Redox remobilization and the heavy metal record in lake sediments: a modelling approach

The issue of whether heavy metal concentration profiles in lake sediments are vulnerable to redox mediated post-burial alteration stubbornly refuses to go away. Such behaviour is well-known in iron and manganese, due to great contrasts in the solubility of the reduced and oxidized chemical forms. A numbers of trace elements that also exhibit more than one oxidation state in lake sediments have also been shown to undergo diffusive migration in response to redox gradients (e.g., As, Co, Cr, V). What remains unclear is whether elements like Cd, Cu, Pb and Zn, that have only one stable oxidation state in sediments, are also subject to alteration. This paper reviews existing evidence and presents a model, based on current knowledge, to constrain the likely effects. It is concluded that some migration is likely, but that this is only significant at extremely low sediment mass accumulation rates. The formulation of the model highlights the need for further information about the physical form of iron and manganese oxyhydroxides in lake sediments.     

Within-lake variability of subfossil chironomid assemblages in shallow Norwegian lakes

Subfossil chironomids in the surface sediments of five small and shallow Norwegian lakes were studied to determine the within-lake variability of fossil assemblages, changes in chironomid assemblages with respect to water depth, and the representativeness of single samples for the entire chironomid fauna of a lake. In each of the lakes studied, six short sediment cores in the deepest part of the lake basin and two littoral to deep-water transects of seven cores each were obtained using a gravity corer, and chironomid assemblages in the uppermost centimetre of sediment were analysed. In three of the five lakes, chironomid concentrations were highest in the deepest parts of the lake basins. In the remaining two lakes, concentrations were either very variable or, in a lake with clear indications of anoxia in the bottom waters, highest at intermediate water depth. Chironomid assemblages tended to be dominated by the same taxa within a lake basin. However, in each of the lakes studied there was a clear and statistically significant shift in chironomid assemblages with respect to water depth. The organic content of the sediments was statistically significant in explaining the variance in the chironomid assemblages only in lakes where organic matter content was closely related to water depth. Only a few chironomid taxa were restricted to the shallowest parts of the lake basins, whereas a number of chironomids were found exclusively in deep-water sediments. Chironomid head capsules of running water taxa and simuliid remains were generally found in sediments close to lake tributaries and in the deepest parts of the lake basins. Although any individual sample contained only a part of the total subfossil chironomid fauna (21–63% of the total taxa per lake), chironomids dominant in any section of the study lakes were found in most of the transect and mid-lake samples.     

Porewater geochemistry and solute flux from bottom sediments,Devils Lake,North Dakota

Sediment cores were collected for pore-water analysis from the eastern end of Devils Lake, located in northeastern North Dakota, to determine diagenetic reactions occurring in surficial bottom sediments and to evaluate the impact of these reactions on chemical concentrations in the overlying lake water. Sediment pore waters are enriched in major ions and nutrients relative to lake water. The principal sources of major ions to pore water are saline sediments located in the upper 1 m of bottom sediment. The principal source of titration alkalinity and nutrients to pore water is microbial decomposition of sedimentary organic matter by sulfate reduction. Sediment pore waters in the eastern part of Devils Lake have higher major-ion concentrations and solute-flux rates than the sediment pore waters in the central part of the lake. In contrast, sediment pore waters in the central part of Devils Lake have significantly higher nutrient concentrations and solute-flux rates. Major-ion concentrations and solute-flux rates in sediment pore water increase from west to east. These trends indicate that bottom-sediment diagenetic processes are, in part, responsible for the observed concentration gradient in the lake. The higher nutrient concentrations and the higher nutrient diffusional-flux rates in Main Bay are the result of more labile sedimentary organic matter and the occurrence of sulfate reduction. Environmentally-reactive trace-metal concentrations (Cu, Pb, Zn, and Fe) in bottom sediments decrease from west to east with distance from the surface-water sources and with increasing surface-water salinity.     

Bacterial dominance of phototrophic communities in a High Arctic lake and its implications for paleoclimate analysis

The phototrophic communities in meromictic, perennially ice-covered Lake A, on Ellesmere Island in the Canadian High Arctic, were characterized by pigment analysis using high performance liquid chromatography. Samples were taken to determine the vertical changes down the water column as well as a variation between years. These analyses showed that Lake A had distinct phototrophic communities in its oxic and anoxic layers. The pigment analyses indicated that phototrophic biomass in the upper, oxic waters was dominated by picocyanobacteria, while in the lower, anoxic layer photosynthetic green sulphur bacteria were dominant. Interannual variation in pigment concentrations was related to the penetration of photosynthetically active radiation in the water column, suggesting that light availability may be limiting the net accumulation of photosynthetic bacterial biomass in Lake A. Pigment analysis of the surface sediments indicated that deposition was dominated by the photosynthetic sulphur bacterial contribution. The sedimentary record of bacterial pigments in polar meromictic lakes offers a promising tool for the reconstruction of past changes in ice cover and therefore in climate.     

The drowning of a siliciclastic shelf: insights into oceanographic reconstructions of the northern Arabian Platform during the Early Cretaceous

Barremian‐Aptian sedimentary successions along the northern Arabian margin have been described as a transition from a siliciclastic to a carbonate‐dominated marine environment, deposited upon a low‐relief shelf or platform formed as a consequence of continuous regional subsidence. A long (360 m) core from northern Israel offers a unique look at this transition, providing valuable insights for the palaeoceanography, geometry and ventilation conditions that lead to Oceanic Anoxic Event 1 (OAE1) in this region. Results from high‐resolution elemental, mineralogical, sedimentological and petrophysical analyses carried out revealed the emplacement of abundant mass‐transport deposits (MTDs) during the Late Barremian and the Aptian. The transplanted units are characterized by fine grained calcareous shales with elevated organic matter, sulphur and iron contents. The scarcity or absence of bioturbation in the disturbed sequences provides a hint to the sediment/water interface conditions. However, a decrease in sulphur and iron occurring at the contact between the shales and the MTDs is explained as increased oxic conditions at the sediment‐water interface as a result of turbulence and mixing associated with the descending sediment masses. Such recurrent events ventilation of the low‐energy basinal environment during the Late Barremian and Aptian, predate the wide‐scale establishment of OAE1 in the northern Arabian margin. Moreover, the identification of coarse‐grained MTDs within deep‐water calcareous sediments indicates a much steeper gradient of the northern Arabian margin, challenging previous studies.     

Controls on loss-on-ignition variation in cores from two shallow lakes in the northeastern United States

Loss-on-ignition analysis of Quaternary lake sediments provides an inexpensive and easy way to investigate past environmental changes. The mass loss on ignition at 550 °C (LOI) from lake sediment cores may vary because of temporal changes in: (1) sediment composition controlled by factors such as productivity, inorganic inputs, and decomposition; and (2) the patterns of sediment accumulation controlled by factors such as basin morphology and water level. Climatic changes can alter both. Here, modern surface samples and transects of sediment cores, collected across small (<10 ha), shallow (<4 m) lakes in the northeastern United States, show that LOI varies little (2–5%) across the deep portions of these small lakes at a given time. Large changes in LOI occur only at the transition into the littoral (shallow) zone. LOI variations in sediment cores that exceed 2–5%, therefore, appear to represent meaningful environmental changes. However, because of the many possible controls, changes in the LOI of a single core are often hard to interpret. Multiple cores increase the interpretability. At lakes studied here, similar LOI trends among several cores confirm that some LOI changes resulted from basin-wide shifts in sediment composition. Differences among cores, however, developed during the early- and mid-Holocene and indicate that the edge of the littoral zone moved towards the centers of the lakes during two periods of low lake levels, at ca. 11 000–8000 and ca. 5400–3000 cal yr B.P. The basin-wide balance of sediment sources controlled the LOI from deep-water sediments, but sedimentation patterns, which changed as lake levels changed, were also important. LOI differences among cores may therefore help identify past lake-level changes in other lakes.     

Geochemical signatures of two different coastal depositional environments within the same catchment

The geochemistry of lake (Renstradträsket) and estuarine (Pieni Pernajanlahti Bay) sediment was investigated in a medium sized watershed draining to the Gulf of Finland, Baltic Sea. Catchment land-use types were compared and found similar. Sediment cores were dated using ²¹⁰Pb- and ¹³⁷Cs-chronologies and analyzed for Al, K, Cu, Zn, Fe, Mn, phosphorus fractions, TN, TC and biogenic silica (BSi). Differences between the sediment cores were studied by using linear regression analysis and principal components analysis (PCA). Despite similarities in catchment land-use and history, the sediment geochemical profiles of the sites varied significantly. Some of the differences could be related to differences in chemical sedimentation environment (lacustrine versus estuarine). TP concentration was found to be positively correlated with sediment iron content in estuarine sediment but negatively correlated with Fe in lake sediment. In the estuarine core sedimentary iron was not correlated to lithogenic potassium and aluminum but in the lake core the iron seemed to be lithogenic in origin, as suggested by the strong positive correlations (r ² = 0.95–0.96) between these three variables. Most similarities among the cores were found in Al concentrations. Estuarine nutrient profiles appeared relatively monotonous compared to the lake core. This is probably due to more vigorous mixing of the sediments that may ensure more rapid and complete consumption of the organic matter deposited on the bottom of the estuary. Therefore the lake sediment appeared to preserve the historical record of eutrophication better. Biologically less active and more particle-bound materials like the trace metals Cu and Zn seemed to retain good records of anthropogenic impact also in the estuarine core. The study highlights the need to take the sedimentation environment into account when interpreting geochemical record.     

Historical changes in sediments of Pyramid Lake,Nevada, USA: consequences of changes in the water balance of a terminal desert lake

Sediment cores from the shallow and deep basins of Pyramid Lake, Nevada, revealed variations in composition with depth reflecting changes in lake level, river inflow, and lake productivity. Recent sediments from the period of historical record indicate: (1) CaCO₃ and organic content of sediment in the shallow basin decrease at lower lake level, (2) CaCO₃ content of deep basin sediments increases when lake level decreases rapidly, and (3) the inorganic P content of sediments increases with decreasing lake volume. Variations in sediment composition also indicate several periods for which productivity in Pyramid Lake may have been elevated over the past 1000 years. Our data provide strong evidence for increased productivity during the first half of the 20th Century, although the typical pattern for cultural eutrophication was not observed. The organic content of sediments also suggests periods of increased productivity in the lake prior to the discovery and development of the region by white settlers. Indeed, a broad peak in organic fractions during the 1800's originates as an increase starting around 1600. However, periods of changing organic content of sediments also correspond to periods when inflow to the lake was probably at extremes (e.g. drought or flood) indicating that fluctuations in river inflow may be an important factor affecting sediment composition in Pyramid Lake.     

A minimal lag time and continuous sedimentation in alpine lakes near Mount Rainier, Cascade Range, Washington, USA

Alpine lakes near Mount Rainier are dammed by late-glacial moraines. Multiple cores show that sedimentation started less than 350 years after deglaciation. Numerous tephra layers provide excellent marker horizons that allow comparison of the stratigraphy without the errors associated with radiocarbon dating. Cores from different locations within a lake display the same stratigraphy and basal sediments. Core location therefore is not crucial to obtaining the oldest sediments. Continuous sedimentation in all cores is interpreted as evidence for stable lake levels throughout the Holocene. The cores are valuable tools for reconstruction and dating of glacier advances and volcanic activity.     

Laghi di Monticchio (Southern Italy,Region Basilicata): genesis of sediments—a geochemical study

The sedimentation record of Lago Grande di Monticchio (LGM) is one of the most prominent paleoclimatic archives in the non-glaciated areas of Europe. However, the modern lake system has never been the subject of intense limnological studies. On the basis of hydrochemical water profiles, detailed investigations of sediment short cores and in situ pore water profiles from the littoral to the profundal zone, we elucidate spatial variations of sediment genesis within the lake basin and the importance of various depth sections for the lake’s internal nutrient cycling. Sediments from the smaller meromictic Lago Piccolo di Montichio are discussed as a reference. Our study demonstrates: (i) distinctly higher sediment accumulation for the centre of the lake basin by focussing of the settling particle flux; (ii) decline of carbonate from the littoral to the profundal zones; (iii) non-synchronous change of calcite net-accumulation for various water depths; (iv) exceptionally high cation release from sediments covering the steeply inclining sector of the lake basin; (v) relatively constant dissolved silica concentrations in the pore waters (SiO₂ ∼42 mg/l) independent of water depth and sediment composition; (vi) influx of oxygen-bearing groundwater into the anoxic hypolimnion after heavy rainfall and the associated precipitation of Fe-oxihydroxides; (vii) higher release of NH₄ by anaerobic degradation of organic matter at a water depth of 23 m than for sediments at a maximum water depth of 32 m, whereby the latter reflects the importance of seasonal sediment re-oxidation for anaerobic degradation of organic debris; (viii) although seasonal re-oxidation of sediments from various water depths is quite different, Oxygen Index values of LGM sediments fall in a small range, which reflects rapid microbial consumption of seasonally re-generated easily bio-degradable organic molecules.     

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

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

A 1500-year record of climatic and environmental change in ElkLake, Clearwater County, Minnesota II: geochemistry, mineralogy, and stableisotopes

Most of the sediment components that have accumulated in ElkLake, Clearwater County, northwestern Minnesota, over the past 1500 years areauthigenic or biogenic (CaCO₃, biogenic SiO₂, organicmatter, iron and manganese oxyhydroxides, and iron phosphate) and are deliveredto the sediment–water interface on a seasonal schedule where they are preservedas distinct annual laminae (varves). The annual biogeochemical cycles of thesecomponents are causally linked through the carbon pump, and are recapitulatedin longer-term cycles, most prominently with a periodicity of about 400 years.Organic carbon is fixed in the epilimnion by photosynthetic removal ofCO₂, which also increases the pH, triggering the precipitation ofCaCO₃. The respiration and degradation of fixed organic carbon inthe hypolimnion consumes dissolved oxygen, produces CO₂, and lowersthe pH so that the hypolimnion becomes anoxic and undersaturated with respectto CaCO₃ during the summer. Some of the CaCO₃ produced inthe epilimnion is dissolved in the anoxic, lower pH hypolimnion and sediments.The amount of CaCO₃ that is ultimately incorporated into thesediments is a function of how much is produced in the epilimnion and how muchis consumed in the hypolimnion and the sediments. Iron, manganese, andphosphate accumulate in the anoxic hypolimnion throughout the summer.Sediment-trap studies show that at fall overturn, when iron-, manganese-, andphosphate-rich bottom waters mix with carbonate- and oxygen-rich surfacewaters, precipitation of iron and manganese oxyhydroxides, iron phosphate, andmanganese carbonate begins and continues into the winter months.Detrital clastic material in the sediments of Elk Lake depositedover the last 1500 years is a minor component (<10% by weight) that ismostly wind-borne (eolian). Detailed analyses of the last 1500 years of the ElkLake sediment record show distinct cycles in eolian clastic variables (e.g.aluminum, sodium, potassium, titanium, and quartz), with a periodicity of about400 years. The 400-yr cycle in eolian clastic material does not correspond tothe 400-yr cycles in redox-sensitive authigenic components, suggesting that theclastic component is responding to external forcing (wind) whereas theauthigenic components are responding to internal forcing (productivity),although both may ultimately be forced by climate change. Variations in theoxygen and carbon isotopic composition of CaCO₃ are small but appearto reflect small variations in ground water influx that are also driven byexternal forcing.     

Multi-proxy Reconstruction of Trophic State,Hypolimnetic Anoxia and Phototrophic Sulphur Bacteria Abundance in a Dimictic Lake in Northern Germany over the past 80 Years

During monthly investigations from 1996 to 2000, a hypolimnetic layer of phototrophic sulphur bacteria (Chromatium spp.) were observed in Lake Dudinghausen, a small dimictic lake in northern Germany. This paleolimnological study was initiated to detect if the occurrence of sulphur bacteria was related to cultural eutrophication or reflected natural conditions. Therefore, diatoms, algal pigments, okenone, geochemical proxies, and ²¹⁰Pb and ¹³⁷Cs were used in four sediment cores to investigate historical changes in trophic development, hypolimnetic redox conditions, anoxia and phototrophic sulphur bacteria abundances. Fossil diatoms, pigments, the ratio of chlorophyll derivatives to total carotenoids and the ratio of chlorophyll a to its derivatives suggest two phases of eutrophication coupled with hypolimnetic anoxia over the last ~80 years: a first phase from about 1923–1932 and a second from 1952 to 1982. In the first phase the ratios of Fe–Mn as well as Fe–Ca increased, suggesting seasonal anoxia. However, hypolimnetic anoxia was only weak because low levels of okenone suggest no mass development of sulphur bacteria. In contrast, sulphur bacteria increased during the early stages of the second eutrophication phase, suggesting increased temporal and spatial hypolimnetic anoxia. Surprisingly, the ratios of Fe–Mn as well as Fe–Ca decreased during this time. Possibly Fe, Mn and Ca were equally reduced through the intense anoxia. In the final stage, sulphur bacteria decreased again. As these bacteria need both anoxic conditions and a certain amount of light, the increased nutrient load probably led to low Secchi depth and therefore insufficient light conditions. In more recent years, diatoms and pigments suggest a decrease in nutrient levels. A second mass development of sulphur bacteria occurred, probably due to improved light conditions and continued anoxia in the upper hypolimnion. We conclude that the recent development of phototrophic sulphur bacteria do not represent natural conditions in Lake Dudinghausen. Furthermore, the upper sediments contain a completely new diatom flora that never occurred in older sediments of Lake Dudinghausen. Therefore, nutrient levels may eventually reach natural conditions, however they may not represent biological background reference conditions.     

Arsenic contamination of lake sediments in Florida: evidence of herbicide mobility from watershed soils

Organic arsenical herbicides, which include monosodium methylarsonate (MSMA), have been applied to golf courses and lawns throughout Florida, USA, since the 1950s. These products convert rapidly to inorganic forms of arsenic (As) in soils and are mobilized readily. Leachates have been known to contaminate groundwater and surface waters, although past studies have not examined whether use of these products has led to significant As accumulation in lake sediments. We used paleolimnological methods to document the depositional history and inventories of total As in sediments and porewaters of Little Lake Jackson in Florida, which is adjacent to three golf courses. Six sediment cores, four of which were ²¹⁰Pb dated, showed porewater total As concentrations as high as 435 μg l⁻¹, and dry-sediment total As concentrations as high as 148 mg kg⁻¹. Approximately 537 kg of total As is present in >19,000 metric tons of sediment (dry mass), and an additional 18 kg of As is dissolved in 10.8 × 10⁴ m³ of porewaters. Total As content in surface sediments (mean = 47.3 mg kg⁻¹) exceeds the consensus-based sedimentary concentration for probable toxicity effects in freshwater benthic fauna. Surface and subsurface waters flow to the lake from topographically higher areas to the west, where golf courses and residential areas are located. Total As concentrations were elevated highly in monitoring wells and in a stream that flows between the golf courses and lake, but As was below detection limits in wells that were located at the distal perimeter of the golf courses. Subsurface and surface waters exit the lake towards topographically lower areas to the east. Nearly all As in sediments remains bound in the solid phase, indicating that As sedimentary profiles largely reflect depositional history. Sedimentary As concentrations are correlated strongly with aluminum and iron, which suggests that As was scavenged from lake waters during the past. Sedimentary As concentrations increased until the 1980s, then declined somewhat to the present time. Dissolved As was scavenged efficiently from the water column when hypolimnetic waters were oxygenated persistently, but after eutrophication led to a seasonally anoxic hypolimnion in the 1980s, apparently less As was co-precipitated, and more was lost to hydrological outflow. Arsenic accumulation in sediments might be common in areas where As derived from organic arsenical herbicide applications is directed by shallow water tables towards adjacent lakes.     

Observations on the diagenetic behaviour of arsenic in a saline lake: Pyramid Lake,Nevada

We present, to our knowledge, the first dissolved arsenic (As) data from the pore waters of a closed-basin, saline lake, Pyramid Lake, USA. The As concentrations first decrease across the sediment-water interface to values below lake water concentrations and then rapidly increase to values as high as 5.85mol L^–1 at a depth of 15 cm. This maximum value is reached at the top of the sulfate reduction zone. The profile suggests that As is remobilized in the suboxic and anoxic zones of the sediments and is reprecipitated near the sediment-water interface under oxic conditions.     

Variability of diatom-inferred phosphorus profiles in a small lake basin and its implications for histories of lake eutrophication

Diatom analyses were undertaken of sediment cores covering a range of water depths in a small eutrophic lake (Lough Augher, Co. Tyrone, N. Ireland). The significance of between-core variability in diatom relative frequency stratigraphy was assessed by Canonical Correspondence Analysis (CCA) where the ordination axes were constrained to external environmental variables (sediment depth, core location coordinates, water depth, effective fetch, distance-from-shore and distance-from-inflow). After the removal of the effect of sediment age by partialling it out, the resultant first two axes from the partial-CCA were significantly correlated with water depth and distance-from-shore, indicating non-uniform diatom stratigraphies across the lake. Despite this variability, all cores show the same succession of species and, therefore, record the eutrophication of the lake. Diatom-inferred total phosphorus (DI-TP) was inferred for six cores using weighted averaging regression and calibration. Apart from considerable differences of DI-TP in surficial sediment samples, there was good between-core repeatability of DI-TP profiles. These data support the use of DI-TP for establishing background nutrient concentrations for lakes, and associated implications for lake restoration schemes using single cores. Comparisons of DI-TP profiles and total diatom accumulation rate data for the individual cores indicate that diatom production peaked prior to the maximum TP concentrations in the lake.     

A rock magnetic record from Lama Lake, Taymyr Peninsula, northern Central Siberia

An 11 m long sediment core ftorn Lama Lake, Northern Siberia, has been subjected to intense sedimentological, geochemical and rock magnetic analyses. According to a palynologic investigation the recovered sediments cover the whole Holocene and the late Pleistocene reaching back to about 17 ka. IRM acquisition experiments, hysteresis loop and back field as well as thermomagnetic measurements revealed magnetite in the pseudo-single domain range as the only remanence carrier. Sharp rock magnetic boundaries occur at 20 and 745 cm sub-bottom depth that are clearly linked to shifts in the median grain size of the magnetite. These boundaries are close to the present boundaries that bracket an anoxic zone between the subrecent and a late Pleistocene oxic section of the sediments. Within the anoxic section, magnetites are characterized by significantly larger median grain sizes but within a very narrow grain size range. The shift from fine grained magnetite within the oxic sediments to coarse grained magnetite is interpreted as the result of dissolution of the finest magnetite grains within the anoxic sediments. A significant shift of the Ti/Fe-ratio of the bulk sediment at a sub-bottom depth of 735 cm does not correspond to thermomagnetic properties, i.e. Curie-temperatures do not follow the variable Ti-content of the sediment.     

Palaeoenvironmental Reconstructions Based on Geochemical Parameters from Annually Laminated Sediments of Sacrower See (northeastern Germany) Since the 17th Century

The history of hardwater lake Sacrower See (Brandenburg, northeastern Germany) was reconstructed back to the 17th century based on a multi-proxy study of five short sediment cores dated by varve chronology, ²¹⁰Pb and ¹³⁷Cs isotopes. We were able to distinguish three main phases: The lake was mesotrophic prior to the 1830s with an oxic hypolimnion. From the early 19th century on, δ¹³C of organic matter indicates that primary productivity starts to increase slowly. Between the 1830s and 1872, the lake went through a transition towards eutrophy. Low calcite contents in the homogeneous sediment are caused by dissolution connected to increasing primary productivity and growing importance of decomposition processes. After 1873, and accelerated since 1963, Sacrower See is characterised by growing nutrient input, and thus further increasing primary productivity. The lake is eutrophic, and decomposition of organic matter causes high oxygen consumption in the hypolimnion, which becomes regularly anoxic during thermal summer stratification. Biogenic varves are preserved in the sediment, characterised by layers of autochthonous, biochemically precipitated calcite crystals. In this study, we were able to demonstrate that Sacrower See is an example of exceptional slow increase of anthropogenically enhanced nutrient input, and of the imprint which these processes have on sediments of a hardwater lake.     

Paleolimnological investigation of the effects of post-1970 reductions of acidic deposition on an acidified Adirondack lake

A 40-cm sediment core from Big Moose Lake (Adirondack Mountains, New York, U.S.A.) was analyzed for recent changes in chrysophyte composition of chrysophyte species to assess if legislated reductions of sulphur emissions have resulted in changes in biological composition and inferred lakewater pH in this lake. This core, taken in September 1994, had a record of stratigraphic changes in chrysophyte assemblages remarkably similar to those in 210Pb-dated sediment cores taken in 1983 and 1988. Abrupt changes in chrysophyte stratigraphy were used to date the newest sediment core by stratigraphic correlation. We observed relatively constant composition of chrysophyte species and inferred-pH values in post-1970 sediments, which suggests that there has been little change in lakewater pH values despite the reductions in sulfate deposition since 1970 in the Adirondacks. These results are consistent with recent monitoring measurements in the Adirondacks. Further research is necessary to ascertain the applicability of these findings to other lakes in the Adirondacks and to determine the reasons for the lack of recovery.