Paleo-ecotoxicology: What Can Lake Sediments Tell Us about Ecosystem Responses to Environmental Pollutants?

Diatom species responses along gradients of dissolved inorganic carbon, total phosphorus, and lake depth from lakes across Canada

Diatoms are key components of freshwater ecosystems and are regularly used for paleolimnological reconstructions, in which defining species optima and tolerances is fundamental for interpreting assemblage shifts in a sediment record. Here, we examined responses of diatoms across three major environmental gradients-dissolved inorganic carbon (range: 0.1-230.5 mg · L-1), total phosphorus (range: 3-326 μg · L-1), and maximum lake depth (range: 0.9-55.0 m)-taken from 158 lakes from across Canada. The lakes were sampled as part of the LakePulse Network, which conducted a standardized sampling of lakes spanning 12 Canadian ecozones. Hierarchical logistic regression was used to model the species responses of 37 common taxa, and species optima and tolerances were calculated with weighted average modeling. The most common response detected was the symmetrical unimodal model, suggesting we likely captured the full environmental ranges for many species, although skewed unimodal responses were also common. Indicator species analyses identified taxa with high predictive values and fidelities to particular ecozones, with high-nutrient-adapted taxa such as Stephanodiscus spp. and Cyclotella meneghiniana characteristic of the agriculturally productive Prairie region. The Prairies stood out in the dataset as the region with the most unique flora from the local contribution to beta diversity analysis. Overall, the autecological data provided by our study will allow for improved interpretations of paleolimnological records and other biomonitoring efforts, addressing management concerns and contributing to a better understanding of our changing environment.

Subfossil Chironomid Assemblages as Indicators of Remedial Efficacy in the Historically Contaminated St. Lawrence River at Cornwall, Ontario

Long-term data are required to quantify the impacts of historic industrial pollution and subsequent remedial action on the nearshore benthic community in the St. Lawrence River Area of Concern at Cornwall, Ontario. Specifically, high-quality temporal records are needed to understand changes in benthic invertebrate assemblages in response to multiple possible drivers including industrial pollution, environmental heterogeneity, and climate warming. We compare long-term records of subfossil chironomid assemblages and geochemical variables among sediment cores from two Cornwall sites with differing pollution histories and a minimally disturbed downstream reference site. Chironomids were functionally absent from the Cornwall sediment cores when mercury and zinc concentrations were elevated. As metal concentrations decreased in more recent sediment intervals, chironomid abundance and the relative abundance of pollution-sensitive taxa increased. Recently deposited sediment in all three sediment cores display increased relative abundance of warm-water, macrophyte-associated taxa. We conclude that these temporal changes in chironomid assemblages provide evidence for ecological recovery for both of the impacted sites, consistent with the objectives of the current management strategy. These findings advance our understanding of industrial impacts on fluvial chironomid ecology, directly inform local management strategies, and further develop the application of chironomids as bioindicators for contaminated sediments.

Paleolimnological evaluation of metal(loid) enrichment from oil sands and gold mining operations in northwestern Canada

Abundant reserves of metals and oil have spurred large-scale mining developments across northwestern Canada during the past 80 years. Historically, the associated emissions footprint of hazardous metal(loid)s has been difficult to identify, in part, because monitoring records are too short and sparse to have characterized their natural concentrations before mining began. Stratigraphic analysis of lake sediment cores has been employed where concerns of pollution exist to determine pre-disturbance metal(loid) concentrations and quantify the degree of enrichment since mining began. Here, we synthesize the current state of knowledge via systematic re-analysis of temporal variation in sediment metal(loid) concentrations from 51 lakes across four key regions spanning 670 km from bitumen mining in the Alberta Oil Sands Region (AOSR) to gold mining (Giant and Con mines) at Yellowknife in central Northwest Territories. Our compilation includes upland and floodplain lakes at varying distances from the mines to evaluate dispersal of pollution-indicator metal(loid)s from bitumen (vanadium and nickel) and gold mining (arsenic and antimony) via atmospheric and fluvial pathways. Results demonstrate 'severe' enrichment of vanadium and nickel at near-field sites (≤20 km) within the AOSR and 'severe' (near-field; ≤ 40 km) to 'considerable' (far-field; 40-80 km) enrichment of arsenic and antimony due to gold mining at Yellowknife via atmospheric pathways, but no evidence of enrichment of vanadium or nickel via atmospheric or fluvial pathways at the Peace-Athabasca Delta and Slave River Delta. Findings can be used by decision makers to evaluate risks associated with contaminant dispersal by the large-scale mining activities. In addition, we reflect upon methodological approaches to be considered when evaluating paleolimnological data for evidence of anthropogenic contributions to metal(loid) deposition and advocate for proactive inclusion of paleolimnology in the early design stage of environmental contaminant monitoring programs.

Paleoecotoxicology: Developing methods to assess the toxicity of lake sediment records influenced by legacy gold mining

The contamination of lakes by industrial emissions is an issue of international concern. Traditional paleolimnology examines sedimentary micro-fossils to infer the biological response to natural and anthropogenic stressors over time. Here, we calculate a theoretical biological effect for historic sediment sections using Probable Effect Concentration Quotient (PEC-Q) and arsenic specific quotient methods and develop novel time-constrained sediment toxicity test methods using a cultured Daphnia sp. combined with a whole cell microbial biosensor to assess the toxicity of past industrial contamination with modern testing methods. These methods were developed using sediments collected from Pocket Lake (Northwest Territories, Canada), a lake known to have exhibited a significant ecological shift following input from nearby gold smelter emissions during the mid 20th century. We then applied these methods to near-, mid-, and far-field sites to assess the response of Daphnia sp. to varying contaminant load. Daphnia sp. mortality exposed to dated sediments indicated a strong concordance with the timing of mining activities, and a strong concordance with PEC-Q and arsenic specific toxicity quotients. In contrast, a decrease in Daphnia mortality was observed during pre-, and post-mining periods when the contaminant burden was lower. Initial assessments of bioavailability using a microbial biosensor indicated that arsenic in porewater is 72-96% bioavailable, and limited evidence that oxidative stress may contribute to the Daphnia sp. toxic response. These results indicate that lake sediment archives can be used to infer missing biomonitoring data in sites of legacy anthropogenic influence, which will be useful for those seeking to conduct cost-effective and efficient preliminary environmental risk assessments.

Seabird-mediated transport of organohalogen compounds to remote sites (North West Greenland polynya)

The role of sea birds as carriers of pollutants over long distances was evaluated by analyzing organochlorine and organobromine compounds in lake sediment cores from three remote sites around the North Water polynya (North West Greenland). One lake, NOW5, was in the vicinity of a little auk (Alle alle L.) bird colony, whereas the other two lakes, NOW14 and Q5, were undisturbed by seabirds. The former was strongly acidic (pH = 3.4) but the latter had a pH close to 8. Due to the guano loading, NOW5 exhibited higher chlorophyll concentrations (74 μg/L) than the other two lakes (1.6-3.4 μg/L), higher content of total phosphorous (0.34 mg/L vs. 0.007-0.01 mg/L) and total nitrogen (3.75 mg/L vs. 0.21-0.75 mg/L). The concentrations of all organohalogen compounds were substantially greater in NOW5 than in the other lakes, indicating the strong influence of these seabirds in the transport and deposition of these compounds to remote sites. However, not all compounds showed the same increases. Hexachlorocyclohexanes and endosulfans were more than 18 times higher in NOW5, the drin pesticides and hexachlorobenzene (HCB), between 9.5 and 18 times and DDTs, polybromodiphenyl ethers (PBDEs), polychlorobiphenyls (PCBs) and chlordanes about 2.7-6 times. These differences demonstrated that the bird-mediated deposition has preservation effects of the less stable and more volatile compounds, e.g. those with log Kaw < -2.4, log Koa < 9 and/or log Kow < 6.8. The sedimentary fluxes of PCBs, HCHs, drins, chlordanes, PBDEs, HCB and endosulfans were highest in the upper sediment layer of the polynya lake (year 2014). In contrast, the highest DDT fluxes were found in 1980. These trends indicate that despite restrictions and regulations, bird transport continues to introduce considerable amounts of organohalogen pollutants to the Arctic regions with the exception of DDTs, which show successful decline, even when mediated by bird metabolism.

Salinity Is a Key Determinant for the Microeukaryotic Community in Lake Ecosystems of the Inner Mongolia Plateau, China

The arid and semiarid areas experienced remarkable lake shrinkage during recent decades due to intensive human activities and climate change, which would result in unprecedented changes of microeukaryotic communities. However, little is known about how climate change affects the structure and ecological mechanisms of microeukaryotic communities in this area. Here, we used an 18S rRNA gene-based high-throughput sequencing approach to explore the structure, interspecies interaction, and assembly processes of the microeukaryotic community in lake ecosystems of the Inner Mongolia Plateau. As a direct result of climate change, salinity has become the key determinant of the lacustrine microeukaryotic community in this region. The microeukaryotic community in this ecosystem can be divided into three groups: salt (Lake Daihai), brackish (Lake Dalinuoer) and freshwater lakes. Co-occurrence network analysis revealed that salinity shapes the interspecies interactions of the microeukaryotic community. This causes interspecies interactions to change from antagonistic to cooperative with an increase in salinity. Phylogenetic-based β-nearest taxon distance analyses revealed that stochastic processes mainly dominated the microeukaryotic community assembly in lake ecosystems of the Inner Mongolia Plateau, and salinity stress drove the assembly processes of the microeukaryotic community from stochastic to deterministic. Overall, these findings expand the current understanding of interspecies interactions and assembly processes of microeukaryotic communities during climate change in lake ecosystems of the Inner Mongolia Plateau.

Metal contamination in alkaline Phantom Lake (Flin Flon, Manitoba, Canada) generates strong responses in multiple paleolimnological proxies

The copper-zinc smelter at Flin Flon (Manitoba) operated between 1930 and 2010 and emitted large amounts of metal(loid)s and sulphur dioxide into the atmosphere, damaging the surrounding terrestrial landscapes and depositing airborne industrial pollutants into aquatic ecosystems. However, the extent of biological impairment in regional lakes is largely unknown. Here, we analysed biological and geochemical proxies preserved in a dated sediment core from Phantom Lake, collected seven years after the smelter closed in 2010. Our objectives were to determine how smelting history affected long-term trends in (1) sedimentary elements, (2) biota across multiple trophic levels, and (3) spectrally-inferred chlorophyll a and lake-water total organic carbon. The effects of smelting activities were clearest in the diatom record, in concordance with modest responses in chironomid and cladoceran assemblages. Several metal(loid)s were naturally high and exceeded sediment quality guidelines during the pre-smelting era. With the opening of the smelter, metal(loid) concentrations in sediments increased through the 1930s, peaked in the 1960s, and declined thereafter with technological improvements but remained above background to this day. Although modest declines in inferred lake-water total organic carbon indicate reduced terrestrial carbon supply following sulphate deposition in the catchment, the diatom record showed no evidence of acidification as the lake was and remained well-buffered. Pre-smelting diatom and invertebrate assemblages were diverse and indicated oligo-mesotrophic conditions. Smelting was associated with the loss of metal-sensitive biological indicators and the emergence of assemblages dominated by metal-tolerant, generalist taxa. Diatoms tracked substantial reductions in aerial emissions since the 1990s, particularly after the smelter closed, but also indicated that the biological effects of metal pollution persist in Phantom Lake. Examining the effects of a base metal smelter on a well-buffered lake offered insights into multi-trophic level responses to severe metal contamination and potential recovery without the confounding effects of concurrent changes in lake acidity.

Organic pollutants in deep sea: Occurrence, fate, and ecological implications

The deep sea - an oceanic layer below 200 m depths - has important global biogeochemical and nutrient cycling functions. It also receives organic pollutants from anthropogenic sources, which threatens the ecological function of the deep sea. In this Review, critically examined data on the distribution of organic pollutants in the deep sea to outline the role of biogeochemical and geophysical factors on the global distribution and regional chemodynamics of organic pollutants in the deep sea. We found that the contribution of deep water formation to the influx of perfluorinated compounds reached a maximum, following peak emission, faster in young deep waters (< 10 years) compared to older deep waters (> 100 years). For example, perfluorinated compounds had low concentrations (< 10 pg L^-1) and vertical variations in the South Pacific Ocean where the ocean currents are old (< 1000 years). Steep geomorphologies of submarine canyons, ridges, and valleys facilitated the transport of sediments and associated organic pollutants by oceanic currents from the continental shelf to remote deep seas. In addition, we found that, even though an estimated 1.2-4.2 million metric tons of plastic debris enter the ocean through riverine discharge annually, the role of microplastics as vectors of organic pollutants (e.g., plastic monomers, additives, and attached organic pollutants) in the deep sea is often overlooked. Finally, we recommend assessing the biological effects of organic pollutants in deep sea biota, large-scale monitoring of organic pollutants, reconstructing historical emissions using sediment cores, and assessing the impact of deep-sea mining on the ecosystem.

Arsenic and mercury contamination and complex aquatic bioindicator responses to historical gold mining and modern watershed stressors in urban Nova Scotia, Canada

Beginning in the late-1800s, gold mining activities throughout Nova Scotia, eastern Canada, released contaminants, notably geogenic arsenic from milled ore and anthropogenic mercury from amalgamation, to local environments via surface water flows through tailings fields. We investigated recovery from and legacy effects of the tailings field at the Montague Gold District (~1863-1940) on nearby urban lake ecosystems using geochemical measures and zooplankton remains archived in dated sediment cores from an impact (Lake Charles) and a reference (Loon Lake) lake. Sedimentary levels of total arsenic and total mercury were used to assess mining-related inputs. Arsenic concentrations remain elevated at nearly 300 times above sediment guidelines in Lake Charles surface sediments, due to its upward mobilization from enriched sediment intervals and sequestration by iron oxyhydroxides in surficial sediments. Peak mercury concentrations at Lake Charles were eight times above sediment guidelines during the mining period, and since ~1990 have recovered to levels observed before mining began. Legacy mining impacts at Lake Charles and non-mining related environmental changes in the post-1950 sediments at both lakes have thus combined to structure assemblage compositions of primary consumers. At both lakes, assemblages of pelagic-dominated Cladocera differed (p ≤ 0.05) during the mining period compared to periods before and after mining. Taxon richness differed (p ≤ 0.01) only between the pre- and post-mining periods at mining-impacted Lake Charles and reflects long-term declines of substrate-dwelling littoral taxa. Geochemical and biological recovery have not completely occurred at Lake Charles despite the mine district's closure ~80 years ago. Our findings demonstrate that impacts of ore processing and amalgamation from historical gold mining, combined with recent watershed stressors, continue to affect sedimentary arsenic geochemistry and intermediate trophic levels of nearby, downstream aquatic habitats.

Extracts from Dated Lake Sediment Cores in the Athabasca Oil Sands Region Alter Ethoxyresorufin-O-deethylase Activity and Gene Expression in Avian Hepatocytes

Increases in oil sands mining operations in the Athabasca oil sands region have resulted in increased concentrations of polycyclic aromatic compounds (PACs) and heavy metals in aquatic systems located near surface mining operations. In the present study, sediment cores were collected from 3 lakes with varying proximity to surface mining operations to determine the differences in PAC concentrations. Sediment cores were separated into 2 sections-current mining (top; 2000-2017) and premining (bottom; pre-1945)-and extracts were prepared for in vitro screening using a well-established chicken embryonic hepatocyte (CEH) assay. Concentrations and composition of PACs varied between sites, with the highest ∑PACs in Saline Lake, 5 km from an active oil sands mine site. The proportion of alkylated PACs was greater than that of parent PACs in the top sediment sections compared with the bottom. Ethoxyresorufin-O-deethylase activity in CEH permitted the ranking of lake sites/core sections based on an aryl hydrocarbon receptor-mediated end point; mean median effect concentration values were lowest for the top cores from Saline Lake and another near-mining operations lake, referred to as WF1. A ToxChip polymerase chain reaction (PCR) array was used to evaluate gene expression changes across 43 target genes associated with numerous toxicological pathways following exposure to top and bottom sediment core extracts. The 2 study sites with the greatest ∑PAC concentrations (Saline Lake and WF1) had the highest gene expression alterations on the ToxChip PCR array (19 [top] and 17 [bottom]/43), compared with a reference site (13 [top] and 7 [bottom]/43). The avian in vitro bioassay was useful for identifying the toxicity of complex PAC extracts associated with variably contaminated sediment cores, supporting its potential use for hotspot identification and complex mixture screening. EnvironToxicol Chem 2021;40:1883-1893. © 2021 SETAC.

Determining the effects of past gold mining using a sediment palaeotoxicity model

Ore processing techniques used in Yellowknife's largest mining operation, Giant Mine, is responsible for the atmospheric release of approximately 20,000 t of particulate arsenic trioxide and other heavy metal(loids). This rapid deposition of heavy metal(loids) may have caused ecological disturbances to aquatic food webs. Here we use ²¹⁰Pb and ¹³⁷Cs dated lake sediment cores from 20 lakes within a 40 km radius of Yellowknife to examine the spatial-temporal distribution of arsenic, antimony and lead. Further, we model the toxicity of the sediment to aquatic biota pre-, during, and post-mining using palaeotoxicity modelling, enrichment factor assessment, and comparisons to national sediment quality guidelines. We found that metal(loid) profiles in sediment peaked during the height of mining operations. These peak metal(loid) concentrations were highest in lakes near the mine's roaster stack, and decreased with distance from the historic mine. Palaeotoxicity modelling of lake sediment archives indicate that there is no significant difference in the mean predicted toxicity of pre- and post-mining samples (p = 0.14), however mining activities in the region significantly increased the predicted toxicity of sediments to aquatic organisms during mining operations (p < 0.001). In the years since roasting processes ceased, the mean palaeotoxicity of all lakes has decreased significantly (p < 0.05), indicating a projected pattern of biological recovery. Importantly, some lakes remain at an elevated risk, indicating that aquatic ecosystems in Yellowknife may continue to have lingering effects on aquatic biota despite the closure of the mine two decades ago.

Aquatic community structure as sentinel of recent environmental changes unraveled from lake sedimentary records from the Atacama Desert, Chile

The Atacama Desert (21-26°S) is currently one of the driest places on Earth and metal(loid)s are of special concern for this region, which hosts the largest-known porphyry copper deposits produced in Chile. Evidence of past environmental conditions is commonly preserved in natural archives, such as lacustrine sediments. Sediment records obtained from Inca Coya Lake (22°20'S-68°35'W, 2534 m.a.s.l.), a small lake located in the Atacama Desert, reflected the evolution of regional mining activity during the 20th century and sedimentation associated with decadal climate variability. We studied the aquatic community structure changes recorded in sediment records from Inca Coya Lake. By analysis of magnetic properties (susceptibility, hysteresis curves and Curie temperatures), grain size and geochemical composition of the sediments, we identified environmental periods and changes in the community of benthic and planktonic organisms (diatoms and diapausing egg bank). We identified three detrital episodes that we interpret as dry/wet phases during the last 90 years associated with the increase of flash flood events promoting hypoxia oscillations; anthropogenic (mining activity) signals were also identified. Invertebrate community structure (primary consumers) reflected the metal exposure, measured as changes in assemblage composition through species turnover. Diatom community composition was best associated with variables related to wetter/drier alternation and consequent changes in oxygen availability. Bioindicators analyzed (diatoms, diapausing egg bank and invertebrate community) demonstrated to be excellent indicators of the bioavailability of compounds in the aquatic ecosystem of Inca Coya Lake, allowing the environmental impact assessment of the water resources due to flash floods and mining activity in the driest desert of the world.

Sedimentary DNA reveals over 150 years of ecosystem change by human activities in Lake Chao, China

Understanding the extent and directionality of the impact of human activities on ecosystems is directly related to their management and protection. However, the lack of historical data limits our understanding of ecosystem changes with long-term exposure to human activities. Recently, lake sedimentary DNA (sedDNA) has become a powerful tool for revealing changes in ecosystems at the century and millennium scales. Here, we used sedDNA to reveal the dynamic of the microbial community (including bacteria and micro-eukaryotes) in Lake Chao over the past 150 years, and further explored the effects of long-term nutrient and heavy metal loads on these communities. Our data show that nutrient and heavy metal loads in Lake Chao have increased by ca. 2 to 4-fold since the 1960s. In response, the community structure, diversity, and ecological network of bacteria and micro-eukaryotes changed significantly during the 1960s, the 1980s and the 2010s. Importantly, community structure was more sensitive to human activities than diversity. We also found that the relative abundance of some taxa associated with nitrification and algal blooms (e.g., taxa in Nitrospira sp., Peridinales) has increased ca. 100-fold since the 1960s. Nutrient could better explain the variation in the bacterial community (ca. twice as much as heavy metal), while heavy metal explained micro-eukaryotes better (ca. 3 or 5-fold as much as nutrient). In particular, based on parsimonious models from distance-based linear model (distLM), we further identified that Pb is the key factor affecting the bacterial and micro-eukaryotes community in Lake Chao in addition to nutrient. Our study reveals the impacts of long-term human activities on lake ecosystems from multiple perspectives of nutrient and heavy metal loads, community structure, diversity and ecological network, these findings will contribute to the management and conservation of lakes in the future.

Ecological Legacy of DDT Archived in Lake Sediments from Eastern Canada

Historic forest management practices led to widespread aerial application of insecticides, such as dichlorodiphenyltrichloroethane (DDT), to North American conifer forests during ∼1950-1970. Lake basins thus may provide an important archive of inputs and aquatic responses to these organochlorines. We use dated sediment cores from five study lakes in multiple watersheds in New Brunswick (NB), Canada, to provide a regional paleo-ecotoxicological perspective on this potential legacy stressor in remote lake ecosystems. Peak sedimentary levels of p, p'- and o, p'-DDT (ΣDDT) and breakdown products ΣDDE (dichlorodiphenyldichloroethylene) and ΣDDD (dichlorodiphenyldichloroethane) generally occurred during the 1970s to 1980s. Sediments exceeded probable effect levels (PELs) by ∼450 times at the most impacted lake. Modern sediments in all study lakes still contained levels of DDT-related compounds that exceed PELs. For the first time, we show that aerial applications of DDT to eastern Canadian forests likely resulted in large shifts to primary consumers within several lake food webs, principally through lake-specific impacts on zooplankton community composition. Modern pelagic zooplankton communities are now much different compared to communities present before DDT use, suggesting that a regional organochlorine legacy may exist in the modern food webs of many remote NB lakes.

Past and recent state of sediment contamination by persistent organic pollutants (POPs) in the Rhône River: Overview of ecotoxicological implications

Twenty-one sediment samples were taken from five dated sediment cores collected along the Rhône River from 2008 to 2011. A total of 17 polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), 7 polychlorinated biphenyls (PCBs), 8 polybrominated diphenyl ethers (PBDEs), 3 polybrominated biphenyls (PBBs), 3 hexabromocyclododecanes (HBCD) and 31 organochlorine pesticides (OCPs) were investigated to provide information on deposition dynamics in time and space, but also regarding the ecotoxicological risks associated with these contaminants. Median concentrations of total PBDEs are nine times lower than the levels of total PCBs along the entire studied stretch of the Rhône River. The results show that total PBDEs concentrations range from 0.06 to 239 μg·kg^-1 DW with a median value of 3.81 μg·kg^-1 DW and a maximum concentration measured in the years 2000s. These maximum concentrations are identical to those measured for total PCBs at the end of the 1990s, but show a different pattern of distribution. Abnormal dichlorodiphenyltrichloroethane (DDT) levels were also detected in the downstream section of the river, with a peak concentration of 147.5 μg·kg^-1 DW measured at the GEC site from 2005 onwards. Analyses of the enantiomeric fractions reveal a fresh input resulting from a technical formulation. Sediments from the core sampled at the most downstream site (GEC) are found to be highly toxic to organisms living nearby, particularly because of the total PCDD/Fs, DDE and DDT levels. In addition, based on available sediment quality guidelines, there may be a potential bioaccumulation risk for humans not only for these three compounds of concern but also for total PCBs and 7 out of the 8 analysed PBDEs.

Sodium chloride incites reactive oxygen species in green algae Chlorococcum humicola and Chlorella vulgaris: Implication on lipid synthesis, mineral nutrients and antioxidant system

In the present study, microalgae Chlorococcum humicola and Chlorella vulgaris were grown in different concentrations of NaCl (25-1000 mM) to elucidate its impact on morphology, lipid synthesis, minerals status and antioxidative responses. Scanning Electron microscopy showed distorted cell morphology and increased cell size by 33.52% (C. humicola) and 27.79% (C. vulgaris) at 100 mM NaCl. Energy Dispersive Spectroscopy data revealed reduction in mineral contents (C, S, Fe, Mg, Si, Mn and Zn) by 14-54% in both algae. Further, C. humicola was found to have high lipid content than C. vulgaris under NaCl regime. The activities of superoxide dismutase, catalase and glutathione reductase were increased by 2.5-5 folds in both algae as compared to control. The increased level of ascorbate, cysteine and proline in both algae indicated tolerance against salinity. Thus, C. humicola and C. vulgaris may exhibit dual benefits viz., high lipid production and reclamation of sodic soil.

Palaeotoxicity: reconstructing the risk of multiple sedimentary pollutants to freshwater organisms

'Real-world' contaminant exposure of sediment-dwelling biota is typically long-term, low-level and to multiple pollutants. However, sediment quality guidelines, designed to protect these organisms, relate only to single contaminants. This study uses radiometrically dated sediment cores from 7 English lakes with varying contamination histories to reconstruct temporal changes in likely risk to biota (herein termed 'palaeotoxicity'). The Probable Effects Concentration Quotient (PEC-Q) approach was used to combine sediment concentrations from multiple contaminants (trace metals; PCBs; PBDEs) to determine risk allocated to metals and persistent organic pollutants (POPs) separately as well as combined (PEC-Q Mean-All). Urban-influenced lakes were considerably more contaminated, exceeding PEC-Q thresholds of 0.5 and 2.0 over long durations (some since the nineteenth century). This has been mainly due to metals (principally lead) and by factors of up to 10 for individual metals and by > 2 for PEC-Q Mean-Metals. In 6 out of 7 lakes, considerable reductions in risk associated with trace metals are observed since emissions reductions in the 1970s. However, at all lakes, PEC-Q Mean-POPs has increased sharply since the 1950s and at 5 out of 7 lakes now exceeds PEC-Q Mean-Metals. These organic pollutants are therefore now the dominant driver behind elevated contaminant risk to sediment-dwelling biota and recent temporal trends in PEC-Q Mean-All remain above threshold values as a result. Finally, PEC-Q Mean-All values were compared to standard biological toxicity tests for surface sediments at each site. While chironomid growth and daphniid reproduction were significantly reduced compared to controls at 5 out of 7, and all lakes, respectively, the scale of these reductions showed only limited quantitative agreement with predicted risk.

Rock glaciers in crystalline catchments: Hidden permafrost-related threats to alpine headwater lakes

A global warming-induced transition from glacial to periglacial processes has been identified in mountainous regions around the world. Degrading permafrost in pristine periglacial environments can produce acid rock drainage (ARD) and cause severe ecological damage in areas underlain by sulfide-bearing bedrock. Limnological and paleolimnological approaches were used to assess and compare ARDs generated by rock glaciers, a typical landform of the mountain permafrost domain, and their effects on alpine headwater lakes with similar morphometric features and underlying bedrock geology, but characterized by different intensities of frost action in their catchments during the year. We argue that ARD and its effects on lakes are more severe in the alpine periglacial belt with mean annual air temperatures (MAAT) between -2°C and +3°C, where groundwater persists in the liquid phase for most of the year, in contrast to ARD in the periglacial belt where frost action dominates (MAAT < -2°C). The findings clearly suggest that the ambient air temperature is an important factor affecting the ARD production in alpine periglacial environments. Applying the paleoecological analysis of morphological abnormalities in chironomids through the past millennium, we tested and rejected the hypothesis that unfavorable conditions for aquatic life in the ARD-stressed lakes are largely related to the temperature increase over recent decades, responsible for the enhanced release of ARD contaminants. Our results indicate that the ARDs generated in the catchments are of a long-lasting nature and the frequency of chironomid morphological deformities was significantly higher during the Little Ice Age (LIA) than during pre- or post-LIA periods, suggesting that lower water temperatures may increase the adverse impacts of ARD on aquatic invertebrates. This highlights that temperature-mediated modulations of the metabolism and life cycle of aquatic organisms should be considered when reconstructing long-term trends in the ecotoxicological state of lakes.