Relationships between hydrological and limnological conditions in lakes of the Slave River Delta (NWT,Canada) and quantification of their roles on sedimentary diatom assemblages

Water chemistry and surface sediments were analyzed from 41 shallow lakes representing three previously-defined hydrological categories in the Slave River Delta, Northwest Territories, Canada, in order to identify relationships between hydrological and limnological conditions and their associations with recently deposited diatom assemblages. Evaporation-dominated lakes are physically removed from the influence of the Slave River, and are characterized by high alkalinity and high concentrations of nutrients and ions. In contrast, flood-dominated lakes tend to receive a pulse of floodwater from the Slave River during the spring thaw and have low alkalinity and low concentrations of most nutrients and ions. Exchange-dominated lakes are variably influenced by floodwaters from the Slave River and seiche events from Great Slave Lake throughout the spring thaw and open-water season, and are characterized by a broad array of limnological conditions that are largely dependent on the strength of the connection to these sources of floodwater. Specific diatom ‘indicator’ taxa have been identified that can discriminate these three hydrological lake categories. Evaporation-dominated lakes are associated with high relative abundance of common epiphytic diatom taxa, while diatoms indicative of flood- and exchange-dominated lakes span a wide range of habitat types (epiphytic, benthic) but also include unique planktonic diatoms (Stephanodiscus and Cyclostephanos taxa) that were not found in surface sediments of evaporation-dominated lakes. The planktonic diatom taxa originate from the Slave River, and thus are indicative of river influence. In complex, remote, freshwater ecosystems like the Slave River Delta, integration of results from hydrological and limnological approaches provides a necessary foundation to assess present, past and future hydroecological responses to changes in river discharge and climate.     

Local physiographic controls on the responses of Arctic lakes to climate warming in Sirmilik National Park,Nunavut, Canada

Twenty high Arctic lakes and ponds were sampled for water chemistry and modern diatom assemblages in two distinct physiographic sectors of Sirmilik National Park, Nunavut, Canada. Sites on southwestern Bylot Island were warmer, more alkaline, less dilute, and had higher concentrations of nutrients, DOC and Chl-a (mesotrophic to oligo-mesotrophic), whereas sites on Qorbignaluk Headland on northern Baffin Island were deeper, very dilute, mostly oligotrophic and had lower pH. Diatom assemblages differed markedly between these two regions as a consequence of limnological differences between them. Paleolimnological records, spanning > 200 years and dated by ²¹⁰Pb activity, were produced from each region to compare biological responses to recent warming inferred from glaciological studies on Bylot Island and regional syntheses for the Arctic. Diatom assemblages began to change around AD 1900 at both sites. At Qorbignaluk Headland, marked shifts in diatom community composition occurred during the twentieth century, with large increases in the abundance of planktonic diatoms. At Bylot Island, diatom community changes began around the same time, and involved modest decreases in planktonic diatoms and increases in inferred specific conductance, likely because of a decrease in the areal extent of the small lake as a response to warming. The study confirms that responses of freshwater ecosystems to climate warming vary depending on local physiographic factors.     

Late Holocene environmental changes inferred from diatoms in a lake on the western Taimyr Peninsula, northern Russia

Changes in the diatom assemblages preserved in a sediment core taken from a small lake located north of arctic treeline on the western Taimyr Peninsula, Russia, were examined in order to investigate late Holocene (i.e., ca 5000 cal yr BP to present) climatic and environmental changes within the region. Early diatom assemblages were dominated by benthic Fragilaria taxa and indicate a transitional phase in the lake history, most likely reflecting lake development and environmental change associated with treeline retreat to the south of the study site. Concurrent with pollen and macrofossil evidence of a vegetation shift to shrub tundra in the catchment basin at ca 4200 cal yr BP, an increase in cold-water taxa, followed by little change in diatom assemblages until ca 2800 cal yr BP, suggests that conditions were relatively cool and stable at this time. The last 2000 years of the Middendorf Lake record have been marked by fluctuating limnological conditions, characterized by striking successional shifts between Fragilaria pinnata and Aulacoseira distans var. humilis. Recent conditions in Middendorf Lake indicate an increase in diatom taxa previously rare in the record, possibly associated with twentieth-century climatic warming. The Middendorf Lake record indicates that significant limnological change may occur in the absence of catchment vegetation shifts, suggesting late-Holocene decoupling of aquatic and terrestrial responses to climatic and hydrological change. Our study results represent one of the few paleoecological records currently available from northern Russia, and highlight the need for further development of calibration data sets from this region.     

Assessment of drought over the past two millennia using near-shore sediment cores from a Canadian boreal lake

Lack of long-term studies on drought in the boreal region of northwest Ontario limits our ability to assess the vulnerability of this region to climate change. We investigated changes in diatoms, scaled chrysophytes, and sedimentary pigments in two near-shore cores from Gall Lake to infer limnological and water-level changes over the last two millennia. The two coring locations, at modern water depths of 7.5 and 11.5 m, were selected to contrast inferences for past lake level based on distance from the modern water-depth boundary between predominantly benthic and planktonic diatom assemblages in surface sediments (6.0 m). Diatom-inferred depth inferences were more variable in the core from 7.5-m water depth, consistent with the hypothesis that the greatest changes occurred at water depths closest to the benthic:planktonic boundary. Both sites revealed a pronounced drought from ~AD 950 to 1300, synchronous with the medieval climate anomaly (MCA). This finding suggests a northeast expansion of the arid MCA into northwest Ontario, extending the known spatial extent of this megadrought. Scaled chrysophytes and sedimentary pigments suggest a recent increase in thermal stratification. Our findings indicate this region is more susceptible to changes in moisture than was previously suspected based on the instrumental record for the past century.     

Use of sedimentary diatoms from multiple lakes to distinguish between past changes in trophic state and climate: evidence for climate change in northern Germany during the past 5,000 years

Fossil diatoms from lake sediments have been used to infer both past trophic state and climate conditions. In Europe, climate reconstructions focused on northern and alpine regions because these areas are climatically sensitive and anthropogenic impact was low. In contrast, anthropogenic impact was often high in the central European lowlands, such as northern Germany, beginning in the Neolithic Age, ~3700 BC. Since that time, trophic state change was the main factor that affected diatom assemblages in central European lowland lakes. Therefore, it was considered difficult or impossible to identify climate changes in the region using sedimented diatoms. We used diatom assemblage changes, diatom-inferred total phosphorus concentrations and the relative abundance of planktonic diatoms from sediments of three lakes that differ in their location, size, morphology, catchment area and current trophic state to test whether we could distinguish between trophic state and climate signals over the past 5,000 years in northern Germany. In this study, changes in trophic state and climate were well differentiated. In the study lakes, relative abundance of planktonic diatoms seems to be linked to the length of lake mixing phases. Planktonic diatom abundance decreased during years with shorter mixing duration, and these shorter mixing times probably reflect colder winters. The diatom-inferred periods of short mixing phases from 1000 BC to AD 500 and from AD 1300 to 1800 coincide well with two known cooling phases in Europe and the North Atlantic region.     

Diatom responses to late Quaternary vegetation and climate change,and to deposition of two tephras in an alpine and a sub-alpine lake in Yoho National Park,British Columbia

The late Quaternary diatom records from alpine Opabin Lake (altitude 2285 m a.s.l.) and sub-alpine Mary Lake (altitude 2054 m a.s.l.), located in Yoho National Park, British Columbia (lat. 51 ° 21N; long. 116 ° 20), have been analyzed, and changes in these records have been used to reconstruct lake histories. The results have also been related to independently inferred vegetation and climate changes. Following deglaciation, when both lakes were receiving high inputs of clastic materials, benthic diatom taxa dominate the records of these two shallow lakes with small species ofFragilaria being particularly prominent. During the early to mid-Holocene period, when treeline was at a higher elevation than today, the diatom flora of both lakes became more diverse with previously minor species becoming more prominent.Cyclotella radiosa occurs in cores from both Mary Lake, and much deeper, neighbouring Lake O'Hara during the warm early Holocene, and may reflect this warmer climate, a longer ice-free season than presently, and perhaps less turbid water, or its presence may reflect a subtly higher nutrient status of the lake water during this period. The Neoglacial is marked by increased amounts of sediments originating from glacial sources in Opabin Lake, which undoubtedly led to very turbid water, and by the presence ofEllerbeckia arenaria f.teres andCampylodiscus noricus v.hibernica in Opabin Lake; however, these species are absent from Mary Lake which has not been influenced by either glacial activity since the recession of the glaciers prior toc. 10 000 years BP or water originating from Opabin Lake. The impact of the two tephras during the Holocene was dramatic in terms of increased diatom production, as exemplified by the increases in diatom numbers, but there was little effect upon species composition. The diatom records and changes in the diatom:cyst ratio suggest that the chemical status of these two small, shallow lakes has changed little during the Holocene, other than after deposition of the two tephras. These results provide evidence that shallow alpine and high sub-alpine lakes are sensitive recorders of past environmental changes.     

Lake-Sediment Records of Recent Environmental Change on Svalbard: Results of Diatom Analysis

Surface sediments from 23 lakes on Svalbard were analysed for diatoms. About 182 taxa were found but samples generally have a low richness, with the majority of sites dominated by benthic genera such as Fragilaria, Navicula, and Achnanthes. Centric Cyclotella species occur at only three sites. Modern relationships between diatom abundance and water chemistry and other environmental variables were explored numerically and a preliminary transfer function for pH was developed. Lakes fell into three groups on the basis of their diatom assemblages: (1) high pH, high cation, high conductivity sites characterised by Amphora libyca, (2) shallow sites with relatively high nutrient values characterised by Fragilaria species, and (3) more acid and dilute sites with high amounts of snow cover in the catchments characterised by small Achnanthes species and Navicula digitulus. Five sediment cores representing the recent past were also analysed for diatoms. Three shallow sites were dominated by Fragilaria species throughout the period represented by the cores and no shifts in inferred pH were found. At two deeper sites (Arresjøen, Birgervatnet) major assemblage shifts are found which are unrelated to independent evidence for atmospheric contamination. Early (ca. 1200 AD) changes found at both sites are possibly related to the onset of the ‘Little Ice Age’. Later changes are neither synchronous nor similar in nature and might be best explained as individual responses to the recovery from the ‘Little Ice Age’ and subsequent climatic warming.     

Diatoms indicate that calcium decline,not acidification,explains recent cladoceran assemblage changes in south-central Ontario softwater lakes

In recent decades, softwater lakes across Canada have experienced a wide array of anthropogenic influences, with acidification and climate warming of particular concern. Here, we compare modern and pre-industrial sedimentary diatom assemblages from 36 softwater lakes located on the Canadian Shield in south-central Ontario to determine whether lake acidification or reduced calcium availability was the main stressor responsible for recent declines in Ca-sensitive cladoceran taxa. Regional surveys of south-central Ontario water chemistry have identified the pH recovery of many formerly acidified lakes, and our fossil diatom-inferred pH analyses indicate that modern lakewater pH in the 36 study lakes is similar to (or higher than) pre-industrial levels, with diatom assemblages from both time periods dominated by taxa with similar pH preferences. In addition, modern diatom assemblages compared to pre-industrial assemblages contained higher relative abundances of planktonic diatom taxa (e.g. Asterionella formosa and the Discostella stelligera complex) and lower relative abundances of heavily silicified diatoms (e.g. Aulacoseira taxa) and benthic fragilarioid taxa. These taxonomic shifts are consistent with warming-induced changes in lake properties including a longer ice-free period, decreased wind speed and/or increased thermal stability. We conclude that recent changes observed within the cladoceran assemblages of these lakes are not a response to acidification, but are likely a consequence of Ca declines. In addition, our data suggest that regional climate warming is now responsible for the diatom changes observed in this region.     

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.     

Faunal (Chironomidae, Cladocera) responses to post-Little Ice Age climate warming in the high Austrian Alps

Present climate warming strongly affects limnological and ecological properties of lakes and may cause regime shifts that alter structure and function in the water bodies. Such effects are especially pronounced in climatologically extreme areas, e.g. at high altitudes. We examined a sediment core from Lake Oberer Landschitzsee, Austrian Alps, which spans the period from the Little Ice Age (LIA) to present. We investigated whether post-LIA climate warming altered aquatic invertebrate communities and limnological status in this sensitive high Alpine lake. Fossil Cladocera (Crustacea) and Chironomidae (Diptera) and organic matter in the core were analyzed. Chironomids were used to assess the lake??s benthic quality (i.e. oxygen availability). An instrumental Alpine temperature record was used to assess whether changes in the biotic assemblages correspond to post-LIA temperature trends. The planktonic and macro- and microbenthic invertebrate communities exhibit almost complete and simultaneous species turnover after the LIA, from about AD 1850 onward, when Sergentia coracina-type replaced oxyphilous Micropsectra contracta-type as the dominant macrobenthic taxon, and phytophilous Acroperus harpae outcompeted Alona affinis and Alona quadrangularis in the microbenthos. These directional community shifts corresponded with a period of reduced benthic quality, higher sediment organic content, and progressive climate warming, superimposed on Alpine land-use changes, until the early twentieth century. Detected changes suggest increased productivity and lower benthic oxygen availability. Faunal shifts were even more pronounced during the late twentieth century, simultaneous with enhanced warming. A new planktonic Cladocera species, Bosmina longirostris, typically absent from high Alpine lakes, colonized the lake and gradually became dominant toward the core top. Results show that post-LIA climate warming, coupled with increasing benthic and planktonic production, substantially altered the limnological and ecological status of this remote Alpine lake. Observed faunal turnovers provide evidence that temperature-driven ecological thresholds, whether associated directly or indirectly with greater human activity, have been crossed. Species abundances and distributions have changed in response to post-LIA and late twentieth century climate warming.     

The Holocene–Anthropocene transition in lakes of western Spitsbergen,Svalbard (Norwegian High Arctic): climate change and nitrogen deposition

Lake sediments from four small lakes on western Spitsbergen (Svalbard Archipelago, Norwegian High Arctic) preserve biostratigraphic and isotopic evidence for a complex suite of twentieth century environmental changes. At Lake Skardtjørna and Lake Tjørnskardet on Nordenskiöldkysten, there is a marked diatom floristic change coupled to increased diatom concentrations beginning around 1920. At Lake Istjørna and Lake Istjørnelva, 25 km southwest of Longyearbyen, both diatom total valve and chrysophyte stomatocyst concentrations have increased dramatically since the beginning of the 1900s. The early twentieth century changes are probably related to climate warming after the Little Ice Age. However, the most pronounced changes in diatom assemblages seem to have occurred in the last few decades. At the same time, nitrogen stable isotopes in sediment organic matter in two of the lakes became progressively depleted by ~2‰, which is consistent with diffuse atmospheric inputs from anthropogenic sources and attendant fertilization. These data suggest that climate change and nitrogen deposition may be acting together in driving these lakes towards new ecological states that are unique in the context of the Holocene.     

Lake Jezero v Ledvici (NW Solvenia) – changes in sediment records over the last two centuries

Lake Jezero v Ledvici (NW Slovenia) is a 14 m deep mountain lake at an elevation of 1860 m, situated on limestone bedrock. It is an oligotrophic, alkaline and hard-water lake with a transparency of about 14 m and has suffered several times over recent centuries from strong earthquakes. In 1996 five sediment cores, between 35 and 45 cm long, were collected from the deepest part of the lake and analysed to reconstruct environmental changes over the last few centuries. The data indicate changes induced by pollution and climate change during the last two centuries similar to those in other European mountain and remote lakes. However, at this site earthquakes have also affected the lake and partly obscure the interpretation of the sediment record. From 1780 to 1890, sediment records show low abundance of diatoms and relatively high abundance of Cladocera. After 1890, the number of diatoms started to increase coinciding with a strong earthquake. From the beginning of the 20th century, concentrations of spheroidal carbonaceous particles (SCP), diatom valves and head capsules of chironomids gradually increased whilst in Cladocera the main difference observed was a change in the proportion of benthic taxa. After 1960, in parallel with a rise in air temperature, a further increase in accumulation rate of diatoms started, but there was a decrease in Cladocera.     

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.     

Holocene environmental change at Lake Njulla (999 m a.s.l.), northern Sweden: a comparison with four small nearby lakes alongan altitudinal gradient

We assess Holocene environmental change at alpine Lake Njulla(68°22N, 18°42E, 999 m a.s.l.) innorthernmost Sweden using sedimentary remains of chironomid head capsules anddiatoms. We apply regional calibration sets to quantitatively reconstruct meanJuly air temperature (using chironomids and diatoms) and lake-water pH(using diatoms). Both chironomids and diatoms infer highest temperatures(1.7–2.3°C above present-day estimates, includinga correction for glacio-isostatic land up-lift by0.6°C) during the early Holocene (c.9,500–8,500 cal. yrs BP). Diatoms suggest a decreasing lake-waterpH trend (c. 0.6 pH units) since the early Holocene. Usingdetrended canonical correspondence analysis (DCCA), we compare the Holocenedevelopment of diatom communities in Lake Njulla with four other nearby lakes(Lake 850, Lake Tibetanus, Vuoskkujávri, Vuolep Njakajaure) locatedalong an altitudinal gradient. All five lakes show similar initial DCCA scoresafter deglaciation, suggesting that similar environmental processes such ashigh erosion rates and low light availability associated with high summertemperature appear to have regulated the diatom community, favouring highabundances of Fragilaria species. Subsequently, the diatomassemblages develop in a directional manner, but timing and scale ofdevelopment differ substantially between lakes. This is attributed primarily todifferences in the local geology, which is controlling the lake-waterpH. Imposed on the basic geological setting, site-specific processessuch as vegetation development, climate, hydrological setting andin-lake processes appear to control lake development in northernSweden.     

A Holocene paleosalinity diatom record from southwestern Saskatchewan, Canada: Harris Lake revisited

Fossil diatoms were analysed from a 10.3 m core from Harris Lake, Cypress Hills, Saskatchewan, and a diatom-salinity transfer function was used to construct a history of Holocene salinity changes for the lake. The diatom paleosalinity record indicates that Harris Lake remained fresh <0.5 g l-¹ throughout the Holocene, with only slight increases in salinity between approximately 6500 and 5200 years BP. This interval corresponds to the only period in the lake's history when planktonic diatoms were abundant; benthic Fragilaria taxa, mainly F. pinnata, F. construens and F. brevistriata were dominant throughout most of the Holocene. The shift from a benthic to a planktonic diatom flora between 6500 and 5200 years BP may be an indirect response to a warmer climate that reduced forest cover in the watershed and allowed greater rates of inorganic sedimentation. The small salinity increase that accompanies the floristic change is probably not the result of lower lake levels; in fact the lake was probably deeper at this point than in the later Holocene. This paleosalinity record indicates that Harris Lake did not experience episodes of hypersalinity during the mid-Holocene, as suggested by a previous study, and that the lake may have been fresh during the early Holocene as well.     

A reference typology of low alkalinity lakes in the UK based on pre-acidification diatom assemblages from lake sediment cores

This paper has two aims: (1) to show for the first time how a natural typology can be established using palaeoecological methods; and (2) to show how it can be used in lake restoration studies with respect to the definition of recovery targets for acidified lakes. By defining the characteristic reference assemblages for low alkalinity site types rather than for a specific site it allows success to be measured more broadly, unconstrained by the specific composition of the pre-acidification flora. We analyse statistically the pre-acidification diatom assemblages of sediment cores from 121 low alkalinity lakes in the UK in order to assess whether a reference typology for such lakes can be defined on the basis of their diatom floras. We use samples dating to ~1850 AD to represent pre-acidification conditions. The results show that three main clusters can be identified, two dominated by benthic taxa (Clusters 1 and 3) and one dominated by planktonic taxa (Cluster 2). Cluster 1 is characterised by taxa such as Brachysira vitrea, Cymbella microcephala and Fragilaria spp., Cluster 2 by Cyclotella comensis, C. radiosa, Asterionella formosa, Aulacoseira subarctica and Achnanthes minutissima and Cluster 3 by Eunotia incisa, Frustulia rhomboides var. saxonica, Fragilaria virescens var. exigua, and Cymbella perpusilla. Although environmental data for 1850 AD are not available it is apparent from the contemporary distribution of the taxa in the different clusters that Cluster 2 represents the most alkaline pre-acidification conditions. Some sites in this cluster have been acidified, but some, especially the larger, deeper lakes have been enriched. Cluster 1 includes sites that contain diatoms with relatively high pH optima (pH 6.4–7.4) whereas Cluster 3 sites contain diatoms with the lowest pre-acidification pH conditions in the data-set. Sites in this cluster also have the lowest base cation concentrations at the present day and include the sites in the UK that have been most affected by acid deposition.     

Water depth is a strong driver of intra-lake diatom distributions in a small boreal lake

There has been much debate over the relative importance of environmental selection and spatial variation on community organization in microorganisms. To assess the importance of environmental or spatial variables in diatom species assemblages in Gall Lake, northwest Ontario, 41 surface-sediment samples were collected in a two-dimensional gridded pattern along and across depth contours. A depth-constrained cluster analysis separated the diatom flora into three communities: a shallow-water benthic zone (B1); a deeper-water benthic zone (B2); and a planktonic zone (P). Redundancy analysis (RDA) confirmed that water depth was a major predictor of variation in the flora. Further RDAs and variation partitioning using orthogonal polynomials and Moran’s eigenvector maps showed that spatial location had minimal effect on the diatom assemblages. Principal components analysis grouped the diatom flora not only by assemblage, but also by water depth, regardless of two-dimensional spatial separation, suggesting the importance of the environmental gradients associated with lake depth. These findings indicate that environment is a more important explanatory variable than spatial variables for diatoms within lakes, suggesting dispersal plays a limited role in intra-lake diatom distributions.     

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 Paleolimnological Record of Holocene Climatic and Environmental Change in the Altai Mountains, Siberia

The sedimentary diatom records of three shallow lakes in the Altai Mountains, southern Siberia, were examined to assess the nature and timing of Holocene environmental changes. Few paleoenvironmental records, especially reconstructions not based on pollen, have been reported from this region. The lakes differ in elevation, annual precipitation, and catchment vegetation. Diatom assemblages in all lakes were dominated for the entire period of record by small benthic species of Pseudostaurosira Williams & Round, Staurosira Ehrenberg, and Staurosirella Williams & Round. Planktonic taxa only occur in very low abundances (<5%). The most diverse diatom flora was found in Dzhangyskol, which is situated at the lowest elevation within a forested catchment. A lack of detailed information on the ecological preferences of the dominant taxa and the complexity of environmental drivers make direct interpretation of the diatom record difficult. However, other proxies suggest that dramatic shifts in dominance between Staurosira elliptica and Staurosirella pinnata in Grusha Ozero reflect millennial-scale variability in climate. Together, chironomids and diatoms provide evidence of a cooling possibly correlative to the Younger Dryas Stade and subsequent early-Holocene warming consistent with pollen evidence of afforestation, which also is likely linked to increased humidity. By ~6000 cal year BP, the transition to a cooler, more continental climate had begun. The diatom record of Akkol shows significantly less variation in diatom community composition, but biogenic silica accumulation rates, a proxy for diatom productivity, appear to reflect climatic variability driven by insolation trends over the past 8000 years. Long-term variability in Dzhangyskol is not clearly linked to climate.     

Diatom-based Holocene paleoenvironmental records from continental sites on northeastern Ellesmere Island, high Arctic, Canada

Stratigraphic changes in diatom assemblages from four small lakes on northeastern Ellesmere Island, high Arctic, Canada, provide a proxy lake-ice cover and paleoenvironmental record. Low absolute diatom abundances and a benthic Fragilaria (sensu lata) dominated assemblage during the postglacial (< 7.6 ka B.P.) to mid-Holocene record the moderating effects of locally retreating glaciers. Around 5.5 ka B.P. diatom concentrations begin to rise, reaching their highest levels (10⁹ valves per g dry sediment) between 4.2 and 3 ka B.P., interpreted to be the warmest period in this region. Topoclimatic differences between lakes on Hazen Plateau and those lower in Lake Hazen Basin account for the initial decline in diatom abundances in the upper lakes after 3 ka B.P. This change is thought to reflect a lowering of the regional snowline, accordant with widely recognized Neoglacial advances on Ellesmere Island and Greenland. Lakes in lower Lake Hazen Basin maintained extensive summer ice free conditions until ~ 1.9 ka B.P., after which diatom abundances declined, suggesting prolonged summer lake-ice cover through the remainder of the recovered Holocene record. Differences between the records presented here and those from coastal areas of the Canadian high Arctic highlight the unique topoclimatic characteristics and continentality of the Lake Hazen region, and possible effects that local marine environments may have had on coastal records. Such differences serve to demonstrate the inherent geographic variability of paleoenvironmental records from the high Arctic.