Sedimentological and geochemical records of past trophic state and hypolimnetic anoxia in large,hard-water Lake Bourget,French Alps

Sedimentological, geochemical and particle-size analyses were used to reconstruct the evolution of both trophic state and hypolimnetic anoxia in Lake Bourget (French Alps) during the last century. Radionuclide dating (²¹⁰Pb, ¹³⁷Cs and ²⁴¹Am) confirmed the annual rhythm of laminations in the upper sediment profile. In Lake Bourget, biochemical varves are triplets composed of a diatom layer (spring lamina), a bio-precipitated calcite-rich layer (spring/summer lamina), and a layer rich in organic matter and detrital particles (winter lamina). The onset of eutrophication and the first appearance of an anoxic facies occurred simultaneously and were dated by laminae counting to AD 1943±1 year. Persistent anoxic conditions began in AD 1960. Eutrophication is characterised by drastic increases in the flux of biogenic silica (mostly diatoms), lacustrine organic matter, and larger calcite crystals (15–30 μm). The increase of organic matter also represents a marker of the onset of anoxic conditions in the hypolimnion. Our results show that eutrophication was the main factor controlling anoxia in the hypolimnion. This eutrophication was caused mostly by the inflow of untreated sewage effluents, and to a lesser extent, by input of fertilizer-derived phosphorus during floods of the Rhone River and run-off from the lake catchment. The Rhone River, however, can also be a source of re-oxygenation via underflows that originate during flood events. Oxygenation of the hypolimnion is also controlled by low winter temperatures, which enable turnover of the lake. Thus, global warming, associated with a forecasted reduction in precipitation, might reduce the efficiency of hypolimnetic re-oxygenation in Lake Bourget.     

Quantitative inferences of past hypolimnetic anoxia and nutrient levels from a Canadan Precambrian Shield lake

Paleolimnological analyses were used to infer limnological changes during the past ~ 300 yrs in the west basin of Peninsula Lake, a small (853 ha) Precambrian Shield lake in Ontario, Canada, that has been subjected to moderate cultural disturbances (forest clearance, cottage and resort development). This study represents a pioneering attempt to use sedimentary chironomid assemblages and weighted-averaging models to quantify past hypolimnetic anoxia (expressed as the anoxic factor, AF). Impacts of forest clearance and human land-use on deepwater oxygen availability and surface water quality were assessed by comparing chironomid-inferred AF and diatom-inferred total phosphorus concentration ([TP]) to changes in terrestrial pollen and historical data. This study also discusses the ability of chironomids to quantitatively infer changes in AF.Pre-disturbance chironomid assemblages were stable and dominated by taxa indicative of oxygen-rich hypolimnetic conditions (e.g., Protanypus, Heterotrissocladius, Micropsectra type), while diatoms indicated oligotrophic lake status (diatom inferred [TP] = 5-7 g·l^-1). Chironomids characteristic of lower oxygen availability (e.g., Chironomus, Procladius) increased following land-clearance, road construction, establishment of a grist mill and lakeshore development beginning ca. 1870. Increased abundances of Tanytarsus s. lat., a multigeneric group of mainly littoral chironomids, since 1900, indicated that littoral chironomids may have comprised a greater proportion of fossil assemblages during periods of eutrophication and prolonged anoxia. Abundances of meso-eutrophic diatom taxa (e.g., Fragilaria crotonensis, Asterionella formosa, Aulacoseira ambigua, A. subarctica) increased concurrent with European settlement (ca. 1870) and diatom-inferred [TP] doubled (~ 6-12 g·l^-1), further indicating that naturally-oligotrophic Precambrian Shield lakes were extremely sensitive to initial land-clearance activities.Recent increases in oligotrophic diatom taxa (e.g., Cyclotella stelligera) indicate a shift to more oligotrophic conditions since ca. mid-1960s, with greatest changes since ca. 1980. The chironomids Heterotrissocladius and Micropsectra type also increased at this time suggesting greater deepwater oxygen availability. These recent water-quality improvements, possibly in response to enhanced nutrient removal from detergents and sewage, climate-related reductions in external phosphorus loads, and catchment (but not lake) acidification and reforestation, suggest that habitat for commercially-valuable cold-water fishes has improved in recent decades despite greater recreational lake-use.Paleolimnological assessment of trophic status changes in Peninsula Lake using fossil diatom and chironomid assemblages were in good agreement. Diatom inferences of [TP] and chironomid inferences of AF both suggest that Peninsula Lake was historically oligotrophic, became oligo-mesotrophic after European settlement, and returned to oligotrophy in recent yrs. Chironomid inferences of [TP] consistently underestimated the trophic status of Peninsula Lake, possibly due to its relatively large hypolimnion. These results suggest that AF represents a useful tool for quantitatively reconstructing the past trophic status of deeper, stratified lakes.     

A high resolution multi-proxy record of pronounced recent environmental change at Baker Lake,Nunavut

Arctic aquatic systems are considered to be especially sensitive to anthropogenic disturbance, which can have cascading effects on biological communities as aquatic food-web structure is altered. Bio-indicators that respond to major limnological changes can be used to detect and infer major environmental change, such as climate warming, with the use of paleolimnological techniques. A multi-proxy approach was used to quantify recent environmental changes at Baker Lake, Nunavut, Arctic Canada. Analyses of fossilized remains of chironomids and diatoms were conducted on a sediment core of 20 cm in length sampled at 0.5-cm intervals. A new surface sediment training set of subfossil chironomid assemblages from 65 lakes across the eastern Canadian Arctic generated a robust (r _jack² = 0.79) surface water paleotemperature transfer function. The transfer function was applied to stratigraphic intervals from the Baker Lake sediment core to generate a paleotemperature reconstruction of sub-decadal resolution. The surface water temperature reconstruction inferred a 2°C increase in mid-summer surface water temperature for Baker Lake over the last 60 years, which was corroborated by the local instrumental record spanning the period of 1950–2007 AD. The chironomid record shows a recent decline of several cold-water taxa and appearance of warm-water indicators. This shift in community structure began circa 1906 AD, and intensified after 1940 AD. The corresponding fossil diatom record showed an increase in small planktonic Cyclotella taxa over the past 60 years, intensifying in the last 5 years, which also suggests a warmer climate and longer ice-free periods. The shifts in the diatom assemblages began later than the shifts in the chironomid assemblages, and were of lower magnitude, reflecting differences in the mechanisms in which these two indicators respond to environmental change.     

Response of diatom community in Lugu Lake (Yunnan–Guizhou Plateau,China) to climate change over the past century

A mean annual temperature increase has been recorded on the Yunnan–Guizhou Plateau of China during the last century. This temperature increase has been significantly greater since the 1950s. Thus, paleolimnological analyses may be utilized to better understand ecological responses to recent changing climate over decadal to centennial timescales, especially in regions with sparse lake monitoring data. Here, we present paleolimnological results from a ²¹⁰Pb/¹³⁷Cs-dated sediment core spanning approximately the last ~250 years from a remote, alpine, semi-closed oligotrophic lake (Lugu Lake) on the northwestern Yunnan–Guizhou Plateau. Sediment profiles of diatoms, geochemical variables (LOI₅₅₀, TOC and C/N) and median grain size were analyzed and compared with the climate data (1951 AD–2010 AD) from the Lijiang weather station. Endogenous productivity of Lugu Lake has increased gradually over the last 30 years. The majority of diatom taxa encountered in the core are typical of alkaline oligotrophic lakes. Diatom assemblages were dominated by Cyclostephanos dubius, Cyclotella taxa, and fragilarioid taxa. Diatom species composition has changed significantly with three assemblage shifts at different scales over the ~250-year period. Diatom species diversity reveals a distinct increase before ~1970 AD, followed by a decline. In addition, a decreasing trend in diatom cell-size was consistent with recent warming trends. Redundancy analysis (RDA) shows that regional air temperature trends (annual, spring, summer, and winter) have played a significant role (p < 0.05) in determining diatom compositional changes over the past six decades. Results of this study suggest that regional warming is the main driving force behind recent changes in diatom composition at Lugu Lake, while nutrients may also have impact on the diatom change in recent 10 years.     

Diatom assemblage response to Iroquoian and Euro-Canadian eutrophication of Crawford Lake,Ontario, Canada

Diatom and geochemical data from Crawford Lake, Ontario, have been used to document limnological responses to periods of cultural disturbance resulting from native Iroquoian occupation of the watershed (1268–1486 AD) and Euro-Canadian agriculture and deforestation (1867 AD–present). Here, we further develop the high-resolution nature of the Crawford Lake sediment record to examine the physical, chemical and biological aspects of limnological response to human disturbances in the lake catchment area with exceptional detail. We report detailed diatom abundance and flux data for individual taxa from Crawford Lake, and further describe the relationship between assemblage composition and environmental conditions using canonical correspondence analysis (CCA). Diatom assemblage data are used to calculate diatom inferred-total phosphorus (DI-TP) concentrations for the past ∼1,000 years. We also examine the diatom community response during and after periods of disturbance by Iroquoian and Euro-Canadian populations, and compare this response to existing geochemical proxies of lake production and new elemental geochemical indicators of catchment area erosion. In particular, we explore the differing limnological response to the two distinct periods of cultural eutrophication and examine the limnological processes that occurred during the period of␣low (or no) human activity (1487–1866 AD), when geochemical indicators of lake production recovered to pre-disturbance conditions, but diatom assemblages notably did not. Our results illustrate the highly susceptible nature of diatom communities to periods of anthropogenic disturbance, and emphasize that ecological indicators (such as diatom assemblages) should be included with other proxies (such as nutrient concentrations and physical characteristics) when assessing disturbance and recovery in lake systems.     

Changes in submerged macrophyte abundance altered diatom and chironomid assemblages in a shallow lake

Submerged macrophyte abundance strongly influences aquatic ecosystems. Because of a lack of monitoring data, however, the long-term dynamics of such aquatic plants are poorly understood. Increasingly, paleolimnologists use changes in subfossil algae and invertebrates to infer past submerged macrophyte dynamics and assess how human activities have altered this important primary producer component of aquatic ecosystems. We evaluated the sensitivity of subfossil diatom and chironomid assemblages to historically documented changes in macrophyte abundance in Chenango Lake, New York, USA, where macrophyte cover has been monitored since 1978. We also tested the ability of a semi-quantitative diatom-based macrophyte-abundance inference model to detect the pronounced macrophyte decline that was observed between 1993 and 2001. Diatoms responded to the recent loss of macrophytes, with a decline in the relative abundance of macrophyte-associated taxa. Estimates of macrophyte abundance fluctuated according to the diatom-based inference model. Chironomid changes were coherent with the diatom-inferred macrophyte zones. The largest shifts in subfossil assemblages occurred before the start of the monitoring record and coincided with construction of a ~4.3-m-high dam on the lake, which substantially expanded the littoral habitat. Even in heavily managed systems, large reductions in macrophyte abundance can be detected with paleolimnological approaches.     

Responses of Fragilarioid-dominated diatom assemblages in a small Arctic lake to Holocene climatic changes, Russell Island, Nunavut, Canada

We present a paleolimnological record spanning the Holocene from a small lake on Russell Island (Lake PW02), in the central Canadian Arctic Archipelago (74.07° N, 97.77° W, 182 m asl). Fragilarioid diatom types in the genera Pseudostaurosira, Staurosira and Staurosirella constitute >90% of valves in fossil samples. Using modern biogeographic data which specify the temperature optima of the Fragilarioid diatom taxa, we present new inferences about the timing of paleoclimatic changes in the central Arctic islands. The early Holocene was characterized by maximum values for sediment organic matter, and lower ratios of Staurosirella pinnata to Staurosira construens v. venter, suggesting warm summer air temperatures between about 9500−6500 cal year BP. Influxes of biogenic silica and diatom valves decreased following 4000 cal year BP, the sediment accumulation rate slowed and diatom taxa of the littoral zone diversified, suggesting cooler summers and more persistent lake ice. Variations in the species composition of the assemblages indicate paleoclimatic changes that are in broad agreement with other paleoenvironmental records from the Arctic including melt records from the Agassiz Ice Cap. Although autecological data remain incomplete for Fragilarioid taxa, our results indicate differences in these taxa in responses to paleoenvironmental change and underline the potential for the increased use of these taxa in paleoenvironmental reconstructions. The record from Lake PW02, as in other records from Arctic lakes with low algal diversity throughout the Holocene, shows a pronounced increase in diatom diversity since the 1920s, and diatom production since the 1970s far exceeds any recorded during the Holocene.     

Use of subfossil <Emphasis Type="Italic">Chaoborus</Emphasis> mandibles in models for inferring past hypolimnetic oxygen

Assemblages of subfossil Chaoboridae mandibles from 80 thermally-stratified shield lakes in southern central Canada were examined to explore the influence of subfossil Chaoborus on subfossil Chironomidae-based paleolimnological inference models of deepwater oxygen, as volume-weighted hypolimnetic oxygen (VWHO). Inclusion of subfossil Chaoborus in subfossil Chironomidae-based VWHO models only improved model performance modestly, however it produced substantively better inferences of hypolimnetic oxygen in anoxic lakes, because Chaoborus had a much stronger positive relationship with low VWHO compared to chironomid taxa indicative of anoxic conditions, such as Chironomus. A Chaoborus mandible:Chironomidae head capsule ratio (chaob:chir) may be a useful index in paleolimnological studies, as chaob:chir in a surface sediment training set was significantly related to VWHO, and displayed little co-variation with other limnological variables such as trophic status (e.g. TP, TN) or lake depth (e.g. Z _max). Chaob:chir values in a stratigraphic analysis tracked chironomid-inferred VWHO, however the use of chaob:chir in regional ‘top–bottom’ paleolimnological studies must be used with caution.     

Diatoms and pollen as indicators of water quality and land-use change: a case study from the Oak Ridges Moraine,Southern Ontario,Canada

Swan Lake is a small kettle lake located on the Oak Ridges Moraine; a moraine that is recognized as an important source of ground water for the nearby and rapidly expanding Greater Toronto Area. A paleolimnological reconstruction using pollen and diatoms from the lake sediments showed significant changes in biological community composition through the last ∼400 years. Alterations in the diatom and pollen assemblages were most dramatic ca. A.D. 1850, correlating with the highest sediment flux in the lake between the period ca. A.D. 1850 and A.D. 1870. These changes were directly linked to regional deforestation and agricultural activities associated with European settlement. The pollen record from ca. A.D. 1850 to present day indicated that tree species (e.g. Pinus spp., Tsuga canadensis) were declining, while grass (Poaceae) and invasive species (e.g. Ambrosia) were increasing. Around A.D. 1850, the diatom flora changed from an assemblage dominated by large, benthic species (e.g. Sellaphora pupula, Pinnularia cf. maior, and Stauroneis phoenicenteron) to an assemblage characterized by smaller, tychoplanktonic (e.g. Fragilaria tenera, Staurosirella pinnata) and epiphytic (e.g. Achnanthidium minutissimum, Rossithidium linearis) taxa. This diatom community change supports the intermediate disturbance hypothesis which predicts a high level of diversity and richness following an intermediate to intense disturbance of short duration. Phosphorus concentrations in Swan Lake were inferred using a diatom-based regional calibration model, and the results indicated marked changes in lake water chemistry through time (from below detection limits before land clearance and settlement to 19.3 μg l⁻¹ in the current sediments), which were concurrent with episodes of regional deforestation and land-use change. Although the sediment and biological records indicate that the lake ecology has stabilized over the last 30–50 years, paleolimnological records show that the water quality and biology of Swan Lake has changed dramatically and not returned to pre-settlement conditions. Swan Lake presents a detailed record of the impact created by deforestation and urban development with a population of <50 individuals per km². Detailed paleolimnological studies like Swan Lake, in tandem with global human footprint studies, can create realistic estimates of land-use impacts at the global scale.     

Effects of long term nutrient and climate variability on subfossil Cladocera in a deep,subalpine lake (Lake Garda,northern Italy)

Albeit subfossil Cladocera remains are considered as a reliable proxy for tracking historical lake development, they have been scarcely studied in large subalpine lakes south of the Alps. In this study, subfossil Cladocera remains from Lake Garda in northern Italy were analyzed to track the lake’s environmental changes since the Middle Ages. One core was retrieved from the largest sub-basin of Lake Garda (Brenzone, 350 m deep) and two cores were retrieved from the profundal and littoral zone of the smaller lake sub-basin (Bardolino, 80 and 40 m deep, respectively). The species distribution of Cladocera remains in recent sediment layers was similar to that observed in contemporary water samples. The deepest sections of the three cores were characterized by species sensitive to water temperature, which suggest a key role of major climatic events in driving the lake response during the late Holocene. The most evident change in Cladocera assemblages in the studied cores was observed during the 1960s, when planktonic taxa such as Daphnia spp. and Bosmina spp. became dominant at the expense of littoral taxa. Despite the highly coherent general pattern of subfossil Cladocera, the cores showed a set of minor differences, which were attributed to different morphological and hydrological features of the two basins forming Lake Garda. Multivariate analysis revealed a clear relation of Cladocera to climate variability during periods of low lake nutrient levels (i.e. before the 1960s). This provides additional information on past ecological responses of Lake Garda, as contrast previous data from subfossil diatom and pigment analyses did not fully capture effects of climate change during the same period. Since the 1960s shifts in assemblage composition of Cladocera and diatoms were highly coherent, in response to nutrient increase and the following, less pronounced, decrease in nutrient concentrations. During the last five decades, the response of the Cladocera to climate variability appeared to be masked by nutrient change. This work points up the potential of the multi-proxy approach for disentangling the multifaceted biological responses to multiple environmental stressors in large and deep lakes.     

Chironomid assemblages in cores from multiple water depths reflect oxygen-driven changes in a deep French lake over the last 150 years

We sampled modern chironomids at multiple water depths in Lake Annecy, France, before reconstructing changes in chironomid assemblages at sub-decadal resolution in sediment cores spanning the last 150 years. The lake is a large, deep (z_max = 65 m), subalpine waterbody that has recently returned to an oligotrophic state. Comparison between the water-depth distributions of living chironomid larvae and subfossil head capsules (HC) along three surface-sediment transects indicated spatial differences in the influence of external forcings on HC deposition (e.g. tributary effects). The transect with the lowest littoral influence and the best-preserved, depth-specific chironomid community characteristics was used for paleolimnological reconstructions at various water depths. At the beginning of the twentieth century, oxygen-rich conditions prevailed in the lake, as inferred from M. contracta-type and Procladius sp. at deep-water sites (i.e. cores from 56 to 65 m) and Paracladius sp. and H. grimshawi-type in the core from 30 m depth. Over time, chironomid assemblages in cores from all three water depths converged toward the dominance of S. coracina-type, indicating enhanced hypoxia. The initial change in chironomid assemblages from the deep-water cores occurred in the 1930s, at the same time that an increase in lake trophic state is inferred from an increase in total organic carbon (TOC) concentration in the sediment. In the 1950s, an assemblage change in the core from 30 m water depth reflects the rapid expansion of the hypoxic layer into the shallower region of the lake. Lake Annecy recovered its oligotrophic state in the 1990s. Chironomid assemblages, however, still indicate hypoxic conditions, suggesting that modern chironomid assemblages in Lake Annecy are decoupled from the lake trophic state. Recent increases in both TOC and the hydrogen index indicate that changes in pelagic functioning have had a strong indirect influence on the composition of the chironomid assemblage. Finally, the dramatic decrease in HC accumulation rate over time suggests that hypoxic conditions are maintained through a feedback loop, wherein the accumulation of (un-consumed) organic matter and subsequent bacterial respiration prevent chironomid re-colonization. We recommend study of sediment cores from multiple water depths, as opposed to investigation of only a single core from the deepest part of the lake, to assess the details of past ecological changes in large deep lakes.     

Importance of diffuse nutrient loading and lake level changes to the eutrophication of an originally oligotrophic boreal lake: a palaeolimnological diatom and chironomid analysis

The recent environmental history of Lake Lappajärvi in western Finland (63°00 N, 23°30 E, area 149 km²), a humic, brown water lake with an average phosphorus content of ca. 20 g l^–1, was studied from short core sediment samples taken from the two main basins of the lake. Based on the stratigraphy of diatoms and chironomids and the sediment quality it was possible to distinguish four developmental stages during the past century: (1) a pre-industrial stage covering the time up to about 1935; (2) a stage of increasing nutrient loading (ca. 1936–1960); (3) a stage of pronounced erosion from lake level regulation and extensive ditching of the catchment area (ca. 1960–1970); and (4) a meso-eutrophic stage from ca. 1970 onwards.Acidophilous Aulacoseira distans coll. and other species typical of dystrophic, nutrient-poor lakes characterized the diatom assemblages during the first stage, and the profundal zoobenthic assemblages, characterized by Heterotrissocladius subpilosus and Micropsectra, indicated good hypolimnetic oxygen conditions and a low sedimentation of organic matter (approx. less than 50 g m^–2 a^–1). The increased loading rapidly led to changes both in diatoms and chironomids (e.g., to an early extinction of H. subpilosus in the 1950s). The process finally led to eutrophication with a successive proliferation of diatom species such as Asterionella formosa followed by Aulacoseira ambigua, Fragilaria crotonensis, and finally Melosira varians. The relative proportion of alkaliphilous species reached a maximum in the final stage and the original profundal chironomid fauna was replaced by Chironomus anthracinus gr. and C. plumosus which are typical of profundal areas suffering from temporal oxygen deficit. It is notable that the considerable decrease in waste water loading from the point sources (80–86% ) during the past two decades has not led to a recovery in the lake. This highlights the importance of diffuse loading from agriculture, forestry and other human activities even to this comparatively large lake.     

Spatial variability of chironomid death assemblages in the surface sediments of a fluctuating tropical lake (Lake Naivasha,Kenya)

Studies addressing within-lake variability of fossil chironomid assemblages are very few, and all deal with hydrologically stable temperate lakes where the question of spatial integration mostly relates to the mixing of faunal assemblages associated with shallow, warm-water habitat and those associated with deeper, cold-water habitat. Here we study within-lake variability of surface-sediment chironomid assemblages in the fairly large (∼100–170 km² since 1983) and shallow (Z _max = 5–8 m) fluctuating tropical lake basin of Lake Naivasha, Kenya, and compare the patterns observed with those in two smaller adjacent basins, one similarly shallow (Lake Oloidien, 5.1–5.7 km2, 5–8 m), the other deep and stratified (Crescent Island Crater, 1.9 km², 14–17 m). Chironomid assemblages were analysed in core-top samples and surface sediments along inshore to offshore transects, and how well individual samples represented the total (basin-wide mean) subfossil assemblage was considered both in terms of taxon richness and taxon percent composition. Within-lake variability of subfossil chironomid concentrations (with generally higher absolute values in nearshore samples) could be explained by effects of sediment winnowing and focusing, whereas between-lake variability reflected their relative susceptibility to wind-driven sediment disturbance or bottom anoxia. In all study lakes, but most significantly in lakes Naivasha and Oloidien, species distribution in the subfossil chironomid assemblages showed a strong nearshore to offshore gradient, which in these shallow lakes, reflects the dominant control of substrate and food quality on species distribution in the living community. Particularly in the larger basins, nearshore samples better represented the total lake assemblage than offshore samples, because the former always contained a component of mud-dwelling species whereas the latter often lacked a component of macrophyte-dwelling species. Our results show that although sedimentation dynamics in the shallow, wind-stressed Lake Naivasha is dominated by frequent resuspension and random sediment redistribution, the near- to offshore gradient in chironomid habitat remains imprinted on subfossil assemblages. We conclude that also in shallow fluctuating lakes, given sufficient size, incomplete pre-burial spatial integration of habitat-specific chironomid assemblages can be exploited for within-lake calibration of environmental gradients.     

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.     

A record of hypolimnetic oxygen conditions in a temperate multi-depression lake from chemical evidence and chronomid remains

A multiproxy paleolimnological study of Douglas Lake, Michigan, was undertaken to elucidate the history of productivity and oxygen depletion in three basins of this multi-depression lake. Indicators investigated in three dated cores included chlorophyll a, Fe and Mn stratigraphy, and fossil chironomid assemblages. The coring sites were chosen to correspond to modern studies of oxygen depletion rates, and to determine if conclusions reached in these studies were supported by paleolimnological evidence. Stratigraphies of chlorophyll a, Fe and Mn indicate that two of the basins, South Fishtail Bay and Fairy Island, have been eutrophic and anoxic for a long period of time, predating European settlement. The third basin, Grapevine Point, has been consistently less productive, and had less severe oxygen depletion. Results of the chironomid analysis agree with these conclusions, including a change from mesotrophic to eutrophic indicator taxa in the Grapevine Point basin. All three cores show evidence of increasing trophic state in the most recent sediments, supporting some of the conclusions reached in the modern studies. It is also demonstrated that deforestation of the watershed had profound effects on littoral chironomid assemblages. Paleolimnological investigations also demonstrated the individual nature of the separate basins in Douglas Lake.     

Pollution history and recovery of a boreal lake exposed to a heavy bleached pulping effluent load

We examined the effects of heavy pulp mill discharges on the Lake Lievestuoreenjärvi ecosystem and the later recovery of diatom and chironomid communities from age-dated short core samples. Beginning in 1927 the lake received a heavy effluent load from a sulphite pulp mill. Except for the recession during the Second World War and the temporary closure of the mill from 1967 to 1971, the industrial load, containing large quantities of nutrients, organic matter and toxic compounds, increased continuously. In the early 1980s, laboratory documents were falsified by the directors of the mill and the systematic illegal effluent overload led to a collapse of the whole lake ecosystem. In 1985, the outdated plant was finally closed down. Based on the assessment of chemical properties and biological remains of the sediment, we distinguished five developmental phases in the ecological state of the lake. In the pre-industrial phase, the pelagic and profundal benthic communities were dominated by species preferring ultraoligotrophic or oligotrophic lakes. Concomitant with the increasing discharge and deposition of chlorine compounds, resin acids, and mercury, as well as strong acidity and hypolimnetic and epilimnetic anoxia, the ecological status changed in a short period from excellent to bad. Finally, in the early 1960s, the majority of the lake was virtually dead and the aquatic life survived only in the uppermost littoral zone. Since 1985, a fast recovery in the water quality has led to a strong, but temporary eutrophy in pelagic communities. The main peak of eutrophication was caused by the invasion of a species new to the lake,Aulacoseira granulata var.angustissima. Later, the pelagic communities shifted towards oligotrophy, but the original, pre-industrial status has not been re-established. The profundal benthic communities have not achieved the pre-industrial structure, but at present indicate mesotrophy.     

Late Holocene diatom assemblages in a lake-sediment core from Central Kamchatka,Russia

Fossil diatom assemblages in a sediment core from a small lake in Central Kamchatka (Russia) were used to reconstruct palaeoenvironmental conditions of the late Holocene. The waterbody may be a kettle lake that formed on a moraine of the Two-Yurts Lake Valley, located on the eastern slope of the Central Kamchatka Mountain Chain. At present, it is a seepage lake with no surficial outflow. Fossil diatom assemblages show an almost constant ratio between planktonic and periphytic forms throughout the record. Downcore variations in the relative abundances of diatom species enabled division of the core into four diatom assemblage zones, mainly related to changes in abundances of Aulacoseira subarctica, Stephanodiscus minutulus, and Discostella pseudostelligera and several benthic species. Associated variations in the composition and content of organic matter are consistent with the diatom stratigraphy. The oldest recovered sediments date to about 3220 BC. They lie below a sedimentation hiatus and likely include reworked deposits from nearby Two-Yurts Lake. The initial lake stage between 870 and 400 BC was characterized by acidic shallow-water conditions. Between 400 BC and AD 1400, lacustrine conditions were established, with highest contributions from planktonic diatoms. The interval between AD 1400 and 1900 might reflect summer cooling during the Little Ice Age, indicated by diatoms that prefer strong turbulence, nutrient recycling and cooler summer conditions. The timing of palaeolimnological changes generally fits the pattern of neoglacial cooling during the late Holocene on Kamchatka and in the neighbouring Sea of Okhotsk, mainly driven by the prevailing modes of regional atmospheric circulation.     

Middle to late Holocene chironomid-inferred July temperatures for the central Northwest Territories,Canada

We analyzed subfossil chironomids, sediment organic matter and sediment particle size data from a 1.11-m-long freeze core collected from Carleton Lake (unofficial name), located approximately 120 km north of the modern treeline. This well-dated core spans the last ca. 6,500 years. Two chironomid transfer functions were applied to infer mean July air temperatures. Our results indicated that the chironomid-inferred temperatures from this lake sediment record did not pass a significance test, suggesting that other factors in addition to temperature may have been important in structuring the chironomid community through time. Although not statistically significant, the chironomid-inferred temperatures from this site do follow a familiar pattern, with highest inferred temperatures occurring during the Holocene Thermal Maximum (~6–4 cal kyr BP), followed by a long-term cooling trend, which is reversed during the last 600 years. The largest change in the chironomid assemblage, which occurred between ca. 4,600 and 3,900 cal yr BP is possibly related to the well-documented northward advance and subsequent retreat of treeline in this region.     

Natural and anthropogenic aquatic environmental changes reconstructed by paleolimnological analyses in Lake Kitaura,central Japan

In order to assess the recent anthropogenic environmental changes in Lake Kitaura, central Japan, changes during the past few centuries were reconstructed from results of radiometric and tephrochlonological age determination, magnetic susceptibility measurements, total organic carbon analyses, total nitrogen analyses and fossil diatom analyses on a sediment core from the lake. A total of six major and sub-zones are recognized according to the diatom fossil assemblages, and we discuss aquatic environmental change in Lake Kitaura mainly based on these diatom assemblage change. Zone Ia and Zone Ib (older than AD 1707) are marine to brackish. In Zone IIa (AD␣1707–AD 1836), most of the brackish diatoms disappeared, and were replaced by freshwater species indicating a decrease in salinity. We interpret the salinity decrease in Zone I–IIa as a sea-level fall during the Little Ice Age. The salinity of the lake decreased to near freshwater conditions in Zone IIb (AD 1836–AD 1970), which could arise from alteration in River Tone or development of a sandspit in the mouth of River Tone in addition to sea-level change. In Zone IIIa (AD 1970–AD 1987), the diatom assemblage indicates a freshwater environment, and sedimentation rates increase rapidly. These changes reflect sedimentary environment change and an ecosystem transition due to the construction of the tide gate. In Zone IIIb (AD 1987–AD 2002), the diatom flux (valves cm⁻² y⁻¹) increased and species composition changed. The changes in Zone IIIb show a good agreement with limnological monitoring data gathered from the lake. These paleolimnological data suggest that the recent human-induced changes of the aquatic environment of the lake after the 1970s exceed rates during the period concerned in this study.     

Diatom-based interpretation of sediment banding in an urbanized lake

Sediment stratigraphy and diatom succession were studied in an 80.5-cm core taken from the deepest part of Third Sister Lake, a small kettle hole in a recently urbanized landscape of southeastern Michigan. Alternating light clay and dark organic bands documented sporadic inputs of clay from outside the basin during rain events, rather than annual laminations. Urban construction activity also disrupted the inflow stream bed and facilitated transport of clay into the lake to generate non-rhythmic banding in the lake's deep hole. Diatom analysis revealed dramatic changes in predominant taxa with sediment depth verifying the non-annual nature of the sediment bands. Observation of halophilic diatom taxa also documented effects of human activity such as road salting on this small, urban lake.