Pre-industrial Sediments Research Articles

Accumulation of total mercury in deep-sea sediments and biota across a bathymetric gradient in the Southeastern Mediterranean Sea

This study explores the accumulation of total mercury (THg) in deep-sea sediments and demersal megafauna of the ultra-oligotrophic Southeastern Mediterranean Sea (SEMS) across bathymetric gradients in the range 35–1900 m, sampled in seven cruises during 2013, 2017–2021, and 2023. Measurements of THg were conducted in surficial (0.0–0.5 cm) and subsurface (9.0–10 cm) sediments, demersal sharks, demersal teleost fish, and benthic crustaceans. Sedimentary organic carbon and biota δ13C and δ15N values were determined to explore possible foraging habitats and dietary sources of THg. The results exhibit an increasing trend of THg in surficial sediments with increasing bottom depth, while in the subsurface, pre-industrial sediments, THg remains lower, slightly increasing with depth. Having no major terrestrial point sources in this area, this increasing trend of THg in surficial sediments across bathymetric gradients is controlled by atmospheric mercury deposition, scavenged by the biological pump, and by lateral transport of particulate Hg in winnowed fine particles from the shelf. Similarly, the THg in benthic crustaceans and demersal fish ranged between 0.02 and 2.71 μg g−1 wet weight (0.06 and 10.8 μg g−1 dry weight) and increased with muscle δ13C as a function of distance offshore, while presenting a low THg-δ15N bio-magnification power. Our results suggest that foraging habitats, longevity, and species-specific depth distribution control their muscle THg bioaccumulation. Despite this complexity, the pooling of THg in megafauna into specific deep zones reflected the trend of increasing anthropogenic THg across bathymetric gradients. Furthermore, many of the biota measurements exceeded safe consumption thresholds for Hg and therefore, should be considered carefully in the development and regulation of deep-sea trawling in this region.

Sediment records from Lake Nipissing (ON, Canada) register a lake-wide multi-trophic response to climate change and reveal its possible role for increased cyanobacterial blooms

Recurring cyanobacterial blooms have been reported in Lake Nipissing (Ontario, Canada) since at least 2000. However, monitoring data indicate that surface water nutrient concentrations have not increased over the past ∼45 years in this large, shallow, mesotrophic lake. Applying paleolimnological methods, we establish baseline environmental conditions and extend the Lake Nipissing water quality data to pre-industrial times. Lake-wide assessment of changes in nutrient status, algal production, and effects of climate warming are provided by examining: 1) modern and pre-industrial sedimentary diatom assemblages from nine locations; 2) temporal trends in sedimentary photosynthetic pigments over ∼200 years from five locations; and 3) detailed sedimentary profiles of diatom and chironomid assemblage composition from Callander Bay, spanning the past ∼200 years. A pronounced shift in diatom assemblage composition was registered at all sites with pre-industrial sediments dominated by Aulacoseira spp. and modern samples dominated by Discostella stelligera. The detailed diatom analysis for Callander Bay indicated that this shift occurred ca. 1990. Since ca. 1975, cyanobacterial production (spectrally inferred from sediments) exceeded the ∼200-year mean and matches the observed increase in bloom reports. Twentieth century shifts in chironomid assemblages in Callander Bay from Procladius to Chironomus spp. dominance indicate lower hypolimnetic oxygen in recent decades. Collectively, these changes suggest that previously polymictic Lake Nipissing is now experiencing periods of thermal stratification and development of transient anoxia in bottom waters. These conditions are more prevalent and widespread in modern Lake Nipissing, consistent with a response to recent warming, decreased wind speed, and a longer ice-free period.

Long-term environmental changes in the Canadian boreal zone: Synthesizing temporal trends from lake sediment archives to inform future sustainability

Covering 55% of Canada’s total surface area and stretching from coast to coast to coast, the Canadian boreal zone is crucial to the nation’s economic and ecological integrity. Although often viewed as relatively underdeveloped, it is vulnerable to numerous stressors such as mining, forestry, and anthropogenic climate change. Natural archives preserved in lake sediments can provide key insights by quantifying pre-disturbance conditions (pre-1850 CE) and the nature, magnitude, direction, and speed of environmental change induced by anthropogenic stressors over the past ∼150 years. Here, we paired a review of paleolimnological literature of the Canadian boreal zone with analyses of published sediment core data to highlight the effects of climate change, catchment disturbances, and atmospheric deposition on boreal lakes. Specifically, we conducted quantitative syntheses of two lake health indicators: elemental lead (Pb) and chlorophyll a. Segmented regressions and Mann–Kendall trend analysis revealed a generally increasing trend in elemental Pb across the boreal zone until ∼1970 CE, followed by a generally decreasing trend to the present. Snapshot comparisons of sedimentary chlorophyll a from recent and pre-industrial sediments (i.e., top-bottom sediment core design) revealed that a majority of sites have increased over time, suggesting a general enhancement in lake primary production across the boreal zone. Collectively, this body of work demonstrates that long-term sediment records offer a critical perspective on ecosystem change not accessible through routine monitoring programs. We advocate using modern datasets in tandem with paleolimnology to establish baseline conditions, measure ecosystem changes, and set meaningful management targets.

Pre-industrial sediment concentrations of metals: insights from the Venice lagoon (Italy).

This study investigated the occurrence of selected metals and metalloids (As, Cd, Cr, Cu, Hg, Ni, Pb, Zn) in radio-dated sediment cores from a coastal lagoon (Lagoon of Venice, Italy) by critically reviewing and grouping available data. Pre-industrial concentrations (pICs, estimated for the period before the early 1900s) for the Venice lagoon are identified according to the 2σ statistical procedure (2σ-pICs), i.e., the upper bound values of the dataset distribution. Results show the following 2σ-pICs (µg/g, d.w.): As 15,9; Cd 0,6; Cr 38,0; Cu 18,1; Hg 1,1; Ni 32,9; Pb 29,4; Zn 94,5. Most of the estimated 2σ-pICs are comparable to previously assessed background values. In the case of Hg, on the contrary, 2σ-pIC is remarkably higher than background values, reflecting a significant anthropogenic contribution also in the pre-industrial period. The results of this work may support the evaluation of the temporal evolution of metal concentrations in sediments of the Venice lagoon. Results are compared with background concentrations (BC) observed in previous studies conducted in the lagoon and in other areas of the Adriatic Sea, as well as with benchmarks set in Italy for sediment assessment.

Pervasive changes in algal indicators since pre-industrial times: A paleolimnological study of changes in primary production and diatom assemblages from ~200 Canadian lakes

Anthropogenic stressors affect lakes around the world, ranging in scale from catchment-specific pollutants to the global impacts of climate change. Canada has a large number and diversity of lakes, yet it is not well understood how, where, and when human impacts have affected these lakes at a national scale. The NSERC Canadian Lake Pulse Network sought to create the first nationwide database of Canadian lake health, undertaking a multi-year survey of 664 lakes spanning 12 ecozones across Canada. A key objective of the network is to determine where, by how much, and why have Canadian lakes changed during the Anthropocene. To address this objective, we compared sedimentary chlorophyll a and diatoms from modern and pre-industrial sediment intervals of ~200 lakes. The lakes spanned a range of sizes, ecozones, and degrees of within-catchment land use change. We inferred the quantity of chlorophyll a, its isomers and main diagenetic products using visible reflectance spectroscopy. We found widespread increases in primary production since pre-industrial times. Primary production increased, on average, across all ecozones, human impact classes, and stratification classes. Likewise, an increase in planktonic diatom taxa over time was detected in the majority of sampled lakes, likely due to recent climate warming. However, regional factors (ecozones) explained the most variation in modern diatom species assemblages as well as their temporal turnover. Furthermore, lakes with high human impact (i.e., higher weighted proportions of human land use in the catchment) exhibited greater taxonomic turnover than lakes with a low human impact class. The greatest diatom turnover was found in the agriculture-rich Prairies and the lowest in the sparsely populated Boreal Shield and Taiga Cordillera ecozones. Overall, our study highlights that drivers operating at different geographic scales (i.e., climatic and land-use changes) have led to significant alterations in algal indicators since pre-industrial times across the country.

Quantifying historical releases and pre-operation levels of metals and radionuclides

Assessing the recovery of aquatic ecosystems from metal and radionuclide contamination requires knowledge of the concentration of radionuclides and metals before anthropogenic releases. Pre-operational conditions, or baseline, are often unknown for many mining operations initiated decades ago. The objectives of this study were to quantify baseline levels of metals and radionuclides and describe historical releases of an industrialised watershed in Northern Ontario where mining operations were carried out from 1955 to 1996. For this purpose, water and surface sediment samples were collected from this watershed and in an adjacent non-industrialised watershed every 2 km. Using metal and radionuclide concentrations in the non-impacted watershed, we calculated water and sediment baseline concentrations as upper 95th percentile values. Baseline pH, 226Ra and uranium in water of lakes and rivers were similar at pH of 6.8, 10 mBq·l−1, and 2.5 μg·l−1, respectively. For sediments, baseline lake sediment exhibited concentrations of radionuclides that were higher than river sediments. We calculated baseline concentrations in lake sediment at 2,115 Bq·kg−1 210Pb, 535 Bq·kg−1 210Po, 218 Bq·kg−1 226Ra, 235 Bq· kg−1 228Th, 184 Bq·kg−1 230Th, and 223 Bq·kg−1 232Th. Baseline concentrations of metal levels in lakes were at 98 mg kg−1 Ni, 119 mg kg−1 Cu, 2300 mg kg−1 Zn, 112 mg kg−1 Pb and 19 mg kg−1 U. In Lake Huron, we collected two sediment core profiles along with surface sediment to estimate baseline radionuclide activities and metal concentrations and quantify historical releases from the industrialised watershed. The sediment core profiles reflected baseline conditions prior to releases from the uranium mining operation and contamination from its onset in 1955 to its closure in 1996. Concentration of metals in pre-industrial sediment layers were lower than in surface sediment of Lake Huron, suggesting atmospheric depositions. Our study indicates that collecting surface sediment in this non-impacted watershed may yield baseline concentrations for uranium and radionuclides. For metal, collecting surface sediment may yield ambient metal concentrations because of long-range atmospheric transport from remote sources. By comparison, sedimentary profiles can provide baseline concentrations of both metals and radionuclides. In the case of the Serpent River watershed, we report that water quality has recovered downstream of Quirke Lake as of 1993 and that additional sediment cores would better assess sediment recovery.

Regional changes in Cladocera (Branchiopoda, Crustacea) assemblages in subarctic (Yellowknife, Northwest Territories, Canada) lakes impacted by historic gold mining activities

In Yellowknife (Northwest Territories, Canada), roaster stack emissions from historic gold mining activities, particularly Giant Mine (1948–2004), have left a legacy of arsenic contamination in lakes. We examined Cladocera (Branchiopoda, Crustacea) subfossil remains in the recent and pre-industrial sediments of 23 lakes (arsenic gradient of 1.5–750 µg/l) within a 40 km radius of Giant Mine to provide a snapshot of regional change in Cladocera since pre-1850. We found that littoral and benthic taxa dominated the recent assemblages in high-[As] lakes (surface water [As] > 100 µg/l), while pelagic Bosmina was dominant in lakes with lower [As]. Cladocera richness and diversity were positively correlated with [As] (P = 0.004, R2 = 0.39; and P = 0.002, R2 = 0.46, respectively), except for four lakes with [As] > 100 µg/l. The lakes that showed the most pronounced changes in Cladocera since pre-1850 were those affected by both metal(loid) pollution and urban development, where complete shifts in the dominant taxa occurred. Lakes that were most heavily impacted by arsenic emissions did not experience notable shifts in Cladocera assemblages. Our study suggests that changes in Cladocera assemblages in mining-impacted subarctic lakes are modulated by local, lake-specific limnological conditions and the interaction of multiple stressors.

Sediment Metagenomes as Time Capsules of Lake Microbiomes.

The reconstruction of ecological time series from lake sediment archives can retrace the environmental impact of human activities. Molecular genetic approaches in paleolimnology have provided unprecedented access to DNA time series, which record evidence of the microbial ecologies that underlaid historical lake ecosystems. Such studies often rely on single-gene surveys, and consequently, the full diversity of preserved microorganisms remains unexplored. In this study, we probed the diversity archived in contemporary and preindustrial sediments by comparative shotgun metagenomic analysis of surface water and sediment samples from three eastern Canadian lakes. In a strategy that was aimed at disentangling historical DNA from the indigenous sediment background, microbial preservation signals were captured by mapping sequence similarities between sediment metagenome reads and reference surface water metagenome assemblies. We detected preserved Cyanobacteria, diverse bacterioplankton, microeukaryotes, and viruses in sediment metagenomes. Among the preserved microorganisms were important groups never before reported in paleolimnological reconstructions, including bacteriophages (Caudovirales) and ubiquitous freshwater Betaproteobacteria (Polynucleobacter and Limnohabitans). In contrast, ultramicroscopic Actinobacteria ("Candidatus Nanopelagicales") and Alphaproteobacteria (Pelagibacterales) were apparently not well preserved in sediment metagenomes even though they were numerically dominant in surface water metagenomes. Overall, our study explored a novel application of whole-metagenome shotgun sequencing for discovering the DNA remains of a broad diversity of microorganisms preserved in lake sediments. The recovery of diverse microbial time series supports the taxonomic expansion of microbiome reconstructions and the development of novel microbial paleoindicators.IMPORTANCE Lakes are critical freshwater resources under mounting pressure from climate change and other anthropogenic stressors. The reconstruction of ecological time series from sediment archives with paleolimnological techniques has been shown to be an effective means of understanding how humans are modifying lake ecosystems over extended timescales. In this study, we combined shotgun DNA sequencing with a novel comparative analysis of surface water and sediment metagenomes to expose the diversity of microorganisms preserved in lake sediments. The detection of DNA from a broad diversity of preserved microbes serves to more fully reconstruct historical microbiomes and describe preimpact lake conditions.

Assessing the viability of pre-industrial sediment prior to remediation using primary producer (Zostera marina and Spartina alterniflora) growth and survival

Boat Harbour, Nova Scotia, Canada, has served as a polishing pond for treated pulp and paper effluent since 1967. In 2020, the effluent flow ceased, and the site will be remediated. The focus of the remediation is the removal of a layer of contaminated sediment, shown to contain high levels of metals and dioxins and furans. Our primary objective was to test whether the underlying pre-industrial sediment could support growth and survival of estuarine plants. A large-diameter (15 cm) corer was used to extract cores from underneath the contaminated layer. These cores were inserted into a reference estuary, along with cores of reference estuarine sediment. Four 3 × 3 subtidal plots were used to test transplants of the estuarine plant Zostera marina, and five 1 × 9 marsh edge plots were used to test Spartina alterniflora. No significant differences in plant growth or survival were observed between Boat Harbour and reference sediment after 2 months. Postexperiment analysis of contaminants (metals and polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans (PCDD/Fs)) in both types of sediment and plant tissues showed similarly low levels of contaminants. Findings indicate that pre-industrial sediment in the harbor should be able to support plant growth after removal of overlying contaminated sediment.

Dataset of natural metal background levels inferred from pre-industrial palaeochannel sediment cores along the Rhône River (France).

Natural metal background levels in sediments are critical to assess spatial and temporal trends of contamination in hydrosystems and to manage polluted sediments. This is even more sensitive that multi-factors such as geogenic basement, depositional context, and past or long-term pollution can affect the level of metals in sediments. This article provides natural metal background levels and ancillary data (location, chronology, grain-size, total organic carbon – TOC) in pre-industrial sediments along the Rhône River (France). Two distinct areas were selected to take into account the geological variability of the watershed: the Dauphiné Lowlands (Upper Rhône River) and the Tricastin Floodplain (Middle Rhône River). On each area, the sediment cores were retrieved from palaeochannel sequences and the sampled sections were dated by radiocarbon from the Roman to the Modern Times (AD 3–1878). Regulatory metals (Al, Fe, Cd, Cr, Cu, Ni, Pb, and Zn) and other trace elements (Ba, Co, Li, Mg, Mn, Na, P, Sr, Ti, V) were analysed following both Aqua Regia (AR) and Total Extraction (TE) procedures. Classically, TE provides metal concentrations greater than AR because TE includes crystalline lattice, while AR is close to the potentially bio-accessible part of metals (used for ecotoxicological purposes). Due to the small number of samples and to the non-normal distribution of the results, a median-based approach was chosen to establish the geochemical background values and ranges (MGB) for each sample and area. These MGBs are valuable to identify pollution sources, to characterise a contamination (spread and timing), and to estimate the state of rivers regarding pollution legacy. Along the Rhône River, these two continental MGBs were used to reconstruct the metal geo-accumulation trajectories in river sediments from 1965 to 2018 [1].

Investigating the Potential Impact of Louisiana Coastal Restoration on the Trace Metal Geochemistry of Constructed Marshlands

Coastal restoration through diversion of suspended sediments from the Lower Mississippi River (LMR) into hydrologically isolated marshlands of Mid-Barataria Bay and Mid-Breton Sounds in southern Louisiana has the potential to mobilize lead (Pb), and other trace elements. We investigate the potential impact(s) of the diversion on marsh porewater through analysis of modern riverbank and suspended sediments, compared to sediments from pre-industrial deltaic deposits of LMR. Sequential extraction methods were used to evaluate Pb, cobalt (Co), copper (Cu), nickel (Ni), and zinc (Zn) in the sediments. Our results show that metal contents are higher (e.g., 8- to 10-fold for Pb) in the modern sediments relative to pre-industrial deposits. Also, the reducible fraction, presumably iron/manganese (Fe/Mn) oxides/oxyhydroxides, is the chief reservoir of environmentally available metals. The substantially higher trace metal contents of the modern relative to pre-industrial sediments suggest that the modern sediments contain a sizeable amount of anthropogenic contributions. Furthermore, the concentration of the trace metals in the reducible fraction suggests bioavailability to marsh organisms upon reductive dissolution within the planned, constructed coastal marshes. Still, additional sediment samples from the marshlands during the diversion implementation phase will be necessary to support the preliminary findings in this contribution as it affects coastal marshes and vital local fisheries.

Holocene sedimentation in the southwestern Black Sea: Interplay between riverine supply, coastal eddies of the Rim Current, surface and internal waves, and saline underflow through the Strait of Bosphorus

An extensive grid of high-resolution Huntec deep-tow seismic reflection profiles (~9150 line-km), complemented by 72 gravity cores and 25 piston cores reveals that the sedimentary architecture of the southwestern Black Sea above a prominent shelf-crossing unconformity α is characterized by three allostratigraphic units (allounits 1b, 1c, 1d), separated by two regional unconformities and their correlative conformities, α1 and α2. A fourth allounit (1a) occurs as patchy remnants between the α unconformity and a deeper Pleistocene erosional unconformity, α0. The chronology is established using 56 radiocarbon dates in five key piston cores. The α unconformity developed as a ravinement surface during the transgression associated with the transition from the Last Glacial Maximum to Holocene, thus defines the base of the Holocene successions. Allounit 1b is bounded at its base and top by the α and α1 unconformities/correlative conformities. It was deposited between ~12.5 and ~7.4 cal ka and shows a notably patchy but widespread distribution across the southwestern shelf. Allounit 1c is bounded at its base and top by the α1 and α2 unconformities/correlative conformities. It was deposited between ~7.4 and ~5.3 cal ka. Allounit 1d accumulated above the α2 unconformity/correlative conformity and was deposited since ~5.3 cal ka.Interpretation of radiometrically-dated cores and several detailed regional maps reveal that the widespread occurrence of ~12.5–7.4 cal ka deposits (allounit 1b) across the middle and outer shelf provides no possibility for a Holocene evaporative drawdown to a lowstand elevation of −120 m at 9.4 cal ka as suggested by Yanchilina et al. (2017). Instead, the volume of detritus in allounit 1b can only be explained if base level was sufficiently high to allow major distant rivers in Bulgaria and Romania (e.g., Kamchiya, Danube) to contribute mud through long-distance marine transport along a submerged shelf. There is no evidence in any cores which sample allounit 1b that the sites were located in limans, rather than on a transgressed shelf which had been flooded by post-glacial meltwater and enhanced river runoff as the climate became more humid in the early Holocene. A prolonged stillstand at the spill depth controlled by the sill in the Strait of Bosphorus is confirmed by the elevation of a drowned recurved spit downdrift from Cape Emine, Bulgaria. The development of the subaqueous unconformities/correlative conformities α1 and α2 was facilitated by storm-induced wave erosion; transport, bypass and erosion under the Rim Current and its anticyclonic eddies, and possibly breaking internal waves. Volumetrics based on isopach maps and pre-industrial sediment discharge rates of various rivers entering the western and southwestern Black Sea suggest that the total sediment yields of Bulgarian and Turkish rivers can only account for ~13–14% of the total Holocene succession (allounits 1b–1d), and that major input from the Danube River via along-shelf long-distance transport must have occurred through the entire Holocene.

Cladoceran assemblage changes across the Eastern United States as recorded in the sediments from the 2007 National Lakes Assessment, USA

Cladoceran zooplankton subfossils have been widely applied as indicators of lake health and used to infer past environmental change. However, our understanding of how the environment controls the abundance and distribution of Cladocera, as recorded in lake sediments, is still developing; only a few large landscape studies have been conducted in temperate North American lakes. Here we examine cladoceran remains from across 72 lakes and through time by analysing sediments that were collected as part of the US Environmental Protection Agency 2007 National Lakes Assessment. Using both taxonomic and functional-trait approaches, we related both taxonomic and functional-trait subfossil cladoceran assemblages to a suite of biological, chemical, and physical variables of lake health by applying Redundancy Analyses and Multivariate Regression Trees. Primary gradients identified as predictors of the cladoceran assemblages were: calcium and pH, depth and measures associated with pelagic and littoral habitat, and climate. Notably, calcium was identified as the primary driver of the Bosminidae, with greater relative abundances of Bosmina longirostris at concentrations ≥3.99 mg L−1, and with Eubosmina spp. associated with lower concentrations. Comparison of surface and pre-industrial sediments on a northern (temperate) lake subset suggested that the primary agents of change were localized, as we did not detect consistent directional shifts in Cladocera. In particular, the quality and complexity of the littoral habitat were associated with changes in key cladoceran groups through time. Overall, our study identified insightful relationships between cladoceran subfossil assemblages and environmental variables across spatial and temporal gradients, which are critical for refining the utility of Cladocera as ecological indicators in paleolimnological studies.

Environmental background values of trace elements in sediments from the Jiaozhou Bay catchment, Qingdao, China

Selected trace elements (As, Cr, Zn, Cu, Cd, Co, Pb and Ni) in 76 surface sediment samples collected from the rivers and the intertidal zone of Jiaozhou Bay (JZB) were evaluated to assess their environmental background values in the JZB catchment. Overall, the sediment quality in the area meets the China Marine Sediment Quality criteria. The background values (ranges) of the elements As, Cr, Zn, Cu, Cd, Co, Pb and Ni were, respectively, 8.28 (4.10–12.46), 67.96 (38.40–97.52), 56.80 (16.42–196.51), 19.13 (5.71–64.06), 0.10 (0.02–0.42), 6.51 (2.08–20.40), 17.97 (12.26–55.84) and 20.69 (10.43–30.95)mg/kg. The background values of most of the trace elements were lower than those in Chinese soil, the upper continental crust, global shales and global preindustrial sediments. The results may assist in defining future coastal and river management measures specifically targeted at monitoring trace element contamination in the JZB catchment.

Historical reconstruction of anthropogenic mercury input from sedimentary records: Yeongsan Estuary, South Korea

The rapid economic growth of the Republic of Korea (S. Korea) within the last half-century has resulted in a pronounced increase in anthropogenic Hg emission from coal combustion, oil refining, cement production, and waste incineration. The record of increasing atmospheric sources have been investigated with a historical reconstruction of Hg accumulation in 30 sediment cores collected from the Yeongsan Estuary. Within the last several decades, this region has undergone severe anthropogenic alteration, including the construction of an estuarine dam forming the Yeongsan Lake, and installation of numerous seawalls that eliminated vast tidal flats and restricted estuarine circulation. Total mercury concentrations (T-Hg) measured in sediments deposited after 1980 (23.2 ± 9.6 ng g−1; n = 273), were significantly higher than those reported for pre-industrial sediments (i.e. background values: 8.6 ± 2.7 ng g−1; n = 274). An extensive survey of surface samples show that T-Hg concentrations are highest above the dam, with a gradient to lower values further offshore. The concomitant timing of enrichment of T-Hg within the sedimentary record and increased National emissions in Korea suggests that regional sources dominate the input to the Yeongsan Estuary. This indicates that with sufficient regional historic emission data, T-Hg might be utilized as a geochronologic tool to aid in corroborating traditional radioisotopic methods.

Long-term stability of cladoceran assemblages in small, shallow, Canadian Shield lakes experiencing marked calcium declines

A greater appreciation of biotic responses to environmental changes is warranted in small, shallow lakes because of the high number of these habitats, and their unique contributions to regional biodiversity. Furthermore, recent water chemistry monitoring data show that shallow lakes in Ontario are sensitive and have responded significantly to environmental stressors such as acid deposition and lake water calcium decline. Here, we use paleoecological techniques to examine cladoceran assemblages in modern and pre-industrial sediments of 30 shallow lakes to determine the key environmental gradients that influence present-day assemblages, and to assess how assemblage structure has changed since pre-industrial times (pre-1850s). Redundancy analysis of present-day cladoceran assemblages and key environmental variables identified lake surface area and Secchi depth as significant predictors of assemblage composition. In our data set, Secchi depth was not correlated to water clarity but rather to macrophyte cover, suggesting that cladoceran assemblages were highly influenced by habitat structure. In contrast to nearby, deeper lakes, where pelagic cladoceran taxa have changed significantly in relative abundance over time, cladoceran assemblages in present-day and pre-industrial sediments of shallow lakes did not differ significantly in composition. While the specific reasons for this muted response are unknown, we hypothesize that: (1) littoral taxa may be less sensitive to low Ca concentrations, or ecological thresholds have not yet been crossed or are lower for littoral taxa; (2) calcium availability may vary spatially within shallow lakes, and this is not captured in a single measure of water chemistry from the centre of the lake; and/or (3) habitat structure is more important than water chemistry as a predictor of assemblage composition in these study lakes, and this has not changed significantly over time.

Reconstructing the ecological impacts of eight decades of mining, metallurgical, and municipal activities on a small boreal lake in northern Canada

As a result of long-term metal mining and metallurgical activities, the sediment of Ross Lake (Flin Flon, MB, Canada) is highly contaminated with metals and other elements. Although the effluents likely were discharged into Ross Lake as early as the late 1920s, lake biophysical data were not collected until 1973, more than 4 decades after the onset of mining and municipal activities. The early influence of these activities on the ecology of Ross Lake is unknown, as are the effects of improvements to metallurgical effluent quality and discontinuation of municipal wastewater discharge into the lake's north basin. To address this knowledge gap, analyses typical of paleolimnological investigations were applied to cores of sediment collected in 2009 from the south basin of Ross Lake. Stratigraphic analyses of physicochemical sediment characteristics (e.g., the concentrations of metals and other elements, organic C, total N, and δ(13)C and δ(15)N values) and subfossil remains (diatoms, Chironomidae, Chaoborus, and Cladocera) were used to infer historical biological and chemical changes in Ross Lake. With the onset of mining activities, concentrations of various elements (e.g., As, Cr, Cu, Zn, and Se) increased dramatically in the sediment profile, eventually declining with improved tailings management. Nevertheless, concentrations of metals in recent sediments remain elevated compared with pre-industrial sediments. Constrained cluster analyses demonstrated distinct pre-industrial and postindustrial communities for both the diatoms and chironomids. The biodiversity of the postindustrial diatom assemblages were much reduced compared with the pre-industrial assemblages. The postindustrial chironomid assemblage was dominated by Chironomus and to a lesser extent by Procladius, suggesting that Ross Lake became a degraded environment. Abundances of Cladocera and Chaoborus were severely reduced in the postindustrial era, likely because of metals toxicity. Overall, improvements to the management of both metallurgical and municipal effluent are reflected in the physicochemical sediment record; nevertheless, the ecology of Ross Lake remains impaired and shows minimal signs of returning to a pre-industrial state. Recommendations are made regarding possible future investigations at this site and the need for a framework to help assess causation using paleolimnological and other site data.

Upper Mississippi Pb as a mid-1800s chronostratigraphic marker in sediments from seasonally anoxic lakes in Eastern Canada

Sediment cores from eight headwater lakes located in Southern Québec, Eastern Canada, were analyzed for Pb, stable Pb isotopes, and the radioelements 210Pb, 137Cs, 241Am and 226Ra. The depth profiles of stable Pb isotope ratios show, for the post-19th century period, the influence of several isotopically distinct anthropogenic lead sources, mainly including emissions from two Canadian smelters and from leaded gasoline combustion in Canada and in the United States. A most interesting feature of the profiles, however, is the presence of sharp stable Pb isotope ratio peaks near the depth horizon, where excess 210Pb becomes undetectable. Using a binary mixing model and assuming that natural Pb concentrations and isotopic compositions from the catchment are given by the pre-industrial sediments at the bottom of the cores, we find that a significant part of the anthropogenic Pb supplied to the sediments at this horizon originated from smelting activities in the Upper Mississippi Valley. We assess that the Pb isotope ratio peaks, also observed in the laminated sediments of the Pettaquamscutt Estuary, Rhode Island, USA, are an accurate chronostratigraphic marker for the validation of mid-19th century 210Pb-derived dates. Given that the study lakes are located up to 2000km from the Mississippi Valley, we conclude that this isotopic Pb signal provides a widely distributed time-marker that is key to validate 210Pb chronologies in environmental archives from Eastern North America.

Spatial Trends of Trace-Element Contamination in Recently Deposited Lake Sediment Around the Ni–Cu Smelter at Nikel, Kola Peninsula, Russian Arctic

A large copper-nickel smelter complex is located at the Kole Penninsula, Russia, close to the Norwegian border. Trace-element concentrations in surface sediments (0-0.5cm) and pre-industrial sediments from 45 lakes in the region were used to uncover spatial deposition patterns and contamination factor of sediments. Elevated concentrations were found, especially for Ni and Cu, but also for Pb, Co, Hg, As, and Cd. Highest concentrations were found up to 20 km from the smelter, but the concentrations decreased exponentially with distance from the smelter. Increasing Ni, Cu, As, and Hg concentrations from sub-surface to surface sediments were found for lakes at intermediate distances (20-60km). This may reflect recent changes in atmospheric depositions, as shown in nearby Norwegian areas. However, we cannot rule out that this also may have been caused by diagenetic processes, especially for the most redox-sensitive elements such as As.

Copper in Galician ria sediments: natural levels and harbour contamination

Copper distribution, natural background and contamination levels in sediments of the Galician rias were assessed from 389 surface sediment samples and five sediment cores. The natural reference concentration of copper can be estimated using the Cu-Fe relationship obtained from preindustrial sediments of rias: [Cu (mg kg –1 )] / [Fe (g kg –1 )] = 0.55. Severe copper contamination was mainly observed in the harbour, fishing ports and shipyards near shore areas of the Galician coast. Therefore, the rias of Ferrol and Vigo were the most environmentally disturbed, while the northern Galician rias were the least disturbed. The Galician case can be seen as the type of copper contamination due to maritime activities observed in other world harbours.