Sediment Organic Content Research Articles

Organic carbon composition and microbial transformation of seagrass meadow and its responses to eutrophication

<p indent="0mm">Carbon neutrality is an important concept in global climate change governance. Seagrass beds rank among the most productive autotrophic ecosystems on the planet, and are receiving increasing attention as globally-significant hotspots of organic carbon (“blue carbon”) sequestration. The blue carbon stored in seagrass beds has important implications for climate change mitigation. The protection of seagrass beds will also protect their organic carbon sequestration potential, consequently helping to achieve the goal of carbon neutrality. Unfortunately, about 29% of the world’s seagrass beds have been destroyed, with a loss rate of 7% a⁻¹ since 1990, mostly due to eutrophication. The organic carbon sequestration potential of seagrass beds could be influenced by the subsequent changes in organic carbon composition, and microbial transformation processes. We systematically reviewed the relevant local and international research to summarize the composition and microbial transformation of organic carbon and its responses to eutrophication. Organic carbon is present in various forms in seagrass beds, and can be roughly divided into primary producer living biomass carbon, litter organic carbon, water organic carbon, and sediment organic carbon according to its storage area. Organic carbon can also be divided into labile organic carbon (LOC), which is susceptible to microbial decomposition, and recalcitrant organic carbon (ROC), which is resistant to microbial decomposition. The total ROC (cellulose and lignin) content in seagrass is present in the range of <sc>153−561 mg/g,</sc> and accounts for more than 28.7% of the organic carbon in the seagrass plants. The ROC content of seagrass plants is affected by the species, morphology, tissue and organ types, and climatic zones. In the water column, color dissolved organic matter (CDOM) and neutral sugars are indicators of LOC, while the lignin phenol and humic-like fractions of fluorescent dissolved organic matter (FDOM) are potential tracers of ROC. The DOM in water can be divided into dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and dissolved organic phosphorus (DOP). The DOC/DON and DOC/DOP ratios are used to reflect DOC lability and microbial bioavailability, with high DON and DOP concentrations indicating high DOC lability. In sediments, extractable organic carbon and microbial biomass carbon are important indicators of LOC, while aromatic hydrocarbons and alkanes are indicative of recalcitrant components. There are significant differences in the sediment organic carbon contents of temperate and tropical regions. Microorganisms play a vital role in the transformation of organic carbon. There are geographic differences in microbial composition, which are mainly influenced by the available habitats and climate zones, but there are no differences between seagrass species within a fine-scale seagrass bed. Different microorganisms have different decomposition capacities. <italic>R</italic>-strategist microbes, including Clostridiales<italic> </italic>and<italic> </italic>Gammaproteobacteria, are able to rapidly decompose LOC, while <italic>k</italic>-strategists, such as Verrucomicrobiae<italic> </italic>and Anaerolinae, can break down the more recalcitrant compounds. There is a succession of <italic>r</italic>- to <italic>k</italic>-strategists microbes during the decomposition of fresh organic matter (including fresh seagrass litter, DOC, and SOC). Eutrophication may lead to the transformation of the community structure of primary producers in seagrass beds from seagrass to macroalgae, which will increase the input of labile carbon. This consequently causes an increase in LOC in the plant biomass, water column, and sediments, but a decrease in ROC. The nutrient loading can affect the microbial community structure and enzyme activity, which will affect the microbial transformation of organic carbon. Furthermore, nutrient and LOC inputs may induce a “priming effect” that promotes the remineralization of stored organic carbon. Several future research directions for determining the transformation of seagrass beds and their organic carbon were proposed. (1) Extensive studies of the organic carbon components (LOC and ROC) in seagrass beds are required. (2) There is a need to better understand the composition, function, and gene expression of microbial communities in seagrass beds. (3) The eutrophication mechanisms in microbial communities and the related extracellular enzyme activities need to be determined. (4) The response of the organic matter decomposition process to nutrient loading and the interaction of nutrients and algae need to be studied. There is also a need to strengthen local research on the carbon storage potential of seagrass beds to mitigate global climate change and provide theoretical support for achieving carbon neutrality.

Spatial and temporal heterogeneity of methane ebullition in lowland headwater streams and the impact on sampling design

Headwater streams are known sources of methane (CH4) to the atmosphere, but their contribution to global scale budgets remains poorly constrained. While efforts have been made to better understand diffusive fluxes of CH4 in streams, much less attention has been paid to ebullitive fluxes. We examine the temporal and spatial heterogeneity of CH4 ebullition from four lowland headwater streams in the temperate northeastern United States over a 2‐yr period. Ebullition was observed in all monitored streams with an overall mean rate of 1.00 ± 0.23 mmol CH4 m−2 d−1, ranging from 0.01 to 1.79 to mmol CH4 m−2 d−1 across streams. At biweekly timescales, rates of ebullition tended to increase with temperature. We observed a high degree of spatial heterogeneity in CH4 ebullition within and across streams. Yet, catchment land use was not a simple predictor of this heterogeneity, and instead patches scale variability weakly explained by water depth and sediment organic matter content and quality. Overall, our results support the prevalence of CH4 ebullition from streams and high levels of variability characteristic of this process. Our findings also highlight the need for robust temporal and spatial sampling of ebullition in lotic ecosystems to account for this high level of heterogeneity, where multiple sampling locations and times are necessary to accurately represent the mean rate of flux in a stream. The heterogeneity observed likely indicates a complex set of drivers affect CH4 ebullition from streams which must be considered when upscaling site measurements to larger spatial scales.

Relationship Between Microplastics Abundance and Sediment Organic Content in Dumai Coastal Waters

Plastics in the environment would eventually be degraded into plastic granules which are called microplastics. In recent years, microplastics have been spread widely not only inland but also in aquatic habitats, where averagely 5% of the plastic produced each year will end up in marine waters. The purpose of this study which was conducted in October to November 2020 was to identify the type and abundance of microplastics in the sediment as well as its possible relationship with the sediment organic matter in Dumai waters. Sediment samples were taken from Dumai coastal waters by using purposive sampling method. Identification of the types and abundance of microplastics as well as the analysis of sediment organic content were carried out in the Laboratory of Chemical Oceanography, Department of Marine Sciences, Universitas Riau. Microplastic and sediment particles were separated using 150 ml saturated NaCl solution. The types of microplastics found are fiber, fragment and films, where fiber was found to be the most commonly found (36.43%), followed by fragments (31.97%) and films (31.60%), respectively. No significant correlation found between microplastic abundance and the sediment organic content in Dumai coastal waters

Redox-dependent biotransformation of sulfonamide antibiotics exceeds sorption and mineralization: Evidence from incubation of sediments from a reclaimed water-affected river

Trace levels of sulfonamide antibiotics are ubiquitous in reclaimed water, yet environmental pathways to completely remove those chemicals are not well understood when such water is used to restore flows in dried rivers. This study investigated sulfonamide sorption-desorption, biodegradation, and mineralization processes with seven sediments from a reclaimed water-dominant river. Batch experiments were conducted under oxic and anoxic (nitrate-reducing) conditions, and each removal process of sulfamethazine, sulfadiazine, and sulfamethoxazole (SMX) was evaluated individually at environmentally relevant concentrations (≤ 10 μg/L). Over 28 days, 44 ± 32% of sulfonamides were biodegraded, while the full mineralization to carbon dioxide was < 1%. Around 5% of sulfonamides were removed via sediment sorption, with a positive correlation with sediment organic contents. Detailed investigation of SMX biodegradation revealed that although its transformation appeared to be faster in anoxic than oxic tests by day 2, it reversed over 28 days with a longer apparent half-life in anoxic tests (69 ± 25 days) than that in oxic tests (12 ± 11 days). This is attributed to the formation of reversible metabolites at denitrifying conditions, such as DesAmino-SMX of which the production was affected by nitrite concentrations. Despite measurements of three frequently reported metabolites, > 70% biotransformation products remained unknown in this study. The findings highlight the persistency of sulfonamides and their derivatives, with research needed to further elucidate degradation mechanisms and to perform risk assessment of reclaimed water reuse.

The threat of freshwater input on sandy beaches: A small-scale approach to assess macrofaunal changes related to salinity reduction

Increased rainfall has become a key threat in recent decades for subtropical coastal regions. On sandy beaches that are associated with streams and rivers, the intensification of freshwater inputs is expected to reduce the salinity of interstitial waters and affect benthic biodiversity. Large freshwater gradients are promoted by river mouths and also change beach morphodynamic and sediment organic contents, which are covariates that have hindered the understanding of salinity-specific effects on benthic species in previous studies. Here, we aimed to assess how salinity reduction affects macrobenthic communities at small spatial scales to control the effects of environmental covariates. We assessed the macrofaunal spatial changes across few-meters gradients of freshwater influence (30 m) that were promoted by small streams (~2 m wide) within three subtropical beaches in southeastern Brazil. Our results showed that salinity was the only environmental factor that explained the macrofaunal variations across such small-scale gradients. We noted that salinity reductions decrease the overall macrobenthic abundance and richness in the first 15 m close to streams. Such variations are associated with changes in polychaete abundance (mainly Scolelepis squamata), which form larger patches with 1200–2800 individuals/m2 only at sites with high salinity (greater than 25). We also found that a salinity reduction from 33 to 20 may promote a decrease of 85% in polychaete abundance near the streams. Nevertheless, salinity reductions did not affect crustacea abundance across the gradient. In the current global change context, these results suggest that freshwater input has great potential to impact polychaete patches, reduce macrobenthic biomass and secondary production and thus threaten important beach functions and services, such as bioturbation and food provision for the top predators.

Sea-Air Exchange of Methane in Shallow Inshore Areas of the Baltic Sea

We report sea-air fluxes of methane in physically and biologically distinct inshore habitats of the Baltic Sea with the goal to establish empirical relationships that allow upscaling of local site-specific flux measurements. Flux measurements were conducted using floating chambers with and without bubble shields, and by using a boundary layer gas transfer model before, during, and after an annually occurring algal bloom from June to October 2019. Water and air temperature, salinity, wind, sediment organic content, and organic content of floating algal biomass were found to successfully discriminate the different habitats in terms of methane flux, both over periods of days and over a season. Multivariate statistical analysis was used to establish the relative environmental forcing of methane emissions over one growth season for each flux method. Floating algal biomass carbon and sediment organic content were identified as the most important controlling factors for methane emissions based on flux chamber measurements over a period of days to weeks, whereas water and air temperature and wind velocity were the most important factors based on the gas transfer model on these time scales. Over the season, water and air temperature were the most important controlling factors with both methods. We present a first attempt how our observations can be extrapolated to determine the coastal methane emission along the coastline.

Seasonal implications of the benthic ecological status assessment in a protected Mediterranean coastal lagoon: Stagno Longu of Posada (Sardinia, Italy)

Understanding how biotic indicators respond to natural variability is essential for the health state evaluation of the benthic ecosystems. This study analysed the composition and structure of the benthic communities in the coastal lagoon Stagno Longu of Posada (Sardinia) to investigate the seasonal response of the biological indicator M-AMBI. Multivariate analysis and biodiversity descriptors showed temporal (winter–summer) and spatial variability (across the three basins of the lagoon).The ecological status (ES) of Stagno Longu was assessed using M-AMBI index: results mainly showed the lagoon to be in “good” and “moderate” status across seasons and basins, while “poor” status was recorded in the North basin during summer.Despite the absence of direct human pressures, sediment organic content (OC) varied seasonally and spatially and M-AMBI revealed winter to summer shifts on the ES of 3 out of 6 sampling stations. Overall, this study reveals negative significative correlation on the variations on M-AMBI with the increase of sediment OC across the studied sites, none the less discrepancies on the ES of certain stations were detected causing over/underestimation.Results suggested that natural seasonal variability is a relevant factor to be considered while performing ES assessments and implementing monitoring programmes in Mediterranean transitional ecosystems when applying indicators such as M-AMBI.

Relationship of Organic Content in Sediment with Morphometric Mangrove Leaves (Rhizophora apiculata) in the West Mangrove Ecosystem, Dumai City, Riau Province

The damage rate of mangrove forests in Indonesia is considered as the fastest and largest in the world. The reduced density of mangroves can be influenced by the water quality and the organic matter content around the mangrove ecosystem, causing changes in the mangrove leaves both in the shape of the length and width. This study was conducted in the mangrove ecosystem in the western part of Dumai City, with the aim of determining the morphometric size of the leaves of Rhizophora apiculata and its relationship with the content of organic matter in the sediment. The method used in this study was survey method, where observations and sampling were performed directly in the field and some of samples were analyzed in the laboratory. Mangrove density obtained from the research results ranges from 1966.66 - 4700 ind/ha, organic matter content 10.88 -17.21%. The morphometric size of the leaf length varies from 13.03 - 15.05 cm, the leaf width varies from 4.65 - 5.42 cm, and the length of the petiole varies from 1.59 - 1.91 cm. There is no significant relationship between the mangrove leaf morphometric with sediment organic matter in the studied area.

Source or sink? Quantifying beaver pond influence on non-point source pollutant transport in the Intermountain West

Non-point source (NPS) pollution remains high in many watersheds despite strategies aimed at reducing such pollution. Beaver (Castor canadensis) activity converts lotic systems to semi-lentic by impounding stream flow and trapping sediments, which have a high affinity for NPS pollutants such as nitrogen (N), phosphorus (P), and heavy metals. This study aimed to identify environmental conditions under which beaver ponds influence the fate and cycling of NPS pollutants. Dissolved and particulate nutrients were sampled upstream and downstream of three headwater beaver ponds differing in age and character through the summer season. Sedimentation rates and sediment concentrations of nutrients and metals were also determined. Results from this study suggest that beaver ponds can attenuate heavy metals at a rate 2 to 4 times greater than a riffle reach (p < 0.05). Metal sequestration scaled with pond age and sediment organic matter content. The oldest and youngest ponds had no significant effect on dissolved nutrients (NO3−, TDN and SRP) or total P (TP). The middle age pond was a significant TN sink in summer (0.6–0.8 g N m−2 d−1 [p = 0.03]) and influenced dissolved nutrient concentrations differently in spring (21% NO3− sink [p = 0.03], 61% SRP source [p = 0.05]) compared to summer (34% NO3− source, 7% SRP sink). This pond had little apparent effect on TP loads during the study period but accumulated a total of 146 g m−2 of phosphorus in the sediments suggesting that beaver ponds may reach their phosphorus sequestration potential within the first few years of pond development and then subsequently act as a weak SRP source. We use a theoretical relationship describing sediment-water interactions to show that biogeochemical processing in a beaver pond is optimized at intermediate levels of pond nutrient supply and residence time. If beaver ponds are to be considered as an option for landscape scale restoration, this theoretical relationship may be useful for predicting the effects of beaver ponds on water chemistry, and aid in the interpretation of variable water quality results from inherently heterogeneous environments.

Insight into the adsorption mechanisms of ionizable imidazolinone herbicides in sediments: Kinetics, adsorption model, and influencing factors

To reveal the adsorption mechanisms of imazamox, imazapic, and imazethapyr on sediment and batch experiments were carried out in this study. The adsorption kinetics of three imidazolinone herbicides on sediment were accurately described by the pseudo-second-order kinetic model(R2 > 0.9004). The values of adsorption capacity (Qe.cal) were ranged from 0.0183 to 0.0859 mg kg−1 for three herbicides. Adsorption equilibrium was reached within 24 h for three herbicides on sediment, and well fitted by the Freundlich model(R2 > 0.9561). The KF of values for adsorption obtained sediment samples were ranged from 0.2501 to 1.322 L1/n mg1-1/n kg−1for three herbicides. These results indicated that intraparticle diffusion and external mass transport were the main rate controlling steps of the adsorption of herbicides on sediment and that the chemical adsorption was dominant during the adsorption processes. The calculated hysteresis coefficient H were 0.9422,0.7877 and 0.744 for imazmox, imazapic and imazethapyr in raw sediment, respectively, indicating that there is a hysteresis in desorption. The influences of solution pH and sediment organic carbon content on the imidazolinone herbicide adsorption behaviors were also examined. Which shown that the adsorption process for herbicides was highly pH-dependent and adsorption efficiency was closely related to the organic matter content of the sediment, suggesting that electrostatic interactions played crucial roles in the adsorption behavior between sediment and imidazolinone herbicides, and the herbicides were mostly absorbed by the amorphous materials of sediment. These research findings are important for assessing the fate and transport of imidazolinone herbicides in water–sediment systems.

Influence of habitat on meiofaunal abundance and distribution in a New Zealand temperate estuary

ABSTRACT Meiofaunal assemblages are often overlooked, yet they are important ecological components of estuarine ecosystems. To evaluate variations in meiofaunal communities in estuarine habitats, we conducted a comprehensive analysis of meiofaunal density, distribution and relationships with environmental parameters in a mangrove-dominated ecosystem at Mangawhai, Northern New Zealand, in summer (March) and winter (October), 2013. Five habitat types were studied including mangrove stands, pneumatophore zones, sandflats, mudflats and saltmarshes. In addition, data were recorded for sediment grain size, organic content, and primary benthic productivity (chlorophyll a). Twelve major taxa were identified, with meiofaunal densities ranging between 269 and 8090 ind. 10 cm−2. Highest abundances were found in fine-grained organic-rich sediments compared to coarse-grained organically poor sediments; the former were generally associated with mangroves stands and pneumatophore zones. Nematodes dominated all observed habitats. Foraminifera ranked second in fine-grained organically rich sediments, whereas copepods ranked second in coarser-grained sediments. Results suggest that meiofaunal taxonomic abundance and distribution are highly sensitive to local physical and biological parameters (e.g. sediment grain size, organic content, benthic primary productivity). The findings revealed that environmental characteristics have an important influence within this temperate estuary.

Sediment Deposition and Availability in Riparian Wetlands

Eulie, D.O.; Leonard, L., and Polk, M., 2021. Sediment deposition and availability in riparian wetlands. Journal of Coastal Research, 37(2), 302–315. Coconut Creek (Florida), ISSN 0749-0208.Coastal flooding and sea-level rise have the potential to negatively influence the ability of coastal wetlands to accumulate sediment, threatening the persistence of coastal wetlands. This study examines sediment availability, deposition rates, and elevation change in two types of tidally-dependent riparian wetlands to understand their ability to persist in the wake of rising sea level. Tidal marshes and riparian swamp habitats in blackwater and brownwater river systems were studied over a 32-month period between 2004 to 2007. Wetlands along the brownwater river system exhibited significantly greater and more variable deposition rates than those located along the blackwater river system (p < 0.05). The brownwater marsh lost –1.7 cm of elevation and the blackwater marsh lost –1.1 cm of elevation, while the brownwater swamp site had an increase of 7.0 cm and the blackwater swamp site exhibited an increase in elevation of 0.7 cm. Within the Cape Fear River Estuary, a combination of sediment availability, organic content, and wetland type (tidal brackish marshes, riparian freshwater swamp forests) are what most strongly influenced patterns of deposition but are strongly temporally and spatially influenced. Marshes in the Cape Fear River are losing elevation, while riparian swamps are, at the time of this study, maintaining their current elevation. Coastal tidal wetlands are under increasing pressure from climate change, sea-level rise, and urban development resulting in vertical drowning. Tidal marshes in the Cape Fear River are not able to maintain elevation, and riparian swamp forests may also be unable to as the magnitude of these pressures increase.

Metal contaminations in sediment and associated ecological risk assessment of river Mahanadi, India.

Mahanadi is one of the major rivers of peninsular India. Like other Indian rivers, it is contaminated with sewages, industrial discharges, and agricultural runoff. Thus, necessity was felt to monitor its pollution status. Present work was part of that program and aimed to assess the sediment contamination due to the trace metals Cd, Cr, Cu, Mn, Pb, and Zn during 2012-2015. Sediment pollution status and ecological risks were evaluated calculating contamination factor (CF), geo-accumulation (Igeo), pollution load index (PLI), potential ecological risk (EiR), etc. The recorded metal concentrations were Cd BDL of flame mode of AAS; Cr BDL - 73.9; Cu BDL- 44.4; Mn 37.2-1887.0; Pb BDL- 29.5; and Zn BDL- 92.5 mg kg-1. As per US EPA guidelines, Cr concentrations at many locations were in the moderately polluted range. Igeo, CF, mCd, PLI, and EiR indicated low pollution levels and low ecological risks due to the trace metals assessed. The sediment quality guidelines (SQGs) indicated that Cr and Cu concentrations exceeded (16% sample) the threshold effect concentrations and may occasionally exhibit adverse biological effects. The association of sediment organic matter, conductivity and content of Cu, and their grouping in component 1 of PCA revealed that the anthropogenic input was dominant and so also the component 2 where Cr exhibited moderately good correlation with organic matter. Cluster analysis of the sampling sites based on pollution status yielded 3 groups: relatively uncontaminated (S3, S4), low to moderately contaminated (S2), and moderately contaminated (S1, S5, S6) stretches.

Integrated analysis of petroleum biomarkers and polycyclic aromatic compounds in lake sediment cores from an oil sands region.

We examined polycyclic aromatic compounds (PACs) and petroleum biomarkers (steranes, hopanes, and terpanes) in radiometrically-dated lake sediment cores from the Athabasca oil sands region (AOSR) and the Peace-Athabasca Delta (PAD) region in Alberta (Canada) to determine whether contributions from petroleum hydrocarbons have changed over time. Two floodplain lakes in the PAD (PAD 30, PAD 31) recorded increased flux of alkylated PACs and increased petrogenic (petroleum-derived) hydrocarbons after ∼1980, coincident with a decline of sediment organic carbon content and a rise of bulk sedimentation rate, likely due to increased Athabasca River flow. A large expansion of upstream oilsands mining, upgrading, and refining may also have contributed to the observed shift to more petrogenic hydrocarbons to sediments since the 1980s. Alkylated PAC flux increased in the floodplain lake analyzed within the AOSR (Saline Lake) since the 1970s-1980s, coincident with a sharp rise in sediment organic carbon content and increased contributions of petrogenic hydrocarbons. These changes identify increased supply of petrogenic PACs occurred as Athabasca River floodwaters waned, and may implicate aerial contributions of petrogenic hydrocarbons from oilsands activity. PACs and petroleum biomarkers (steranes, hopanes, and terpanes) in sediment cores from Saline Lake, PAD 30 and PAD 31 revealed a predominance of petrogenic hydrocarbons in these lakes. In contrast, we recorded minimal petrogenic hydrocarbons in the reference lakes outside the surface minable area of the AOSR and PAD (Mariana Lake and BM11), though we noted slight increases in petrogenic contributions to modern (2010-2016) sediments. We show how a combined analysis of PACs and petroleum biomarkers in sediments is useful to quantify petrogenic contributions to lakes with added confidence and highlight the potential for petroleum biomarkers in lake sediment cores as a novel and effective method to track petroleum hydrocarbons in lake sediment.

Biological effects on incipient motion behavior of sediments with different organic matter content

The organic content in urban sewer sediment can significantly influence sediment erosion behavior because of biological activity. In this experimental study, the erosion behavior of sediments with different organic content was investigated to develop a mathematical relationship between the critical shear stress and biological activities. Before testing, sediment samples were incubated under two environmental conditions: aerobic conditions with high biological activities and sterile conditions with low biological activities. With different incubation periods, samples were subjected to the scouring test in an open channel flume, by which the incipient motion of sediments was determined. Experimental results show that the sediment erosion resistance is simultaneously subject to the strengthening and the weakening effect induced by the biological activity, which is dependent on the incubation period and organic content. For low organic content sediment (volatile solids content < 4%), the biological activity can strengthen the sediment erosion resistance and increase τc, while τc of higher organic content sediment is distinctly reduced. The biological activity can both strengthen and weaken the sediment erosion resistance. The strengthening effect is attributed to the biofilm formation in the biological activity, whereas the weakening effect is due to the sediment volume expansion by the gas bubble formation. These effects depend on the incubation period and organic content within sediments, which can be used to predict the incipient motion of sediments by modifying the biological influence coefficient. Finally, Shields curves were modified to account for the biological activity.

Significance of 2-methylhopane and 22,29,30Trisnorhop17(21)-ene biomarkers in holocene sediments from the Gulf of Tunis - Southern Mediterranean Sea

The investigation of lipid biomarkers in sediments deposited during the Holocene in the Lagoon of Tunis, located at the southern edge of the Mediterranean Sea, revealed two important hopanoid biomarkers: 2-methylhopaneand 22,29,30 trisnorhop-17(21)-ene. These compounds are considered of special interest because they reveal information about climate conditions during the Late Holocene sediment deposition. These hopanoids, or bacteriohopanepolyols (BHPs), indicate a considerable bacterial contribution to the organic matter preserved in these sediments.The occurrence of 2-methylhopanein substantial amounts indicates that cyanobacteria, Rhodopseudomonas palustris bacteria, and genus Desulfovibriobacteria communities made large contributions to the sediment's organic content. In addition, the presence of these bacteria communities have been interpreted as reliable indicators of hydrologic modifications associated with the development of wetland systems during the Late Holocene.The occurrence of 22,29,30trisnorhop17(21)-ene also suggests a considerable bacterial contribution to the organic matter and indicates an immature diagenesis stage. It also reflects the development of favourable conditions to the preservation and accumulation of organic matter within an anoxic depositional environment.

Enhanced middle Holocene organic carbon burial in tropical floodplain lakes of the Pantanal (South America)

Wetland carbon storage is an important and environmentally sensitive ecosystem service. Carbon burial in the floodplain lakes of the Pantanal (tropical South America) appears to have varied during the late Quaternary, but several paleolimnological studies have recorded unusually high sediment organic carbon content from ~ 7.3 to 6.0 cal kyr BP in lakes connected to the Upper Paraguay River. We conducted a multi-indicator (phytoliths, sponge spicules, and geochemistry) study of a sediment core from Lake Caceres (Bolivia), and found evidence for enhanced organic carbon burial during the middle Holocene that provides insights into the flooding history of the Upper Paraguay River. δ13Corg and C/N data suggest that organic matter deposited at that time in Lake Caceres was from macrophytes. Similar datasets from three other floodplain lakes are consistent with this finding. We suggest that enhanced carbon burial occurred when lake levels declined under relatively dry climate conditions, which increased the littoral area at the expense of open water and captured floating macrophyte islands. This study sheds new light on hydroclimate controls on carbon cycling in the Pantanal wetlands, and improves interpretations of geochemical measures on bulk organic matter in floodplain lake cores.

Site exposure, substrate, depth, and the thermocline affect the growth of native unionid mussels in a stratified lake

The distribution of the native mussel Elliptio complanata (Lightfoot, 1786) in lakes is limited to nearshore areas above the thermocline. Their distribution appears to be constrained by wind-driven physical forces, and if so, we expect: 1) lower growth at the edges of their depth range, at shallow exposed sites, and at deep sites close to the thermocline; and 2) long-term changes in growth with lake warming and changing thermocline depth. This study compares the growth of juvenile and adult E. complanata over their depth range in a stratified lake and across sites with different exposures and sediment types. I used thin shell sections covering as long of a period as possible (7–54 y) to measure growth of 195 mussels. The average growth, measured as the von Bertalanffy growth rate coefficient, k, of juveniles ranged more than 3-fold (k = 0.06–0.27) around a single lake basin and that of adults 2-fold (0.3–0.6 mm/y). Mussel growth decreased with increasing depth and with increasing site exposure, but a positive relationship with sediment organic content suggests that sediments provide a refuge for adult mussels. Coherence between growth series was strongest at the deepest sites (6 m; expressed population signal = 0.82), where mussel growth responded to thermocline depth. Longer stratification periods and shallower thermoclines are expected to compress the range of these mussels in lakes and may similarly affect the wider littoral benthic communities.

Impact of the Sediment Organic vs. Mineral Content on Distribution of the Per- and Polyfluoroalkyl Substances (PFAS) in Lake Sediment.

Contamination of the water and sediment with per- and polyfluoroalkyl substances (PFAS) was studied for the lake impacted by the release of PFAS-containing aqueous film forming foam (AFFF). PFAS concentrations were analyzed in lake water and sediment core samples. ΣPFAS concentrations were in the range of 95–100 ng L−1 in the lake water and 3.0–61 µg kg−1 dry weight (dw) in sediment core samples, both dominated by perfluorohexane sulfonate, perfluorooctane sulfonate; 6:2 fluortelomer sulfonate was inconsistently present in water and sediment core samples. The sediment–water partitioning coefficients (log Kd) were estimated and ranged 0.6–2.3 L kg−1 for individual perfluoroalkyl carboxylates (PFCAs) and 0.9–5.6 L kg−1 for individual perfluoroalkane sulfonates (PFSAs). The influence of the sediment inorganic content and organic matter on PFAS distribution was investigated. In studied sediments, the mineral content (corresponding to <5% of the bulk media mass) was mainly represented by sulfur, iron and calcium. The PFAS distribution was found strongly connected to the sediment mineral content (i.e., Fe, Pb, Rb and As), whereas the sediment organic carbon content did not to have a direct influence on the PFAS distribution. The aim of this study was to improve our understanding of the PFAS distribution in the natural heterogeneous media.

Variable Oxygen Levels Lead to Variable Stoichiometry of Benthic Nutrient Fluxes in a Hypertrophic Estuary

Harmful blooms of cyanobacteria may extend over long time spans due to self-sustaining mechanisms. We hypothesized that settled blooms may increase redox-dependent P release and unbalance the stoichiometry of benthic nutrient regeneration (NH4+:SiO2:PO43− ratios). We tested this hypothesis in the hypertrophic Curonian Lagoon, the largest in Europe. During summer, at peak chlorophyll and water temperatures, sediment cores were collected over 19 stations representing all the lagoon sedimentary environments. Sediment organic content, granulometry, aerobic respiration, and oxic and anoxic fluxes of dissolved inorganic nutrients and metals—Fe2+ and Mn2+—were measured. Loads and stoichiometry of regenerated nutrients were compared with those from the watershed. Analyzed sediments had elevated oxygen demand (−1.90 to −5.66 mmol O2 m−2 h−1), generally uncoupled to their variable organic matter content (1–23%) and median particle size (30–300 μm). Under oxic conditions, summer internal recycling equaled (SiO2) or exceeded, by a factor of ~66 and ~ 2, external loads of NH4+ and PO43−, respectively. Transient anoxia produced a general decrease of NH4+ and SiO2 regeneration, likely due to decreased macrofauna activity or inefficient mineralization, whereas it doubled average PO43− fluxes. In sandy, well-flushed areas, anoxia had a minor effect on PO43−, but stimulated a large production of Mn2+. Muddy sediments in lagoon areas with slow water renewal displayed large redox-dependent PO43− mobility, coupled to Fe2+ release. Settled algal blooms and hypoxic conditions might unbalance benthic regeneration stoichiometry and sustain blooms. The sedimentary pool of Mn4+ may represent a natural buffer preventing iron reduction and PO43− mobility.