Terrestrial organic matter biomarkers as tracers of Hg sources in lake sediments

Terrestrial organic matter (TOM) plays a key role in mercury (Hg) dynamics between watersheds and lakes. In this study we attempts to determine the role of TOM source and quality and not only quantity, in the fate and transport of total Hg (T-Hg) to boreal lakes. Integrating the watershed complexity is a daunting task. Within the scope of this project, we characterized this organic matter at a molecular level in order to determine Hg transfer conditions to the sediments. We sampled ten lakes in the Quebec boreal forest. In each lake, we took a sediment core at the deepest point in addition to analyzing T-Hg and a set of terrigenous biomarkers in recent sediments. Our results show no relationship between TOM quantity and T-Hg concentration in lake sediments. However, [T-Hg] variation is well explained by the increase of 3,5Bd/V ratios (R²?=?0.84; p?<?0.0002) and the decrease of C/V ratios (R²?=?0.5; p?<?0.0227). Our study shows that TOM source and quality are determinant for Hg loadings in lake sediments. More precisely, increasing TOM derived from humified soil horizons explains most of Hg level variation within sediments.     

Carbon isotopic composition in suspended organic matter and bottom sediments of the East Arctic seas

The samples of water and bottom sediments of the East Siberian and Chukchi Seas collected during the second Russian-American RUSALCA expedition were used to analyze patterns of the isotopic composition of carbon in the organic matter (OM) of suspended material (SOM) and bottom sediments (BOM). Similar to other marine environments, the SOM isotopic composition depended on the ratio between the terrigenous and planktonic OM, both in the water body as a whole and in its parts. Thus, in the East Siberian Sea the carbon of SOM was poorer in ¹³C (??¹³C = ?24.51??) than the open part of the more productive Chukchi Sea (??¹³C = ?22.16??). In the less productive coastal waters of the Chukchi Sea, the ratio of terrigenous OM increased, resulting in a ??¹³C shift to lower values (?23.40??). Due to the influx of reduced products of anaerobic diagenesis of the sediments, elevated total number of microorganisms and dark CO₂ fixation were found in the near-bottom water at the water-sediment biogeochemical barrier. The newly formed biomass of autotrophic microorganisms shifted the carbon isotopic composition of the near-bottom suspended material to more positive ??¹³C values, with the average values of ?23.39 and ?20.37?? for the East Siberian and Chukchi Sea, respectively. Changes in the carbon isotopic composition of OM resulting from microbial activity continued in the upper sediment layers. When the rate of biomass synthesis increased that of biomass consumption, the ¹³C content increased further. At higher rates of OM mineralization, ¹²C accumulated in its remaining part.     

Carbon sources for lake food webs in the Canadian High Arctic and other regions of Arctic North America

We investigated the role of autochthonous and terrestrial carbon in supporting aquatic food webs in the Canadian High Arctic by determining the diet of the dominant primary consumer, aquatic chironomids. These organisms were studied in fresh waters on 3 islands of the Arctic Archipelago (~74–76°N) including barren polar desert watersheds and a polar oasis with lush meadows. Stomach content analysis of 578 larvae indicated that chironomids primarily ingested diatoms and sediment detritus with little variation among most genera. Carbon and nitrogen stable isotope mixing models applied to 2 lakes indicated that benthic algae contributed 68–95% to chironomid diet at a polar desert site and 70–78% at a polar oasis site. Detritus, originating from either phytoplankton or terrestrial sources, also contributed minor amounts to chironomid diet (0–32%). Radiocarbon measurements for the 2 lakes showed that old terrestrial carbon did not support chironomid production. Carbon stable isotope ratios of chironomids in other High Arctic lakes provided further dietary evidence that was consistent with mixing model results. These findings indicate that, in the Canadian High Arctic, chironomids (and fish that consume them) are supported primarily by benthic algae in both polar desert and oasis lakes. In contrast, our review of carbon flow studies for lakes in other Arctic regions of North America shows that terrestrial carbon and phytoplankton can be important energy sources for consumers. This study provides a baseline to detect future climate-related impacts on carbon pathways in High Arctic lakes.     

Thermophilic anaerobes in Arctic marine sediments induced to mineralize complex organic matter at high temperature

Marine sediments harbour diverse populations of dormant thermophilic bacterial spores that become active in sediment incubation experiments at much higher than in situ temperature. This response was investigated in the presence of natural complex organic matter in sediments of two Arctic fjords, as well as with the addition of freeze‐dried Spirulina or individual high‐molecular‐weight polysaccharides. During 50°C incubation experiments, Arctic thermophiles catalysed extensive mineralization of the organic matter via extracellular enzymatic hydrolysis, fermentation and sulfate reduction. This high temperature‐induced food chain mirrors sediment microbial processes occurring at cold in situ temperatures (near 0°C), yet it is catalysed by a completely different set of microorganisms. Using sulfate reduction rates (SRR) as a proxy for organic matter mineralization showed that differences in organic matter reactivity determined the extent of the thermophilic response. Fjord sediments with higher in situ SRR also supported higher SRR at 50°C. Amendment with Spirulina significantly increased volatile fatty acids production and SRR relative to unamended sediment in 50°C incubations. Spirulina amendment also revealed temporally distinct sulfate reduction phases, consistent with 16S rRNA clone library detection of multiple thermophilic Desulfotomaculum spp. enriched at 50°C. Incubations with four different fluorescently labelled polysaccharides at 4°C and 50°C showed that the thermophilic population in Arctic sediments produce a different suite of polymer‐hydrolysing enzymes than those used in situ by the cold‐adapted microbial community. Over time, dormant marine microorganisms like these are buried in marine sediments and might eventually encounter warmer conditions that favour their activation. Distinct enzymatic capacities for organic polymer degradation could allow specific heterotrophic populations like these to play a role in sustaining microbial metabolism in the deep, warm, marine biosphere.     

A molecular marker-based assessment of sedimentary organic matter sources and distributions in Florida Bay

Seven surface sediment samples covering the general geographical area of Florida Bay were examined through the measurement of ¹³C isotopic abundance and lipid classes to assess the distributions and sources of organic matter (OM) in this estuarine environment. The bulk δ¹³C_org value shifted from a more isotopically depleted (−19.9‰) to a more isotopically enriched (−13.5‰) signal along the NE to SW transect. Two geochemical proxies (Paq and C₂₅/C₂₇ n-alkan–2-ones) indicative of seagrass-derived OM significantly increased from near-shore to offshore areas, while taraxerol, a biomarker for mangroves, substantially decreased from 7200 to 284 ng/g along that transect. A clear spatial variation of OM sources was observed in Florida Bay. Generally, the sites in the NE contained mixed OM sources of terrestrial (mangrove) and seagrass-derived OM, where the terrestrial component accounted for over 60% of the OM. In contrast, the sites in central and SW Florida Bay were strongly dominated by seagrass-derived OM. Other lipid fractions such as fatty acids, n-alcohols and sterols revealed an important contribution of algae and bacteria especially in the central and SW section of the Bay. Relatively abundant C₂₅ HBIs suggest important marine diatom inputs, while the presence of C₂₀ HBIs particularly in central Florida Bay possibly reflects the contribution of cyanobacterial mats. The molecular proxies developed in this study to assess OM sources in Florida Bay are promising tools for the characterization and seasonal variability assessment of OM in this and other similar subtropical and tropical estuaries and for paleoenvironmental studies. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Depth profiles of plankton,particulate organic matter and microbial activity in the eastern Canadian Arctic during summer

Summary Deep profiles of particulate organic matter, microplankton (phytoplankton and bacteria), zooplankton and their metabolic activities were investigated during two summer voyages to the eastern Canadian Arctic. Magnitudes and depth distributions were similar in many respects to observations from temperate and tropical waters. Strong gradients in most properties were observed in the upper 50–100 m and subsurface maxima were generally associated with the upper mixed-layer (>50 m). In addition to the general vertical decreases in plankton biomass and metabolic activity there was evidence for both rapid transport (sinking) of organic matter and for enhanced (above background) levels of microbial metabolic activity in deep waters (>500 m). Zooplankton depth distributions differed from the pattern generally observed at lower latitudes; in the Arctic, zooplankton abundance decreased to a lesser degree with depth than particulate organics and microplankton. The overwintering behavior of high-latitude zooplankton appeared to be the best explanation for their relatively high abundance at depth. Despite this, however, zooplankton apparently contributed little to the total column community metabolism.     

Bacterial decomposition of organic matter in lacustrine sediments

The investigations were carried out in lakes situated in the forest zone (Karelian Isthmus, Baltic zone, South Ural) and in the forest-steppe (South Ural) of the USSR. The lakes differ in their bioproductivity and in the intensity of their human influence. The amount of organic matter accumulating in the sediments is closely related to production and decomposition processes in the trophogenic layer. Processes of organic matter transformation were found to be most active in the uppermost sediment. The quantity of bacteria shows no correlation with the organic matter content of the sediment. Increase in organic matter, up to 70–80% of dry weight, is often accompanied by a decrease in bottom bacteria. The intensity of aerobic decomposition of the labile organic matter can be judged from the oxygen demand of the sediments. However, it is important to differentiate between chemical and biological oxidation processes. The quantity of bottom bacteria correlates closely with the value of oxygen consumption only in cases of high sediment redox potentials.     

Sources of organic matter and methylmercury in littoral macroinvertebrates: a stable isotope approach

The main objective of this study was to assess organic matter (OM) and methylmercury (MeHg) sources for freshwater littoral macroinvertebrate primary consumers. The carbon and nitrogen stable isotope ratios (δ¹³C, δ¹⁵N) of sources (epiphytes, macrophytes, suspended particulate matter _SPM) and of macroinvertebrate consumers were measured in a fluvial lake with extensive macrophyte beds (emergent and submerged). To determine the relative contribution of each OM source to macroinvertebrate diets we used the IsoSource model that examines all possible combinations of solutions for each source. Total and MeHg concentrations of consumers were also measured. Results show that epiphytes and macrophytes are dominant in the diet of macroinvertebrates, especially in early summer (July). In mid-summer (August), SPM constitutes a non-negligible OM source to the primary consumers. Hg concentrations were higher in epiphytes than in the other OM sources. The proportion of epiphytes in macroinvertebrate diet was positively correlated with the percentage of MeHg in their tissues. There was no relationship between SPM assimilation and Hg concentration in macroinvertebrate consumers. These results suggest that epiphytes and macrophytes constitute the main pathway of Hg bioaccumulation in littoral food webs.

Evidence of spatial and temporal changes in sources of organic matter in estuarine sediments: stable isotope and fatty acid analyses

We investigated spatial and temporal changes in sources of organic matter in sediments within an estuarine environment in South Africa using fatty acids (FA) and stable isotopes (SI). Samples of sediments and sources of organic matter [i.e., particulate organic matter, microphytobenthos (MPB), macrophytes, salt marsh plants, and terrestrial leaves] were collected during spring and summer 2012, and autumn and winter 2013 from the upper, middle, and lower reaches. A Stable Isotope Analysis in R (SIAR) mixing model was used to identify the organic matter sources contributing to sediments in each estuarine reach and season. We found that diatom-associated fatty acids (20:5ω3; 16:1ω7) increased toward the upper reaches, while long-chained terrigenous fatty acids (24:0) tended to be more prevalent in lower reach sediments. In support of the FA results, the SI mixing model showed a substantial contribution from the marsh grass Spartina maritima in sediments of the lower estuary during periods of low-freshwater discharge (autumn and winter), while MPB was the main component in sediments from the upper and middle reaches during all seasons. Our results have implications for evaluating estuarine food webs since the spatial and seasonal variability in the organic matter deposited can influence estuarine community structure.     

Loss-on-ignition estimates of organic matter and relationships to organic carbon in fluvial bed sediments

Fluvial bed sediments represent an important sink and source for a variety of organic and inorganic compounds. Their most important constituent is organic matter (OM) and its primary component organic carbon (OC). Few studies have been conducted in fluvial environments examining bed-associated OM or OC. This is surprising given the recent interest in global carbon cycling and the importance of bed-associated organics as ecosystem energy sources. The objective of this study was to examine the relationship between OM, determined by loss-on-ignition (LOI), and OC in fluvial bed sediments determined by a dry combustion analyzer. The wide adoption of the LOI method in soil science reflects its ease of use, it is inexpensive, it is rapid, requires no specialized training, and strong statistical relationships commonly exist between OM and OC estimated by standard dry combustion procedures. Regression models were developed between OC and OM for six bed sediment size fractions (≤2.0 mm) for 113 sample sites in a tropical stream on Oahu, Hawaii. All models were highly significant (p < 0.0001), with coefficients of determination ranging from 35 to 79%. Measurement of LOI explained 64% of the variation in OC for all grouped data. The black-box LOI approach may be useful for rapid reconnaissance surveys of drainage systems. Examination of OM to OC conversion factors for Manoa bed sediments indicates that values typically observed in the soils literature (1.7–2.2) are far too low. Values of OM/OC were found to increase with increasing grain size, and decrease with increasing LOI percentage. Conversion factors obtained for grouped data had a mean of 14.9, a coefficient of variation of 21%, and a range of values between 6.2 and 27.4. It is suggested that these high conversion factors reflect significant water loss by dehydration of Fe, Al, and Mn oxides at a muffle furnace temperature of 450 °C. Therefore, the blind application of conversion factors developed from soils should be avoided when converting from OM to OC for fluvial bed sediments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Temperature regulation of anaerobic degradation of organic matter

Anaerobic degradation of organic matter follows similar pathways in digesters and anaerobic freshwater sediments. The responsible microorganisms are linked in a complex food web, where short chain fatty acids and H₂ are important intermediates. Degradation of short-chain fatty acids is endothermic under standard conditions and is only possible at low H₂ partial pressures maintained by exothermic methanogenesis. The coupling between these endothermic and exothermic processes is delicate, and hence sensitive to environmental changes such as temperature variations. The effect of temperature on thermodynamics and on kinetics of these and other anaerobic degradation processes with emphasis on freshwater ecosystems is discussed.The author is with the Department of General Microbiology, Institute of Molecular Biology, University of Copenhagen, Sølvgade 83 H, DK-1307 Copenhagen K, Denmark     

Breeding biology and provisioning of nestling snow buntings in the Canadian High Arctic

We examined basic breeding biology, as well as the effects of parental sex, brood size, nestling age, and nest habitat on foraging distances and parental food delivery rates of snow buntings (Plectrophenax nivalis) on Devon Island, NT, Canada, in the summers of 2003–2006. Clutch sizes and initiation dates were similar to those found in previous studies, although nest density was much higher. Feeding rates and foraging distances of buntings differed depending on the size and age of the brood, but were similar between nest habitats. Unlike in earlier studies, we found that male buntings made fewer feeding visits, but traveled longer distances to forage for food than females. Whether these differences between the sexes relate to reducing competition for prey, or to differing sex roles in parental care due to the poorly insulated nest cavities of this species, remains to be determined.     

Zooplankton impact on the organic matter flux of Siberian Arctic seas

Role of zooplankton in the organic matter flux was studied in the Kara and Laptev seas using sediment traps in the course of voyage 63 of the Academician Mstislav Keldysh research vessel in August–October 2015. The values of the total flux and that of organic matter were at least an order of magnitude higher than those obtained before the 2000s. A hypothesis is proposed on an increasing influence of the river runoff on the Kara Sea area under climatic changes. Zooplankton contribution into C_org flux varied broadly, averaging 50%. The highest flux values and contribution of zooplankton in them (up to 96%) were recorded in the frontal (gradient) zones.     

乌梁素海沉积物中有机质和全氮含量分布特征

针对乌梁素海富营养化日趋严重和湿地面积逐渐萎缩,系统地研究了其生态环境地球化学效应.结果表明,乌梁素海表层沉积物中的全氮含量存在明显的经向和纬向分异特征;沉积物中全氮养分含量与有机质含量显著相关(r＞0.93);沉积物中C/N的平均值介于12.07～19.95之间,表明有机质主要来源于湖中水生植物,水体富营养化具有显著的内源性.TN和有机质在不同粒级表层沉积物中的粒度效应明显,且TN和有机质在IV粒级的含量分别为I粒级的3.1～7.6倍和2.5～8.0倍.     

Determination of total destruction of organic matter in sediments of water bodies

A new scheme for determining the destruction of organic matter in bottom sediments with an account of methanogenesis and the dark assimilation of CO₂ is suggested. The values of the total destruction calculated according to the new methodical scheme increases by 30–60%.     

Contrasting above‐ and belowground organic matter decomposition and carbon and nitrogen dynamics in response to warming in High Arctic tundra

Tundra regions are projected to warm rapidly during the coming decades. The tundra biome holds the largest terrestrial carbon pool, largely contained in frozen permafrost soils. With warming, these permafrost soils may thaw and become available for microbial decomposition, potentially providing a positive feedback to global warming. Warming may directly stimulate microbial metabolism but may also indirectly stimulate organic matter turnover through increased plant productivity by soil priming from root exudates and accelerated litter turnover rates. Here, we assess the impacts of experimental warming on turnover rates of leaf litter, active layer soil and thawed permafrost sediment in two high‐arctic tundra heath sites in NE‐Greenland, either dominated by evergreen or deciduous shrubs. We incubated shrub leaf litter on the surface of control and warmed plots for 1 and 2 years. Active layer soil was collected from the plots to assess the effects of 8 years of field warming on soil carbon stocks. Finally, we incubated open cores filled with newly thawed permafrost soil for 2 years in the active layer of the same plots. After field incubation, we measured basal respiration rates of recovered thawed permafrost cores in the lab. Warming significantly reduced litter mass loss by 26% after 1 year incubation, but differences in litter mass loss among treatments disappeared after 2 years incubation. Warming also reduced litter nitrogen mineralization and decreased the litter carbon to nitrogen ratio. Active layer soil carbon stocks were reduced 15% by warming, while soil dissolved nitrogen was reduced by half in warmed plots. Warming had a positive legacy effect on carbon turnover rates in thawed permafrost cores, with 10% higher respiration rates measured in cores from warmed plots. These results demonstrate that warming may have contrasting effects on above‐ and belowground tundra carbon turnover, possibly governed by microbial resource availability.     

Methanogenic potential of Arctic and Antarctic subglacial environments with contrasting organic carbon sources

Subglacial environments are largely anoxic, contain organic carbon (OC) overridden by glacier ice during periods of advance, and harbour active microbial communities. This creates favourable conditions for OC degradation via methanogenesis. It has been hypothesized that OC beneath ice sheets is converted to methane (CH₄) and may be released to the atmosphere during retreat. However, there are limited data available to support this contention. Here, we present new data on the abundance, diversity and activity of methanogenic archaea and the amount and character of OC in subglacial sediments from Arctic and Antarctic glacial systems based on different substrate types. We employed long‐term laboratory incubations to quantify the CH₄ production potential in different subglacial settings. Significant numbers of methanogens (up to 7 × 10⁴ cells g^?1) were detected in the samples and clones of Methanomicrobiales and Methanosarcinales were identified in clone libraries. Long lag periods (up to >200 days) were observed before significant CH₄ concentrations were measured. We report order of magnitude differences in rates of CH₄ production (10¹–10⁵ fmol g^?1 d^?1) in different subglacial sediments, reflecting contrasts in the origin of the sediment and the OC character. Hence, we predict that contrasting rates of CH₄ production are likely to occur beneath glaciers and ice sheets that overran different types of substrate. We subsequently estimated the potential for CH₄ production beneath the Laurentide/Inuitian/Cordilleran and Fennoscandian Ice Sheets during a typical 85 ka Quaternary glacial/interglacial cycle. CH₄ production from lacustrine‐derived OC is likely to be an order of magnitude higher (~6.3–27 Pg C) than that from overridden soils (~0.55–0.68 Pg C), possibly due to a difference in lability between lacustrine and soil OC. While representing a fraction of the entire OC pool (~418–610 Pg C), this finding highlights the importance of considering the character of different OC pools when calculating subglacial CH₄ production.     

Illuminating microbial species-specific effects on organic matter remineralization in marine sediments

Marine microorganisms play a fundamental role in the global carbon cycle by mediating the sequestration of organic matter in ocean waters and sediments. A better understanding of how biological factors, such as microbial community composition, influence the lability and fate of organic matter is needed. Here, we explored the extent to which organic matter remineralization is influenced by species-specific metabolic capabilities. We carried out aerobic time-series incubations of Guaymas Basin sediments to quantify the dynamics of carbon utilization by two different heterotrophic marine isolates (Vibrio splendidus 1A01; Pseudoalteromonas sp. 3D05). Continuous measurement of respiratory CO₂ production and its carbon isotopic compositions (¹³C and ¹⁴C) shows species-specific differences in the rate, quantity and type of organic matter remineralized. Each species was incubated with hydrothermally-influenced versus unimpacted sediments, resulting in a ~2-fold difference in respiratory CO₂ yield across the experiments. Genomic analysis indicated that the observed carbon utilization patterns may be attributed in part to the number of gene copies encoding for extracellular hydrolytic enzymes. Our results demonstrate that the lability and remineralization of organic matter in marine environments is not only a function of chemical composition and/or environmental conditions, but also a function of the microorganisms that are present and active.