Diatom-inferred pH Research Articles

Responses of diatom composition and teratological forms to environmental pollution in a post-mining lake (SW Poland)

Various types of environmental stressors, such as heavy metals, lignite residues, or extremely low water pH, cause the formation of teratological forms of diatoms during the reproductive cycle. The damage mainly includes an abnormal diatom valve outline, changes in the striation pattern, anomalies in the raphe line/sternum, reduction in the cell size, metabolism alterations, or combined deformities. Diatom remains with such morphological deformities occur in the sediments of post-mining lake ŁK-46 (SW Poland). This lake was formed by excavation after the end of lignite exploitation in the mid-1960s. The percentage of teratological valves in the studied lake ranged from 12 to 33% of the relative abundance. The majority of abnormal diatoms have been observed in species of Eunotia. The reconstruction of the diatom-inferred pH (DI-pH), together with the concentration of heavy metals in the sediments and contemporary measurements of the water pH, indicates that the lake was contaminated to varying degrees and was acidic from the beginning of lake creation until today. The lake is located in an area of acid mine drainage (AMD), and in its vicinity, there are overburden heaps containing lignite residues, which are eroded and supplied to the lake, constantly acidifying the water. In the youngest sediments, the concentration of heavy metals increased, while the percentage of abnormal diatoms decreased. Determining which factor, i.e., extremely low water pH and the presence of lignite or heavy metals, was responsible for the formation or reduction of teratogenic forms of diatoms is very difficult.

Developing diatom-based inference models to assess lake ecosystem change along a gradient of metal smelting impacts: Sudbury lakes revisited.

Mining and smelting activities have strongly influenced the Sudbury region (Ontario, Canada) since the late 19th century, leading to acidification and metal contamination in many local ecosystems. Regulations on restricting acidic emissions were enacted in the 1970s, after which a considerable volume of paleolimnological work was completed to study the impacts of acidification on Sudbury-region lakes and their subsequent biological recovery. Twenty years after the last regional diatom-based assessment, many lakes have undergone large changes in limnological variables, including increases in pH and dissolved organic carbon concentrations, as well as decreases in metal concentrations. Additionally, these lakes are under the potential impacts of newly emerging environmental stressors such as climate warming and road salt contamination. Here, we revisited a suite of Sudbury-region lakes (n = 80) to examine the relationships between their current water chemistry and diatom assemblages preserved in surface sediments using a canonical correspondence analysis. Although the pH gradient in our study lakes is shorter (pH ~1.4) than in earlier calibration studies conducted in this region, lake water pH was still identified as the strongest environmental variable shaping diatom distributions and was used to construct a robust inference model (R2boot = 0.73; RMSEP = 0.32). By assessing ecological changes experienced by a subset of these Sudbury-region lakes (n = 33) over the past few decades, we identified two major trends: an overall increase in diatom-inferred pH and a rise in the relative abundance of planktonic taxa. Our study provides useful insights into the autecology of major diatom taxa in acidified waters and highlights the importance of considering other anthropogenic stressors when assessing the recovery response of acid-impacted systems.

Paleolimnological evidence for lacustrine environmental evolution and paleo-typhoon records during the late Holocene in eastern Taiwan

The late Holocene lacustrine environmental evolution and paleo-typhoon records are poorly understood around lowland Liyu Lake and mountain Tunlumei Pond in eastern Taiwan. In this study, we use records of diatom populations, magnetic susceptibility, total organic carbon, carbon-to-nitrogen ratio, and δ13C from two lacustrine sediment cores in eastern Taiwan to reconstruct the paleoenvironment and the paleo-typhoon records during the late Holocene. Significant changes in lithology and in multi-proxy data in the record from lowland Liyu Lake (LYL) show that LYL became hydrologically isolated by 2850 cal BP, possibly as a result of landslide-induced alluvial fan formation. Synchronous increases of the diatom-inferred lake level proxy for LYL and regional precipitation proxies reflect a strengthened East Asia summer monsoon since 1600 cal BP. Eutrophication of the water in LYL is also inferred, and this is interpreted to be related to agricultural activities, which provides evidence for changes in land use during the past 200 years. The occurrence of diatom valves in sediment of the mountain Tunlumei Pond (TLM) indicates that the pond area has been stable since 760 cal BP, which reflects an increase in the local precipitation. The decrease of diatom-inferred pH indicates an increase in the input of acid runoff from the watershed during typhoon-triggered heavy rainfall. Both lacustrine records suggest that the typhoon intensity increased during the early Little Ice Age. The sediment records in northeastern Taiwan also suggest an increase in typhoon activity during the late Little Ice Age. The asymmetric pattern of typhoon intensity during the last 1000 years is interpreted to reflect the control exerted by anomalies in both global temperature and the El Nino-Southern Oscillation intensity on typhoon tracks over several centuries.

Diatom-based reconstructions of hydrological variations and the underlying mechanisms during the past ∼520 years in the central Tianshan Mountains

In order to deepen our understanding of water environment-related issues in the central Tianshan Mountains (TCTMs), it is necessary to study the change processes of hydrological variables and the underlying mechanisms within a longer time window that goes beyond the instrumental period. This study focuses on a high-resolution lacustrine sediment core drilled from an alpine lake in TCTMs with a hope that palaeohydrological changes inferred from diatom assemblages may provide an insight on details of water environment evolutions. The diatom inferred temperature was broadly consistent with the temperature records in its adjacent areas, representing a temperature trough during ∼1490 to ∼1835 CE and a general rising trend since ∼1835 CE. The diatom inferred pH (DI-pH) was generally low during ∼1490 to ∼1835 CE, and was high with a declined trend during ∼1835 to ∼2013 CE, being negatively correlated with TN or TOC and positively correlated with carbonate in sediments significantly. The diatom inferred conductivity (DI-Cond) was characterized with a significantly high salinity interval during ∼1490 to ∼1835 CE than that during ∼1835 to ∼2013 CE. The salinity during the past ∼520 years was modulated by interplays among precipitation driven by the Atlantic Multidecadal Oscillations, temperature-dictated evaporation, and temperature-led glacier melting. The water environment changes (lakewater temperature, DI-pH, DI-Cond and diatom assemblages) were identified with a remarkable shift at ∼1835 CE when the temperature began to climb up in TCTMs, implying that climate warming not only exerted influences on water resources but also on water properties in TCTMs.

The diatom-inferred pH reconstructions for a naturally neutralized pit lake in south-west Poland using the Mining and the Combined pH training sets

Many artificial lakes in various parts of the world were formed as a result of the exploitation of lignite and other mineral deposits. Usually, lakes created after the end of lignite mine activity have extremely acidic or very acidic water, with pH<3.0, caused by the oxidation of pyrite and marcasite and other sulfide compounds. After the closure of mines, the lakes undergo natural neutralization. However, this process is long-term and depends on many different factors, such as the amount of carbonates, the geological structure of bedrock, hydrology and hydrobiology, climate and the method of lignite exploitation. In some types of anthropogenic lakes (i.e., clay pits), water pH is almost neutral or alkaline due to the adsorption properties of clay minerals. Along the Polish-German border in the Łuk Mużakowa Geopark, over 100 post-mining lakes were formed as a result of lignite and clay exploitation. The Mining pH training set of modern diatom samples and water chemistry was created from 69 lakes to quantify the relationship between the diatom assemblages living in these water bodies and the lakes' chemical composition. Then, this database was applied to the reconstruction of diatom-inferred pH in the sediments of a post-mining lake TR-33. The results of reconstruction was compared to similar analysis based on the Combined pH training set containing more variety of diatom taxa from a higher number of lakes. Although the second dataset was larger and gave a smaller root mean square error, the Mining pH training set has a longer gradient of pH including lakes with extremely acidic water and gives more reliable results. The reconstruction of DI-pH in TR-33 which had very acidic water in the past and currently neutralized from acidification was more precise and reliable using the database sensu stricto, which consists of only the post-mining lakes.

Evolution of Lake Ailike (northern Xinjiang of China) during past 130 years inferred from diatom data

We have studied diatom assemblages of a 42-cm-long sediment core obtained from Lake Ailike in northern Xinjiang of China to reconstruct the hydrological and ecological variations of the past 130 years. Three environmental parameters of lake water (i.e., diatom-inferred pH, diatom-inferred total phosphorus and diatom-inferred conductivity) were estimated by comparing the fossil diatom assemblages from the Lake Ailike core with the modern diatom assemblages in the European Diatom Database. The reconstruction exhibited two major stages: stage A (1884–1960) was a “natural” stage and stage B (1960–2013) was a “human” stage. The “natural” stage can be further divided into two sub-stages: A-1 (1884–1920) was characterized by an averagely low lake area and A-2 (1920–1960) was a stably high lake-area period. The comparison between the reconstructed lake-area variations from Lake Ailike with the treering-recorded Palmer Drought Severity Index (PDSI) variations from nearby Hutubi shows that the low lake area from 1884 to 1920 was correspondent with a dry period and that the high lake area from 1920 to 1960 with a wet period. The similarity between PDSI from Hutubi and the Atlantic Multidecadal Oscillation (AMO) implies a climatic linkage. The linkage is revealed by the tight in-phase relationship between AMO and SWP (Siberian warm-season precipitation), that is, the AMO-promoted SWP might have extended its influence to northern Xinjiang. The “human” stage can also be divided into two sub-stages: B-1 (1960–2000) was a regressing stage and B-2 (2000–2013) was a transgressing stage. The man-made regression led to dramatic increases in TP of the lake water, TOC and TN of the sediments and also in the resulted decrease in lake water pH. Fortunately, the Irtysh-Karamay Canal started to inject water into the lake in 2001, resulting in a constant expansion of lake-covered area and also in a constant improvement of ecological conditions.

Limited acid deposition inferred from diatoms during the 20th century — A case study from lakes in the Tatra Mountains

Mountain lakes are usually sensitive to the effects of global and regional environmental changes. Since the second half of the 20th century, surface-water acidification has become a significant ecological problem, and many lakes in Europe and North America have anthropogenically acidified. Additionally, following reduction in emissions of sulfur (S) and nitrogen (N) compounds, recovery from acidification has been observed in many lakes. In this study, we used changes in diatom communities to reconstruct the pH histories based on changes recorded in nine Tatra lakes (Western Carpathians, Poland) since approximately 1850AD. Overall, results indicate that acidic precipitation had little influence on lake-water pH in the Tatra Mountain lakes. Changes in diatom-inferred pH (DI-pH) generally were small and showed little evidence of acidification during the time of the highest air pollution (since the 1960s), and have shown little change since the reduction of acidic deposition since the 1990s. Lakes that showed some evidence of acidification included dystrophic lakes with low acid neutralizing capacity. However, as illustrated by the PCA trajectories of the diatom assemblages, the majority of the lakes currently contain diatom assemblages that are unlike the diatom floras that existed ca. 1850.

The relative influences of climate and volcanic activity on Holocene lake development inferred from a mountain lake in central Kamchatka

A sediment sequence was taken from a closed, high altitude lake (informal name Olive-backed Lake) in the central mountain range of Kamchatka, in the Russian Far East. The sequence was dated by radiocarbon and tephrochronology and used for multi-proxy analyses (chironomids, pollen, diatoms). Although the evolution of Beringian climate through the Holocene is primarily driven by global forcing mechanisms, regional controls, such as volcanic activity or vegetation dynamics, lead to a spatial heterogeneous response. This study aims to reconstruct past changes in the aquatic and terrestrial ecosystems and to separate the climate-driven response from a response to regional or localised environmental change. Radiocarbon dates from plant macrophytes gave a basal date of 7800calyr BP. Coring terminated in a tephra layer, so sedimentation at the lake started prior to this date, possibly in the early Holocene following local glacier retreat. Initially the catchment vegetation was dominated by Betula and Alnus woodland with a mosaic of open, wet, aquatic and semi-aquatic habitats. Between 7800 and 6000calyr BP the diatom-inferred lake water was pH4.4–5.3 and chironomid and diatom assemblages in the lake were initially dominated by a small number of acidophilic/acid tolerant taxa. The frequency of Pinus pumila (Siberian dwarf pine) pollen increased from 5000calyr BP and threshold analysis indicates that P. pumila arrived in the catchment between 4200 and 3000calyr BP. Its range expansion was probably mediated by strengthening of the Aleutian Low pressure system and increased winter snowfall. The diatom-inferred pH reconstructions show that after an initial period of low pH, pH gradually increased from 5500calyr BP to pH5.8 at 1500calyr BP. This trend of increasing pH through the Holocene is unusual in lake records, but the initially low pH may have resulted directly or indirectly from intense regional volcanic activity during the mid-Holocene. The chironomid-inferred July temperature reconstruction suggests cool periods between 3200–2800calyr BP and 1100–700calyr BP and a warmer period between 2800 and 1100calyr BP. Chironomid and diatom DCA scores decline from ca. 6000calyr BP, indicating compositional changes in these aquatic assemblages. In comparison declines in pollen PCA scores are delayed, starting ca. 5100calyr BP. The results suggest that while catchment vegetation was responding primarily to climate change, the biota within the lake and lake water chemistry were responding to localised environmental conditions.

Establishing realistic management objectives for urban lakes using paleolimnological techniques: an example from Halifax Region (Nova Scotia, Canada)

To determine pre-disturbance limnological conditions, evaluate the impact of environmental stressors (surface water acidification, nutrient inputs, climate change, and winter deicing salt), and set realistic recovery targets for lake management strategies, a rapid assessment paleolimnological approach was used to determine the amount (and likely causes) of environmental changes over the past ∼100–150 years in 51 urban lakes from Halifax, Nova Scotia (Canada). Diatom assemblages from lake sediment cores were used with “top” (recently deposited, surface) samples being matched to measured limnological conditions, and “bottom” (generally from >15 cm deep) samples used to infer pre-disturbance limnological conditions such as pH, total phosphorus (TP), specific conductance, and shifts due to changing climatic conditions. Environmental change was assessed by calculating the metric of change in species composition between present-day and pre-disturbance diatom assemblages, and inferences from quantitative estimates of diatom-inferred pH, specific conductance, and TP. All 51 study lakes have experienced floristic changes in diatom species composition since pre-disturbance times, but different environmental stressors were implicated: 8 of the 51 lakes underwent significant (i.e., >2 times the root mean-squared error [2X RMSE] of the inference model) decreases in diatom-inferred pH; 8 lakes had significant increases in diatom-inferred TP; and 19 otherwise relatively pristine lakes had increases in planktonic taxa consistent with observations linked to changes in lake seasonality and limnological changes most closely linked to climate warming in Nova Scotia and other regions. The remaining 16 lakes did not have large and consistent changes in diatom flora or changes in diatom-inferred TP or pH >2X RMSE of the prediction of the models. Lake-specific factors were related to these inferences, and the lakes that acidified were mainly the currently most acidic sites, whereas those that experienced issues related to eutrophication were generally among the most alkaline sites. Of our 51 lakes, 22 (including some experiencing pH, TP, or showing floristic changes linked to climate changes) had increases in measured conductivity (1980–2002) and, correspondingly, increased relative abundances of halophilic diatom taxa. These lakes, often with catchments containing high surface areas of impervious surfaces, are examples of a trend of increasing salinity in northeastern North American lakes likely related to winter application of deicing (road) salt. The application of this paleolimnological approach enabled us to identify which lakes have undergone significant changes in diatom assemblages, as well as which environmental stressor(s) were most probable. This information can help lake managers develop more targeted and effective management strategies.

Late Holocene environment of subalpine northeastern Taiwan from pollen and diatom analysis of lake sediments

We used multi-decadal pollen and diatom records from sediment core TFL-1 from Tsuifong Lake to reconstruct the vegetation dynamics and hydroclimate in northeastern Taiwan during the past 3500cal BP. Coarse grained sediments in association with higher percentages of wetland pollen (Cyperaceae) and upper conifer pollen (Tsuga and Pinus) in the lower part of the core indicate low lake levels and a relatively cold/dry climate between 3500 and 2030cal BP, reflecting a decline of the East Asian summer monsoon (EASM). Muddy sediments coupled with reduction of wetland pollen represent the rise of lake levels, implying the re-strengthening of the EASM during the past 2000years. Paleotemperature was inferred from the variation of pollen origin from the upper and lower mountain forest, indicating the global temperature anomalies of the Medieval Warm Period (MWP) and the Little Ice Age (LIA). In comparison to the main climate forces in the North Pacific, we suggest that the long-term climatic trend in Taiwan was controlled by variations in EASM intensity, while increased precipitation over the past 2000years may also be linked to warmer sea surface temperature (SST) of the western Pacific Warm Pool (WPWP) and increased El Niño-Southern Oscillation (ENSO) events, which increased typhoon intensity. Higher diatom-inferred pH during 2930–2030cal BP and the LIA suggest strong hydrological disturbances, reflecting more typhoons passing over Taiwan. The frequent typhoon events could be linked by an abrupt shift of typhoon track, due to the reduction of the WPWP and expansion of the Northwestern Pacific High, which move typhoons in a more westerly direction.

Post-glacial diatom-inferred aquatic changes in Sicamous Creek Lake, British Columbia, Canada

Diatom analyses of sediments from a high elevation lake situated in an Engelmann Spruce - Subalpine Fir (ESSF) forest of south-central British Columbia, Canada, reveal long-term climate and water chemistry change. During the transition from the late-glacial / Pleistocene to the xerothermic early Holocene, small, benthic Fragilaria diatoms species that grew under low light conditions in Sicamous Creek Lake gave way to planktonic Cyclotella species that require open-water conditions. Warm temperatures in the mesothermic Holocene are indicated by smaller Cyclotella species and large, benthic pennate diatoms. Diatom communities reflected Neoglacial cooling in the late Holocene, with abundant Nitzschia fonticola and Achnanthes minutissima. Small, benthic Fragilaria regained abundance, suggesting cooling and conditions similar to the late-glacial interval. Diatom community composition responded to the deposition of the Mt. Mazama and Mt. St. Helens tephras, though the Mazama eruption caused greater change in relative abundance of various taxa within the assemblage. Correspondence analysis shows distinct communities have occurred since the initiation of sedimentation, likely due to climate controlled landscape and vegetation changes; diatom-inferred pH values using various models and training sets show limited acidification change occurred through the lake’s history.

Recent environmental change in an upland reservoir catchment: a palaeoecological perspective

Reservoirs provide important water resources and require careful management, through ecological monitoring, to identify and mitigate changes in water quality. Long-term data on vegetation changes and the impacts of human activities on reservoir water chemistry, however, are often limited. Setting restoration targets can therefore be problematic. Palaeoenvironmental research has made little use of reservoir sediments and there is great potential for palaeoecological data to be incorporated into management planning. Diatoms and pollen were analysed in sediment cores from Venford Reservoir, southwest England, to infer pH and land-use changes, respectively, over the last century. Diatom-inferred (DI) pH indicates that reservoir pH declined from ~pH 6.0 in the early part of the record and reached a low between AD 1920 and 1940 (~pH 5.6), which was likely associated with fossil fuel combustion and acid deposition. DI-pH then increased, but values remained relatively low, even in the most recent sediments (~pH 5.7), and the magnitude of inferred pH change over time was small. Land-use changes, such as increased grazing intensity and erosion, and establishment of pine plantations, also likely influenced reservoir water chemistry changes over time. Understanding the impacts of such factors on water chemistry has implications for future catchment land-use planning, which is essential for managing water resources. The pollen record indicates a shift from heather-dominated to grass-dominated vegetation since ~AD 1935–1950, which could be related to increased grazing intensity. The palaeoecological dataset is valuable as a long-term record against which short-term monitoring datasets and future changes can be assessed.

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.

Paleolimnological assessment of limnological change in 10 lakes from northwest Saskatchewan downwind of the Athabasca oils sands based on analysis of siliceous algae and trace metals in sediment cores

The extraction of bitumen from the Athabasca oil sands is rapidly expanding, and emission of sulphur and nitrogen oxides has substantially increased. To determine whether lakes downwind of this development in northwest Saskatchewan have been detrimentally impacted since development of the oil sands, a paleolimnological assessment of ten lakes was carried out. Analysis of diatom valves and inferences of diatom-inferred pH indicated that emissions have not resulted in widespread chronic acidification of acid-sensitive lakes ~80–250 km east and northeast of the oil sands development around Fort McMurray and Fort Mackay. However, one of the closest sites to the development indicated a slight decline in diatom-inferred pH, but the two next closest sites, both of which had higher alkalinity, did not show any evidence of acidification. There were also no consistent trends in the concentration or flux of total or individual priority pollutants including lead, mercury, copper, zinc and vanadium. The sedimentation rates in most lakes increased since the mid-1900s, along with increased flux of both diatoms and scaled chrysophytes. Subtle changes in the species assemblages of diatoms and increased flux of diatoms and chrysophyte scales are consistent with recent climate change in this region.

Contrasts between dystrophic and clearwater lakes in the long‐term effects of acidification on cladoceran assemblages

Summary1. Most studies on zooplankton responses to acidification have focused on clearwater lakes with a dramatic acidification history. The role of dissolved organic carbon (DOC) in moderating zooplankton responses to acidification in naturally acidic, dystrophic lakes is less well understood and is partially impeded by a lack of baseline data.2. Cladocera leave identifiable remains preserved in lake sediments that can be used to provide information on pre‐industrial species assemblages and their responses to environmental stressors such as acidification. Therefore, we used palaeolimnological approaches to track cladoceran assemblage responses to acidification since c.1850 (inferred from sedimentary diatom assemblages) in three acidified lakes in Kejimkujik National Park (Nova Scotia, Canada) that differ markedly in DOC content. These include two highly dystrophic lakes (Kejimkujik and Pebbleogittch lakes), and one clearwater lake (Beaverskin Lake).3. In dystrophic Pebbleogittch Lake, an increase in the acid‐tolerant, jelly‐clad, pelagic taxon Holopedium glacialis occurred coincident with diatom‐inferred pH (DI‐pH) declines, but no other notable cladoceran assemblage shifts occurred. Similarly, Cladocera assemblages did not appear to respond to lakewater acidification in dystrophic Kejimkujik Lake.4. In contrast, in the clearwater Beaverskin Lake, several observed shifts in cladoceran assemblage corresponded to DI‐pH declines, including an increase in the proportion of littoral taxa and an increase in Hill’s N2 species diversity. This may indicate increased water clarity as a result of acidification‐related decreases in DOC, which may have enhanced growth of emergent aquatic macrophytes and improved visibility for planktivorous fish, leading to increased predation on pelagic taxa. Species shifts within the littoral assemblage of Beaverskin Lake may reflect the differing tolerances of littoral taxa to low pH and aluminium toxicity.5. Overall, our results suggest that cladoceran assemblages in naturally acidic, dystrophic lakes may be resilient against additional pH declines related to industrial emissions of acidifying agents, as dystrophic lakes are less vulnerable to increased aluminium toxicity and acidification‐induced increases in water clarity and often have a pre‐industrial cladoceran assemblage already adapted to acidic conditions.

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.

From cold to cool in northernmost Norway: Lateglacial and early Holocene multi-proxy environmental and climate reconstructions from Jansvatnet, Hammerfest

A multi-proxy palaeoecological study of the lateglacial and early Holocene sediments of Jansvatnet, Hammerfest, northernmost Norway (70°39′ N) showed that cold and arid conditions prevailed in both the lateglacial interstadial and the Younger Dryas. Terrestrial proxies are macrofossils and pollen. Aquatic proxies are plant and invertebrate macrofossils, pollen, diatoms, and chironomids. Mean July temperatures were reconstructed using pollen and chironomid calibration functions and ecological knowledge of the fossil flora and fauna. Lake-water pH was reconstructed using a diatom pH-calibration function. Above sterile basal deglacial silts, biotic activity was detected around 14600 years ago in the interstadial (chronologically equivalent to the Bølling-Allerød in the Greenland Ice-Core Chronology). Catchment vegetation resembled polar desert and ultra-cold stenothermic chironomids lived in the lake. However, diatom assemblages were diverse and dynamic. In the Younger Dryas stadial, conditions deteriorated. In the early Younger Dryas chironomid-inferred air temperatures (CI-Tjul) fell about 1 °C. Pollen-inferred temperatures (PI-Tjul) did not fall and the terrestrial vegetation hardly changed because of the extreme aridity. The lake water was turbid from suspended clay which diminished aquatic life. Later in the Younger Dryas (ca 12400 cal yr BP) reconstructed mean July temperatures fell by a further 3 °C and were close to the minimum to support life, at around 3–4 °C. However, decreased turbidity allowed moss growth on the lake bottom that provided habitats for invertebrates and diatoms. In the last 200 years of the Younger Dryas temperatures increased by 2–3 °C and terrestrial and aquatic organisms responded quickly. At the start of the Holocene a rapid increase of more than 3 °C in PI-Tjul to 9.5 °C initiated the replacement of sparse arctic tundra by low-arctic dwarf-shrub heath. Simultaneously, a further 2 °C increase in CI-Tjul to 10–11 °C reflected a regime shift in the lake. Aquatic macrophytes rapidly replaced the moss carpet, diatom assemblages stabilised and diatom plankton developed, and cold stenotherm chironomids were replaced by cool-temperate taxa. Productivity increased as CI-Tjul reached a steady maximum of around 12 °C and PI-Tjul reached 10.5 °C at about 10000 cal yr BP. At this time, tree-birch arrived and woodland developed over the catchment. Birch could have immigrated from the south up the west coast or from the east along the Barents Sea coast. Acidophilous aquatic taxa increased as diatom-inferred pH gradually decreased from 7.3 to 6.6. After ca 9380 cal yr BP, the density and extent of birch woodland decreased and species of exposed rocky areas increased, reflecting increased windiness, precipitation, and possibly a small decrease in July temperatures. Birch now formed the arctic tree-line at Jansvatnet. The lateglacial climatic pattern at Jansvatnet resembles patterns detected elsewhere in northern Norway and Svalbard and in marine records from the Norwegian and Barents Seas but contrasts with patterns in southern Norway.

The complementary power of pH and lake-water organic carbon reconstructions for discerning the influences on surface waters across decadal to millennial time scales

Abstract. Lysevatten, a lake in southwest Sweden, has experienced both acidification and recent changes in the amount of lake-water organic carbon (TOC), both causing concern across Europe and North America. A range of paleolimnological tools – diatom-inferred pH, inferred lake-water TOC from visible-near-infrared spectroscopy (VNIRS), multi-element geochemistry and pollen analysis, combined with geochemical modeling were used to reconstruct the lake's chemistry and surroundings back to the most recent deglaciation 12 500 years ago. The results reveal that the recent anthropogenic impacts are similar in magnitude to the long-term variation driven by natural catchment changes and early agricultural land use occurring over centuries and millennia. The combined reconstruction of both lake-water TOC and lithogenic element delivery can explain the major changes in lake-water pH and modeled acid neutralizing capacity during the past 12 500 years. The results raise important questions regarding what precisely comprises "reference" conditions (i.e., free from human impacts) as defined in the European Water Framework Directive.

A history of acid mine contamination, recovery, and eutrophication in Sandy Lake, Pennsylvania

Sandy Lake, PA, was subject to acid mine contamination in the 1860s, which led to complete extirpation of fish. The mine effluent was soon diverted, but the watershed experienced continued deforestation, agricultural development, and population growth to the beginning of the twentieth century. Paleolimnological analyses of metals and diatoms clearly show the onset of acid mine drainage with rapid increases in concentrations of iron, manganese and sulfur, and decreases in diatom-inferred pH in the sedimentary profile, and a very rapid recovery following remediation. Diatom-inferred phosphorus suggests a period of oligotrophication following diversion of the acid mine drainage and reduction in lake catchment area. However, analyses of nutrients and algal pigments suggest continued eutrophication to the present in spite of increasing forest cover and a stable population over the last century. Pigments indicate a tenfold increase in phytoplankton, with cyanobacteria becoming a more significant portion of the biomass. Accumulation of sedimentary phosphorus has increased by a factor of 4 and sedimentary organic carbon by a factor of 5 since the mid-1800s.

Palaeolimnological assessment of lake acidification and environmental change in the Athabasca Oil Sands Region, Alberta

Exploitation of the Athabasca Oil Sands has expanded hugely over the last 40 years. Regional emissions of oxidised sulphur and nitrogen compounds increased rapidly over this period and similar emissions have been linked to lake acidification in other parts of North America and Europe. To determine whether lakes in the region have undergone acidification, 12 lakes within the Regional Municipality of Wood Buffalo and the Caribou Mountains were selected to cover chemical and spatial gradients and sediment cores were obtained for palaeolimnological analyses including radiometric dating, diatom analysis, isotopic analysis of bulk sediment 13C and 15N, and spheroidal carbonaceous particles (SCPs). All lake sediment cores show evidence of industrial contamination based on SCPs, but there is no clear industrial signal in stable isotopes. Most lakes showed changes in diatom assemblages and sediment C:N ratios consistent with nutrient enrichment over various timescales, with potential drivers including climatic change, forest fires and anthropogenic nitrogen deposition. Only one of the 12 lakes investigated showed strong evidence of acidification with a decline in diatom-inferred pH from 6.3 to 5.6 since 1970 linked to increasing relative abundances of the acidophilous diatom species Actinella punctata, Asterionella ralfsii and Fragilariforma polygonata. Analysis of mercury (Hg) in the acidified lake showed increasing sediment fluxes over the last 20 years, a possible indication of industrial contamination. The acidified lake is the smallest of those studied with the shortest residence time, suggesting a limited capacity for neutralisation of acid inputs in catchment soils or by inlake processes.