Identifying sources of organic matter in sediments of shallow lakes using multiple geochemical variables

We analyzed ²¹⁰Pb-dated sediment cores from four relatively shallow lakes (z_max < 10 m) in the Upper Ocklawaha River Basin, Florida, USA to compare primary producer community structure before and after anthropogenic impacts. We measured physical and chemical sediment variables including density, organic matter (OM), water-soluble phosphorus, polyphosphate (Poly-P), total P (TP), total carbon to total nitrogen mass ratios of OM (TC:TN), biogenic silica (diatoms, sponge spicules), total amorphous silica, and stable carbon and nitrogen isotope ratios of bulk OM. Principal component analysis showed that diatom biogenic silica, TC:TN, Poly-P and TP displayed discernible stratigraphic changes associated with the shift in the primary producer community. We applied k-means cluster analysis to these variables to identify macrophyte-derived, transitional, and phytoplankton-derived sediments. Our approach provides an objective method for identifying sediment sources that may be applied to shallow lakes in other regions. The four study lakes shifted from a macrophyte-dominated state to a transitional state before major anthropogenic disturbances, and became phytoplankton-dominated after ~1950.     

Determination of carbon, carbonate, nitrogen, and phosphorus in freshwater sediments by near-infrared reflectance spectroscopy: Rapid analysis and a check on conventional analytical methods

Sediments are typically analyzed for C, N, and P for characterization, sediment quality assessment, and in nutrient and contaminant studies. Cost and time required for analysis of these constituents by conventional chemical techniques can be limiting factors in these studies. Determination of these constituents by near-infrared reflectance spectroscopy (NIRS) may be a rapid, cost-effective method provided the technology can be applied generally across aquatic ecosystems. In this study, we explored the feasibility of using NIRS to predict total C, CO₃ ^–2 organic C, N, and P in deep-water sediment cores from four Canadian lakes varying over 19 degrees of latitude. Concentration ranges of constituents in the samples (dry weight basis) were total C, 12-55; CO₃ ^–2, 6-26; organic C, 7-31; N, 0.6-3.1; and P, 0.22-2.1 mg g^–1. Coefficients of determination, r2, between results from conventional chemical analysis and NIR-predicted concentrations, based on calibrations across all the four lakes, were 0.97-0.99 for total C, organic C, and N. Prediction for CO₃ ^–2 was good for the hard water lake from a calibration across all four lakes, but this constituent in the three soft water lakes was better predicted by a calibration across the soft water lakes. The NIR calibration for P fell below acceptable levels for the technique, but proved useful in the identification of outliers from the chemical method that were later removed with the re-analysis of several samples. This study demonstrated that NIRS is useful for rapid, simultaneous, cost-effective analysis of total C, CO₃ ^–2, organic C, N, and P in dried sediments from lakes at widely varying latitudes. Also, this study showed that NIRS is an independent analytical tool useful for the identification of outliers that may be due to error during the analysis or to distinctive composition of the samples.     

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.     

Inputs of dissolved and particulate 226Ra to lakes and implications for 210Pb dating recent sediments

Gamma spectroscopy was used to measure radioisotope (²¹⁰Pb, ²²⁶Ra, ¹³⁷Cs) activities in sediment cores from 20 lakes and a wetland in Florida, USA. Nine profiles display relatively low (<5 dpm g^–1) and constant ²²⁶Ra activities, whereas 12 show high (>5 dpm g^–1) and variable ²²⁶Ra activities. In the latter group, most display up-core increases in activity. Upper sediments from two lakes (Round and Rowell) possess very high (>20 dpm g^–1) ²²⁶Ra activities that exceed total ²¹⁰Pb activities, clearly illustrating disequlibrium between ²²⁶Ra and supported ²¹⁰Pb. Supported ²¹⁰Pb activity is generally thought to come from in situ, ²²⁶Ra-containing detrital mineral particles, and is typically assumed to be in secular equilibrium with ²²⁶Ra activity. Since 1966, Round Lake has been augmented hydrologically with ²²⁶Ra-rich (6.2 dpm L^–1) groundwater pumped from the local deep aquifer. Adsorption of dissolved ²²⁶Ra to recent Round Lake sediments probably accounts for the high measured ²²⁶Ra activities and the pronounced disequilibrium between ²²⁶Ra and supported ²¹⁰Pb in topmost deposits. We suspect that many Florida waterbodies receive some ²²⁶Ra-rich runoff and seepage from groundwater pumped for irrigation, residential use, industrial applications, and mining. This may account for up-core increases in ²²⁶Ra activity measured in sediment cores from some Florida lakes. Significant groundwater pumping began within the last century, and there has been insufficient time for supported ²¹⁰Pb to come into equilibrium with adsorbed ²²⁶Ra in uppermost deposits. Input of ²²⁶Ra-rich groundwater to lakes may occur in any geographic region where local bedrock contains ²³⁸U and its daughters. When dissolved ²²⁶Ra adsorbs to recent sediments, it complicates accurate estimation of supported ²¹⁰Pb activity, and confounds calculation of unsupported ²¹⁰Pb activity that is used in dating models.     

Mechanisms for Organic Matter and Phosphorus Burial in Sedimentsof a Shallow, Subtropical, Macrophyte-Dominated Lake

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.     

Stable isotope (δ13C and δ15N) signatures of sedimented organic matter as indicators of historic lake trophic state

We explored the use of carbon and nitrogen isotopes (¹³C and ¹⁵N) in sedimented organic matter (OM) as proxy indicators of trophic state change in Florida lakes. Stable isotope data from four ²¹⁰Pb-dated sediment cores were compared stratigraphically with established proxies for historical trophic state (diatom-inferred limnetic total phosphorus, sediment C/N ratio) and indicators of cultural disturbance (sediment total P and ²²⁶Ra activity). Diatom-based limnetic total P inferences indicate a transition from oligo-mesotrophy to meso-eutrophy in Clear Lake, and from eutrophy to hypereutrophy in Lakes Parker, Hollingsworth and Griffin. In cores from all four lakes, the carbon isotopic signature of accumulated OM generally tracks trophic state inferences and cultural impact assessments based on other variables. Oldest sediments in the records yield lower diatom-inferred total limnetic P concentrations and display relatively low ¹³C values. In the Clear, Hollingsworth and Parker records, diatom-inferred nutrient concentrations increase after ca. AD 1900, and are associated stratigraphically with higher ¹³C values in sediment OM. In the Lake Griffin core, both proxies display slight increases before ~1900, but highest values occur over the last ~100 years. As Lakes Clear, Hollingsworth and Parker became increasingly nutrient-enriched over the past century, the ¹⁵N of sedimented organic matter decreased. This reflects, in part, the increasing relative contribution of nitrogen-fixing cyanobacteria to sedimented organic matter as primary productivity increased in these waterbodies. The Lake Griffin core displays a narrow range of both ¹³C and ¹⁵N values. Despite the complexity of carbon and nitrogen cycles in lakes, stratigraphic agreement between diatom-inferred changes in limnetic total P and the stable isotope signatures of sedimented OM suggests that ¹³C and ¹⁵N reflect shifts in historic lake trophic state.     

A mineral magnetic and scaled-chrysophyte paleolimnological study of two northeastern Pennsylvania lakes: records of fly ash deposition, land-use change, and paleorainfall variation

A combined mineral magnetic and scaled chrysophyte study of lake sediments from Lake Lacawac and Lake Giles in northeastern Pennsylvania was conducted to determine the effects of land-use and sediment source changes on the variation of pH, conductivity, and alkalinity inferred from biotic changes. Ten 30–40 cm long gravity cores were collected from Lake Lacawac and three from Lake Giles. Isothermal remanent magnetizations (IRMs) were given to the lake sediments in a 1.3 T magnetic field to measure magnetic mineral concentration variations. IRM acquisition experiments were conducted to identify magnetic mineralogy. The bedrock, soils and a peat bog on the shores of Lake Lacawac were also sampled for magnetic analysis to determine possible lake sediment sources. The top 10 cm of sediment collected from Lakes Lacawac and Giles was two to four times more magnetic than deeper sediment. ²¹⁰Pb dating suggests that this intensity increase commenced circa 1900. SEM images of magnetic extracts from the highly magnetic sediments indicates the presence of magnetic fly ash microspheres from fossil fuel burning electric power generation plants. The similarity in magnetic coercivity in the top 8 cm lake sediments and in the peat bog supports an atmospheric source for some of the magnetic minerals in the youngest lake sediments. The highly magnetic sediments also contain an antiferromagnetic mineral in two cores closest to Lake Lacawacs southeastern shore. This magnetic mineral is only present deep in the soil profile and would suggest erosion and significant land-use changes in the Lacawac watershed as another cause for the high magnetic intensities (concentrations) in the top 10 cm of the lake sediments. The most significant changes in the scaled chrysophyte flora occurred immediately above the 10 cm level and were used to infer a doubling of the specific conductivity between circa 1910 and 1929. These variations also support land-use changes in the Lacawac catchment at this time. A similar shift in the scaled chrysophte flora was not observed in the top of Lake Giles, however, distinct changes were found in the deeper sections of the core coupled with a smaller peak in magnetic concentration. Fourier analysis of the ²¹⁰Pb-dated lake sediment magnetics indicates the presence of a 50 year period, low amplitude variation in the Lake Lacawac, Lake Giles, and Lake Waynewood (Lott et al., 1994) magnetic concentration records. After removal of the land-use/fly ash magnetic concentration peak by Gaussian filtering, the 50 year variation correlates strongly from lake to lake even though the lakes are in different watersheds separated by up to 30 km. When this magnetic variation is compared with Gaussian-filtered rainfall variations observed in New York City and Philadelphia over the past 120–250 years there is a strong correlation suggesting that magnetic concentration variations can record regional rainfall variations with an approximately 50 year period. This result indicates that magnetics could be used to document regional variations in climatic change.     

Siliceous microfossil succession in the recent history of two basins in Lake Baikal, Siberia

As part of the international cooperative Baikal Drilling Project, siliceous microfossil assemblage succession was analyzed in two short ( 30-cm) sediment cores from Lake Baikal. One core was recovered from the north basin (Core 324, 55°15N, 109°30E), a second from between the central and southern basins (Core 316, 52°28N, 106°5E). The northern core had higher amounts of biogenic silica (40 g SiO₂ per 100 g dry weight sediment) compared to the southern core, and increased deposition in the more recent sediments. Weight percent biogenic silica was lower in the southern core, ranging from approximately 20–30 g SiO₂ per 100 g dry weight sediment throughout the entire core. Trends in absolute microfossil abundance mirror those of biogenic silica, with generally greater abundance in the northern core (86–275×10⁶ microfossils g^–1 dry sediment) compared to the southern core (94–163×10⁶ microfossils g^–1 dry sediment).Cluster analyses using relative abundance of the dominant diatom and chrysophyte taxa revealed four zones of microfossil succession in each core. Microfossil assemblage succession in the north basin may be reflecting shifts in nutrient supply and cycling driven by climatic changes. The most recent sediments in the northern basin (Zone 1,c. 1890's–1991 A.D.) were characterized by an increased abundance ofAulacoseira baicalensis andAulacoseira spore. Zone 3 (c. 1630's–1830's A.D.) was dominated by the endemicCyclotella spp. and reduced abundance of theAulacoseira spp. Zone 3 corresponds approximately to the Little Ice Age, a cooler climatic period. The microfossil assemblages between Zones 1 and 3 (Zone 2,c. 1830's–1890's A.D.) and below Zone 3 (Zone 4,c. 830's–1430's A.D.) are similar to one another suggesting they represent transitional intervals between warm and cold periods. Southern basin sediments record similar changes in the endemic taxa. However, the increased abundance of non-endemic planktonic taxa (e.g.Stephanodiscus binderanus, Synedra acus, Cyclostephanos dubius) during two periods in recent history (post World War II and late 1700's) suggests evidence for anthropogenic induced changes in southern Lake Baikal.     

Modern and historic accumulation rates of phosphorus in Lake Okeechobee, Florida

Phosphorus accumulation rates in depositional zone sediments of Lake Okeechobee were determined in 11 mud-zone cores and two peat-zone cores dated by ²¹⁰Pb. Although difficulties were encountered in interpreting ²¹⁰Pb data from some sites, reliable dating of sediments from the mud zone of this shallow lake is possible. Sediment accumulation rates in this zone have increased during the present century by an average of about twofold, and accumulation of organic sediments in the lake during pre-settlement times apparently was much slower than during the past century. Concentrations of all forms of sedimentary P but especially nonapatite inorganic-P and organic-P also have increased since pre-settlement times and especially since about 1940. Annual P accumulation rates in the lake's sediments have increased about fourfold during the 1900s, with most of the increase occurring in the past 40–50 years. The recent accumulation rate of sedimentary P (past ~ 10 years) agrees within a factor of 1.5 with the net retention of P in the lake calculated from published input-output mass balances.     

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.     

Spatial pattern of recent sediment and diatom accumulation in a small,monomictic, eutrophic lake

Spatial variability of sediment and diatom deposition was assessed in a small monomictic, eutrophic lake in Northern Ireland (Lough Augher, Co. Tyrone) using measurements from 17 sediment cores. Loss-onignition profiles in water depths >6 m showed good repeatability, while littoral cores were more variable with localised profiles. Dry mass accumulation rates, derived by biostratigraphic correlation to a ²¹⁰Pb dated master core, were variable and not correlated with water depth. Basin mean dry mass accumulation rate was 0.068 g cm^-2 yr^-1 (range 0.036–0.09) prior to 1900, and 0.19 g cm^-2 yr^-1 (range 0.11–0.3) after 1974. Post-1940 cumulative fluxes were estimated for dry mass (range 3.49–916 g cm^-2) and diatoms (range 16.9–113.8×10⁷ frustules cm^-2). Cumulative dry mass was inversely correlated (r=–0.64) with distance from the inflow, indicating its localised influence. No variable was correlated with water depth except frustules of planktonic diatoms (r=0.66). However, high cumulative fluxes of diatoms and dry mass away from the inflow suggest that the expansion of the littoral macrophyte community may be responsible for decreased resuspension in shallow water, and together with increased sediment trapping, has enhanced sediment accumulation in shallow water over recent time periods.     

Anthropogenic impacts recorded in recent sediments from Otisco Lake,New York,USA

Geochemical analysis of ²¹⁰Pb-dated sediment cores from Otisco Lake (New York) combined with analysis of recent land cover change in the watershed revealed the history of land use change, lake management, and industrial pollution since European settlement in the northeastern US. Clearance of forestland for agriculture characterized the early settlement era that was marked in the Otisco Lake sediments by a decline in organic carbon (OC) and OC:N ratios, which indicate a change in the sources of organic matter to the lake. Agricultural land use reached its greatest areal extent in the Otisco watershed around 1900, followed by field abandonment and reforestation over the last century. In the 1920s sediment accumulation rates began to increase coincident with residential development along the lake shore. Nitrogen and organic carbon, which are transported from the watershed, show increased fluxes to the lake in response to this change. Deposition of inorganic carbon increased markedly over a short period from the 1940s to the 1980s, which is consistent with enhanced mobilization of watershed calcium by increased acidic deposition, followed by alkalinization of the lake waters and calcite precipitation. An increase in copper content in the sediments reflects application, since 1942, of the algicide copper sulfate to the lake waters to control algal blooms. This CuSO₄ marker confirmed the accuracy of the ²¹⁰Pb chronology. Atmospheric mercury (Hg) fluxes to the lake were affected by increased sediment transport from the watershed. The maximum Hg peak in the sediment record, however, was dated to the early 1970s and coincides with maximum Hg emissions to the atmosphere in the Great Lakes region.     

Magnetic properties of recent sediments in Lake Baikal, Siberia

Mineral magnetic measurements of six ²¹⁰Pb-dated surface cores from different basins of Lake Baikal, Siberia, show temporal records controlled by a range of internal and external processes. With the exception of sediments on the Academician Ridge, there is clear evidence for widespread reductive diagenesis effects on the ferrimagnetic component coupled with neo-formation of paramagnetic iron minerals. Greigite formation, bacterial magnetosome accumulation and turbidite layers may affect the properties of some sediment levels. Concentrations of canted antiferromagnetic minerals (eg. haematite) appear to increase from the 19th century onwards. These minerals are less affected by dissolution processes and probably represent detrital minerals delivered by catchment fluvial processes. The magnetic evidence for recent atmospheric pollution by fossil-fuel combustion processes is weak in all the cores, and supports the findings from studies of spherical carbonaceous particles (SCPs) and heavy metals that pollution is largely restricted to the southern basin. Correlations between recent sediments based on magnetic data may be insecure over long distances or between basins.     

Interpretation of210Pb profiles and verification of the CRS dating model in PIRLA project lake sediment cores

This paper reports results and analysis of²¹⁰Pb-activity measurements in 51 lake-sediment cores from 32 lakes in the four PIRLA (Paleoecological Investigations of Recent Lake Acidification) project regions (Adirondack Mountains [New York], Northern New England, Northern Florida, and the Northern Great Lakes States). General application of the Constant Rate of Supply (Constant Flux) model for²¹⁰Pb dating is valid for lakes in the PIRLA study, although application of the model is equivocal in a few lakes.²¹⁰Pb inventories and profiles are replicable among closely spaced cores within a lake. Specific²¹⁰Pb activity in surface sediments is negatively correlated with bulk sediment accumulation rate in seepage lakes, but not in drainage lakes. Drainage lakes with lower pH have lower unsupported²¹⁰Pb inventories in sediments, but the relationship does not occur in seepage lakes.²¹⁰Pb profiles in only seven of the cores, all from either the Adirondacks or the northern Great Lakes states, exhibit exponential decay curves. Deviations from an exponential profile include a flattening of the profile in the top few cm or excursions of one or a few measurements away from an exponential curve.²¹⁰Pb dates typically agree with other chronostratigraphic markers, most of which are subject to greater uncertainty. Several hypotheses, including sediment mixing, hydrologic regime, sediment focusing, and acidification, are proposed to explain variation of²¹⁰Pb distribution among lakes and regions. Hydrologic factors exert control on unsupported²¹⁰Pb inventories in PIRLA lakes, and there is a strong focusing effect in drainage lakes but a weak focusing effect in seepage lakes.This is the third of a series of papers to be published by this journal following the 20^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr P. G. Appleby is guest editing this series.     

Sensitivity of tropical-African aquatic invertebrates to short-term trends in lake level and salinity: a paleolimnological test at Lake Oloidien,Kenya

Aquatic invertebrates are intrinsically capable of rapid and sensitive response to changes in their lacustrine habitat. Fossil invertebrate assemblages preserved in the sediments of a climate-sensitive lake can thus produce high-resolution proxy records of past climate. In shallow lakes, however, a potential conflict exists between the sensitivity of biota to frequent habitat change in their fluctuating environment and the increased probability of disturbance of selected proxy records by bioturbation and physical mixing of sediments. I investigated this problem with tropical-African aquatic invertebrate faunas in a paleolimnological sensitivity study that incorporates both the response of biota to short-term habitat change and the taphonomic integrity of fossil assemblages in a small, shallow, and hydrologically closed lacustrine basin. Analysis of chironomid, cladoceran, and ostracode remains in a²¹⁰Pb-dated short core from Lake Oloidien (Kenya) indicates that habitat changes accompanying the late 19th- and 20th-century fluctuations in lake level (Z _max range: 3 to 18 m) and salinity (conductivity range:c. 400 toc. 2000–4000 µS cm^–1) were sufficient to drastically alter the composition of local benthic and planktonic invertebrate faunas. This response remained relatively unaffected by taphonomic phenomena during its incorporation into the sediment record. Results indicate that tropical-African aquatic invertebrate faunas in suitable climate-sensitive lakes are a valuable tool to resolve paleoclimatic fluctuations on a timescale of decades.This paper was presented at the VI Palaeolimnology Symposium, held at Canberra in April 1993. Dr. Mark Brenner guest edited this contribution.     

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.     

Historical loading record of sulfur in an Adirondack Lake

A model for the distribution of sulfur in the sediments of a lake in the Adirondack Mountains, New York, is developed to gain insight into the timing and magnitude of anthropogenic increases in sulfur loading. Surficial sulfur concentrations are about 3500 µg g^–1, increase to 6000–7000 µg g^–1 at a depth of about 6 cm, and then decrease downcore to background levels of about 2500 µg g^–1. Sulfate concentrations are about 60–80 µM in the overlying water and decrease rapidly below the sediment-water interface. This reflects assimilatory reduction and/or dissimilatory reduction of sulfate and incorporation into the inorganic and/or organic solid-phase fractions of the sediment. A mathematical model is constructed assuming sulfur is incorporated into sediments by burial of detrital organic matter and by diffusion of sulfate from the overlying water with subsequent fixation in the sediment. Several historical scenarios of atmospheric sulfate loading rates were examined as model boundary conditions. Model results are compared with observed sedimentary sulfur profiles. The observed sediment sulfur profiles are best described using model boundary conditions showing increases in lake water sulfate concentrations since about 1940. Assuming as much as a decade of retention of sulfate within the terrestrial portion of the ecosystem, this suggests that significant increases in sulfate deposition rates began sometime after 1930 in this relatively remote area.     

A multi-proxy trophic state reconstruction for shallow Orange Lake, Florida, USA: possible influence of macrophytes on limnetic nutrient concentrations

We retrieved four sediment cores from shallow, eutrophic, macrophyte-dominated Orange Lake (A = 51.4 km2, zmax <5 m, zmean < 2 m), north-central Florida, USA. The 210Pb-dated profiles were used to evaluate spatial and temporal patterns of bulk sediment and nutrient accumulation in the limnetic zone and to infer historical changes in lake trophic state. Bulk density, organic matter, total carbon, total nitrogen, total phosphorus and non-apatite inorganic phosphorus (NAIP) concentrations displayed stratigraphic similarities among three of four cores, as did accumulation rates of bulk sediment, organic matter and nutrients. Accumulation rates were slower at the fourth site. Nutrients showed generally increasing rates of accumulation since the turn of the century. Percentages of periphytic diatom taxa increased progressively in the cores after ~ 1930. Diatom-inferred limnetic total P trends were similar among profiles. Eutrophic conditions were inferred for the period prior to the turn of the century. The lake was hypereutrophic in the early decades of the 1900s, but inferred limnetic total P values declined after ~ 1930. Declining inferred limnetic total P trends for the last 60--70 years were accompanied by concomitant increases in accumulation rates of total P and NAIP on the lake bottom. Several lines of evidence suggest that after ~ 1930, phosphorus entering Orange Lake was increasingly utilized by submersed macrophytes. Paleolimnological records from Orange Lake highlight the importance of using multiple sediment variables to infer past trophic state and suggest that aquatic macrophytes can play a role in regulating water-column nutrient concentrations in shallow, warm-temperate lakes.     

How reliable is the210Pb dating method? Old and new results from Switzerland

We present a historical overview of applications of²¹⁰Pb dating in Switzerland with a special emphasis on the work performed at the University of Bern. It is demonstrated that the average specific activity of²¹⁰Pb in the lower atmosphere is very constant and does not show seasonal variations. We then concentrate on new results from Lobsigensee, a very small lake, and on published and new data from Lake Zurich. Several²¹⁰Pb profiles from these lakes show obvious disturbances and a disagreement of the resulting sedimentation rate when compared to that for the 23 years defined by¹³⁷Cs peaks of 1986 (Chernobyl) and 1963 (bomb fallout).A mean sedimentation rate of about 0.14 g cm^–2 y^–1 is found in the oxic and suboxic center part of Lake Zurich. In the oxic locations, the²¹⁰Pb flux to the sediments was close to the atmospheric input of about 1/60 Bq cm^–2 y^–1. In other parts of the lake a significant deficit in the inventory of²¹⁰Pb was found in the sediments. This could be due to a chemical redissolution of²¹⁰Pb together with Mn under reducing conditions. In contrast, in the suboxic part of the lake (135 m depth) the flux of²¹⁰Pb was about twice the atmospheric input. This excess is not caused by allochthonous contributions and is tentatively explained by the transport of sediment material resulting from small slides at the very steep lake shores or more probably by reprecipitation of²¹⁰Pb together with Mn when the conditions in the lake water become locally and seasonally more oxidizing. Dissolved²¹⁰Pb may migrate from locations with reducing conditions and reprecipitate under more oxic conditions. Indeed, a correlation of Mn and²¹⁰Pb in sediments of Lake Zurich was found.This is the first of a series of papers to be published by this journal following the 20^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr P. G. Appleby is guest editing this series.     

A chronological assessment of Lake Okeechobee (Florida) sediments using multiple dating markers

Reconstructing recent limnological history often relies on lead-210 dating to accurately ascribe a chronology to a sediment profile. In Lake Okeechobee, Florida, a large, shallow subtropical lake that may experience severe mixing, multiple dating methods are required to confirm that conformable sedimentation has been preserved and that the assumptions of the ²¹⁰Pb method are satisfied. This study uses stratigraphic profiles of heavy metals, ¹³⁷Cs, PCBs and pollen as independent dating markers to validate the sediment chronology as determined by ²¹⁰Pb for three cores from the central mud zone of the lake. Unsupported ²¹⁰Pb and most dating markers show distinct concentration/depth profiles, suggesting that the sediments have not been severely mixed for at least the last 75 years. Onset and maximum activity of the radioisotope ¹³⁷Cs in the cores coincides with the ²¹⁰Pb-dated interval of 1945–1970. This agrees well with the known timing of atmospheric deposition of ¹³⁷Cs that resulted from above-ground nuclear testing during late 1940s until 1963. Sediment core profiles of atmospherically deposited metals such as Zn and Pb, which reflect regional increases during industrialization and decreases after regulation in the 1970s, exhibit expected concentration increases and peaks coinciding within 5–15 years of the predicted ²¹⁰Pb dates. Uranium, a contaminant in some phosphate fertilizers, shows large concentration increases at core depths dated to be about 1950 by ²¹⁰Pb, matching the intensification of agriculture after WWII. PCBs, which are expected to peak in the early 1970s, were measured in one core, and the observed peak corresponds to a ²¹⁰Pb date of about 1960. Pollen makers were unable to verify specific events, but increases in disturbance taxa and declines in native types correspond generally with the expected dates assigned by ²¹⁰Pb dating. Conformity between the ²¹⁰Pb defined dates and independent markers of < ±15 years confirm that Lake Okeechobee sediments do preserve a sequential and reliable stratigraphic history of the lake, useful for reconstructing past limnological conditions.