Shifts In Diatom Assemblages Research Articles

Holocene paleoenvironmental and paleoclimatic variability in a high mountain lake in Sierra Nevada (Spain): Insights from diatom analysis

This study examines the Holocene history of Río Seco Lake (3040 m a.s.l; Sierra Nevada, Southern Spain) by analysing diatom remains and other paleoenvironmental data. The aim is to understand the impact of long-term environmental and climatic variability on the aquatic ecosystem over the past 21,000 years. Our results suggest that shifts in diatom assemblages were mainly climate-driven in terms of temperature and water availability. The absence of diatom remains during the Late Pleistocene indicated low temperatures and prolonged lake snow cover. Five distinct periods were identified during the Holocene. The high abundance of epiphytic and bog-inhabiting taxa and tychoplanktonic Tabellaria flocculosa in the period 11,000–6700 cal yr BP were indicative of a humid climate. The onset of the tychoplanktonic Aulacoseira alpigena between 6700 and 5100 cal yr BP indicated a drop in temperature. These changes intensified during the period 5100–3300 cal yr BP, when the most significant changes in diatom assemblages took place with the dominance of A. alpigena and an abrupt increase in the abundance of the epiphytic Fragilaria radians. During the subsequent period (3300–1500 cal yr BP), the significant declines in A. alpigena and in epiphytic taxa were indicative of increased aridity and higher alkalinity values due to increased aridity and Saharan dust input during this period. The last period (1500–256 cal yr BP) was characterized by a rise in the abundance of Staurosirella pinnata, indicative of warmer temperatures and higher alkalinity values coincident with a marked increase in proxies of temperature and aridity. The increase in aridity and temperature during the last period, which has led to changes in diatom assemblages, is a matter of great concern in an ecosystem that is particularly susceptible to global warming.

Cyclical browning in a subtropical lake inferred from diatom records

Changing climate and land use activity are altering inputs of colored dissolved organic carbon (cDOC) into lakes. Increased cDOC reduces water transparency (browning) and changes lake physicochemistry, with biological consequences. Identifying the drivers and effects of changing cDOC inputs is critical for mitigating the consequences of climate change on lake ecosystems through adaptive watershed management. This study focused on determining the drivers of lake browning by evaluating shifts in diatom assemblages in subtropical, oligotrophic Lake Annie (FL, United States), which is known to experience climate-driven oscillations in transparency associated with watershed inputs of cDOC. We combined long-term limnological monitoring data and paleolimnological techniques to determine how diatoms respond to changes in cDOC and to infer past cDOC fluctuations relative to records of past climate and land use changes in the watershed. Diatom assemblage composition in a 14-year phytoplankton dataset was strongly correlated with cDOC-driven transparency fluctuations. Likewise, diatom assemblages in the upper 35 cm of the sediment core, which corresponded to a 35-year lake monitoring record, were also strongly related to past water transparency, yielding a strong transfer function (paleo model, R 2 = 0.72). When the model was applied to the diatom record from a 166-cm sediment core, diatom-inferred transparency and rates of nutrient input showed that localized ditching in the 1930s enhanced the effect of climate oscillations on water transparency, intensifying cyclical browning thereafter. Integration of long-term monitoring and paleoecological data provided valuable insights into the history of the aquatic ecosystem, enabling implementation of adaptive management strategies to contend with a changing climate.

Impacts of Anthropogenic Disturbances on Diatom Diversity in a Shallow Spring-Fed Pool

Human impacts cause significant problems for shallow lakes in the karst regions of South China. In recent years, the ecological issues related to managing and restoring these critical aquatic ecosystems have come to the fore. Lingshui Pool (LSP) is one of the most-studied spring-fed lake ecosystems in south China, and it has well-preserved chronological profiles. The LSP was a very shallow lake before dam construction in 1962. Due to the dam’s construction, rapid development and urbanization, this important freshwater ecosystem has suffered severe eutrophication and environmental degradation. In order to determine the environmental effects of recent anthropogenic disturbances and establish conservation recommendations for this lake ecosystem, we analyzed diatoms and weak-acid leachable elements from a well-dated sediment core from LSP to reconstruct the changes in aquatic conditions and diatom diversity during the period AD 1960–2013. This revealed shifts in diatom assemblages, diversity, and geochemistry that were strongly linked to the construction of a dam in 1962 and rapid eutrophication after 1990. Diatom diversity was stable and relatively high when submerged macrophytes were present between 1962 and 1990. Thus, the reintroduction of endemic macrophytes could benefit the ecological stability and diversity of this shallow lake.

Mountain aquatic Isoëtes populations reflect millennial-scale environmental changes in the Bohemian Forest Ecosystem, Central Europe

In this study we aim to investigate millennial-scale dynamics of Isoëtes, a type of macrophyte well adapted to oligotrophic and clear-water lakes. Despite its wide distribution during the Early Holocene, nowadays Isoëtes is considered as vulnerable or critically endangered in many Central European countries. Using a multi-proxy palaeoecological reconstruction involving Isoëtes micro- and megaspores, pollen, plant macrofossils, macro-charcoal, diatoms and chironomids from four lakes (Prášilské jezero, Plešné jezero, Černé jezero, Rachelsee) located in the Bohemian Forest Ecosystem mountain region in Central Europe, we reconstruct Isoëtes dynamics and discuss how local environmental factors impacted its distribution and abundance during the Holocene. Our results show regionally concurrent patterns of Isoëtes colonisation across all lakes beginning 10,300–9300 cal yr BP, and substantially declining around 6400 cal yr BP. Results from Prášilské jezero imply that Isoëtes decline and collapse in this lake reflect gradual dystrophication that led to the browning of lake water. This is evidenced by a shift in diatom assemblages towards more acidophilous taxa dominated by Asterionella ralfsii and by a decrease in total chironomid abundance and taxa sensitive to low oxygen levels. Dystrophication of Prášilské jezero was linked with the immigration of the late-successional tree taxa ( Picea abies and later Fagus sylvatica and Abies alba), peatland expansion, and decreasing fire activity. Multi-site comparison of pollen records suggest that these vegetation-related environmental changes were common for the whole region. Our study demonstrates the sensitivity of Isoëtes to millennial-scale natural environmental changes within the surrounding lake catchment.

Climate and land-use as the main drivers of recent environmental change in a mid-altitude mountain lake, Romanian Carpathians.

Recent decades have been marked by unprecendented environmental changes which threaten the integrity of freshwater systems and their ecological value. Although most of these changes can be attributed to human activities, disentagling natural and anthropogenic drivers remains a challenge. In this study, surface sediments from Lake Ighiel, a mid-altitude site in the Carpathian Mts (Romania) were investigated following high-resolution sedimentological, geochemical, environmental magnetic and diatom analyses supported by historical cartographic and documentary evidence. Our results suggest that between 1920 and 1960 the study area experienced no significant anthropogenic impact. An excellent correspondence is observed between lake proxy responses (e.g., growth of submerged macrophytes, high detrital input, shifts in diatom assemblages) and parameters tracking natural hydroclimate variability (e.g., temperature, NAO). This highlights a dominant natural hydroclimatic control on the lacustrine system. From 1960 however, the depositional regime shifted markedly from laminated to homogenous clays; since then geochemical and magnetic data document a trend of significant (and on-going) subsurface erosion across the catchment. This is paralleled by a shift in lake ecosystem conditions denoting a strong response to an intensified anthropogenic impact, mainly through forestry. An increase in detrital input and marked changes in the diatom community are observed over the last three decades, alongside accelerated sedimentation rates following enhanced grazing and deforestation in the catchment. Recent shifts in diatom assemblages may also reflect forcing from atmospheric nitrogen (N) deposition, a key recent drive of diatom community turnover in mountain lakes. In general, enhanced human pressure alongside intermittent hydroclimate forcing drastically altered the landscape around Lake Ighiel and thus, the sedimentation regime and the ecosystem’s health. However, paleoenvironmental signals tracking natural hydroclimate variability are also clearly discernible in the proxy data. Our work illustrates the complex link between the drivers of catchment-scale impacts on one hand, and lake proxy responses on the other, highlighting the importance of an integrated historical and palaeolimnological approach to better assess lake system changes.

A 1600-year record of human impacts on a floodplain lake in the Mississippi River Valley

In North America, land use practices of the last two centuries have strongly influenced aquatic communities and freshwater quality, but the impacts of prehistoric land use on freshwater resources remain poorly documented. Here we investigate the influence of prehistoric and historical land use on Horseshoe Lake, Illinois, USA, an oxbow lake in a floodplain of the Mississippi River that is adjacent to Cahokia, the largest prehistoric indigenous population center north of Mexico. Diatom assemblages from Horseshoe Lake’s sedimentary record track shifts in aquatic environmental conditions over the last ca. 1600 years. During the period of prehistoric population growth and agricultural intensification associated with Cahokia’s emergence (ca. 600–1200 CE), the relative abundance of Aulacoseira granulata—a planktonic diatom associated with shallow eutrophic lakes—increased. Following the abandonment of Cahokia in the 14th century CE, the diatom flora of the lake shifted from planktonic Aulacoseira taxa to the epiphytic taxa Cocconeis and Gomphonema. This shift in diatom assemblages is consistent with a reduction of nutrient inputs to the lake and/or reduced fishing pressure as prehistoric populations abandoned the area. Following the intensification of historic settlement after 1800 CE, diatom assemblages shift to epipelic species of small Staurosira and Fragilaria, indicating a reduction in aquatic macrophytes and increased turbidity. Our results document prehistoric indigenous impacts on a freshwater system beginning nearly 1000 years before European colonization of the Americas and demonstrate the antiquity of human impacts on freshwater resources in North America.

Major climatic influences on Yellowstone-region lake ecosystems suggested by synchronous transitions in Late-Glacial and early-Holocene diatom assemblages

Diatom assemblages that span the late-glacial and early-Holocene period were analyzed from three small lakes in the Greater Yellowstone region. The sites vary in surficial geology, elevation, vegetation, and precipitation seasonality, and these differences provide a tool to evaluate the importance of large-scale climatic change relative to localized catchment proces%s on aquatic ecosystem evolution. All assemblages underwent synchronous shifts in community structure in the intervals of 13.9-13.8, 13.2-13.1, 11.3-11.0 and 8.8-8.7cal ka. The shift at 11.3-11.0cal ka also is evident in other proxy records from these sites, such as pollen and charcoal, which suggests large-scale climatic influences on both aquatic and terrestrial ecosystems. Because the shifts in diatom assemblages are synchronous in lakes with variable geological settings and local precipitation patterns, we infer that large-scale climatic change was the principal driver of aquatic ecosystem evolution in this region during the late-Glacial and early Holocene periods.

Ice-cover is the principal driver of ecological change in High Arctic lakes and ponds.

Recent climate change has been especially pronounced in the High Arctic, however, the responses of aquatic biota, such as diatoms, can be modified by site-specific environmental characteristics. To assess if climate-mediated ice cover changes affect the diatom response to climate, we used paleolimnological techniques to examine shifts in diatom assemblages from ten High Arctic lakes and ponds from Ellesmere Island and nearby Pim Island (Nunavut, Canada). The sites were divided a priori into four groups (“warm”, “cool”, “cold”, and “oasis”) based on local elevation and microclimatic differences that result in differing lengths of the ice-free season, as well as about three decades of personal observations. We characterized the species changes as a shift from Condition 1 (i.e. a generally low diversity, predominantly epipelic and epilithic diatom assemblage) to Condition 2 (i.e. a typically more diverse and ecologically complex assemblage with an increasing proportion of epiphytic species). This shift from Condition 1 to Condition 2 was a consistent pattern recorded across the sites that experienced a change in ice cover with warming. The “warm” sites are amongst the first to lose their ice covers in summer and recorded the earliest and highest magnitude changes. The “cool” sites also exhibited a shift from Condition 1 to Condition 2, but, as predicted, the timing of the response lagged the “warm” sites. Meanwhile some of the “cold” sites, which until recently still retained an ice raft in summer, only exhibited this shift in the upper-most sediments. The warmer “oasis” ponds likely supported aquatic vegetation throughout their records. Consequently, the diatoms of the “oasis” sites were characterized as high-diversity, Condition 2 assemblages throughout the record. Our results support the hypothesis that the length of the ice-free season is the principal driver of diatom assemblage responses to climate in the High Arctic, largely driven by the establishment of new aquatic habitats, resulting in increased diversity and the emergence of novel growth forms and epiphytic species.

Drivers of Change in a 7300-Year Holocene Diatom Record from the Hemi-Boreal Region of Ontario, Canada.

A Holocene lake sediment record spanning the past 7300 years from Wishart Lake in the Turkey Lakes Watershed in the Hemi-Boreal of central Ontario, Canada, was used to evaluate the potential drivers of long-term change in diatom assemblages at this site. An analysis of diatom assemblages found that benthic and epiphytic taxa dominated the mid-Holocene (7300–4000 cal yr BP), indicating shallow, oligotrophic, circum-neutral conditions, with macrophytes present. A significant shift in diatom assemblages towards more planktonic species (mainly Cyclotella sensu lato, but also several species of Aulacoseira, and Tabellaria flocculosa) occurred ~4000 cal yr BP. This change likely reflects an increase in lake level, coincident with the onset of a more strongly positive moisture balance following the drier climates of the middle Holocene, established by numerous regional paleoclimate records. Pollen-inferred regional changes in vegetation around 4000 yrs BP, including an increase in Betula and other mesic taxa, may have also promoted changes in diatom assemblages through watershed processes mediated by the chemistry of runoff. A more recent significant change in limnological conditions is marked by further increases in Cyclotella sensu lato beginning in the late 19th century, synchronous with the Ambrosia pollen rise and increases in sediment bulk density, signaling regional and local land clearance at the time of Euro-Canadian settlement (1880 AD). In contrast to the mid-Holocene increase in planktonic diatoms, the modern increase in Cyclotella sensu lato likely indicates a response to land use and vegetation change, and erosion from the watershed, rather than a further increase in water level. The results from Wishart Lake illustrate the close connection between paleoclimate change, regional vegetation, watershed processes, and diatom assemblages and also provides insight into the controls on abundance of Cyclotella sensu lato, a diatom taxonomic group which has shown significant increases and complex dynamics in the post-industrial era in lakes spanning temperate to Arctic regions.

A quantitative framework for analysis of regime shifts in a Galápagos coastal lagoon

Regime shifts are often used to describe sharp changes between two or more ecological states, each characterized by their own dynamics, stochastic fluctuations, or cycles. Ecological theory indicates they can occur either as a result of an abrupt environmental forcing (extrinsic regime shift), or be indicative of complex responses to local‐scale dynamics and thresholds (intrinsic regime shift). One important area of ecological research is to develop quantitative tools to analyze regime shifts, but there are few studies that have applied these methods to the long‐term ecological record. In this study, we introduce a framework to investigate regime shifts in diatom assemblages and mangrove ecosystem dynamics in a coastal lagoon from the Galápagos Islands over the past 2600 years. The framework integrates a set of established statistical methodologies for investigating regime shift dynamics in long‐term ecological records. We use these methods to (1) identify the presence of regime shifts; (2) test for a series of hypothetical relationships (i.e., linear through to threshold) between ecological response and environment using nonlinear regression; and (3) investigate the relative importance of intrinsic and extrinsic dynamics in response to environmental perturbations. The transitions in the diatoms closely track the sequence of disturbance, recovery, and habitat shifts that have occurred in the lagoon over the past 2600 years, demonstrating extrinsic responses to environmental forcing. In contrast, the shift from a mangrove‐ to a microbial mat‐dominated system ∼945 cal yr BP provides potential evidence of an intrinsic regime shift. Our framework enables robust interpretations into the underlying dynamics of regime shifts in the paleoecological record and is widely applicable for investigating abrupt ecological changes in a range of systems.

Diatom flickering prior to regime shift

ARISING FROM R. Wang et al. , 419–422 (2012)10.1038/nature11655 Potential early warning signals for regime shifts are studied intensively in the field of ecology1,2,3,4. Wang and colleagues1 investigated changes in the sediment diatom composition of Lake Erhai, China, and concluded that a regime shift in diatom assemblages that occurred around 2001 was preceded by flickering behaviour for 10 to 30 years. We propose that their results may be more reflective of their data processing than of the diatom data. Although flickering behaviour before regime shifts may be observed in some palaeoenvironmental records, we question whether this behaviour applies to diatoms in Lake Erhai. There is a Reply to this Brief Communication Arising by Wang, R. et al. Nature 498, http://dx.doi.org/10.1038/nature12273 (2013).

Distribution of Siliceous-Walled Algae in Taylor Valley, Antarctica Lakes

The McMurdoDryValleysofAntarcticaare a unique environment characterized by extreme lows in temperature and precipitation, which supports a low diversity microbial and multicellular fauna and flora. Terrestrial biomass is largely limited to soil microbes and mosses, while perennially ice-covered lakes host aerobic and anaerobic microbial communities, algae, and a low diversity eukaryotic fauna. This study provides a scanning electron microscope survey of the distribution of siliceous-walled algae in the water columns and surface sediments of fourTaylorValleylakes. No patterns of distribution of algae, chrysophyte cysts and diatoms, are detected, suggesting that cores taken from perennially ice-covered lakes contain basin-wide records, rather than records specific to the lake depth or other lake-specific criteria. Since Taylor Valley lakes became perennially ice-covered, shifts in diatom assemblages in cores are more likely to record changes to sediment and microfossil transport, e.g. the dominance of eolian vs. stream input, rather than other ecological conditions. Basin-wide records are episodically overprinted by lake-specific events, as demonstrated by a marked increase of the stream diatom genus Hantzschia during a period of increased stream flow into East Lake Bonney.

Using paleolimnology to track Holocene climate fluctuations and aquatic ontogeny in poorly buffered High Arctic lakes

Fossil diatom assemblages, and spectrally-inferred dissolved organic carbon (DOC) and sedimentary chlorophyll-a (SedChla) were analysed on lake sediment cores from two poorly buffered lakes on Pim Island (High Arctic Canada) to assess their responses to Holocene climate changes and to document lake ontogeny. Following deglaciation, diatom assemblages were dominated by small benthic Fragilaria sensu lato taxa. During the mid-Holocene, there was an abrupt shift to more circumneutral and slightly acidophilous taxa dominated by Achnanthes and Navicula taxa. In the most recent sediments, we recorded an increase in the planktonic taxon Cyclotella radiosa. This shift of the last century is the most ecologically unique in the Holocene record and is indicative of longer ice-free summers consistent with modern climate warming. Inferred DOC and SedChla track some of the main Holocene climatic trends documented in the region, including the Holocene Thermal Maximum and Neoglacial period; however, changes in lakewater DOC did not likely drive any of the recorded shifts in diatom assemblages. Compared to nearby well-buffered sites, our poorly buffered lakes recorded a more dynamic diatom response to Holocene environmental change. The decreasing trend in diatom-inferred pH is likely due to changes in the acid neutralizing capacity (ANC) driven by the release of alkalinizing base cations from the easily weathered glacial deposits in the early Holocene and later by climate-driven pH dynamics and within-lake dissolved inorganic carbon (DIC) dynamics. The diatom community composition in our study lakes is different and undergoes greater changes than in nearby well-buffered lakes suggesting that softwater lakes in the high Arctic may respond most sensitively to climate and other environmental stressors.

Long-term water quality changes in a multiple-stressor system: a diatom-based paleolimnological study of Lake Simcoe (Ontario, Canada)

Diatom-based paleolimnological techniques were used to assess long-term changes in the water quality of Lake Simcoe (Ontario, Canada) using210Pb-dated sediment cores from four sites across the lake. Modest lake-wide shifts in diatom community composition occurred in the late 19th and early 20th centuries, suggesting that early cultural disturbances (e.g., land clearance and canal construction) had relatively minor effects on Lake Simcoe water quality. However, starting in the 1930s, phosphorus loading to the lake increased, which was closely tracked by increases in diatom taxa indicative of eutrophic conditions. The most pronounced lake-wide shift in diatom assemblages occurred in the mid- to late 20th century, the nature and timing of which strongly suggested a response to regional climate warming. An additional and marked lake-wide shift in diatom assemblages occurred in the mid-1990s, coinciding with the invasion of dreissenid mussels in Lake Simcoe. Our results indicate that Lake Simcoe water quality has been affected by the interaction of numerous environmental stressors over the past two centuries, the complexity of which has been amplified by recent warming.

Arctic lake ontogeny across multiple interglaciations

Few lakes in the Arctic preserve sediments older than Holocene age because of pervasive glacial scour during the last ice age. Here we present sediment diatom and geochemical records from a lake on east-central Baffin Island (CF8, Nunavut, Canada) that captures three successive interglacial periods within the last 200,000 years: a portion of marine isotope stage (MIS) 7; the last interglacial (MIS 5e); and the Holocene (MIS 1). An additional unit of diatom-rich organic sediment occurs between the latter two intervals, and is ascribed to MIS 5a interstadial conditions. The Lake CF8 paleolimnological record reveals similar ontogenetic trends within each interglacial. Early postglacial environments in both the Holocene and MIS 5e were characterized by a dominance of colonial benthic fragilarioid diatoms that thrived in relatively alkaline waters. These species shifts do not coincide with major changes in base cation delivery to the sediments, suggesting that shifts in diatom assemblages are controlled mainly by climate-driven pH dynamics. Diatom assemblages then transitioned into dominance by tychoplanktonic Aulacoseira species, likely in response to climate-driven pH dynamics. The highest sustained abundances of Aulacoseira occur in sediments ascribed to MIS 5e, and this is the only interval in which thalassiosiroid centric taxa also occur (e.g. Discostella and Puncticulata spp.). Given that these planktonic taxa typically reach high abundances during extended periods of open-water conditions, we surmise that MIS 5e was the warmest interval recorded. The overall similarity of lake ontogenetic trajectories recorded within each interglacial period suggests that climatic and edaphic factors drive a complex succession of environmental changes through indirect effects on lake ice cover, habitat availability, and lake-water pH. In recent decades (past ∼50 yrs), an unprecedented increase in periphytic Eunotia taxa is evident; these diatoms were only present intermittently during previous glacial–interglacial cycles. The expansion of Eunotia is attributed to increased habitat availability associated with declining ice cover during recent 20 th century warming. By integrating data from several distinct interglacials, the CF8 diatom record considerably extends our understanding of Arctic lake ontogeny, and provides an unparalleled natural archive with which to compare recent changes associated with anthropogenic climate warming.

Climate-driven shifts in diatom assemblages recorded in annually laminated sediments of Sacrower See (NE Germany)

Sacrower See is a eutrophic lake with annually laminated sediments extending back to A.D. 1868. Analysis of annual layers revealed multi-decadal periods of distinct diatom assemblages at A.D. 1868–1875, 1876–1940, 1941–1978, and 1979–2000. Detrended correspondence analysis performed on individual seasonal sediment layers showed decadal-scale patterns of turnover in the diatom flora. The spring–summer layers showed higher sample scores until the early 1960s, after which the differences with the autumn–winter layers became smaller. Rates-of-change analysis revealed that the seasonal variability in diatom assemblages was higher than the annual changes. Summer diatom rates of change over the period A.D. 1894–1960 was on average higher than for winter, whereas between the 1960s and 1970s the winter rates of change became higher than the summer ones. Redundancy Analyses showed that seasonal temperatures and wind strength were significant explanatory variables for diatom assemblages in both annual and seasonal layers. These results suggest that meteorological changes indirectly affected diatom assemblages via the mixing regime of the lake. A comparison of the diatom rates of change with the amplitude of inter-annual climate change shows a statistically significant correlation for the spring-summer layers in the period of A.D. 1963–2000, showing that the sensitivity of diatom assemblages to meteorological changes has varied over the past century, with a stronger effect on diatoms registered during the past 40 years.

Watershed-scale reconstruction of middle and late Holocene paleoenvironmental changes on Melville Peninsula, Nunavut, Canada

Arctic regions have heightened sensitivity to climatic changes, however regional scale differences exist in the magnitude and timing of biotic responses. Melville Peninsula in the east-central Canadian Arctic is a transitional area with respect to modern climate, with a continental climate to the west, and more maritime influences to the east. This, and the presence of Thule archaeological sites, make the Peninsula an area of high importance for paleoclimate reconstructions. Lake sediment cores from two adjacent ultra-oligotrophic and very dilute lakes (SP02 and SP04) from interior Melville Peninsula (68°33′N and 83°17′W, 220 m a.s.l.) were analyzed for diatom assemblages (microscopic algae) to track paleoclimatic changes. High magnetic susceptibility, coarse-grained sediments, low organic matter, and lack of diatoms in sediments older than 6000 years BP confirm earlier suggestions of a persistent ice cap on the Peninsula. Peak diatom concentration and biogenic silica, and maximum values for sediment organic matter, between 4400 and 2900 years BP may indicate the Holocene Thermal Maximum (HTM). The timing of the HTM is similar to that of the Baffin region to the east. Decreases in these proxies, as well as small shifts in diatom assemblages following 2900 years BP indicate the advancement of Neoglacial cooling. The changes of the pre-industrial portion of the last millennium were of smaller magnitude than those of the HTM, but subtle changes in diatom assemblages after 750 years BP may be indicative of the Little Ice Age, the timing of which agrees well with that of Boothia Peninsula to the west. The Anthropocene is well marked in these records, with increases in diatom species richness from a pre-industrial maximum of 28–51 diatom species in the modern assemblage at Lake SP02. Planktonic diatoms appear in the 20th century, signalling profound ecological changes associated with longer ice-free seasons.

Diatom changes in two Uinta mountain lakes, Utah, USA: responses to anthropogenic and natural atmospheric inputs

Diatom assemblages in sediments from two subalpine lakes in the Uinta Mountains, Utah, show asynchronous changes that are related to both anthropogenic and natural inputs of dust. These lakes are downwind of sources of atmospheric inputs originating from mining, industrial, urban, agricultural and natural sources that are distributed within tens to hundreds of kilometers west and south of the Uinta Mountains. Sediment cores were retrieved from Marshall and Hidden lakes to determine the impacts of atmospheric pollution, especially metals. Paleolimnological techniques, including elemental analyses and 210Pb and 239+240Pu dating, indicate that both lakes began receiving eolian inputs from anthropogenic sources in the late 1800s with the greatest increases occurring after the early 1900s. Over the last century, sediments in Marshall Lake, which is closer to the Wasatch Front and receives more precipitation than Hidden Lake, received twice the concentrations of metals and phosphorus as Hidden Lake. Comparison of diatom and elemental data reveals coeval changes in geochemistry and diatom assemblages at Marshall Lake, but not at Hidden Lake; however, a major shift in diatom assemblages occurs at Hidden Lake in the seventeenth century. The change in diatoms at Marshall Lake is marked by the near disappearance of Cyclotella stelligera and C. pseudostelligera and an increase in benthic, metal-tolerant diatoms. This change is similar to changes in other lakes that have been attributed to metal pollution. The marked change in diatom assemblages at Hidden Lake indicates a shift in lake-water pH from somewhat acidic to circumneutral. We hypothesize that this change in pH is related to drought-induced changes in input of carbonate-rich desert dust.

Paleolimnologic Evidence for Recent Eutrophication in the Valley of the Great Lakes (Mongolia)

Climate warming and major land-use changes have profoundly affected the Mongolian landscape in the past several decades. Previous studies have recognized the impacts of a warmer, more arid climate and Mongolia’s 1991 transition from a command to a market economy on terrestrial ecosystems, including impaired sustainability of subsistence herding and threats to wild animals. In this study, we examined the combined effects of changing climate and herding practices on lake eutrophication in Western Mongolia. We sampled 65 lakes for modern nutrients and found the majority of lakes were eutrophic to hyper-eutrophic. Sediment cores were taken from five of the lakes to compare current lake status to paleolimnologial measures of lake eutrophication over the past 100–2000 years, including changes in diatom assemblages, diatom-inferred total phosphorus, biogenic silica, organic matter, and sediment accumulation rates. Variance partitioning analysis showed that recent shifts in diatom assemblages were related to changes in both climate and herding practices. The results presented here demonstrate a need for further study and long-term monitoring of water quality in Mongolia to understand the complicated interactions of climate and land use on aquatic resources and to preserve water quality in this remote and ecologically important region.

An approach to the recent environmental history of Pilica Piaski spring (southern Poland) using diatoms

Diatom floras from springs have received little attention until recently, despite the fact that springs provide specific conditions that cannot be found in any other aquatic system, and remain of great importance in terms of general environmental changes. Due to pollution of the Krakow-Czestochowska upland aquifers, the chemical composition of spring waters in the area is anthropogenically modified, and far from the natural state. In order to generate a baseline diatom flora for springs, a paleolimnological study was applied as one of the best recognized and applied methods used to track long-term environmental changes worldwide. Little is known, however, about the direction and nature of such changes in aquatic environments fed by springs. The present article focuses on shifts in diatom assemblages preserved in sediments collected from a small pond, situated close to several spring outlets. They were interpreted as a record of environmental changes that had taken place during the last century. For most of the history of the pond—as recorded in this 84 cm long core—the diatom assemblage was dominated by small Fragilaria spp. The major shifts in species composition began in a core depth of 65 cm with a decline in Fragilaria construens var. venter (Ehrenberg) Grunow and Fragilaria pinnata Ehrenberg complexes, and a concurrent increase in Achnanthidium minutissimum (Kutzing) Czarnecki. The second change was recorded at the depth of 45 cm with a sharp decline of A. minutissimum, which again was replaced by the small Fragilaria spp. In recent years, further changes in the diatom assemblage occurred, with a notable increase in Cyclotella delicatula Hustedt. The assemblage shifts recorded at this site appear to be consistent with environmental changes triggered by land use (e.g. agriculture intensity) and/or possible changes in spring water discharge.