Increasing Sediment Depth Research Articles

Environmental transport and sorting of glass retroreflective microbeads and their potential as proxies for road marking paints

Road marking paints are a potentially important contributor to the global microplastic pool but very little reliable information is available on their erosion or environmental distributions. As potential carriers of or proxies for road paints, we determine the concentrations and sorting of retroreflective glass microbeads in marking materials and in fractionated (< 5 mm) local dusts, soils and sediments. As an aid to our investigation, we also determine the concentrations of metals of geochemical significance or components of road paint pigments in markings and geosolids. Concentrations of beads up to 92,800 kg−1 were observed in street dusts, with a median diameter (350 μm) greater than that in road marking samples (270 μm). Few beads were found in adjacent (< 5 m) or more remote soils (six beads in ten 50-g samples) and none were detected in replicates of a sample of roof dust, suggesting that aeolian transport is limited. Concentrations up to 3700 kg−1 were found in estuarine sediments close to bridges or stormwater runoff effluents, and with increasing sediment depth concentrations and median diameter decreased; beads were not, however, detected in sediments 400 m away from any significant roads or runoff effluents. These observations suggest that bead accumulation is constrained locally but that the precise distance travelled and extent of burial in sediments are inversely related to size. Road marking paints sampled from urban streets readily fragmented into pieces smaller than glass microbeads, suggesting that while beads might carry small quantities of paint, transport and dispersion of the two particle types may not be directly coupled. Environmental ratios of V to Bi and Cr to Pb, as markers for BiVO4- and PbCrO4-pigmented yellow paints, respectively, did not correlate with bead distribution, presumably because these metals have a multitude of additional anthropogenic sources. However, an inverse relationship between bead concentrations and K:Ca suggests that this ratio might be a useful proxy of road marking paint in regions that are geologically similar.

Distribution and response of electroactive microorganisms to freshwater river pollution

Electroactive microorganisms (EAMs) play a vital role in biogeochemical cycles by facilitating extracellular electron transfer. They demonstrate remarkable adaptability to river sediments that are characterized by pollution and poor water quality, significantly contributing to the sustainability of river ecosystems. However, the distribution and diversity of EAMs remain poorly understood. In this study, 16S rRNA gene high-throughput sequencing and real-time fluorescence quantitative PCR were used to assess EAMs in 160 samples collected from eight rivers within the Pearl River Delta of Southern China. The results indicated that specialized EAMs communities in polluted sediments exhibited variations in response to water quality and sediment depth. Compared to clean sediment, polluted sediments showed a 4.5% increase in the relative abundances of EAMs communities (59 genera), with 45- and 17-times higher abundances of Geobacter and cable bacteria. Additionally, the abundance of cable bacteria decreased with increasing sediment depth in polluted sediments, while the abundance of L. varians GY32 exhibited an opposite trend. Finally, the abundances of Geobacter, cable bacteria, and L. varians GY32 were positively correlated with the abundance of filamentous microorganisms (FMs) across all samples, with stronger interactions in polluted sediments. These findings suggest that EAMs demonstrate heightened sensitivity to polluted environments, particularly at the genus (species) level, and exhibit strong adaptability to conditions characterized by high levels of acid volatile sulfide, low dissolved oxygen, and elevated nitrate nitrogen. Therefore, environmental factors could be manipulated to optimize the growth and efficiency of EAMs for environmental engineering and natural restoration applications.

Insights into nitrogen biogeochemical cycling in mangrove wetland from Genome-Resolved metagenomic sequencing

The critical role of nitrogen (N) in sustaining the vitality of mangrove wetlands is widely recognized, yet research elucidating complex microbiome structures and functional genes driving N biogeochemical cycles are markedly limited. This study leverages high-throughput and metagenomic sequencing to investigate N biogeochemical cycling from the Dongzhai Harbor mangrove wetland. Results reveal that Proteobacteria (29.95–49.26%), Chloroflexi (15.68–35.33%), and Acidobacteria (5.18–12.46%) are the predominant phyla, with Deltaproteobacteria (3.71–7.95%), Gammaproteobacteria (0.83–8.73%), and Alphaproteobacteria (1.73–14.13%) leading at the order level. Microbial diversity and the prevalence of functional genes diminish with increasing sediment depth, indicating that N transformation predominantly occurs in surface sediments. The identified N metabolic pathways in mangrove sediments encompass N fixation (nifD, nifK, nifH genes), nitrification (amoA, amoB, amoC, hao, nxrA and nxrB genes), denitrification (narG, nirK, nirS, norB and nosZ genes), dissimilatory nitrate reduction to ammonium (hzo、hzsA and hzsB genes), and organic N mineralization (gdhA and ureC genes). Organic N mineralization and N fixation are most pronounced in mangrove sediments, with N fixation exhibiting greater intensity in the mangrove than mudflat zone. Salinity, TOC, TN, and SO42- predominantly foster N cycling facilitated by microbial activities, while Fe appears to impede it. These insights offer a scientific foundation for the ecological restoration and conservation of mangrove wetlands.

Evaluate climate change and anthropogenic activities influencing geochemical variations in sediment between and within the avulsion period in the Lower Yellow River avulsion channels

The geochemical data from sediments in avulsion channels provide historical evidence of climate change and human-induced alterations in river basin environments. The present study focused on the particle size of sediments in cores and the level of geochemical variation in avulsion channels of the Lower Yellow River Delta (YRD), China. The sediment samples were collected in a depth range of 20–400 cm in avulsion channels. The collected samples were analyzed for sediment particle size and geochemical composition using standard methods. The results demonstrated rapid increases in agriculture practices, rainfall pattern changes, and terrestrial sediment runoff reduction in river basins after the 1960s. The reduced sediment loads in the Sanmenxia Reservoir significantly changed the sediment grain size and geochemical levels in the avulsion channel from August 1960.8 to January −1961.1. In particular, TC, TN, and C/N levels decreased with increasing sediment depth. The C/N values of <12 denoted completely reduced terrestrial sources of organic matter in the channel during the August 1960–January 1961 period compared to the July 1953–August 1960 period. The two-way ANOVA p-values were (p <0.016-p<0.001) strong between the avulsion periods but had no significant variation within the avulsion periods. We emphasize that this study provides a close interplay of different historical periods of geochemical variation in avulsion channel sediments in the alluvial fan YRD, and we argue that the evolution of the middle upstream river basin was subjective by climate change and human developmental actions, which impacted the YRD. In particular, reservoir-interrupted water flow and sediment reduction impacts associated with geochemical fluctuations are documented in the YRD.

Microbial diversity and oil biodegradation potential of northern Barents Sea sediments

The Arctic, an essential ecosystem on Earth, is subject to pronounced anthropogenic pressures, most notable being the climate change and risks of crude oil pollution. As crucial elements of Arctic environments, benthic microbiomes are involved in climate-relevant biogeochemical cycles and hold the potential to remediate upcoming contamination. Yet, the Arctic benthic microbiomes are among the least explored biomes on the planet. Here we combined geochemical analyses, incubation experiments, and microbial community profiling to detail the biogeography and biodegradation potential of Arctic sedimentary microbiomes in the northern Barents Sea. The results revealed a predominance of bacterial and archaea phyla typically found in the deep marine biosphere, such as Chloroflexi, Atribacteria, and Bathyarcheaota. The topmost benthic communities were spatially structured by sedimentary organic carbon, lacking a clear distinction among geographic regions. With increasing sediment depth, the community structure exhibited stratigraphic variability that could be correlated to redox geochemistry of sediments. The benthic microbiomes harbored multiple taxa capable of oxidizing hydrocarbons using aerobic and anaerobic pathways. Incubation of surface sediments with crude oil led to proliferation of several genera from the so-called rare biosphere. These include Alkalimarinus and Halioglobus, previously unrecognized as hydrocarbon-degrading genera, both harboring the full genetic potential for aerobic alkane oxidation. These findings increase our understanding of the taxonomic inventory and functional potential of unstudied benthic microbiomes in the Arctic.

Soil dynamic response mapping and vulnerability analysis of earthquake hazard in Banda Aceh using HVSR method

Banda Aceh is a city with a high level of seismicity and earthquake risk. The seismicity level is mainly caused by two Sumatran Fault segments, i.e., the Aceh and Seulimeum Segments. Soft alluvium deposits aggravate the earthquake hazard in Banda Aceh. The presence of soft alluvium layers above the more rigid soil layer will cause earthquake wave amplification. This amplification phenomenon can increase the amplitude and energy of earthquake waves, making the earthquake more destructive.Therefore, dynamic response mapping and subsurface structure modeling are required to determine the soil characteristics of the research area. Such information is needed for the infrastructure and planning development of Banda Aceh City. This study obtained the subsurface model from microtremor data and a sequence of processing and modeling techniques. The process uses the Horizontal to Vertical Spectral Ratio method (HVSR). The resonance frequency and amplification values are identified from the ratio between the horizontal and vertical spectrum components depicted in the HVSR curves. Based on the obtained results, Banda Aceh has a predominant frequency values range of 0.54-11.72 Hz and an amplification range of 1.22-6.26. Low predominant frequency is mainly located in the study area’s southeast and center, which is associated with large sediment thickness. Meanwhile, high amplification values are mainly located on the study area’s southeast, middle, and northwest sides, which are associated with increased sediment depth and geological features in the form of swamp sediment. Then the value of seismic vulnerability (Kg ) can be obtained based on the predominant frequency and amplification value. In this study, the value of seismic vulnerability ranges between 0.72-31.77 area with relatively high vulnerability values in the middle, southeast, south, and southwest of the study area.

Organic carbon preservation in wetlands: Iron oxide protection vs. thermodynamic limitation

The sequestration of organic carbon (OC) in wetland sediments is influenced by the presence of oxygen or lack thereof. The mechanisms of OC sequestration under redox fluctuations, particularly by the co-mediation of reactive iron (Fe) protection and thermodynamic limitation by the energetics of the OC itself, remain unclear. Over the past 26 years, a combination of field surveys and remote sensing images had revealed a strong decline in both natural and constructed wetland areas in Tianjin. This decline could be attributed to anthropogenic landfill practices and agricultural reclamation efforts, which may have significant impacts on the oxidation–reduction conditions for sedimentary OC. The Fe-bound OC (CBD extraction) decreased by 2 to 10-fold (from 8.3 to 10% to 0.7–4.5%) with increasing sediment depth at three sites with varying water depths (WD). The high-resolution spectro-microscopy analysis demonstrated that Fe (oxyhydr)oxides were colocalized with sedimentary OC. Corresponding to lower redox potential, the nominal oxidation state of C (NOSC), which corresponds to the energy content in OC, became more negative (energy content increased) with increasing sediment depth. Taken together, the preservation of sedimentary OC is contingent on the prevailing redox conditions: In environments where oxygen availability is high, reactive Fe provides protection for OC, while in anoxic environments, thermodynamic constraints (i.e., energetic constraints) limit the oxidation of OC.

Abundance of Live and Dead Bacteriopsammon Inhabiting Sandy Ecosystems of Recreational Marine Beaches of the Southern Baltic Sea

The study was carried out on four non-tidal sandy marine beaches located on the Polish part of the southern Baltic Sea coast. We applied a LIVE/DEAD™ BacLight™ Bacterial Viability Kit (Invitrogen™) method to determine the abundance of live and dead bacteriopsammon. Live psammon bacteria cells constituted 31–53% of the total number of bacteria inhabiting sand of the studied beaches. Abundance of live and dead psammon bacteria generally differed along the horizontal profile in all beaches. The maximum density of bacteria was noted in the dune and the middle part of the beach (dry zones) and the minimum in wet zones, i.e., under seawater surface and at the swash zone. Generally along the vertical profile, the highest numbers of two studied bacterial groups were noted in the surface sand layer, while with increasing sediment depth their numbers significantly decreased. The abundance of live and dead bacteria showed a distinct seasonal variation.

A large-scale study of microplastic abundance in sediment cores from the UK continental shelf and slope

To inform risk assessments, reliable, time efficient and affordable quantification methods are required for creating a microplastic (MP) pollution baseline in the world's oceans. To facilitate this, MP abundance was investigated in sediments of three contrasting areas of the UK continental shelf: North West of Jones Bank, the Canyons in the Celtic Sea and Dogger Bank in the North Sea, utilising the Nile Red tagging method to assess its time efficiency and cost. Average MP abundance in the top 10 cm was 1050–2700 MP kg−1. MP abundance decreased with increasing sediment depth and increased with increasing water depth. The findings emphasise the extent of MP pollution and illustrate the value of Nile Red for large scale mapping at relatively low cost.

Depth distribution of organic carbon sources in Arctic Chukchi Sea sediments

Climate-induced changes in the composition of organic matter sources in Chukchi Sea sediments could have major implications on carbon cycling, carbon sequestration, and food sources for lower benthic trophic levels. The aim of this study was two-fold: (1) to identify the proportional contributions of organic matter from various primary producers (phytoplankton, terrestrial, and bacterial) to depth-stratified sediments (0–5 cm) across the Arctic Chukchi Sea shelf using essential amino acid (EAA) specific stable carbon isotope biomarkers; and (2) to experimentally evaluate sediment bacterial production under different temperature scenarios. Proportional contributions of EAA sources to surface sediments had little relationship with environmental variables across the Chukchi Shelf and only showed noticeably higher terrestrial proportions in surface sediments in a high-deposition region in the southern study area. Across all sediment depth strata, the majority of EAA in sediments (∼76%) originated from terrestrial sources and may be indicative of accumulation over time due to slow degradation processes of this source within sediments. The different EAA sources showed no significant differences in proportional contributions with sediment depth except for phytoplankton-derived EAA, which decreased with increasing sediment depth. These patterns indicate a well-mixed upper sediment horizon, possibly from bioturbation activities by the abundant benthos. One EAA source assumed to respond quickly to changing environmental conditions are bacteria. To evaluate if and how bacterial production would respond to elevated temperatures, sediment bacterial production was measured experimentally using phospholipid fatty acid (PLFA) analysis. Bacterial production was initially (first 24 h) higher at 5 °C than at 0 °C; however, a drawdown of substrate or potential increase in predation activity and viral lysis resulted in bacterial production to subsequently be similar at both temperature settings. Overall results of this study suggest that terrestrial and bacterial carbon sources may become more prominent in a future, warmer Arctic. Identifying current patterns and potential shifts in organic matter sources with changes in temperature can aid in the understanding of the consequences of climate change in terms of organic matter presence and flow through benthic consumers that use these shelf sediments as feeding grounds.

Intra- and inter-spatial variability of meiofauna in hadal trenches is linked to microbial activity and food availability

Hadal trenches are depocenters for organic material, and host intensified benthic microbial activity. The enhanced deposition is presumed to be reflected in elevated meiofaunal standing-stock, but available studies are ambiguous. Here, we investigate the distribution of meiofauna along the Atacama Trench axis and adjacent abyssal and bathyal settings in order to relate the meiofauna densities to proxies for food availability. Meiofauna densities peaked at the sediment surface and attenuated steeply with increasing sediment depth. The distribution mirrored the vertical profile of the microbial-driven oxygen consumption rate demonstrating a close linkage between microbial activity and meiofauna density. Meiofaunal standing-stock along the trench axis varied by a factor of two, but were markedly higher than values from the abyssal site at the oceanic plate. Overall, meiofaunal densities poorly correlated with common proxies for food availability such as total organic carbon and phytopigments, but strongly correlated with the microbial benthic O2 consumption rate. We argue that microbial biomass likely represents an important meiofaunal food source for hadal meiofauna. Observations from three trench systems underlying surface water of highly different productivity confirmed elevated meiofaunal densities at the trench axis as compared to abyssal sites on oceanic plates. Food availability appear to drive elevated abundance and variations in meiofauna densities in hadal sediments.

Microbial metabolism and adaptations in Atribacteria-dominated methane hydrate sediments.

Gas hydrates harbour gigatons of natural gas, yet their microbiomes remain understudied. We bioprospected 16S rRNA amplicons, metagenomes, and metaproteomes from methane hydrate-bearing sediments under Hydrate Ridge (offshore Oregon, USA, ODP Site 1244, 2-69 mbsf) for novel microbial metabolic and biosynthetic potential. Atribacteria sequences generally increased in relative sequence abundance with increasing sediment depth. Most Atribacteria ASVs belonged to JS-1-Genus 1 and clustered with other sequences from gas hydrate-bearing sediments. We recovered 21 metagenome-assembled genomic bins spanning three geochemical zones in the sediment core: the sulfate-methane transition zone, the metal (iron/manganese) reduction zone, and the gas hydrate stability zone. We found evidence for bacterial fermentation as a source of acetate for aceticlastic methanogenesis and as a driver of iron reduction in the metal reduction zone. In multiple zones, we identified a Ni-Fe hydrogenase-Na+ /H+ antiporter supercomplex (Hun) in Atribacteria and Firmicutes bins and in other deep subsurface bacteria and cultured hyperthermophiles from the Thermotogae phylum. Atribacteria expressed tripartite ATP-independent transporters downstream from a novel regulator (AtiR). Atribacteria also possessed adaptations to survive extreme conditions (e.g. high salt brines, high pressure and cold temperatures) including the ability to synthesize the osmolyte di-myo-inositol-phosphate as well as expression of K+ -stimulated pyrophosphatase and capsule proteins.

Sediment release of dissolved organic matter to the oxygen minimum zone off Peru

Abstract. The eastern tropical South Pacific (ETSP) represents one of the most productive areas in the ocean that is characterised by a pronounced oxygen minimum zone (OMZ). Particulate organic matter (POM) that sinks out of the euphotic zone is supplied to the anoxic sediments and utilised by microbial communities, and the degradation of POM is associated with the production and reworking of dissolved organic matter (DOM). The release of DOM to the overlying waters may, therefore, represent an important organic matter escape mechanism from remineralisation within sediments but has received little attention in OMZ regions so far. Here, we combine measurements of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) with DOM optical properties in the form of chromophoric (CDOM) and fluorescent (FDOM) DOM from pore waters and near-bottom waters of the ETSP off Peru. We evaluate diffusion-driven fluxes and net in situ fluxes of DOC and DON to investigate processes affecting DOM cycling at the sediment–water interface along a transect at 12∘ S. To our knowledge, these are the first data for sediment release of DON and pore water CDOM and FDOM for the ETSP off Peru. Pore water DOC accumulated with increasing sediment depth, suggesting an imbalance between DOM production and remineralisation within sediments. High DON accumulation resulted in very low pore water DOC ∕ DON ratios (≤1) which could be caused by an “uncoupling” in DOC and DON remineralisation. Diffusion-driven fluxes of DOC and DON exhibited high spatial variability and ranged from 0.2±0.1 to 2.5±1.3 mmolm-2d-1 and from -0.04±0.02 to 3.3±1.7 mmolm-2d-1, respectively. Generally low net in situ DOC and DON fluxes, as well as a steepening of spectral inclination (S) of CDOM and an increase in humic-like DOM at the sediment–water interface over time, indicated active microbial DOM utilisation. The latter may potentially be stimulated by the presence of nitrate (NO3-) and nitrite (NO2-) in the water column. The microbial DOC utilisation rates, estimated in our study, are potentially sufficient to support denitrification rates of 0.2–1.4 mmolm-2d-1, suggesting that the sediment release of DOM may on occasion contribute to nitrogen loss processes in the ETSP off Peru.

Horizontal and vertical distribution of microplastics in the Wuliangsuhai Lake sediment, northern China

Microplastics are ubiquitous in the environment, sediments in the water body are considered as the sink of microplastics, but its accumulation and change trend have received little attention. We assessed the concentrations of microplastics in sediment samples collected in Wuliangsuhai Lake. The abundance of microplastics (0–6 cm) in surface sediment ranged from 16.5 to 72.4 particles 100 g−1 dry weight. The results show that the abundance, type and size of microplastics in surface sediment of different functional areas are different. Four sampling points were selected to investigate the vertical distribution of microplastics. The abundance of microplastics exhibited a clear decreasing trend with increased sediment depth, the abundance of microplastics in deep sediments is much less. This result shows that when estimating the storage of microplastics in sediments, if only considered the surface sediments, the assessment of microplastic storage worldwide might be underestimated.

An Evaluation of Microplastics in Lac Hertel Sediment Over Time

Background: Microplastics, defined as plastic smaller than 5 mm, are pervasive in both marine and freshwater ecosystems. Humans, zooplankton, and fish have been shown to ingest microplastics, which could have detrimental health impacts. Consequently, this project investigated the question: are there microplastics in the sediment of Lac Hertel, located in the Mont Saint Hilaire Biosphere Reserve in Quebec, and if so, how has the amount of microplastics changed over time?Methods: One sediment core was obtained from the centre of the lake and one was obtained from the edge near the mesocosm dock. Next, one section from the top, middle, and bottom of each core was collected. Afterwards, the microplastics were extracted from the sediments, counted with a dissecting microscope under regular light, and a subset of fragments were tested with a hot needle to confirm that they were plastic.Results: A generalized linear model indicated that the number of microplastics in our samples increased significantly over time and that the sediment samples from the mesocosm dock had significantly fewer microplastics than the lake’s centre. Similarly, a Pearson correlation test revealed that an increasing sediment depth had a significantly negative relationship with the number of microplastics at the lake’s centre. However, another Pearson correlation test determined that this trend was not reflected at the mesocosm dock, potentially because of sediment focusing.Limitations: Due to resource and time constraints, we had a small sample size, only analyzed microplastics larger than 250 µm, and counted microplastics instead of weighing them.Conclusion: Our results suggest that there has been a significant increase in microplastics in Lac Hertel sediment over time. Ultimately, our results emphasize the need to mitigate plastic pollution.

Daphnia laevis ephippia cemetery in the sediment of a permanent tropical lake

Aiming to evaluate the importance of egg banks from a tropical lake for the active Daphnia laevis dynamics, we investigated ephippia down to a 28-cm sediment depth, and used data obtained by traps to compare with the in situ production and hatching rates of fresh ephippia. All the ephippia from the sediment were opened for inspection of the presence of resting eggs, and those found were incubated to verify hatching. Ephippia density degradation rate was tested by putting isolated ephippia immersed in the sediment lake for 75 days, and found to exponentially decrease with increasing sediment depth. Higher densities were recorded in 2017 (maximum of 1.43 ephippia per cm2) than in 2016 (maximum of 0.8 ephippia per cm2). The empty ephippia rates were high in all sediment depths ranging from 83% to 100%. In contrast, the hatching rates in situ of fresh ephippia were high, suggesting that those that sink are mostly unviable or empty, forming an unusual egg bank in the sediment. In the laboratory, ephippia degradation signs were observed as early as the fifteenth day after contact with sediment. We conclude that the egg bank is not able to support an active population since most of the ephippia are empty and also rapidly degrade in sediment. Our results contribute to the current understanding of diapause in tropical cladocerans, demonstrating that the presence of Daphnia ephippia in the sediment of permanent lakes do not always correspond to an egg bank for temporal dispersion.

Temporal variations of trace metals and a metalloid in temperate estuarine mangrove sediments.

Mangrove sediments are strong modulators of organic matter (OM) content and pollutant dynamics, acting both as sinks and sources of these components. This study aimed to assess temporal dynamics of OM within temperate mangrove sediments and their ability to sequester pollutants. Specifically, levels of trace metals (Fe, Cu, Zn, Pb, Cd) and a metalloid (As) were examined within mangrove and mudflat sediments located in a high-energy environment in Mangawhai Harbour Estuary, northern New Zealand. Sediment cores were collected from a mangrove stand and adjacent mudflats at three sediment depths during different months over a year. Variations in OM and elements were compared to rainfall and temperature patterns observed during the sampling period. All element concentrations, except for those of As, were significantly higher in mangrove compared to mudflat sediments during the entire sampling period. This is consistent with the well-reported ability of mangroves to trap suspended particles and OM. In addition, we observed a decreasing trend in trace metal concentrations with increasing sediment depth within mangrove habitat, which correlated well with decreasing OM content. Our results also suggested that most elements had different, but significant, temporal variations throughout the year, especially in mangrove sediments. Overall, the concentrations of Cu, Zn, Pb, Cd, and As in mangrove sediments increased during summer, whereas maximum levels of Fe and OM were observed in winter. This temporal pattern was determined to be related to OM and redox cycling as a result of changes in effluent input rates and physical/chemical environments during different seasons.

Patterns of resource allocation in a coastal marsh plant (Schoenoplectus americanus) along a sediment-addition gradient

Reductions in sediment delivery to coastal marshes increase their vulnerability to relative sea-level rise. Sediment pulses from storm events and commercial dredge-spray operations (e.g., beneficial use) represent increasingly important sediment sources to otherwise sediment-deprived marshes. These sediments can stimulate plant growth by providing nutrient and elevation subsidies, with plant growth predicted to peak at intermediate sediment depths. Most sediment subsidy studies have compared plant responses among discrete sediment depths applied across a uniform marsh platform, and often assessed impacts in the years following sediment deposition. Less is known about the immediate effects of sediment additions on plant growth or the mechanisms driving differences in these responses along a sediment-addition gradient. To investigate immediate plant responses to sediment additions (i.e., within 6 months), we added sediment to Schoenoplectus americanus-dominated brackish marsh sods in a greenhouse setting. Sediment was added incrementally along a sediment addition gradient (0–20 cm) to 18 sods, which were placed in clear planting enclosures to permit monitoring of above- and below-ground responses and hydro-edaphic properties. Shoot production, which increased linearly along the sediment addition gradient, was the first observable response to sediment addition and was likely a result of apical dominance interruption. This trend continued throughout the study even as survival of those shoots declined with increasing sediment depth. At the end of the experiment, however, shoot biomass production was biphasic, responding favorably to sediment addition up to intermediate depths before collapsing at higher depths. While fine root production in sods was maximal at intermediate sediment addition levels, root production in new sediment layers was limited. There were limited differences observed for hydro-edaphic properties along the sediment addition gradient. Thus, plant resources are allocated to shoot production immediately following sediment addition at the expense of root colonization of new sediment layers, suggesting that, in certain conditions, sediment deposition can represent a stress rather than a subsidy.

Microbial Selection and Survival in Subseafloor Sediment.

Many studies have examined relationships of microorganisms to geochemical zones in subseafloor sediment. However, responses to selective pressure and patterns of community succession with sediment depth have rarely been examined. Here we use 16S rDNA sequencing to examine the succession of microbial communities at sites in the Indian Ocean and the Bering Sea. The sediment ranges in depth from 0.16 to 332 m below seafloor and in age from 660 to 1,300,000 years. The majority of subseafloor taxonomic diversity is present in the shallowest depth sampled. The best predictor of sequence presence or absence in the oldest sediment is relative abundance in the near-seafloor sediment. This relationship suggests that perseverance of specific taxa into deep, old sediment is primarily controlled by the taxonomic abundance that existed when the sediment was near the seafloor. The operational taxonomic units that dominate at depth comprise a subset of the local seafloor community at each site, rather than a grown-in group of geographically widespread subseafloor specialists. At both sites, most taxa classified as abundant decrease in relative frequency with increasing sediment depth and age. Comparison of community composition to cell counts at the Bering Sea site indicates that the rise of the few dominant taxa in the deep subseafloor community does not require net replication, but might simply result from lower mortality relative to competing taxa on the long timescale of community burial.

Size-selective feeding of Arenicola marina promotes long-term burial of microplastic particles in marine sediments

Despite of their ubiquitous distribution in marine sediments, the role of benthic fauna in microplastic transport at the sea floor has received little attention yet. The present study investigated the influence of bioturbation activity of the polychaete Arenicola marina on microplastic transport and burial in marine sediments. Sediment ingestion was assessed in a long term mesocosm experiment with exposure times ranging from 106 to 240 days, using three particle tracers with different particle diameters (microplastic: 500 and 1000 μm, respectively; luminophores: 130 μm). Sediment grain size distributions were assessed after experiment termination in all feeding layers at 8–12 cm depth to determine the influence of size-selective feeding of A. marina on median grain size and microplastic retention. Burial of microplastic occurred in all mesocosms up to a depth of 20 cm and was strongly dependent on individual sediment feeding rates. For low bioturbation conditions, both microplastic and luminophore concentrations exhibited an exponential decrease with increasing sediment depth, indicating particle burial via feeding funnel transport. Particle concentrations remained high in the uppermost 4 cm of the sediment. At high bioturbation rates, no microplastic particles remained in near-surface sediment layers, but a distinct accumulation of microplastic was observed in the feeding layer, suggesting the discrimination of plastic particles during feeding. In contrast, luminophores displayed a similar accumulation, but additionally showed uniform distributions above feeding layers, indicating ingestion and defecation by polychaetes. In accordance with these findings, an overall coarsening of median grain sizes was observed in all feeding layers, indicating the retention of large microplastic due to size-selective feeding. These findings demonstrate the ability of the conveyor belt-feeding polychaete A. marina to promote unidirectional transports of microplastic ≥500 μm and the potential for the long-term retention of these particles in marine sediments.