Mid-Holocene palaeoclimatic and palaeohydrological conditions in northeastern European Russia: a multi-proxy study of Lake Vankavad

Mid-Holocene changes in vegetation, palaeohydrology and climate were investigated from the sediments of Lake Vankavad in the northern taiga of the Usa Basin, NE European Russia, through the analysis of pollen, plant macrofossils, Cladocera and diatoms. Lake Vankavad was probably formed at ca. 5000 BP (ca. 5600 cal. BP) and initially it was shallow with a littoral cladoceran fauna. Macrofossil and pollen results suggest that dense Betula-Picea forests grew in the vicinity and the shore was close to the sampling point. At ca. 4600 BP (ca. 5400 cal. BP) the water level rose coincident with the decrease in the density and area of forests, probably caused by cooling climate and accelerated spread of mires. There was also a further rise in the water level at ca. 3500 BP (ca. 3800 cal. BP). The initiation of the lake, followed by two periods of rising water-level, as well as the increase in mire formation, was a consequence of a rise in groundwater level. This probably reflects lower evapotranspiration in a cooling mid-Holocene climate and/or higher precipitation in the lowland area. Also the decreased forest density and area may have contributed to the lower evapotranspiration. It is also possible that permafrost aggradation or changes in peat ecosystems might have affected the hydrological conditions in the area.     

Records of environmental and climatic changes during the late Holocene from Svalbard: palaeolimnology of Kongressvatnet

A multi-core, multidisciplinary palaeolimnological study of the partially varved sediment of a deep, meromictic, arctic lake, Kongressvatnet (Svalbard, Western Spitsbergen), provides a record of environmental and climatic changes during last ca. 1800 years. The chronology of sedimentation was established using several dating techniques (¹³⁷Cs, ²¹⁰Pb, varve counts, palaeomagnetic correlation). A multiproxy record of palaeolimnological variability was compiled based on sedimentation rates, magnetic properties, varve thickness, organic matter, geochemistry, pigments from algal and photosynthetic bacteria, mineralogy and biological assemblages (diatoms, Cladocera). The major features recognised in our master core K99-3 include a shift in sediment source and supply (magnetic measurements, geochemistry) probably caused by glaciological changes in the catchment around 38–32 cm core depth (AD 700–820). Additional environmental changes are inferred at 20–18, 8–4.5 and 3–2 cm (AD ca. 1160–1255; 1715–1880; 1940–1963, respectively). During the past ca. 120 years a prominent sedimentological change from brownish-grey, partly laminated silt-clay (varves) to black organic-rich deposits was observed. From AD 1350 to AD1880 the sediment is comprised of a continuous sequence of varves, whereas the earlier sediments are mostly homogeneous with only a few short intercalated laminated sections between AD 860 and 1350. Sedimentation and accumulation rates increased during the last 30 years (modern warming). Pigment concentrations are very low in the lower ca. 32 cm of the core (AD 820) probably because of the high turbidity high energy environment. The high sulphur content in the uppermost 32 cm of sediment has given rise to two horizontally stratified populations of sulphur anaerobic photosynthetic bacteria, as inferred from their specific carotenoids. These bacteria populations are much more abundant during the Little Ice Age (LIA) than during warmer periods (e.g., during the Medieval Warm Period and 20th century). Diatoms are lacking from the core base up to 18 cm (ca. AD 1255); at this level, species indicative of mesotrophic water are present, whereas from 17 cm to the top of the core, oligotrophic taxa such as Staurosira construens/S. pinnata complex dominate, indicating extended ice coverage and more oligotrophic waters during the LIA. The concentration of Cladocera subfossil remains (dominated by Chydorus) are relatively high in the deepest sections (54–32 cm), whereas the upper 32 cm are characterized by a very low concentration of remains, possibly because of the strongly anoxic conditions, and in this upper sediment section rotifer resting eggs become prevalent. We interpret these changes as responses to climate forcing through its impact on glacial melt water, lake ice cover duration and mainly redox conditions in deep water. The observed changes suggest that at least some of our recorded changes may parallel the Greenland Ice core, although our study added more details about the inferred climatic changes. Further aspects are discussed, such as catchment processes, glacial activity, duration of the Medieval Warm Period, the Little Ice Age, local human activity, and limnology.     

Late Glacial and Holocene environmental history of the Pirin Mountains (SW Bulgaria): a paleolimnological study of Lake Dalgoto (2310 m)

Diatoms, Cladocera, and chironomids preserved in the sediments of Lake Dalgoto were studied to reconstruct the history of the lake ecosystem in the context of the vegetation history as represented by the pollen stratigraphy. Younger Dryas silty sediments at the base of the core are characterized by low diversity of aquatic organisms. The transition to the Holocene is indicated by a sharp change from silt to clay-gyttja. The migration and expansion of trees at lower elevations between 10200 and 8500 ¹⁴C-yr BP, along with higher diversities and concentrations of aquatic organisms and the decreased proportion of north-alpine diatoms, point to rapidly rising summer temperatures. After 6500 ¹⁴C-yr BP the expansion of Pinus mugo in the catchment coincides with signs of natural eutrophication as recorded by an increase of planktonic diatoms. In the late Holocene (4000–0 ¹⁴C-yr BP) Pinus peuce and Abies are reduced and Picea expands. Cereal grains and disturbance indicators suggest late-Holocene human modification of the vegetation.     

Holocene paleoclimate changes determined using diatom assemblages from Lake Long,King George Island,Antarctica

Formation of Lake Long, King George Island, Antarctica started about 4,000 years B.P., after which the diatom community changed in response to environmental shifts driven by climatic oscillations (warm/wet and cool/dry). Successive sequences of diatoms in a 7.5-m drill core were divided into 11 assemblage zones by cluster analysis. The most obvious change was an alternation of major dominants, Achnanthes minutissima, Fragilaria alpestris and Fragilaria pinnata v. antarctica according to the climatic oscillations in the late Holocene. Variations in diatom assemblages clearly reflect two warm periods, a single cool period, and three transition periods. The recent warm period (zones 2 and 1) has persisted for approximately 450 years, perhaps sufficiently long to suggest the imminent onset of a new transition period. A recent high TOC (total organic carbon) value in the core reflects a warm period in Antarctica during the late Holocene.     

Holocene environmental history in northwest Finnish Lapland reflected in the multi-proxy record of a small subarctic lake

A 2.5-m-long sediment core was retrieved from Lake Somaslampi, a small lake located in a kame field on the north slope of the Scandes Mountains in Finnish Lapland. Holocene environmental changes were inferred from the lithological, geochemical, pollen, diatom and Cladocera records stored in the lake sediment. The chronology was based on six radiocarbon AMS dates supported by a palynological control chronology. The sediment profile consists of a glacial sedimentary sequence truncated by a lacustrine one. A hiatus, tentatively correlated with climate cooling and advances of glaciers during the 8.2 ka yrs BP “Finse cooling Event”, occurs between these sequences. The glacial sequence was composed of fluvioglacial clastics, smoothly changing into glacio-lacustrine diatomaceous ooze deposited in a meromictic proglacial lake that covered the kame field. The meromixis was probably caused by the greater depth of the lake, the extended ice-cover, and the microbial mats covering large areas of the lake bottom. A distinct change in the biota of the glacio-lacustrine sediments indicates higher trophic conditions than during deposition of the fluvioglacial clastics. The late-Pleistocene vegetation was characterised by subarctic birch tundra vegetation (Betula–Salix–Ericaceae) with low biodiversity gradually changing to Betula–Pinus dominance in the early Holocene. The lake was deep and had a diatom inferred pH ~ 7 indicated also by the dominance of planktonic Cladocera. The base of the lacustrine sediment sequence (6,650–6,300 cal. BP) consisted of loess-rich sediment indicating an increase in eolian activity. This is also supported by the pollen record, which is dominated by more long-distant taxa such as Alnus and Pinus, and by the increased C/N ratio of the sediment. After the initial meromictic phase of the lake, an abrupt lowering of the water level occurred. Lake Somaslampi was isolated from the larger Pre-Lake Somas basin and became holomictic, shallow, much warmer and more productive, until the deterioration of climate around 3,000 yr BP and the increased input of clastics from the tundra soils. The vegetation followed the general climatic trend by gradually changing from the dominance of Betula and Pinus to the dominance of more tundra-related vegetation like Poaceae and Cyperaceae. However, the higher frequencies of planktonic Cladocera and centric diatoms in the most recent sediments indicates higher trophic conditions, increased turbulence and a prolonged ice-free period, which can possibly be linked to the recent climate warming especially in areas of higher altitude and latitude.     

Seasonal variability of Holocene climate: a palaeolimnological study on varved sediments in Lake Jues (Harz Mountains, Germany)

Studies combining sedimentological and biological evidence to reconstruct Holocene climate beyond the major changes, and especially seasonality, are rare in Europe, and are nearly completely absent in Germany. The present study tries to reconstruct changes of seasonality from evidence of annual algal successions within the framework of well-established pollen zonation and ¹⁴C-AMS dates from terrestrial plants. Laminated Holocene sediments in Lake Jues (10°20.7′ E, 51°39.3′ N, 241 m a.s.l.), located at the SW margin of the Harz Mountains, central Germany, were studied for sediment characteristics, pollen, diatoms and coccal green algae. An age model is based on 21 calibrated AMS radiocarbon dates from terrestrial plants. The sedimentary record covers the entire Holocene period. Trophic status and circulation/stagnation patterns of the lake were inferred from algal assemblages, the subannual structure of varves and the physico-chemical properties of the sediment. During the Holocene, mixing conditions alternated between di-, oligo- and meromictic depending on length and variability of spring and fall periods, and the stability of winter and summer weather. The trophic state was controlled by nutrient input, circulation patterns and the temperature-dependent rates of organic production and mineralization. Climate shifts, mainly in phase with those recorded from other European regions, are inferred from changing limnological conditions and terrestrial vegetation. Significant changes occurred at 11,600 cal. yr. BP (Preboreal warming), between 10,600 and 10,100 cal. yr. BP (Boreal cooling), and between 8,400 and 4,550 cal. yr. BP (warm and dry interval of the Atlantic). Since 4,550 cal. yr. BP the climate became gradually cooler, wetter and more oceanic. This trend was interrupted by warmer and dryer phases between 3,440 and 2,850 cal. yr. BP and, likely, between 2,500 and 2,250 cal. yr. BP.     

Phytoplankton response to climate changes in Lake Baikal during the Holocene and Kazantsevo Interglacials assessed from sedimentary pigments

Lake Baikal, an ancient pristine lake in Siberia, has accumulated sediment deposits that span 25 million years. These deposits have the potential to provide a long-term record of climate changes and their interaction with the ecology of the lake. In order to investigate whether sedimentary phytoplankton pigments could be used to reconstruct past changes in total phytoplankton abundance and productivity, we compared the spatial variability in sedimentary pigment distributions in Holocene cores from three separate regions of the lake; Vidrino in the south, Posolski on Selenga Delta and Continent Ridge in the north. Furthermore, we present the first continuous sedimentary pigment and organic carbon sequence of the Kazantsevo interglacial (roughly a time equivalent to the European Eemian, and Marine Isotopic Stage MIS5e) at a resolution of approximately 150 years. Results of the spatial study showed marked differences in the sediment pigment deposition. Lowest chlorophyll a plus its degradation products versus organic carbon ratios (Chlas/TOC) indicating lowest production, but highest variability with time (indicating strongest climatic oscillations) were found at Continent Ridge. The study of sedimentary pigments deposited during the last two interglacial periods at Continent Ridge showed Chlas/TOC ratios 50–1000 times higher during the Kazantsevo Interglacial compared to the glacial periods, whereas the TOC content was only five times higher, thus indicating the significance of the Chlas/TOC ratio for the reconstruction of the phytoplankton abundance and productivity. Strong oscillations occurred during the Kazantsevo Interglacial within centennial time scales. Chlorophyllb plus its degradation products provided additional information on the past development of Chlorophyceae. Highest Chlas/TOC ratios were found during the early Holocene at approximately 9 kyr BP. Indications of short phytoplankton production maxima were also found during the late Atlantic (6 kyr BP) and at the Subboreal/Subatlantic transition (3 kyr BP). From this we conclude that sedimentary chlorophyll a is a reliable indicator of phytoplanktonic response to climate changes and may serve for␣validation of future climate scenarios in continental regions.     

A Late glacial and Holocene diatom record from Clettnadal, Shetland Islands, northern Scotland

The study was undertaken as part of a wider palaeoecological investigation of Late glacial and Holocene lake sediments from a site on the exposed Atlantic coast of the Shetland Islands. The diatom data presented here define a sequence of assemblages, commencing at c. 15.8 cal ka BP, which reflects lithological variation in the section, in particular the Late glacial alternation of minerogenic and more organic horizons. Cliff retreat caused drainage of the lake sometime after c. 4.0 cal ka BP. Almost all taxa recorded are small benthic and tychoplanktonic diatoms: Fragilaria (sensu lato), Achnanthes (s.l.) and some Navicula spp. predominate in the Late glacial. Different benthos become dominant in the Holocene, but no plankton developed. Stauroforma was the commonest genus present, and results indicate a relationship between the occurrence of two types, Stauroforma A and Stauroforma B, and the severity of prevailing environmental conditions. The lithology and associated assemblages suggest a sequence including the classic north European Bølling and Allerød' warmer periods, followed by the Loch Lomond Stadial. Subsequently, the temporal diatom succession resembles the pattern described in modern linear transects across the circumpolar treeline in north America and Asia, both in type of assemblage and some dominant species.     

Tracking Holocene environmental changes in an alpine lake sediment core: application of regional diatom calibration, geochemistry, and pollen

Diatom abundances in the surface sediment samples of 41 mountain lakes in the central Austrian Alps (Niedere Tauern) were related to environmental variables using multi-variate techniques. Canonical correspondence analysis (CCA) revealed that the pH, date of autumn mixing (A _mix), mean August water temperature (T _Aug), dissolved organic carbon (DOC), and relative water depth (Z _rel) made significant contributions to explain the diatom assemblage variation in the lakes of the training set. A weighted averaging partial least square regression and calibration model was used to establish Di-pH (R ² _boot= 0.72, RMSEP_boot= 0.131), and a thermistor measurements-based PLS model for A _mix (R ² _boot= 0.71, RMSEP_boot= 0.006 log₁₀ Julian days). The latter showed a better prediction than T _Aug, and was used in terms of climate change. These transfer functions, together with analyses of loss on ignition (LOI), the total carbon/nitrogen (C/N)-ratios, and selected pollen, were applied to an early to mid-Holocene (11.5–4 cal. ky BP) sediment core section from an Austrian Alpine treeline lake on crystalline bedrock. Additionally, passive sample scores in the CCA of the diatom training set were used to show trends in the variables DOC and Z _rel. During the early Holocene, diatoms indicative of increased pH, extended warm summers, and low water levels dominated. Between 10.2 and 7.6 cal. ky BP it was followed by diatom assemblages that indicated an increase in lake water depth and an earlier A _mix. The multi-proxy data suggest that the A _mix decline is the result of a series of snow-rich summer cool and wet climate fluctuations, which were divided by climate warming at ∼9 cal. ky BP. Increased A _mix, LOI and DOC, and the correspondent decline in the C/N-ratios, show subsequent climate warming between 7.3 and 6 cal. ky BP. The long-term trend in Di-pH indicates the impact of catchment-related processes during the early-Holocene, that were superimposed by climate.     

Holocene environmental changes and development of Figurnoye Lake in the southern Bunger Hills, East Antarctica

The lithology, radiocarbon chronology, granulometry, geochemistry and distribution of diatoms were investigated in three sediment cores from fresh-water Figurnoye Lake in the southern Bunger Hills, East Antarctica. Our paleolimnological data provide a record of Holocene environmental changes for this region. In the early Holocene (prior to 9.0 ± 0.5 kyr BP), warm climate conditions caused intensive melting of either the floating glacier ice mass or glaciers in the immediate lake surroundings, leading to the accumulation of terrigenous clastic sediments and limiting biogenic production in the lake. From ca. 9.0 ± 0.5 to 5.5 ± 0.5 kyr BP, highly biogenic sediments dominated by benthic mosses formed, indicating more distal glaciers or snowfields. A relatively cold and dry climate during this period caused weaker lake-water circulation and, likely, occurrence of lake ice conditions were more severe than present. The distribution of marine diatoms in the cores shows that, sometime between 8 and 5 kyr BP, limited amounts of marine water episodically penetrated to the lake, requiring a relative sea-level rise exceeding 10–11 m. During the last ca. 5.5 ± 0.5 kyr BP, sedimentation of mainly biogenic matter with a dominance of laminated microbial mats occurred in the lake under warm climatic conditions, interrupted by relative coolings: the first one around 2 kyr BP and then shortly before recent time. Between ca. 5.5 and 4 kyr BP, the drainage of numerous ice-dammed lakes took place in the southern Bunger Hills and, as a result, drier landscapes have existed here from about 4 kyr BP.     

Baptiste Lake,Alberta — A late Holocene history of changes in a lake and its catchment in the southern Boreal forest

Lead-210 assay and dating are subject to several sources of error, including natural variation, the statistical nature of measuring radioactivity, and estimation of the supported fraction. These measurable errors are considered in calculating confidence intervals for ²¹⁰Pb dates. Several sources of error, including the effect of blunders or misapplication of the mathematical model, are not included in the quantitative analysis. First-order error analysis and Monte Carlo simulation (of cores from Florida PIRLA lakes) are used as independent estimates of dating uncertainty. CRS-model dates average less than 1% older than Monte Carlo median dates, but the difference increases non-linearly with age to a maximum of 11% at 160 years. First-order errors increase exponentially with calculated CRS-model dates, with the largest 95% confidence interval in the bottommost datable section being 155±90 years, and the smallest being 128±8 years. Monte Carlo intervals also increase exponentially with age, but the largest 95% occurrence interval is 152±44 years. Confidence intervals calculated by first-order methods and ranges of Monte Carlo dates agree fairly well until the ²¹⁰Pb date is about 130 years old. Older dates are unreliable because of this divergence. Ninety-five per cent confidence intervals range from about 1–2 years at 10 years of age, 10–20 at 100 years, and 8–90 at 150 years old.This is the third of a series of papers to be published by this journal which is a contribution of the Paleoecological Investigation of Recent Lake Acidification (PIRLA) project. Drs. D.F. Charles and D.R. Whitehead are guest editors for this series.     

Diatoms from gut contents of museum specimens of an endangered minnow suggest long-term ecological changes in the Rio Grande (USA)

Diatoms consumed by Rio Grande silvery minnows (Hybognathus amarus) collected 104 years apart were used to deduce ecological requirements of this endangered species and to infer a possible cause of its decline based on environmental conditions in the Rio Grande. In 1874, foraged diatoms were largely motile, silt tolerant generalist and epipelic species (e.g. Navicula capitatoradiata, Navicula cryptotenella, Nitzschia palea, Sellaphora pupula) somewhat tolerant to pollution and indicative of eutrophic conditions, low dissolved oxygen, and high biological oxygen demand (BOD). In contrast, diatoms foraged in 1978 were mainly nonmotile epipsammic species (Fragilariaceae) sensitive to pollution and characteristic of low nutrient, oligotrophic conditions with high dissolved oxygen and low BOD. The large-scale difference in composition of the foraged diatom assemblages is consistent with a decline in nutrients and a shift in trophic state commonly associated with river regulation by dams. The results suggest that facilitating allochthonous input of detritus and nutrients into the Rio Grande ecosystem to meet foraging requirements for algivorous and detritivorous fish species such as H. amarus would be a good river restoration strategy.     

Reconstructing long-term changes in Daphnia's body size from subfossil remains in sediments of a small lake in the Himalayas

A combined analysis of modern zooplankton and fossil Cladocera assemblages from a Himalayan lake, Lake 40, revealed that the endemic Daphnia tibetana disappeared in the late-1980s, after persisting as the only Daphnia species for almost 3000 years. Daphnia head shields, which are rarely recovered from the sediments, were the most abundant Daphnia remains in the lake. The remains were of the Ctenodaphnia type; the smaller ones had a large central hole. Head shields of the same type were also found in zooplankton water samples, rich in exuviae as well as of intact specimens of D. tibetana, from a nearby lake. Small individuals had a distinct nuchal organ in the dorsal region of the head. We therefore postulated that the head shields with the hole were from young (newborn or individuals in the two first moults). Up to now, the nuchal organ has been described in laboratory populations of D. (Ctenodaphnia) magna, where it disappears quite early in life. It probably functions as an osmoregulatory organ, essential for the survivorship of late embryos and early juveniles. On the other hand, as far as we know, head shields with a hole have never been recorded in plankton or in sediments. In view of the fact that head shields were representative of Daphnia abundance, we used them to reconstruct changes in density and body size during ca. 3000 years. In fishless mountain lakes, mean Daphnia body size tends to increase toward the end of the productive season. The number of moults and maximum body size depend mainly on the duration of the ice-free period, and on summer temperatures. In cold years, when the productive season is short, the number of moults will be low, and the range of Daphnia body size will be narrow. We used Daphnia body size and abundance estimates, in addition to an analysis of changes in the Cladocera assemblage, to reconstruct past environmental conditions.     

Climatic Changes from 12,000 to 4,000 Years Ago in the Austrian Central Alps Tracked by Sedimentological and Biological Proxies of a Lake Sediment Core

Major and trace elements, minerals, and grain-size were analysed from the early to mid-Holocene (12 to 4 ky BP) period of a sediment core from the Alpine lake Oberer Landschitzsee (ObLAN, 2076 m a.s.l.), which is located on predominantly crystalline bedrock on the southern slopes of the Austrian Central Alps. Geochemistry and mineralogy were compared with diatom-inferred (Di-) ‘date of autumn mixing’ (A_mix), DOC, pH, and selected indicator pollen species from the same sediment core. Principal components analysis (PCA) indicated a positive correlation between processes triggered by temperature and precipitation (e.g., lake mixing, DOC). PCA grouped indicators of physical weathering and enhanced catchment run-off (sand, quartz, feldspar), elements of weathering (e.g., Ti, Rb, Mn) under dryer conditions (clay to silt fractions), and elements that probably were related to changes in redox conditions (Cu, Fe, S, Zn). The duration and height of the snow-pack played an important role in this high-alpine environment, affecting weathering, erosion, pH, and lake stratification. Low Alnus viridis pollen abundance, together with markers for increased elements of erosion, indicated extensive snow-pack. Changes in S coupled with As, and elements indicating increased weathering, reflected climate oscillations. LOI was affected by productivity and erosion. High (late) Di–A_mix coupled with increased Di-DOC indicated prolonged summers with increased productivity. Cold and wet (snow-rich) phases and subsequent melting caused low pH and a decoupling of the significant linear correlation between sedimentary Ca and Di-pH. Weathering and leaching during climate deteriorations opposed the long-term trend in a loss of cations and forced in-lake alkalinity generation during the following lake warming. Overall, the multi-proxy study indicated complex climate-driven processes within different time-scales (long-term trends, climate oscillations, seasonality). The climate oscillations within 12–5 ky BP corresponded well with the cool and wet phases known from central Europe suggesting a dominant common Atlantic climate impact. When Mediterranean climate established between 5 and 4 ky BP, its influence on the southern slopes of the Alps increased.     

Chironomidae (Diptera) remains in surficial lake sediments from the Canadian Cordillera: analysis of the fauna across an altitudinal gradient

The altitudinal distribution of Chironomidae (Diptera) in the southern Canadian Cordillera was analyzed by means of head capsules preserved in surficial sediments of 30 lakes. Taxa characteristic of late-glacial deposits of southern, coastal British Columbia are extant at high elevations, particularly in the Rocky Mountains, and in large, deep, low-elevation lakes. Many chironomid taxa common at low elevations in the southern Canadian Cordillera were not found in alpine and upper subalpine lakes. These faunal differences are probably climatically related. The differences in fauna between high and low-elevation lakes parallel differences between arctic and temperate lakes.     

Chaoborus (Diptera: Chaoboridae) mandibles—paleolimnological indicators of the historical status of fish populations in acid-sensitive lakes

Paleolimnological analyses of Chaoborus mandibles were used to assess the status of fish populations over the previous 150 to 300 years in five lakes from the Adirondack region of New York State. Windfall Pond (pH 6.5) has not acidified and currently has viable fish populations. Big Moose Lake (pH 5.0) has acidified in recent years, and the number of fish populations declined from 14 in the 1930's to seven in the early 1980's. The occurrence of only migratory Chaoborus (subgenus Sayomyia) in the cores indicated the long term presence of fish populations in Windfall Pond and Big Moose Lake. Brooktrout Lake (pH 5.0), Deep Lake (pH 4.7) and Upper Wallface Pond (pH 4.8) have all acidified in recent years, and all three are currently fishless. Chaoborus (Sayomyia) was present throughout the Brooktrout Lake core, but the entirely limnetic species, C. americanus, appeared in the topmost interval in the core. The appearance of C. americanus in the top of the core indicated a recent elimination of fish from Brooktrout Lake, probably during the 1970's. Elimination of fish by the 1940's was inferred for Deep Lake because C. americanus appeared above the 1930 level and replaced C. trivittatus as the dominant. Dominance of C. americanus throughout the Upper Wallface Pond core indicated that planktivorous fish were never present. These results strongly suggest that stratigraphic analyses of Chaoborus mandibles provide a useful assessment of the general status of historical fish populations in Adirondack lakes. The technique should be useful in other regions, as well as for applications other than those concerning lake acidification.This is the eleventh of a series of papers to be published by this journal which is a contribution of the Paleoecological Investigation of Recent Lake Acidification (PIRLA) project. Drs. D.F. Charles and D.R. Whitehead are guest editors for this series.     

The Holocene history of West Basin Lake,Victoria, Australia; chemical changes based on fossil biota and sediment mineralogy

Analyses of diatoms, ostracods, pollen and sediment mineralogy from a 524 cm core from a stratified, hypersaline crater lake, West Basin, Victoria, has revealed clear shifts in the lake's water balance and chemistry and the region's climate over the last 10 000 years. Diatom and ostracod analyses reveal lake water salinity changes which are consistent with the conditions suitable for the precipitation of the carbonate and other minerals identified using x-ray diffraction analysis. The fluctuations in lake water balance deduced from diatom and ostracod inferred lake salinity suggest that the lake began to fill at the beginning of the Holocene and was saline and shallow. Toward the mid-Holocene the water levels rose and yet the lake remained largely saline. The late Holocene is marked by a return to more shallow but fluctuating, water conditions. Through the whole period, the regional dryland vegetation was dominated by open sclerophyll woodland. Both the lacustrine and regional environments interpreted here are consistent with those from Holocene records elsewhere in the region.This is the fourth in a series of papers published in this issue on the paleolimnology of arid regions. These papers were presented at the Sixth International Palaeolimnology Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr A. R. Chivas served as guest editor for these papers.     

Palaeoecological tools for improving the management of coastal ecosystems: a case study from Lake King (Gippsland Lakes) Australia

Since European settlement began over 200 years ago, many southeast Australian coastal lakes and lagoons have experienced substantial human impacts, including nutrient enrichment. Present day management and conservation efforts are often hampered by a lack of data on pre-impact conditions. We used a palaeoecological approach at Lake King, Gippsland Lakes, southeast Australia in order to determine its pre-impact condition and to establish the nature and direction of subsequent environmental changes, including responses to the construction of a permanent entrance to the sea in 1889. A 120 cm sediment core was analysed for diatoms, chlorophyll a, total carbon, nitrogen and sulphur, and dated using ²¹⁰Pb. Past phosphate and salinity concentrations were reconstructed using diatom-phosphate and diatom-salinity transfer functions developed from a calibration set based on 53 sites from 14 southeast Australian coastal lakes and lagoons. Results show changes in the diatom assemblage that record a shift from a brackish-water to marine diatom flora since construction of the permanent entrance. Phosphate concentrations increased at the same time and experienced major peaks in the 1940s and 1950s to >100 μg/l. Chlorophyll a concentrations were generally below 24 μg/l/gTOC in the core, but there has been a clear increase since the 1980s, peaking at 120 μg/l/gTOC, likely associated with a recorded increase in the frequency of nuisance algal blooms. These results indicate that the Lake King environment is now very different to that present during early European settlement. We conclude that by identifying the nature and direction of environmental change, palaeoecological studies can contribute towards developing realistic and well-informed management, conservation and restoration strategies in Australian coastal ecosystems.     

Historical changes in sediment and phosphorus loading to the upper Mississippi River: mass-balance reconstructions from the sediments of Lake Pepin

Long-term changes in sediment and phosphorus loading to the upper Mississippi River were quantified from an array of 25 sediment cores from Lake Pepin, a large natural impoundment downstream of the Minneapolis-St Paul metropolitan area. Cores were dated and stratigraphically correlated using ²¹⁰Pb, ¹³⁷Cs, ¹⁴C, magnetic susceptibility, pollen analysis, and loss-on-ignition. All cores show a dramatic increase in sediment accumulation beginning with European settlement in 1830. Accumulation rates are highest and show the greatest post-settlement increases in the upper end of the lake. Present-day sediment-phosphorus concentrations are roughly twice those of pre-settlement times, and the Fe/Al-bound fraction makes up a greater portion of the total. Diatom assemblages record a marked increase in nutrient availability over the last 200 years, changing from clear-water benthic forms and mesotrophic planktonic taxa in pre-settlement times to exclusively planktonic assemblages characteristic of highly eutrophic conditions today. Lake-water total-phosphorus concentrations, estimated by weighted averaging regression and calibration, increased from 50 to 200 μg l⁻¹ during this period. Sediment loading to Lake Pepin from the Mississippi River has increased by an order of magnitude since 1830. Modern fluxes are about 900,000 metric tons annually, and are more than 80% detrital mineral matter. About 17% of the lake’s volume in 1830 has been replaced by sediment, and at current accumulation rates the remainder will be filled in another 340 years. Phosphorus accumulation in Lake Pepin sediments has increased 15-fold since 1830, rising from 60 to 900 metric tons annually. This rise represents a sevenfold increase in phosphorus loading from the Mississippi River coupled with more efficient retention of phosphorus inflows by bottom sediments. More efficient trapping of phosphorus in Lake Pepin over the last century resulted from higher rates of sediment burial. The most dramatic changes in nutrient and sediment inputs to Lake Pepin have occurred since 1940, although gradual increases began shortly following European settlement. Sediment accumulation rates rose sharply between 1940 and 1970 and then leveled off, while phosphorus inflows record their largest increases after 1970. This is one of eight papers dedicated to the “Recent Environmental History of the Upper Mississippi River” published in this special issue of the Journal of Paleolimnology. D. R. Engstrom served as guest editor of the special issue.     

Holocene pollen record from a peatland in the West Caucasus,Abkhasia, Black Sea region

A pollen sequence spanning over 4000 years was recovered from a small (0.1 ha)Sphagnum-dominated peatland in the mountains near Sukhumi, Abkhasia, West Georgia. The peatland lies atc. 1650 m a.s.l. in denseFagus-Abies forest. The pollen record reveals totally forested surroundings throughout since at least 4000 years BP (90–95% AP). It begins with a complex forest dominated byFagus with large proportions ofCastanea, Acer andUlmus. ThenCastanea became dominant whileFagus was still prominent. This might indicate a warmer climate. Later development shows a dramatic decline ofCastanea. Its pollen drops down to 3–5%. RecentlyAbies has been experiencing an exponential growth. Now it comprises over 50% of the forest composition around the peatland. These changes have possibly been caused by human influence together with climatic change. The basin started as aPotamogeton-dominated shallow lake with ferns andAlisma along the margins. Later it developed into a sedge fen and finally aSphagnum andMenyanthes poor fen with scatteredCarex limosa. The record indicates a progression towards oligotrophy.This publication is the fifth paper in a series of papers presented at the session on Past Climatic Change and the Development of Peatlands at the ASLO and SWS Meetings in Edmonton, Canada, May 30–June 3, 1993. Dr. P. Kuhry and Dr S. C. Zoltai are serving as Guest Editors.