distinguished from their natural background in the sediments using mass accumulation rates. Element mass accumulation rates suggested increased human activities in the lake’s catchment since 1950s, were especially the utilization of artificial fertilizers amplified the anthropogenic input of N and P into the lake. In the course of the improved availability of dissolved nutrients also primary productivity of Longgan Lake increased, resulting in an increase of the Suess-effect corrected organic carbon isotope ratios. δ15N of bulk sediments show a marked shift towards lower values around 1950 that has been attributed to the input of nitrogen from chemical fertilizers characterized by relatively depleted isotopic signatures into the lake.  相似文献   

8.
Zeekoevlei is the largest freshwater lake in South Africa and has a century-long history of anthropogenic impact that caused hyper-eutrophic conditions. We used biomarkers (alkanes and pigments), stable isotopes (δ13C and δ15N), rates of primary palaeoproduction and total inorganic carbon (TIC) accumulation rates in the lake sediments to investigate changes in plankton and macrophyte communities in response to anthropogenic activities in this shallow lake. Specific alkanes (ΣC15,17,19, pristane, phytane and n-C29/n-C17 ratio) and pigment (chlorophyll a, β,β-carotene, echinenone, fucoxanthin and zeaxanthin) concentrations in lake waters indicated the present-day hyper-eutrophic condition and seasonal fluctuations of cyanobacteria, zooplankton and diatom populations. Eutrophic conditions were initiated in the lake with the start of recreational activities and construction of a sewage treatment plant in the early 1920s. The lake transformed from a eutrophic to a hyper-eutrophic waterbody following damming, pondweed eradication and accelerated catchment-derived nutrient input. The change in lake trophic state was recorded by a sharp decline in the terrestrial to aquatic ratio (TAR) of specific n-alkanes, low carbon preference index (CPI) and increased δ13C values in the sediment core. In addition, the aquatic macrophyte n-alkane proxy (Paq) values (~1) indicated a slow takeover by floating macrophytes after the eradication of submerged pondweeds in 1951. Elevated n-alkane (ΣC15,17,19), total alkane and pigment (chlorophyll a, β,β-carotene, zeaxanthin and zeaxanthin to β,β-carotene ratio) concentrations, low δ15N values and low TIC accumulation rates in the upper middle section of the core indicated the beginning of intense cyanobacterial blooms after the dredging in 1983. Although the cyanobacterial population has decreased in recent years, hyper-eutrophic conditions are reflected by low CPI <0.04 and TAR <1 values at the top of the sediment core.  相似文献   

9.
We present a Holocene record of climate and environmental change in central New York (USA) inferred using lithologic and stable isotope data from two sediment cores recovered in Cayuga Lake. The record was divided into three intervals: (1) early Holocene (~11.6–8.8 ka), (2) Hypsithermal (~8.8–4.4 ka), and (3) Neoglacial (~4.4 ka to present). The early Holocene began abruptly, with rising lake level and relatively deep water. Between ~10.8 and 9.2 ka, cool and dry conditions prevailed at a time of maximum solar insolation. This anomaly has been referred to as the “post-Younger Dryas climate interval” and lasted ~1,600 years, the approximate length of one “Bond cycle.” The Hypsithermal was the warmest, wettest and most biologically productive interval of the Holocene in central New York. The Hypsithermal was characterized by centennial to multi-centennial-scale variability. The 8.2 ka event is one such variation. The Neoglacial was an interval of generally cooler and dryer conditions, falling lake levels, and several prominent climate anomalies. At approximately 2.4 ka, δ13C of bulk organic matter increased abruptly by 5‰ as lake level declined, and the lake flora was dominated by Chara sp. during the coldest interval of the Neoglacial. Numerous sediment variables display increased variability ~2.0 ka, which continues today. Archaeological data from the literature suggest that Native American populations may have been large enough to impact land cover by about 2.4 ka and we hypothesize that the “Anthropocene” began at about that time in central New York. We also found paleolimnological evidence for the Medieval Warm Period (~1.4–0.5 ka), which was warmer and wetter than today, and for the Little Ice Age (~500–150 years ago), a period with temperatures colder than today.  相似文献   

10.
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.  相似文献   

11.
Organic geochemical record of environmental changes in Lake Dianchi,China   总被引：1，自引：0，他引：1  
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 δ13Corg and δ15Ntotal 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.  相似文献   

12.
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 δ13Corganic 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 δ15Norganic 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.  相似文献   

13.
Lacustrine sediments in north-eastern Germany have rarely been used as archives to address the effects of climate change and human impact on both lake ecosystem and landscape evolution for this region. Sacrower See, a hardwater lake located in Brandenburg, provides a unique sediment record covering the past 13,000 years which was used to reconstruct climatic and anthropogenic forcing on lacustrine sedimentation. Time control is provided by 12 AMS 14C dates of terrestrial plant remains, the Laacher See Tephra, and the onset of varve formation in AD 1870 (80 cal. BP). Geochemical (including XRF logging of major elements, CNS analyses as well as δ13Corg and δ15N measurements) and pollen analyses allowed detecting detailed environmental changes in the sediment record. During the Younger Dryas cold phase increased soil erosion and hypolimnetic oxygen depletion enhanced the nutrient supply to the lake water causing eutrophic conditions. The beginning of the Holocene is characterized by large changes in C/N ratios, total sulphur, δ13C of bulk organic matter as well as in K, Si, and Ti, reflecting the response of the lake’s catchment to climatic warming. Reforestation reduced the influx of detrital particles and terrestrial organic matter. The first, rather weak evidence of human impact is documented only in the pollen record at 5,500 cal. BP. However, until 3,200 cal. BP sedimentological and geochemical parameters indicate relatively stable environmental conditions. During periods of intense human impact at around 3,200, 2,800, and 900 cal. BP peaks in Ti and K represent phases of increased soil erosion due to forest clearing during the Bronze Age, Iron Age, and Medieval Times, respectively. In general, greater variation is observed in most variables during these perturbations, indicating less stable environmental conditions. The steady rise of biogenic silica accumulation rates during the Holocene reflects an increasing productivity of Sacrower See until diatoms were outcompeted by other algae during the last centuries. The applied multi-proxy approach fosters the interpretation of the sediment record to reveal a consistent picture of environmental change including environmental factors controlling lake ontogeny and the effects of human impact.  相似文献   

14.
West Hawk Lake (WHL) is located within the glacial Lake Agassiz basin, 140 km east of Winnipeg, Manitoba. The small lake lies in a deep, steep-sided, meteorite impact crater, which has been partly filled by 60 m of sediment that today forms a flat floor in the central part of the basin below 111 m of water. Four cores, 5–11 m in length, were collected using a Kullenberg piston gravity corer. All sediment is clay, contains no unconformities, and has low organic content in all but the upper meter. Sample analyses include bulk and clay mineralogy, major and minor elements, TOC, stable isotopes of C, N, and O, pollen, charcoal, diatoms, and floral and faunal macrofossils. The sequence is divided into four units based mainly on thickness and style of lamination, diatoms, and pollen. AMS radiocarbon dates do not provide a clear indication of age in the postglacial sequence; possible explanations include contamination by older organic inwash and downward movement of younger organic acids. A chronological framework was established using only selected AMS dates on plant macrofossils, combined with correlations to dated events outside the basin and paleotopographic reconstructions of Lake Agassiz. The 822 1-cm-thick varves in the lower 8 m of the cored WHL sequence were deposited just prior to 10,000 cal years BP (∼8,900 14C years BP), during the glacial Lake Agassiz phase of the lake. The disappearance of dolomite near the top of the varved sequence reflects the reduced influence of Lake Agassiz and the carbonate bedrock and glacial sediment in its catchment. The lowermost varves are barren of organisms, indicating cold and turbid glacial lake waters, but the presence of benthic and planktonic algae in the upper 520 varves indicates warming; this lake phase coincides with a change in clay mineralogy, δ18O and δ13C in cellulose, and in some other parameters. This change may have resulted from a major drawdown in Lake Agassiz when its overflow switched from northwest to east after formation of the Upper Campbell beach of that lake 9,300–9,400 14C years ago. The end of thick varve deposition at ∼10,000 cal years BP is related to the opening of a lower eastern outlet of Lake Agassiz and an accompanying drop in West Hawk Lake level. WHL became independent from Lake Agassiz at this time, sedimentation rates dropped, and only ∼2.5 m of sediment was deposited in the next 10,000 years. During the first two centuries of post-Lake Agassiz history, there were anomalies in the diatom assemblage, stable O and C isotopes, magnetic susceptibility, and other parameters, reflecting an unstable watershed. Modern oligotrophic conditions were soon established; charcoal abundance increased in response to the reduced distance to the shoreline and to warmer conditions. Regional warming after ∼9,500 cal years BP is indicated by pollen and diatoms as well as C and O isotope values. Relatively dry conditions are suggested by a rise in pine and decrease in spruce and other vegetation types between 9,500 and 5,000 cal years BP (∼8,500–4,400 14C years BP), plus a decrease in δ13Ccell values. After this, there was a shift to slightly cooler and wetter conditions. A large increase in organic content and change in elemental concentration in the past several thousand years probably reflects a decline in supply of mineral detritus to the basin and possibly an increase in productivity.  相似文献   

15.
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 Ntotal accumulation rates. Larger δ13C and δ15N values in sediments deposited during this period imply higher productivity. Fluctuations in Ntotal and Ptotal 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 Paq 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.  相似文献   

16.
We studied the role that submersed aquatic vegetation (SAV) plays in the sedimentation of organic matter (OM) and phosphorus (P) in Lake Panasoffkee, Florida (USA), a shallow, hard-water, macrophyte-dominated water body. Carbon/Nitrogen ratios (C/N) and stable isotope signatures (δ13C and δ15N) in algae, higher plants, and surface sediments were measured to identify sources of OM to the lake mud. Pollen, plant macrofossils, and geochemistry in sediment cores indicated that primary productivity and SAV abundance in Lake Panasoffkee increased in the late 1800s, probably as a response to increased P loading from human settlement and forest clearance. SAV and associated periphyton served as temporary sinks for soluble P, maintaining relatively clear-water, low-nutrient conditions in the lake. P accumulation in Lake Panasoffkee sediments increased together with indicators for greater SAV presence. This suggests that SAV and associated epiphytes promote P burial and retention in sediments. Although it might be assumed that rooted submersed macrophytes are directly responsible for P uptake from water and transfer to sediments, C/N and stable carbon isotope results argue for the importance of other macrophyte growth forms, and perhaps epiphytic algae, in permanent OM and P sequestration. For instance, high rates of photosynthesis by epiphytes in hard-water systems consume CO2 and promote CaCO3 precipitation. Sloughing of accumulated carbonates from macrophyte leaves transfers epiphytes and associated P to the sediment. Our paleolimnological findings are relevant to restoration efforts in the Florida Everglades and support the claim that constructed SAV wetlands remove P from waters effectively.  相似文献   

17.
Tonle Sap Lake, Cambodia, possesses one of the most productive inland fisheries in the world and is a vital natural resource for the country. The lake is connected to the Mekong River via the Tonle Sap River. Flow in the Tonle Sap River reverses seasonally, with water exiting the lake in the dry season and entering the lake during the summer monsoon. This flood pulse drives the lake’s biological productivity. We used Sr, Nd, and Pb isotopes and elemental concentrations in lake sediment cores to track changes in the provenance of deposits in Tonle Sap Lake. We sought to determine when the lake first began to receive water and sediment input via the Mekong River, which initiated flood pulse processes. The transition from a non-pulsing lake to the Mekong-connected system is marked by shifts to values of 87Sr/86Sr, εNd, and 207Pb/204Pb that are characteristic of Mekong River sediments. In addition, magnetic susceptibility increased and sediment elemental composition changed. Elemental (P) measures point to enhanced phosphorus loading and C/N and isotope ratios of bulk organic matter indicate a shift to greater relative contribution of organic material from aquatic versus terrestrial environments, coinciding with the initiation of flood pulse processes. On the basis of radiocarbon dating in two cores, we estimate the initiation of the annual flood pulse occurred between ~4,450 and 3,910 cal year BP.  相似文献   

18.
We investigated how the history of local disturbances in a watershed can influence the regional coherence of ecosystem properties in lakes that have similar morphometry and climatic conditions. We measured sedimentary δ13C, δ15N, C:N and %BSiO2 in Sooke Lake Reservoir (SOL) and Shawnigan Lake (SHL), which are located within 4 km of each other on Vancouver Island, Canada. SOL is an impounded lake whose watershed has been fully protected over the last century, although the lake level has been raised 3 times via impoundment during this period. SHL has a similar limnological regime, but the surrounding watershed has been developed extensively for residential uses. We investigated how a pulse disturbance regime in SOL (i.e. repeated dam raising) and a press disturbance regime in SHL (i.e. persistent development) influenced the variability of paleoindicators in each system over time. We found that these contrasting disturbance regimes reduced the regional temporal coherence of aquatic productivity between the two lakes (indicated by %BSiO2), but did not influence the regional coherence of nutrient status or the main carbon sources of the lakes (indicated by %C, %N and δ13C). In contrast, an indicator of the sources and cycling of nitrogen (δ15N) showed increased coherence. Local disturbances also affected the variability of the paleoindicators within each system over time. In SOL, impoundments led to both declines (%N, δ15N) and increases (δ13C) in the variability of paleoindicators. In SHL, persistent watershed development led to lower variability of two paleoindicators (%N, %BSiO2). Overall, our data suggest that local disturbances can influence the %BSiO2 and C:N ratio of lake sediments, but are less likely to alter the regional coherence of %C, %N and δ13C between lakes. Biplob Das and Anita Narwani contributed equally to this work.  相似文献   

19.
Total organic carbon (TOC) content, total nitrogen (TN) content, stable nitrogen isotope (δ15N) and stable organic carbon isotope (δ13Corg) 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 δ13Corg 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 (δ13Corg, 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.  相似文献   

20.
Stable isotope analyses on cladoceran subfossil exoskeletons retrieved from sediment cores could allow the reconstruction of past changes in lake food webs provided the δ13C and δ15N values of the exoskeletons reflect those of the organisms’ whole body. The relationships between the C and N stable isotope compositions of the exoskeletons and those of the whole body were investigated for two freshwater cladoceran taxa (Bosmina sp. and Daphnia sp.) from modern samples. The C and N stable isotope compositions of the exoskeleton and those of the whole body were strongly correlated. Exoskeleton δ13C was similar to the whole body δ13C for both taxa. Daphnia exoskeletons were strongly depleted in 15N (−7.9‰) compared to the whole body. Stable isotope analyses were thereafter performed on cladoceran remains from five downcore samples from Lake Annecy, France. Results showed that Bosmina δ15N values increased by more than 4‰, between the early twentieth and twenty first centuries. Such changes might be the result of changes in nitrogen sources or cycling in the lake and/or of major shifts in Bosmina trophic position within the lake food web. This study sets up the potential of stable isotope analyses performed on cladoceran subfossil remains for paleo-ecological purposes.  相似文献   

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