Remote Lakes Research Articles

Temporal dynamics of alpha and beta diversity of lake algae: Opposing patterns and influencing factors over the past 200 years

Abstract Better understanding of the responses of algal biodiversity to multiple pressures, such as climate warming and eutrophication, is a key issue in aquatic ecology. Alpha and beta diversity may have various patterns over temporal scales, especially in the Anthropocene, when external pressures became more multifaceted. However, the limited availability of historical data hampers the exploration of algal biodiversity through time. Recently, sediment DNA has emerged as a potential tool for elucidating temporal patterns in algal communities. Here, we used sediment DNA to reconstruct temporal turnover and diversity of algal communities in four remote lakes in northern China over the past 200 years. Furthermore, to distinguish the contributions of possible influencing environmental factors, we conducted structural equation modelling. Our results revealed that algal communities have experienced rapid shifts since the Anthropocene, characterized by increased alpha diversity and decreased temporal beta diversity. Warmer climate and eutrophication were associated with changes in alpha diversity, while temporal environmental variation was associated with temporal beta diversity. This study revealed opposing patterns in alpha and beta diversity for algal communities, possibly caused by warming, eutrophication and lower temporal environmental variation, respectively. While climatic factors played a major role in remote lakes with a natural environment, lakes that are more human impacted may be more structured by nutrient‐related factors. Under climate warming and intensified human activities, remote lakes may encounter complex pressures in the near future. Our findings offer valuable insights into patterns in aquatic biodiversity and possible factors underlying multiple pressures.

Using desiccation-resistant eggs to explore the ecology of giant ostracods (subfamily Mytilocypridinae) in Australian salt lakes

AbstractGiant ostracods in the subfamily Mytilocypridinae are an important component of the biota of Australian salt lakes, which are predominantly temporary, and often located in remote arid areas. Mytilocypridines produce desiccation-resistant eggs to persist in these habitats during unfavourable conditions. In this study, we used mytilocypridine eggs collected from sediments in salt lakes in Western Australia to analyse aspects of the ecology of these ostracods. We conducted two rehydration trials, where sediment was collected from dry lakes or the dry margins of lakes, rehydrated in a laboratory setting, and selected mytilocypridine species hatched and raised in cultures. The first trial found that the eggs of mytilocypridine species were unevenly distributed in a dry lake and could be hatched even after 27 months of dry storage with no discernible loss of viability. The second trial assessed the capacity of selected mytilocypridines to hatch and develop into adults under different salinity treatments. One species, Australocypris insularis, hatched and developed across a very wide salinity range (0–100 g/L). Egg/sediment samples provide a means for testing species’ tolerance to physical and chemical variables throughout their entire life cycle, as well as for surveying the mytilocypridines in remote lakes that rarely hold water.

The global process of eutrophication and its features in Arctic lakes as a consequence of climate warming

An analysis of the problem of water bodies eutrophication as a global process is presented. The volumes increasing use of nitrogen and phosphorus on a planetary scale are shown, the dispersion of which leads to an increase in the content of nutrients in lakes and rivers. The results of original studies of remote lakes in the Arctic zone are presented, which showed an increase in the concentrations of nutrients in lakes in recent decades. A tendency has been revealed for an increase in the contents of nitrogen and phosphorus, as well as organic matter in lake waters, even in the absence of anthropogenic influence. It has been established that an increase in temperature and climate warming in the Arctic regions exert the main influence on the increase in the content of nutrients and the trophic status of lakes.

First report of microplastics in water and sediments of the alkaline Bagno dell’Acqua Lake (Pantelleria Island, southern Italy)

Microplastics are ubiquitous in aquatic ecosystems and lakes are considered important sinks for this contaminant. In this study, we assessed for the first time the occurrence and spatial distribution of microplastics in surficial waters and sediment of a small alkaline lake located in the National Park of Pantelleria Island (Central Mediterranean Sea). The island is small, scarcely populated, not industrialized and ∼70 km far from the southern European and northern African shores. Water samples were collected in triplicates in different sectors of the lake using a 10 L metal bucket, while sediment sampling was carried out at seven stations along the lake shoreline, plus one station at 3.6 m depth. Results show a homogeneous distribution of microplastic in surface waters, dominated by fibers (∼90%), with a mean concentration of 0.13 ± 0.04 MP/L, which is lower than most of the lakes worldwide. Conversely, microplastics in sediment had a higher proportion of fragments and films (50%) and their concentrations range from 0 to 540 MP/kg. The highest abundance was recorded in the western sector, where most of touristic activities concentrate, and at the 3.6 m depth station, thus suggesting that the deeper parts of the lake may retain the majority of microplastics entering the system. This study shows that even in small lakes far from major anthropic pressures, sediment may contain microplastics in striking abundances, pointing out the need to assess rural and remote lakes, at present much understudied compared to lakes in highly populated regions.

Microplastic assessment in remote and high mountain lakes of Gilgit Baltistan, Pakistan

Microplastic (MP) pollution is a critical environmental challenge worldwide, however limited research is reported in remote lakes of Pakistan. This study assessed MPs (>5 mm) prevalence, distribution and risk perspective in water and sediment of eight remote and high-altitude lakes (>1500 m above sea level) of Gilgit Baltistan, Pakistan. The lakes exhibited an average abundance of 152.6 ± 104.6 to 12.1 ± 7 MP/kg of dry sediments and 2 ± 0.9 to 17.1 ± 17.2 MP/L of surface water. MPs <200 μm dominated in both matrices. Surface water predominantly contained polyester and polypropylene, while polypropylene and polyethylene dominated in sediments. The gradient of elevation did not show any pronounced impact on the fiber loading or MP count in both matrices. Backward air mass trajectory revealed that air masses vastly travelled from western-Asia, Arabian sea and Bay of Bengal with an average transmission distance of 2500–3500 km (500 m a.s.l) that can be a potential deposition MP source in the area. Pollution Load Index of the lakes were >1 exhibiting pollution. All other lakes except Batura and Borith manifested a moderate hazard index. Naltar lake along with aforementioned two lakes also manifested high polymer toxicity. Further research should emphasize understanding the mechanisms and biotic interactions in high-mountain ecosystems.

Eutrophication of Lakes: From Global Process to Regional Implication in the Kola Arctic Region

ABSTRACTEutrophication of water bodies is analysed as a global process. The volumes of globally increasing use of nitrogen and phosphorus are demonstrated, with the dispersion of these elements leading to increased nutrient contents in lakes and rivers. Results of original studies on remote lakes in the Arctic zone indicate that the content of nutrients in these lakes has increased over the past decades. Concentrations of nitrogen, phosphorus and organic matter in lake waters tend to increase in the absence of anthropogenic effects. Simultaneously, the silicon concentrations were found to decrease because of the consumption by diatoms. Low concentrations of bioavailable nutrients confirm that these nutrients are rapidly spent in the production processes of ecosystems. The calculated trophic state index (according to R. Carlson) indicates that the number of oligotrophic lakes in the forest tundra zone decreased by 50% by 2010–2018, and these lakes are absent from the northern taiga zone. Temperature increase and climate warming in the Arctic zone first caused the increase in the contents of nutrients in the lakes and their trophic states.

Chronological evaluation of polycyclic aromatic hydrocarbons in sediments of tangxun lake in central China and impacts of human activities.

This study sheds light on the contamination of polycyclic aromatic hydrocarbons (PAHs) in Tangxun Lake sediments, an urban lake reflecting environmental changes in Central China. By analyzing sediment cores from both the inner and outer areas of the lake, we determined the historical trends and sources of PAHs over the past century. The results reveal a significant increase in PAHs concentrations, particularly since the 1980s, coinciding with China's rapid urbanization and industrialization. Using diagnostic ratios and Absolute principal component score-multivariate linear regression (APCS-MLR) methods, we identified petroleum combustion, coal combustion, and biomass combustion as the primary sources of PAHs in the lake sediments. The spatial analysis indicates higher PAHs levels in the inner lake, likely due to its closer proximity to industrial activities. Moreover, by comparing PAH trends in Tangxun Lake with those in other urban, suburban, and remote lakes across China, based on data from 49 sedimentary cores, we highlight the impact of regional socio-economic dynamics on PAH deposition. These insights are crucial for developing effective pollution mitigation strategies and promoting sustainable development in rapidly urbanizing regions.

Heterogenization of microplastic communities in lakes of the Qinghai-Tibetan Plateau driven by tourism and transport activities

The Qinghai-Tibetan Plateau has a booming tourism industry and an increasingly sophisticated road system. There is a paucity of studies quantifying the contributions of anthropogenic and natural factors to microplastic pollution in remote plateau areas. In this study, water and sediment samples were collected from eight lake tourist attractions and four remote lakes in northern and southern regions of the Qinghai-Tibetan Plateau. Microplastics were detected in all samples, with a mean abundance of 0.78 items/L in water and 44.98 items/kg in sediment. The abundance of microplastics in the study area was lower than previously observed in more populated areas of China. Small-sized (<1 mm and 1–2 mm), fiber, and transparent microplastics were predominant, with polyethylene and polypropylene microplastics as the primary polymer types. The compositions of microplastic communities indicated that tourism and road networks were the major sources of microplastics in the lakes. Distance-decay models revealed greater influence of environmental distances on microplastic community similarity than geographic distance. Compared to climate factors, urban spatial impact intensity and traffic flow impact played a leading role in the structuring of microplastic communities in lake water and sediment. Our findings provide novel quantitative insights into the role of various factors in shaping the distribution patterns of microplastic communities in plateau lakes.

Can remotely sensed catchment to lake area ratios predict mercury levels in subarctic fishes?

Mercury concentrations ([Hg]) in fish reflect a complex array of interacting biogeochemical and ecological variables. In northern regions where fish are a critical subsistence food, understanding and predicting fish [Hg] can be particularly difficult, largely due to a paucity of comprehensive data associated with the logistical challenges of field sampling. Building on previous work where we elucidated causal relationships between fish [Hg] and a variety of catchment, water quality, and ecological variables in subarctic lakes, we investigated whether using only ratios of catchment area to lake area (CA:LA) can predict [Hg] in northern freshwater fish species. As CA:LA can be sensed remotely, they may be more feasible and practical to obtain than field data in far northern regions. Our study included thirteen remote lakes that represent a CA:LA gradient of 6.2–423.5 within an ∼66,000 km2 subarctic region of Northwest Territories, Canada. We found that size-standardized [Hg] in three widespread fish species, including Lake Whitefish (Coregonus clupeaformis), Walleye (Sander vitreus), and Northern Pike (Esox lucius), were significantly and positively related to CA:LA (p < 0.007, r2 = 67–80%), indicating higher fish [Hg] in smaller lakes surrounded by relatively larger catchments. Our findings provide compelling evidence that remotely sensed CA:LA can be used to predict [Hg] in northern fishes and aid in prioritizing understudied and subsistence fishing lakes of the Canadian subarctic for [Hg] monitoring programs.

Distribution and risk assessment of microplastics in water, sediment and brine shrimps in a remote salt lake on the Tibetan Plateau, China

Microplastics (MPs) are pervasive environmental contaminants that have infiltrated even the most remote ecosystems. Despite their widespread distribution, the transfer patterns and impacts of MPs in remote lakes remain poorly understood. This study aimed to address the knowledge gap regarding the pathways and consequences of MP pollution in these isolated environments. Focusing on Kyêbxang Co, a remote salt lake in Tibet, this study investigated the transfer patterns, sources and ecological impacts of MPs, providing insights into their mobility and fate in pristine ecosystems. Water, sediment and biota (brine shrimp) samples from Kyêbxang Co, collected during the summer of 2020, were analyzed using µ-Raman spectroscopy to determine MP abundances, polymer types and potential sources. Findings indicated significant MP contamination in all examined media, with concentrations highlighting the role of runoff in transporting MPs to remote locations. The majority of detected MPs were small fragments (<0.5 mm), constituting over 93 %, with polypropylene being the predominant polymer type. The presence of a halocline may slow the descent of MPs, potentially increasing the exposure and ingestion risk to brine shrimp. Despite the currently low ecological risk estimated for MPs, this study underscores the need for long-term monitoring and development of a comprehensive ecological risk assessment model for MPs.

Lake Environmental Data Harvester (LED) for Alpine Lake Monitoring with Autonomous Surface Vehicles (ASVs)

This article introduces the Lake Environmental Data Harvester (LED) System, a robotic platform designed for the development of an innovative solution for monitoring remote alpine lakes. LED is intended as the first step in creating portable robotic tools that are lightweight, cost-effective, and highly reliable for monitoring remote water bodies. The LED system is based on the Shallow-Water Autonomous Multipurpose Platform (SWAMP), a groundbreaking Autonomous Surface Vehicle (ASV) originally designed for monitoring wetlands. The objective of LED is to achieve the comprehensive monitoring of remote lakes by outfitting the SWAMP with a suite of sensors, integrating an IoT infrastructure, and adhering to FAIR principles for structured data management. SWAMP’s modular design and open architecture facilitate the easy integration of payloads, while its compact size and construction with a reduced weight ensure portability. Equipped with four azimuth thrusters and a flexible hull structure, SWAMP offers a high degree of maneuverability and position-keeping ability for precise surveys in the shallow waters that are typical of remote lakes. In this project, SWAMP was equipped with a suite of sensors, including a single-beam dual-frequency echosounder, water-quality sensors, a winch for sensor deployment, and AirQino, a low-cost air quality analysis system, along with an RTK-GNSS (Global Navigation Satellite System) receiver for precise positioning. Utilizing commercial off-the-shelf (COTS) components, a Multipurpose Data-Acquisition System forms the basis for an Internet of Things (IoT) infrastructure, enabling data acquisition, storage, and long-range communication. This data-centric system design ensures that acquired variables from both sensors and the robotic platform are structured and managed according to the FAIR principles.

Dechloranes and chlorinated paraffins in sediments and biota of two subarctic lakes.

Our understanding of the environmental behavior, bioaccumulation and concentrations of chlorinated paraffins (CPs) and Dechloranes (Dec) in the Arctic environment is still limited, particularly in freshwater ecosystems. In this descriptive study, short chain (SCCPs) and medium chain (MCCPs) CPs, Dechlorane Plus (DP) and analogues, and polychlorinated biphenyls (PCBs) were measured in sediments, benthic organisms, three-spined stickleback (Gasterosteus aculeatus), Arctic char (Salvelinus alpinus) and brown trout (Salmo trutta) in two Sub-Arctic lakes in Northern Norway. Takvannet (TA) is a remote lake, with no known local sources for organic contaminants, while Storvannet (ST) is situated in a populated area. SCCPs and MCCPs were detected in all sediment samples from ST with concentration of 42.26-115.29ng/g dw and 66.18-136.69ng/g dw for SCCPs and MCCPs, respectively. Only SCCPs were detected in TA sediments (0.4-5.28ng/g dw). In biota samples, sticklebacks and benthic organisms showed the highest concentrations of CPs, while concentrations were low or below detection limits in both char and trout. The congener group patterns observed in both lakes showed SCCP profiles dominated by higher chlorinated congener groups while the MCCPs showed consistency in their profiles, with C14 being the most prevalent carbon chain length. Anti- and syn-DP isomers were detected in all sediment, benthic and stickleback samples with higher concentrations in ST than in TA. However, they were only present in a few char and trout samples from ST. Dec 601 and 604 were below detection limits in all samples in both lakes. Dec 603 was detected only in ST sediments, sticklebacks and 2 trout samples, while Dec 602 was the only DP analogue found in all samples from both lakes. While there were clear differences in sediment concentrations of DP and Dec 602 between ST and TA, differences between lakes decreased with increasing δ15N. This pattern was similar to the PCB behavior, suggesting the lake characteristics in ST are playing an important role in the lack of biomagnification of pollutants in this lake. Our results suggest that ST receives pollutants from local sources in addition to atmospheric transport.

Ecological response to global changes in Lake Genco, a remote alpine lake on the Southeast Tibetan Plateau, over the past two centuries

The ways in which natural ecosystems respond to global change have become an urgent concern to society. Lakes play a pivotal role in ecosystem changes because of their central position within watersheds and their involvement in various endogenous and cross-system ecological and biogeochemical processes, leaving natural archives accumulated in sediments over time. However, how lake ecosystems have responded to atmospheric changes during the past few centuries is still not fully understood. To analyse the process in more detail, we conducted a case study at Lake Genco, a remote alpine on the southeastern Qinghai-Tibetan Plateau (QTP) and recovered a 170-year history of lake's ecosystem from a 210Pb-dated sediment core. The air temperature increased between 1900 and 1950 CE and 1980 and 2020 CE. Geochemical and physical proxies (magnetic susceptibility, Sr/Ba, K/Al, Na/Al, Ti, Pb, P flux, TN and δ15Norg) indicated greater degrees of weathering and erosion, and higher nutrient loadings as terrestrial systems responded to atmospheric changes after 1900 CE. Higher primary aquatic productivity, richer biodiversity and weaker structural stability were found in Lake Genco between 1852 and 2017 CE, although the ecological response was quiet in terms of the compositional change in the diatom community, which was generally dominated by small planktonic Cyclotella ocellata. During warmer periods, diatom PC1 changed directionally due to the overall increase in the abundance of tychoplanktonic Fragilaria construens and periphytic Achnanthes minutissima and the decreasing abundance (∼10%) of C. ocellata. This study provides empirical evidence that ecological changes in remote lakes rely on interactions between the atmosphere and the landscape and helps us to understand the complexity of the various impacts of global climate change. More local and regional studies are needed to generalise the mechanism of biological responses to atmospheric changes in remote lakes, especially in the QTP.

Predisposing and enabling factors influencing the utilisation of maternal healthcare services at fishing landing sites in Buikwe district, Uganda

Background: Studies have demonstrated that women who get professional healthcare personnel's prenatal, delivery, and postnatal care have better pregnancy outcomes, which lowers the incidence of both neonatal and mother deaths. Uganda is among the sub-Saharan African countries with the highest percentages of maternal deaths of which the majority of the deaths take place in remote, resource-constrained areas, such as fishing landing sites. We investigated the factors influencing the use of maternity healthcare services by women from remote Lake Victoria islands fishing settlements in the Buikwe district of Uganda. Methods: A cross-sectional study design with a quantitative approach was used. One hundred and four mothers were recruited in the study carried out at Ssenyi, Kiyindi and Nkombwe landing site in Buikwe district. Questionnaires were used to obtain information from mothers about predisposing factors, and enabling factors. Multinomial logistic regression was used to determine which factors influenced the use of maternal healthcare services. Results: Marital status (p=0.045, aOR=0.582; 95% CI=0.163-2.082) and paternal occupation (p=0.017, aOR=1.335; 95% CI=0.059-8.348) were the factors influencing maternal healthcare service utilisation among the respondents. Conclusions: Increasing the amount of actions meant to raise socioeconomic standing and involving the men in maternal health seeking might aid in enhancing maternal healthcare services use by women at the landing sites in Uganda.

Identifying the key sources of metal (loid) deposition to remote Tasmanian lakes: is legacy mining a problem?

Reports have shown that even remote lakes have been negatively affected by anthropogenic activities. This study used metal concentrations and enrichment factors to comprehensively determine key metal sources and identify potential impacts of recent anthropogenic activity in remote Central Highland Tasmanian (Australia) lakes. Metal concentrations (Al, As, Cd, Cu, Fe, Pb, and Zn) in these lakes remain below Australian interim sediment quality guidelines but have been slowly increasing since the start of the twentieth century. This increase is likely related to increasing organic matter content, rather than any direct, increased input of metals. The largest anthropogenic impact to these systems was damming, which typically led to an increase in organic matter content either directly, by the trapping of particles and nutrients, or indirectly, by stimulating primary production in the lake. This increased organic matter content, in turn, increased the retention of metals, manifesting as an increase to measured metal concentrations. Minor impacts are the historical use of lead shot, leaded petrol, and industrial processes, which may have contributed to the higher Pb enrichment, compared to the other studied metals, in these lakes. Despite recent concerns of metal contamination in the Tasmanian Wilderness World Heritage Area from distant mining activities, there is no strong evidence in this study indicating that lakes in the Central Highlands (a similar distance away) have been impacted by long-distance atmospheric deposition. This is likely related to the decreased rainfall, and thus decreased rates of wet deposition of aerosols in the east of Tasmania, compared to that of the west and in the Tasmanian Wilderness World Heritage Area.

Geochemical records of human-induced environmental changes in two small remote lakes of Songnen Plain, Northeast China.

The abundance and composition of aliphatic hydrocarbon biomarkers were determined in dated sediment cores from Lakes Qijiapao (QJP) and Huoshaoheipao (HSH) in the Songnen Plain, Northeast China, to investigate historical environmental changes in these lakes and identify likely controlling factors. Based on these results, the recent environmental history of the two lakes can be divided into three periods. Before 1950, low Paq values (avg. 0.23 and 0.27, respectively) and middle-chain n-alkane abundances (normalized to total organic carbon, avg. 14.82 and 16.01µgg-1 TOC, respectively) in both lakes suggested low aquatic productivity and the limited input of submerged macrophyte organic matter (OM). However, the significant increase in the abundance of short-chain n-alkanes in Lake HSH (from 8.34 to 16.68µgg-1 TOC) indicated the emergence of early nutrient enrichment in the lake. From 1950 to 2000, marked increase in the abundance of middle-chain n-alkanes (avg. 21.72 and 22.62µgg-1 TOC in Lakes QJP and HSH, respectively) and Paq values indicated that both lakes had undergone eutrophication because of the population explosion and agricultural intensification. From 2000 to 2013, the abundance of short- and middle-chain n-alkanes in Lake QJP markedly exceeded those in Lake HSH and indicated a larger eutrophication in Lake QJP, which could be caused by the development of ecotourism in Lake HSH and the concomitant increase in aquaculture in Lake QJP in recent years. The highest abundance of C30 αβ-hopane (~ 10.24µgg-1 TOC) and the lowest CPIH values in Lake QJP revealed a possible petroleum pollution since 2008. Taken together, lake eutrophication in the Songnen Plain accelerated after 1950 and was influenced primarily by agriculture and aquaculture. This is in contrast to lakes in other regions of China (such as the Yangtze River Basin and Yunnan Province), where urbanization and industrialization have exerted a dominant influence on the lake environment.

Modeling atmospheric microplastic cycle by GEOS-Chem: An optimized estimation by a global dataset suggests likely 50 times lower ocean emissions

Modeling atmospheric microplastic cycle by GEOS-Chem: An optimized estimation by a global dataset suggests likely 50 times lower ocean emissions

Unexpected enrichment of antibiotic resistance genes and organic remediation genes in high-altitude lakes at Eastern Tibetan Plateau

Elevation has a strong effect on aquatic microbiome. However, we know little about the effects of elevation on functional genes, especially antibiotic resistance genes (ARGs) and organic remediation genes (ORGs) in freshwater ecosystems. In this study, we analyzed five classes of functional genes including ARGs, metal resistance genes (MRGs), ORGs, bacteriophages, and virulence genes between two high-altitude lakes (HALs) and two low-altitude lakes (LALs) in Mountain Siguniang at Eastern Tibetan Plateau by means of GeoChip 5.0. No differences (Student's t-test, p > 0.05) of gene richness including ARGs, MRGs, ORGs, bacteriophages, and virulence genes in HALs and LALs were found. The abundance of most ARGs and ORGs was higher in HALs than in LALs. For MRGs, the abundance of macro metal resistance genes of potassium, calcium, and aluminum was higher in HALs than in LALs (Student's t-test, p < 0.05; all Cohen's d > 0.8). The abundance of some heavy metal resistance genes of lead and mercury was lower in HALs than in LALs (Student's t-test, p < 0.05; all Cohen's d < −0.8). The composition of these functional genes in HALs differed significantly from in LALs. The functional gene network in HALs was also more complex than that in LALs. We speculate that enrichment of ARGs and ORGs in HALs is related to different microbial communities, exogenous ARGs, and enriched persistent organic pollutants through long-range atmospheric transport driven by the Indian monsoon. This study highlights the unexpected enrichment of ARGs, MRGs, and ORGs in remote lakes at high elevations.

Mercury Pollution History in Tropical and Subtropical American Lakes: Multiple Impacts and the Possible Relationship with Climate Change.

Sediment cores obtained from 11 tropical and subtropical American lakes revealed that local human activities significantly increased mercury (Hg) inputs and pollution levels. Remote lakes also have been contaminated by anthropogenic Hg through atmospheric depositions. Long-term sediment-core profiles revealed an approximately 3-fold increase in Hg fluxes to sediments from c. 1850 to 2000. Generalized additive models indicate that c. 3-fold increases in Hg fluxes also occurred since 2000 in the remote sites, while Hg emissions from anthropogenic sources have remained relatively stable. The tropical and subtropical Americas are vulnerable to extreme weather events. Air temperatures in this region have shown a marked increase since the 1990s, and extreme weather events arising from climate change have increased. When comparing Hg fluxes to recent (1950-2016) climatic changes, results show marked increases in Hg fluxes to sediments during dry periods. The Standardized Precipitation-Evapotranspiration Index (SPEI) time series indicate a tendency toward more extreme drier conditions across the study region since the mid-1990s, suggesting that instabilities in catchment surfaces caused by climate change are responsible for the elevated Hg flux rates. Drier conditions since c. 2000 appear to be promoting Hg fluxes from catchments to lakes, a process that will likely be exacerbated under future climate-change scenarios.

Passive water sampling and air–water diffusive exchange of long-range transported semi-volatile organic pollutants in high-mountain lakes

The concentrations of legacy and currently emitted organic pollutants were determined in the freely dissolved phase of water from six high-mountain lakes in the Pyrenees (1619–2453 m) by passive water sampling. Low-density polyethylene (LDPE) and silicone rubber (SR) sheets were exposed for three consecutive periods lasting each one year between 2017 and 2020 for the study of polychlorinated biphenyls (PCBs), organophosphate esters (OPEs), polycyclic aromatic hydrocarbons (PAHs), and other organochlorine compounds (e.g., hexachlorobenzene, HCB). HCB concentrations (1.0–14 pg L−1) remained essentially the same as those measured with pumping systems over two decades ago in the same area. ƩPAHs (35–920 pg L−1) were around half of those observed in the past, which agrees with reductions in European atmospheric emissions. ƩPCB concentrations (1.2–2.2 pg L−1) were substantially lower, although unexpectedly large differences could be due to comparing yearly averages from the present study to seasonally variable (i.e., affected by snowmelt, stratification, and colloidal organic matter) episodic pumping measurements from previous studies. ƩOPEs (139–2849 pg L−1) were measured for the first time in this area and were found at high concentrations in some sites. Concentrations of most compounds obtained with LDPE and SR samplers agreed with each other by ratios generally lower than three or four times, except for a few PAHs and OPEs. Diffusive exchange flux calculations between the atmospheric gas phase and the freely dissolved water phase revealed net deposition of pollutants from air to water, except for some OPEs and PCBs presenting equilibrium conditions, and HCB with volatilization fluxes. Atmospheric degradation fluxes of PAHs and OPEs pointed at competing removal mechanisms that support the air-to-water direction of their diffusive exchange, while PCBs and organochlorines were not affected by photodegradation. In their current state, these remote lakes accumulate many emerging and legacy pollutants subject to long-range atmospheric transport.