Quality Of Sedimentary Organic Matter Research Articles

Spatiotemporal distribution of sedimentary chloropigments and organic carbon in a shelf sea ecosystem, with a focus on the cold water mass

The Yellow Sea is one of the most thoroughly studied shelf seas in the northwestern Pacific. However, many aspects of the northern Yellow Sea sediments remain poorly understood. Sedimentary chloropigments and total organic carbon (TOC) are crucial components that indicate benthic primary productivity and ecosystem functioning in the marginal sea system. The present study investigated the temporal and spatial variations of sediment chloroplastic pigments (Chl-a and Pha-a) and TOC contents in relation to seasons and the northern Yellow Sea Cold Water Mass (NYSCWM). Decadal trends were also examined using linear regression on time-series data from 12 cruises conducted over a nine-year period (2006–2014). The correlations between sedimentary Chorophy-a (Chl-a), Pheophorbide-a (Pha-a), TOC, and their ratios with other abiotic and biotic factors were analyzed to detect any effects of the NYSCWM on the spatiotemporal distribution of sediment Chl-a, Pha-a, TOC contents, as well as the ecological relevance of their ratios as indicators of food quality (freshness) in the benthic system. The NYSCWM substantially influences the spatial and seasonal distribution of sedimentary Chl-a, Pha-a, and TOC contents. High values of Pha-a, TOC, and the ratios of Pha-a/(Chl-a + Pha-a) and TOC/Chl-a were often observed in the central region of the NYSCWM, where Chl-a showed clear seasonal fluctuations while Pha-a and TOC remained relatively constant. While sedimentary TOC appears to negatively mirror the spatial pattern of primary productivity in the water column, the spatial pattern of sedimentary Chl-a in relation to NYSCWM is less evident. However, the contents of Pha-a and TOC in the central NYSCWM sediments were higher than those at surrounding NYSCWM stations. Surface sediment Pha-a/(Chl-a + Pha-a) ratio was negatively correlated with meiofauna abundance and biomass and positively correlated with nematode/copepod ratio. This finding indicates that the quality of sediment organic matter is important to meiofauna. Conversely, the lower organic degradation rate at the lower bottom temperature of the NYSCWM may explain the observed higher Pha-a and TOC contents in its central area. Time-series data collected over nine years at the NYSCWM stations showed a decreasing trend for Chl-a, while TOC and the ratio of Pha-a/(Chl-a + Pha-a) exhibited increasing tendencies. This trend may imply a reduced quality of food supply to the benthic food web in the shelf sea system, along with degraded ecosystem functioning and service under the impacts of global change and anthropogenic disturbances.

Living benthic foraminifera from Almirante Câmara and Grussaí canyons and adjacent slope areas (Campos Basin, Southwest Atlantic): Response to trophic and hydrodynamic conditions

Living (rose Bengal stained) benthic foraminiferal faunas, grain size, geochemical parameters (total organic matter, chlorophyll-a, pheophytin-a, molecular markers, and vertical flux of particulate organic matter) were investigated in 14 stations in two submarine canyons Almirante Câmara and Grussaí considered mature and immature, respectively, and on the adjacent continental slope, from 400 to 1300 m water depth, in the southwestern Atlantic Ocean. Samples were retrieved during two campaigns, in the austral winter 2008 and the austral summer 2009. The density, richness, and species composition of the benthic foraminiferal fauna decrease with depth in the canyons and along the adjacent slope transects, reflecting the availability and quality of sedimentary organic matter. Both canyons act as traps of labile organic matter, mainly towards the head of the Almirante Câmara canyon, promoting high specimen density, species richness, and the occurrence of species indicative of pulses of phytodetritus (e.g., Bolivina pacifica, Nonionella opima, and Nonionella stella). Towards the deeper regions of Almirante Câmara canyon and in the adjacent slope area, the lower abundance, diversity, and richness reflect more oligotrophic conditions and an adverse environment in terms of easily metabolizable organic matter. This multiproxy study demonstrates that the canyons in the southeastern Brazilian continental margin play an important role in accumulating organic matter.

Weak seasonality in benthic food web structure within an Arctic inflow shelf region

The Arctic Ocean is characterized by pronounced seasonality in the quantity and quality of organic matter exported from the surface ocean. While it is well established that changes in food availability can alter the abundance, biomass and function of benthic organisms, the impact on food web structure is not well studied. We used bulk carbon and nitrogen stable isotope analysis to assess the quantity and quality of sediment organic matter and structure of the benthic food web in four seasons within the Northern Barents Sea (76°N − 82 °C). Despite a highly seasonal vertical flux, we found that the organic carbon and chlorophyll-a content of surface sediments was seasonally stable, suggesting a lack of seasonality in food availability at the seafloor. However, organic biomarkers indicate that the quality of sediment organic matter increased to a maximum in August and December, up to 6 months after the spring bloom. The seasonal stability of food quantity was mirrored in food-web structure (e.g., total isotopic range, number of trophic levels) which did not change significantly across sampling periods. We expected that suspension and deposit feeders would respond more readily to seasonal changes in food quality compared to predators. However, we observed no significant seasonal changes in the trophic levels or isotopic niche areas of benthic functional groups. The centroids of isotopic niches of all benthic functional groups shifted seasonally by <2 ‰ along the δ13C-axis, suggesting minimal shifts in carbon resource use. Because the northern Barents Sea experiences significant changes in seasonal sea ice cover, we expected that stable-isotope ratios of benthic organisms would show an increased consumption of sympagic-derived organic matter through less negative δ13C values in early spring and summer. However, only two taxa (the soft coral Gersemia spp. and bivalves in the family Yoldiidae) showed 13C-enrichment in spring or summer consistent with the assimilation of sympagic-derived organic matter, despite previous evidence suggesting widespread use of this carbon source. Overall, our results show that there is an apparent de-coupling in time between pelagic processes and benthic food-webs in which the accumulation and assimilation of high-quality organic matter occurs for benthos during the fall and early winter months when there is little to no fresh organic matter generated at the surface. This temporal mismatch highlights the importance of considering the timescales over which components of the marine ecosystem respond to short-term environmental changes and the methods employed to assess seasonality.

Outcomes of feeding activity of the sea cucumber Holothuria tubulosa on quantity, biochemical composition, and nutritional quality of sedimentary organic matter

IntroductionHolothuria tubulosa is one of the most common sea cucumbers in the Mediterranean Sea, generally associated with organically enriched coastal sediments and seagrass beds. As a deposit-feeder, it is responsible for strong bioturbation processes and plays a putative key role in sedimentary carbon cycling and benthic trophodynamics. With the aim of exploring the potential use of holothuroids as a tool for remediating eutrophicated sediments, we investigated the effects of H. tubulosa on sedimentary organic matter quantity, biochemical composition, and nutritional quality.MethodsHolothuroids and associated samples of ambient sediments were collected in two sites located in the Central-Western Mediterranean Sea (Sardinia, Italy) and characterized by different trophic status backgrounds: the site of Oristano characterized by sandy-muddy sediments and the presence of mariculture plants (ranked as meso-eutrophic) and the site of Teulada characterized by sandy sediments and Posidonia oceanica meadows (ranked as oligo-mesotrophic). We compared the biochemical composition (proteins, carbohydrates, lipids) of ambient sediment vs sea cucumbers feces and the sedimentary protein content vs protein content in the sediments retrieved in different gut sections (esophagus, mid gut, end gut) of the holothuroid.ResultsOur results reveal that holothuroids feeding on meso-eutrophic sediments can increase protein (1.5 times) and lipid (1.3 times) content through their defecation, thus making these substrates a more labile food source for other benthic organisms. We report here that H. tubulosa feeding on meso-eutrophic sediment is most likely able to actively select particles rich in labile organic matter with buccal tentacles, as revealed by the protein content in the esophagus that is up to 2-folds higher than that in the source sediment. According to the inverse relationship between assimilation rates and availability of organic substrates and the optimal foraging theory, H. tubulosa feeding on oligo-mesotrophic sediments showed potential assimilation of proteins ca. 25% higher than that of specimens feeding on meso-eutrophic sediments.DiscussionOur results reveal that H. tubulosa feeding on meso-eutrophic sediments can profoundly influence the benthic trophic status, specifically modifying the biochemical composition and nutritional quality of organic matter, thus paving the way to its possible use in bioremediation actions of eutrophicated sediments and in Integrated Multi-Trophic Aquaculture systems.

Water depth and transparency drive the quantity and quality of organic matter in sediments of Alpine Lakes on the Tibetan Plateau

AbstractIdentifying primary environmental drivers mediating the quantity and quality of sedimentary organic matter (OM) in climate‐sensitive alpine lakes is crucial to understanding the role of alpine lakes in greenhouse gas emissions and Earth's climate system. Here, we characterized various pools of OM of 20 alpine lakes across the Tibetan Plateau, including bulk OM, water‐soluble OM and alkaline‐extracted OM from surface sediments, and dissolved OM (DOM) from surface water. The total organic carbon (TOC) content in sediments was low (&lt; 3%), and δ13C of TOC and C : N ratios indicated limited allochthonous carbon inputs. Sedimentary water‐soluble OM and alkaline‐extracted OM were both dominated by low‐molecular‐weight, low‐aromaticity compounds with low contributions of terrestrial humic substances, suggesting that sedimentary leachable OM was primarily regulated by in‐lake sources and processes. Redundancy analysis showed that water depth, water transparency, and total phosphorus concentration in water column explained ~ 50% variance of sedimentary bulk and leachable OM, substantiating the importance of autochthonous sources and primary productivity in regulating the quantity and quality of sedimentary OM. Compared with lake surface water DOM, water‐soluble OM and alkaline‐extracted OM from sediments had higher proportions of terrestrial humic‐like substances, suggesting preferential preservation of allochthonous materials in sediments. Our results are the first to demonstrate a clear link between physical attributes and sedimentary OM in Tibetan lakes. The associated relations predict that the amount of total OM and autochthonous carbon preserved in sediments would increase due to the lake enlargement under the scenarios of climate warming and precipitation enhancement, which may amplify greenhouse gas emissions from Tibetan lakes.

Quality of Sediment Organic Matter Determines the Intertidal N2O Response to Global Warming

AbstractEstuaries and coasts are areas of intense biogeochemical cycling and are sensitive to global climate change. However, the effect of temperature increase on the emissions of the powerful greenhouse gas nitrous oxide (N2O) in different estuarine and coastal areas is still uncertain. In this study, we used laboratory incubation experiments to investigate increasing temperatures (12, 25, and 35°C) and tidal effects on N2O fluxes in intertidal sediments from the East China coast (ECC). Overall, the ECC acts as a net source of atmospheric N2O and exhibited considerable spatial variability over three orders of magnitude (from −0.17 to 8.4 μmol m−2 h−1). The warming promoted N2O emissions in most intertidal areas, while reducing N2O emissions at some sampling sites. In addition, the overall effect of flooding on N2O emissions changed from a positive to a negative effect with increasing temperature. By combining the sediment properties of all the sampling sites, we found that large differences in N2O emissions at the same amended floodwater nitrogen concentration were due to the quality and quantity of sediment organic matter. Sediment derived mainly from marine sources emitted more N2O than sediment derived from terrestrial sources. This suggests that the mangroves, salt marshes, and intertidal zones of estuaries have a mitigating effect on N2O emissions due to their elevated terrestrial organic matter inputs. This research improves our understanding of the impact of future global climatic changes on intertidal N2O fluxes, which can inform future studies and models and can be used to constrain intertidal N2O emissions.

Multi-proxy approach involving ultrahigh resolution mass spectrometry and self-organising maps to investigate the origin and quality of sedimentary organic matter across a subtropical reservoir

Humic substances (HS) in sediments play an important role in carbon and nutrient biogeochemical cycles and fate of contaminants in the environment. However, information regarding HS quality and transformations that may affect their behaviour in reservoirs is still limited. The aim of this investigation was to track sources and changes in sedimentary HS across a subtropical reservoir, connecting them to in-lake processes and land-use influences. Surface sediments were collected at seven sampling sites in Itupararanga Reservoir (Brazil). Humic (HA) and fulvic (FA) acids (components of HS) were extracted from the sediment samples followed by in-depth characterisation via UV/VIS, fluorescence spectroscopy, elemental (C, N) and isotopic analysis (δ13C, δ15N), nuclear magnetic resonance (13CNMR) and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). All data were analysed by self-organising maps. The results showed that samples from the upstream part of the reservoir were older and more decomposed. They likely originated from C3 land-plants (δ13C varied from −26.3‰ to −22.4‰), having more aromatic, oxygen-poor (O/C < 0.5) and unsaturated compounds (H/C < 1.1). In contrast, near-dam samples were younger and had larger contributions of autochthonous material. This was confirmed by oxygen-rich (O/C > 0.5) and partly more unsaturated compounds for FA as well as oxygen-poor and saturated compounds with H/C > 1.1 for HA. Self-organising maps pointed out these differences between upstream and dam areas and indicated that agriculture lands were related to microbially-derived HS. Changes in HS composition revealed that internal reservoir processes may have influenced HS quality across the reservoir.

Litterfall-derived organic matter enhances mercury methylation in mangrove sediments of South China

Mercury (Hg) contamination in mangrove ecosystems has received increasing attention in recent years. Although many studies have investigated methylmercury (MeHg) contamination and its relationship to a number of environmental factors in mangrove sediments, the production of MeHg in this carbon-rich ecosystem has not been fully evaluated. In this study, we measured the total mercury (THg) and MeHg concentrations in the sediments collected from seven mangrove forests in China. In addition, we examined the origin and quality of sedimentary organic matter (OM), trying to evaluate their influence on the MeHg accumulation in mangrove sediments. We found that litterfall played an important role in regulating THg and MeHg contents in mangrove sediments. THg and MeHg concentrations in the mangrove sediments were positively correlated to OM content and the labile fraction of the OM. Multiple evidence (stable carbon isotopes, monosaccharide compositions, and biogenic silica) suggested that OM in mangrove sediments was dominated by input from litterfall. THg and MeHg concentrations were elevated at the sediments with higher input of mangrove OM. We observed that addition of mangrove litter stimulated the production of MeHg under anaerobic conditions. Overall, our results suggested that litterfall acted as a source of inorganic Hg, labile carbon, and low-molecular-weight OM which greatly favor the Hg methylation. Our study provides new insights into the MeHg production in mangrove sediments.

Fatty acid analyses provide novel insights on hippo defecation and consequences for aquatic food webs

By defecating grasses into aquatic systems at massive scales and intensities, hippos can initiate complex changes to aquatic ecosystems. However, consequent effects on food webs are not well understood, particularly regarding shifts in basal resource contributions to consumer diets and their physiological condition. Here, we use fatty acid analysis to show that dense hippo aggregations and high dung loading are associated with (1) alterations to basal resource pools, (2) reduced quality of sediment organic matter and (3) increases in terrestrial and bacterial biomarker levels, but declines in those of diatoms in estuarine secondary consumers. While hippo defecation can increase boundary permeability between terrestrial and aquatic systems, our findings indicate that this may lead to a shift from a microphytobenthic food web base to one with increasing bacterial contributions to higher consumers. Our findings expand understanding of the mechanisms by which an iconic African megaherbivore indirectly structures aquatic ecosystems.

Reconstructing organic matter sources and rain rates in the southern West Pacific Warm Pool during the transition from the deglaciation period to early Holocene

Transient features in the organic carbon content of deep-sea sediment cores resulting from changes in the flux and/or quality of sedimentary organic matter are not uncommon. We examined the geochemical characteristics of sediments retrieved with a gravity corer from the northwestern Solomon Sea (3908 m water depth), southern West Pacific Warm Pool (WPWP). δ13C and δ15N of sedimentary organic matter, together with TOC/TN data suggest that the organic material is characterized by a mixture of marine and terrestrial origins with a higher contribution from marine algae. The data were analyzed with an inverse non-steady-state reaction-transport model to examine the variability and magnitude of particulate organic carbon (POC) flux to the seafloor during the transition between the deglaciation period and early Holocene. Measured POC content and porewater NO3−, NH4+, DIC and SO42− concentrations were used to constrain the model. Hindcast results revealed that POC flux decreased from 75 to 37.5 μg cm−2 yr−1 during the deglaciation–early Holocene transition. The rate of POC degradation in the present setting is slightly lower compared to that in the pre-Holocene setting. The synchronous decline in the relative contribution of terrestrial organic matter input and rapid sea level rise during this transition suggest that sea level, rather than surface productivity, is the dominant factor controlling the POC deposition flux in the Solomon Sea. This is conceivable because the sampling site is proximal to high-relief islands with high rainfall, a well-developed submarine canyon system and narrow and steep continental margins. Consequently, we suggest that deep-water basins in proximity to similar high-relief mountainous islands in the tropical Pacific may represent important sinks for terrestrial organic material, especially during sea level lowstands.

Fine Sediment Removal Influences Biogeochemical Processes in a Gravel-bottomed Stream.

The transport and processing of nutrients and organic matter in streams are important functions that influence the condition of watersheds and downstream ecosystems. In this study, we investigated the effects of streambed sediment removal on biogeochemical cycling in Fawn River, a gravel-bottomed river in Indiana, U.S.A. We measured stream metabolism as well as nitrogen (N) and phosphorus (P) retention in both restored and unrestored reaches of Fawn River to examine how sediment removal affected multiple biogeochemical functions at the reach scale. We also assessed the properties of restored and unrestored streambed sediments to elucidate potential mechanisms driving observed reach-scale differences. We found that sediment removal led to lower rates of primary productivity and ecosystem respiration in the restored reach, likely due to macrophyte removal and potentially due to changes to sediment organic matter quality. We found minimal differences in N and P retention, suggesting that these processes are controlled at larger spatial or temporal scales than were examined in this study. Denitrification enzyme activity was lower in sediments from the restored reach compared to the unrestored reach, suggesting that restoration may have decreased N removal. Our results indicate that most near-term changes in biogeochemical function following restoration could be attributed to macrophyte removal, although effects from sediment removal may emerge over longer timescales.

Small-scale distribution of metazoan meiofauna and sedimentary organic matter in subtidal sandy sediments (Mediterranean Sea)

While variations in sedimentary organic matter (OM) quantity, biochemical composition and nutritional quality as well as in meiofaunal abundance and assemblage composition at the macro- and mesoscale are relatively well known, information about variations at the microscale is much scarcer. To shed some light on this issue, we tested the null hypothesis by which abundance and composition of the meiofaunal assemblages, and the quantity, biochemical composition and nutritional quality of sedimentary organic matter in coastal shallow environments do not vary within a frame of 1 m2. No significant variation within the frame emerged for OM quantity, nutritional quality, biochemical composition and the abundance of meiofaunal assemblages. On the other hand, the composition of meiofaunal assemblages varied significantly within the frame and exhibited a clear segregation of assemblages farther to the shore, as a likely result of local micro-hydrodynamic conditions. Spatial autocorrelation analysis revealed that lipid and protein sedimentary contents had a random distribution, whereas carbohydrate and biopolymeric C contents and meiofaunal total abundance were characterized by a patchy distribution, with discrete peaks within the sub-frame squares (ca. 0.1 m2). Phytopigments showed a spatial positive autocorrelation distribution, following the micro-hydrodynamic pattern, with patches larger than the sub-frame square, but smaller than the entire one (1 m2). Overall, our results suggest that, within 1 m2 of subtidal sandy sediments, three replicates could be sufficient to assess correctly OM attributes and the abundance of meiofauna, but could be possibly inadequate for assessing meiofaunal assemblages’ composition at a finer scale (&lt;1 m2).

The influence of the quantity and quality of sediment organic matter on the potential mobility and toxicity of trace elements in bottom sediment

Knowledge on the fraction of trace elements in the bottom sediments is a key to understand their mobility and ecotoxicological impact. The purpose of this study was to assess the influence of the content of organic matter fractions on the mobility and ecotoxicity of trace elements in sediments from the Rybnik reservoir. The most refractory fraction of organic matter—Cnh (non-hydrolysing carbon)—dominated in the sediments. The content of organic matter fractions are arranged in the following order: Cnh (non-hydrolysing carbon) > Cfa (fulvic acid) > Cha (humic acid) > DOC (dissolved organic carbon). On the other hand, the highest value of correlation coefficients was found for different fractions of trace elements and DOC content in the bottom sediments. A higher content of TOC in the sediments significantly increased the share of elements in the potential mobile fraction and, at the same time, decreased the binding of elements in the mobile fractions. Moreover, in sediments that contain more than 100 g/kg d.m. TOC, no and medium risk of trace element release from sediments was observed. The Cu, Cd and Ni were potentially the most toxic elements for biota in the Rybnik reservoir. However, the correlation between the content of trace elements and the response of bacteria was insignificant. These results suggested that the complexation of trace elements with organic matter makes them less toxic for Vibrio fischeri. The transformation and sources of organic matter play an important role in the behaviour of trace elements in the bottom sediments of the Rybnik reservoir.

Effect of sedimentary organic matter on species richness of deposit feeders in enclosed bay ecosystems: Insight from fatty acid nutritional indicators

How nutritional quality of dietary resources affects species richness of consumer communities is poorly understood. We used fatty acids as indicators of nutritional quality of sedimentary organic matter to evaluate the effects of highly unsaturated fatty acid (HUFA) content in sediments and fatty acid diversity in sedimentary organic matter on species richness of deposit feeders. We sampled benthic animals and sedimentary organic matter, a potential dietary source for deposit feeders, at 54 locations in two bay ecosystems. The species richness of deposit feeders ranged between 1 and 29 and had a parabolic relationship with the organic carbon content of sediments. At intermediate range of sedimentary organic carbon content, the species richness of deposit feeders was positively related to HUFA content and fatty acid diversity. These findings indicate that nutritional quality is one of the important factors determining species richness. In particular, HUFA content and fatty acid diversity are useful indicators of the nutritional quality of potential diets and good predictors of the occurrences of benthic invertebrates in marine habitats.

Influence of coastal Mediterranean rivers on the organic matter composition and reactivity of continental shelf sediments: The case of the Têt River (Gulf of Lions, France)

River-dominated ocean margins (RiOMars) are areas of high productivity, rapid sediment deposition, and intense recycling. The Gulf of Lions, located in the North occidental Mediterranean Sea, is no exception to this general rule and is under the prevalent influence of the Rhône River. One particularity of this continental shelf is that it is also fed by several small coastal rivers whose inputs actively contribute to the coastal dynamic and export to the deep-sea. Our objective was to gauge the influence of coastal rivers on the quality and reactivity of the organic matter delivered to the continental shelf as these properties control two important functions of coastal areas: benthic productivity and carbon sequestration. We chose for this study, the Têt River, which is typical of coastal Mediterranean rivers with a torrential regime. Sediment cores were sampled five times over a one-year period on four stations located along the dominant trajectory of the Têt River plume. The biogeochemical characteristics of the surface sediments were analysed to highlight spatio-temporal trends in the sedimentary organic matter composition and to identify the major environmental factors controlling its reactivity. Nutrient and dissolved oxygen fluxes at the water-sediment interface were furthermore measured at two stations using ex-situ whole core incubations.Results from this study highlight the seasonal influence of riverine versus autochthonous productions on the quality of sedimentary organic matter in the vicinity of the river mouth. Farther on the mid shelf, a terrestrial imprint is still visible, but the temporal signal is blurred as a consequence of strong mixing during frequent episodes of resuspension/deposition. Organic matter is efficiently remineralised in this system with total oxygen uptake (TOU) rates comparable to those measured off the Rhône River. Finally, organic matter quality and reactivity are also constrained in this system by physical forcings, which promote remineralisation and advection of sediments.

Patterns and drivers of meiofaunal assemblages in the canyons Polcevera and Bisagno of the Ligurian Sea (NW Mediterranean Sea)

Meiofaunal abundance, assemblage structure and richness of higher taxa were investigated for the first time in two submarine canyons (Polcevera and Bisagno) of the Ligurian Sea and along the adjacent open slope, in relation with the quantity and quality of sedimentary organic matter and other environmental variables, including grain size. Meiofaunal abundance and richness of higher taxa decreased with increasing water depth (from ca. 200 down to ca. 2000-m depth) in the open slope and Polcevera canyon, whereas the highest values were observed at 500 m depth in the Bisagno canyon. The comparison between canyons and the adjacent open slope, showed the lack of significant differences in meiofaunal abundance, at the same depth except for samples collected at 200 and 2000-m depth. Overall the biodiversity was higher in canyons than in the open slope. Phytopigments, utilised as a proxy of the input of primary organic matter, were up to 3 times higher in canyon than in slope sediments and, along with grain size, explained a large portion of the variability in all meiofaunal variables. Canyon and slope showed a high beta diversity (83%), mostly due to the presence of a high portion of rare taxa in the canyons. Some taxa, such as Cladocera, Cumacea, Gastrotricha, Nemertina were exclusively encountered in canyon sediments, whereas Tardigrada were encountered only in the adjacent slope. Results reported here indicated that, differences in meiofaunal assemblages between canyons and slopes are primarily driven by quantity and quality of the available food resources and by the presence of specific topographic features.

Spatial and temporal distribution in the biochemical composition of sedimentary organic matter in a tropical estuary along the west coast of India

Distribution of sedimentary organic matter was undertaken in Cochin estuary, the second largest wetland ecosystem in India. Surface sediment samples were collected from twenty-seven stations during 2016 constituting the pre- and post-monsoon periods. The sediment samples were analysed for labile fractions of biochemical constituents such as carbohydrates (CHO), proteins (PRT) and lipids (LPD). Irrespective of the sampling periods, proteins (71%, 38%) constitute the major labile fraction, followed by carbohydrate (23%, 36%) and finally lipids (5%, 24). The application of biochemical index using PRT:CHO ratio revealed the presence of freshly deposited as well as the presence of aged organic matter in the estuary. The LPD:CHO ratio revealed low nutritional quality of sedimentary organic matter during the pre-monsoon and enhanced quality in post-monsoon. The trophic state classification based on biopolymeric carbon (BPC), PRT and CHO values unveils the fact that estuarine sediment nature varied between mesotrophic, eutrophic and meso-oligotrophic status. Low BPC:TOC ratios were observed pointing less availability of food to benthic source, and the organic matter present was mainly refractory in nature.

Source and Biogeochemical Distribution of Organic Matter in Surface Sediment in the Deep Oligotrophic Lake Fuxian, China

The source, quantity and quality of sedimentary organic matter (SOM) were investigated in the surface sediments of Lake Fuxian, a deep oligotrophic lacustrine system in China. Granulometry, biochemical organic composition, bulk organic proxies and their stable isotopes were determined in the surface sediments (0–4 cm). The values of δ13C, δ15N and atomic ratio of total organic carbon to total nitrogen (TOC/TN) indicated that the sediments in the large partial lake were influenced by autochthonous organic matter. The concentrations of TOC, protein (PRT) and total hydrolysable amino acids may be mainly modulated by phytodetritus sinking from euphotic zones since they correlated significantly with chlorins. Otherwise, the lack of correlation between chlorins concentrations and carbohydrate (CHO) and lipid (LIP) indicated that the latter may have an additional terrestrial source. The highest sediment quantity was found in the deepest station NC owing to its fine sediment. Stations near northwestern shore accumulated more SOM than other littoral stations, which was in accordance with sewage discharge strength. Degradation quality indices, such as chlorin index, degradation index, PRT/CHO, and LIP/CHO, were in general agreement in showing the degraded status of SOM in Lake Fuxian. No clear spatial patterns were found in sediment degradation quality, which may be influenced by bottom oxygen concentration in the deep stations.

Effects of small‐scale, shading‐induced seagrass loss on blue carbon storage: Implications for management of degraded seagrass ecosystems

Abstract Seagrass meadows are important global blue carbon sinks. Despite a 30% loss of seagrasses globally during the last century, there is limited empirical research investigating the effects of disturbance and loss of seagrass on blue carbon stocks. In this study, we hypothesised that seagrass loss would reduce blue carbon stocks. Using shading cloth, we simulated small‐scale die‐offs of two subtropical seagrass species, Halodule wrightii and Thalassia testudinum, in a dynamic northern Gulf of Mexico lagoon. The change in quantity and quality of sediment organic matter (OM) and organic carbon was compared among die‐off, control and bare plots before the die‐off treatment, shortly after the die‐off treatment and 11 months after the die‐off treatment. 210Pb age dating was performed on bare and Thalassia plots at 11 months to evaluate the impact of sediment erosion in the absence of vegetation. The small‐scale die‐off led to a 50%–65% OM loss in the sediment in the top 8 cm of Halodule plots. Thalassia plots lost significant portions of OM (50%) and organic carbon (Corg; 21%–47%) in only the top 1 cm of sediment. The 210Pb profiles indicated Thalassia die‐off reduced the Corg sequestration rate by 10%, in addition to a loss of c. 1 year's worth of Corg stocks (c. 22 g/m2). Furthermore, analyses on OM/Corg quality indicated a loss of labile OM/Corg and enhanced remineralisation by microbes. Synthesis and applications. This study provides empirical evidence that small‐scale shading‐induced seagrass die‐offs can reduce seagrass carbon sequestration capacity and trigger losses of blue carbon stocks. While the losses recorded here are modest, these losses in blue carbon storage capacity are notable due to the proximity of shading structures (e.g. boat docks) to seagrass habitats. Thus, policies to avoid or protect seagrass habitats from common small‐scale, shading disturbances are important for optimising both carbon sequestration capacity and coastline development and management.

Sources and degradation of sedimentary organic matter in the mud belt of the East China Sea: Implications from the enantiomers of amino acids

Total organic carbon (TOC), total N (TN) and amino acids (AAs, including the bacterial biomarker D-AAs) were determined in sediment cores from the mud belt of the East China Sea (ECS). The concentration of total hydrolyzable AAs (THAAs) ranged from 3.35 to 13.44 µmol/g dry wt (dw) of the sedimentary organic matter (SOM), exhibiting a decreasing trend downcore. Major constituents of the THAAs were glycine (Gly), L-serine (L-Ser), L-alanine (L-Ala), L-glutamic acid (L-Glu) and L-aspartic acid (L-Asp), whereas the D-aspartic acid (D-Asp), D-glutamic acid (D-Glu), D-serine (D-Ser) and D-alanine (D-Ala), along with non-protein AAs (γ-aminobutyric acid and β-alanine) together accounted for ca. 7% of the THAA pool. Given the C/N ratio values, the OM in three cores was predominantly of marine origin. Based on D-Ala yield, bacterial OM represented on average 18% of TOC and ca. 30% of TN. C-normalized THAAss (THAA-C%) was a sensitive indicator of SOM diagenetic alteration in the upper 40 cm of sediment. However, the degradation index (DI) and reactivity index (RI) values for the cores did not exhibit any definite trend, which may indicate that these two indicators were not sensitive in the early stage of sediment degradation. Negative correlation between the D/L ratio of Ala and THAA-C% suggested a close coupling between the extent of degradation and the accumulation and/or selective preservation of bacterial material in the sediments. In addition, correlation between clay content and THAA-C% suggested that fine grained particles played an important role in affecting the quality of SOM.