Organic Matter In Surface Sediments Research Articles

Spatial and temporal distributions of terrestrial and marine organic matter in the surface sediments of the Yangtze River estuary

Riverine supply of largely terrestrial carbon is important to the carbon cycle chemistry of the oceans. Along a river-ocean continuum, sedimentary organic matter composition and distribution can provide insights to the balance of terrestrial input and in situ production of an estuary. In the Yangtze River estuary (YRE), this balance is especially important to understand as human impacts and climate change continue to modify processes along the continuum. Here we discuss the sources and spatio-temporal distributions of organic matter in the coastal ocean surface sediments and how environmental factors have affected changes there. The isotopic compositions (δ13C and δ15N), total organic carbon (TOC), and total nitrogen (TN) of surface sediments were investigated along with sediment grain size and bottom water temperature, salinity, and total suspended matter. Evidence suggests that fine sedimentary grain size, bottom water temperature, and the circulation dynamics of water masses such as Changjiang Diluted Water (CDW) and Taiwan Warm Current (TWC) were the main factors affecting spatial and temporal distributions, with organic matter concentrations in autumn showing higher than that in spring and nearshore areas higher than offshore. An isotopic-based two end-member mixing model suggests that terrigenous inputs were the main source (about 53%) of organic matter in the YRE, followed by autochthonous marine organic matter, in both seasons. Terrestrial organic matter distribution, controlled by the CDW, decreased seaward from the mouth, as expected, and the proportion of terrestrial organic carbon increased in autumn compared to spring. Intrusions of the TWC water likely contributed to the increase of marine components in a tongue-shaped area during both seasons.

Sources and preservation dynamics of organic matter in surface sediments of Narmada River, India – Illustrated by amino acids

Abstract The preservation process of organic matter (OM) in estuarine environments determines the recycling and sinking of nutrients. This process requires the identification of sources, degradation states and the main processes affecting OM transformations. Unfortunately, our understandingof the sources, degradation and factors affecting OM distribution in tropical rivers experiencing strong seasonality and monsoonal influence is still limited. This study examined the sources, degradation and factors affecting OM distribution along the Narmada River and its estuary during different seasons. Surface waters and sediments were analyzed seasonally for selected physico-chemical parameters and bulk compositions of sediments, together with amino acids (AA, including the bacterial biomarker, D-AA). The sources of OM were soils containing detrital terrestrial plant material, with C4 and C3 plants dominating the estuarine and riverine stations, respectively. The other sources of OM were in-situ production, together with bacteria and their remnants. Strong seasonality and monsoonal conditions control the sources and distribution of OM in the river. Higher concentrations of total hydrolysable amino acids (THAA) were observed in riverine stations, suggesting the presence of relatively fresher OM. The lower OC:SA ratios recorded in the estuarine sediments indicated a limited OM preservation in the studied river. Positive degradation index (DI) values were obtained during the pre-monsoon season, suggesting seasonal changes in OM diagenesis. Physical (strong tidal currents, rainfall, reduced water flow due to seasonal variations and shallow water depth within the estuary) and geochemical (mineral surface adsorption processes) factors control the distribution and transport of OM. Taken together, the sources, preservation and diagenesis of terrestrial OM along the Narmada River was controlled differentially by the strong seasonal variability of the region. Thus, under variable temporal conditions, tropical estuaries and rivers form important realms for examining, determining, evaluating and assessing OM in order to better interpret nutrient budgets of the seas and oceans.

Short-Term Effects of Artificial Reef Construction on Surface Sediment and Seawater Properties in Daya Bay, China

Chen, Q. and Chen, P., 2020. Short-term effects of artificial reef construction on surface sediment and seawater properties in Daya Bay, China. Journal of Coastal Research, 36(2), 319–326. Coconut Creek (Florida), ISSN 0749-0208.The degradation and modification of habitats are dominant drivers of biodiversity fluctuation, which is increasingly threatened in marine ecosystems (especially coral reefs) and can be mitigated by the construction of artificial reefs (ARs). Environmental indices are considered excellent indicators of how disturbances affect organisms, but the effects of AR construction on nearby sediment and seawater environments remain unclear. In the current study, changes in the properties of surface sediment and seawater (surface and bottom) for 2 years following AR construction were investigated in Daya Bay, China. The reef habitat and the nearby nonreef habitat were sampled. The results indicated that surface sediment organic matter significantly increased but sediment texture did not change following AR construction, regardless of habitat. The pH, dissolved oxygen, and salinity significantly declined, but chemical oxygen demand, inorganic nitrogen, and suspended particulate organic matter significantly increased in both surface and bottom seawater, regardless of habitat; temperature, nitrogen/phosphorus ratio, and available phosphate were not consistently altered by AR construction. Considering the intimate relationships between abiotic factors and organisms, these results indicate that long-term assessment of multiple environmental properties is needed to evaluate and predict the effects of AR construction on marine wildlife.

Cross shelf transport of terrigenous organic matter in surface sediments from outer shelf to Okinawa Trough in East China Sea

Identification of sources and distribution of terrigenous organic matter (TOM) are significant to understand the carbon transport in East China Sea (ECS). However, sources of TOM buried in the ECS are diverse and complex, which are likely derived from the Changjiang River, Yellow River and/or small rivers from China mainland and Taiwan. In order to evaluate the fate of TOM offshore in the ECS, the study sampled 80 surface sediments from outer shelf-slope-Okinawa Trough (OT) region in the ECS. Both terrigenous and marine organic biomarkers were detected. The concentration (ng/g dry weight sediment) of terrigenous biomarkers including ƩC27 + C29 + C31 n-alkanes ranged from 12.2 to 990.9; while the content of marine biomarkers varied from 1.3 to 3891.2 for C37 alkenones, 2.6 to 922.6 for brassicasterol and 7.1 to 1535.8 for dinosterol. Bulk organic carbon and TOM distributions were controlled by the hydrodynamic conditions. The terrigenous and marine biomarker ratio (TMBR) (ƩC27 + C29 + C31 n-alkanes)/(ƩC27 + C29 + C31 n-alkanes + (brassicasterol + dinosterol +alkenones)) increased first and decreased from the outer shelf to OT spatially, showing the distribution pattern of relative proportion of TOM. Hotspots of significant deposition of TOM were observed in the ECS by δ13C and TMBR proxies that the mechanism was unclear yet. Binary model revealed higher percentage of TOM based on TMBR than δ13C for both slope (32.1% vs. 20.0%) and trough (23.1% vs. 15.2%) region. An underestimation of C4 vegetation contribution likely biased the bulk carbon model. Carbon preference index suggested a zonation distribution of TOM that outer shelf had different sources from the slope and OT attributed to cross shelf transport of terrigenous organic matter.

Organic Carbon in the Surface Sediments from the Intensive Mariculture Zone of Sanggou Bay: Distribution, Seasonal Variations and Sources

Mariculture has rapidly grown worldwide, which might greatly influence the biogeochemical cycle of organic carbon in coastal seas. In this study, several geochemical parameters, including grain size composition, sedimentary total organic carbon (TOC), total nitrogen (TN), stable carbon (δ13C) and nitrogen (δ15N) isotopic compositions, were analyzed for surface sediments collected from different mariculture zones of Sanggou Bay and in different seasons. We investigated the composition and distribution of organic matter in surface sediments and further evaluated the contribution of mariculture activities to TOC sources. The TOC and TN contents (mass percentage) in the bay were in the range of 0.14% to 1.45% and 0.03% to 0.20%, respectively. The spatial distribution indicated that sedimentary TOC and TN contents in shellfish monoculture and shellfish-kelp polyculture zones were higher than in other mariculture zones, which might be related to grain size composition and mariculture organisms. Seasonal variations of TOC contents were observed in different mariculture zones. The TOC/TN atomic ratio (C/N), δ13C and δ15N were in the ranges of 5.97 to 10.97, −21.76‰ to −13.14‰ and 2.13‰ to 8.08‰, respectively, implying that sedimentary organic matter in Sanggou Bay was the mixture of marine phytoplankton, terrestrial and maricultural sources. A simple mixing model based on δ13C was applied and the results indicated that the relative contributions of organic carbon sources in Sanggou Bay followed the order kelp (36.6%) > marine phytoplankton (28.7%) > shellfish bio-deposition (23.8%) > terrestrial input (10.9%). Surface sediments in Sanggou Bay were dominated by mariculture-derived organic carbon, which on average accounted for 60.4% of TOC.

Salinity disturbance affects faunal community composition and organic matter on a restored Crassostrea virginica oyster reef

Ecological restoration has become a widely recognized tool to ameliorate the effects of habitat loss. Half Moon Reef, once a highly-productive 2 km2Crassostrea virginica oyster reef in Matagorda Bay, Texas, was harvested to depletion in the early 20th century. In 2014, a 0.23 km2 reef complex was created using limestone and concrete substrates to restore oyster populations—one of the largest contiguous oyster reef restoration efforts in the U.S. In the three years post-construction, two large freshwater inflow events provided a unique opportunity to determine the effects of prolonged salinity reductions (<10) on oyster reef faunal community composition and organic matter quality. Oyster size generally increased over the 3-year study period, enhancing habitat provision for reef fauna. Reef-resident and reef-associated species metrics (abundance, biomass, diversity, and species richness) were positively correlated with salinity. Following a large salinity reduction (from ∼26 to ∼9) one year post-construction, the reef-resident faunal community shifted from dominance by small gastropods to larger and more resilient crustaceans. A second, lower magnitude salinity reduction two years post-construction did not elicit the same biotic response. Reef-associated fauna showed no patterns related to distance from the reef (13 m vs. 150 m), indicating limited influence of the restored oyster reef on fauna outside of the physical reef structure. Salinity decreases were associated with 13C-depletion of suspended particulate organic matter, whereas no change in isotopic composition was observed for surface sediment organic matter. Carbon/chlorophyll a and carbon/nitrogen ratios of suspended particulate organic matter indicate the quality of organic matter was higher following low salinity events, likely due to increased autochthonous production facilitated by pulses of freshwater inflow. Oysters assimilated nearly equal proportions of surface sediment organic matter and suspended particulate organic matter. It is important to understand the effects of salinity variability for the effective management of coastal restoration projects because freshwater inflow has a major influence on coastal ecosystems.

Characteristics of sedimentary organic matter and vascular plants in tropical brackish Bolgoda Lake, Sri Lanka: Implications for paleoecology and chemotaxonomy

Tropical brackish ecosystems are characterized by mangrove-dominant communities which are well-adapted to extreme and unique environmental conditions. Organic matter (OM) derived from adjacent vegetation contributes to the chemical composition of sediments. Sedimentary OM can thus be used for paleoecological studies. Surface and core sediments of Bolgoda Lake were analyzed to identify sources of OMs using C, N, S elemental analysis and gas chromatography-mass spectrometry. C/N ratios of raw sediments and protokerogen derivatives illustrate the preferential degradation of mixed OM in surface sediments and mangrove muds. The n-alkanes distributions show that vascular plants are predominantly preserved in sediments due to resistance to microbial decomposition and available biomass in the watershed. Taraxerone provides the most important chemotaxonomical information to identify mangrove-dominant sediments. Ternary n- C27, n- C29 and n- C31 alkanes relationships show that mangrove-dominant sediments are enriched in n-C31along with significant amounts of n-C29. Mangrove and herbaceous plants (grasses and/or floating plants) can be preferentially dispersed in tropical brackish sediments. The relative proportions of n-C27, n- C29 and n- C31n-alkanes can be applied to identify different growth habitats of a particular landmass. The mangrove biomarker (pentacyclic taraxerone) and visualized long-chain n-alkanes proportions (i.e., n-C27, n- C29 and n-C31) can thus be used as chemotaxonomical indicators to reconstruct the paleoecological characteristics of tropical brackish vegetation.

Distribution and Sources of Organic Matter in Surface Sediments of the Northern Bering and Chukchi Seas by Using Bulk and Tetraether Proxies

The organic matter (OM) preserved in Arctic Ocean sediments is of great importance to the global carbon budget. However, works that apply multiple proxies to determine the distribution and concentration of organic carbon (OC) in the surface sediments of the northern Bering and Chukchi Seas remain limited. Here a multiproxy approach based on bulk OM parameters and the branched vs. isoprenoid tetraether (BIT) index was used to investigate the distribution and sources of OM in the surface sediments of the northern Bering and Chukchi Seas. Binary and ternary mixing models were applied to trace the contribution of different OC sources to the total OC in the study area. The δ13C values of the sediments provided by the binary model showed that the proportion of terrestrial OC fell in the range of 27.4%−79.8% (46.2% on average). The BIT index returned the lowest fraction (4.8%−27.3%, 12.0% on average). The ternary mixing model was employed to determine the plant-, soil-, and marine-derived fractions of the total OM. The ternary model showed that 11.5%±6.3%, 31.4%±9.5%, and 57.1%±12.4% of OM in the sediment of the study area was derived from soil, plants, and marine sources, respectively. The differences in OM composition between the west and east sides of the Chukchi Sea were controlled by OM inputs from key water masses (i.e., Anadyr Water and Alaska Coastal Water), river discharge, and the nutrient supply from the Pacific inflow that supports marine productivity.

Geochemical discrimination of bulk organic matter in surface sediments of the Cross River estuary system and adjacent shelf, South East Nigeria (West Africa)

Knowledge of the sources, distribution and fate of organic matter (OM) in estuarine and adjacent shelf sediments are important for the understanding of the global biogeochemical cycles. Bulk organic carbon (C-org), total nitrogen (TN), biogenic silica (BSi), stable carbon (δ13C-org) and nitrogen (δ15N) isotopes, and sediment grain sizes were measured to study the spatial distributions and sources of sediment OM in the Cross River estuary system (CRES) and adjacent shelf. Surface sediments in the CRES were composed of clayey silt and sandy silt, while the adjacent shelf sediments were mainly silty sand. The range of the studied parameters was −28.79‰ to −22.20‰ for δ13C-org, −1.32‰–6.31‰ for δ15N, 6.7–29.2 for C-org/N ratios, 0.08%–0.33% for TN, 0.24‰–0.74‰ for BSi, and 0.47%–5.28% for C-org, and their spatial distributions showed a general decreasing trend in both the terrestrial and estuarine OM from the riverine regions to the adjacent shelf. Based on the three-end-member mixing model using the δ13C and δ15N isotopic values, ~58.01 ± 15.32% of sediment OM are derived from terrestrial sources dominated by C3 vascular plants, while ~26.34 ± 9.71% are attributed to estuarine sources dominated by aquatic macrophytes, and ~15.65 ± 12.37% for marine plankton source. Other sources of OM identified included soils underlain C3 vascular plants and agricultural farms enriched with N, sewage, and petroleum hydrocarbons. The relationship between C-org vs. BSi, and the atomic BSi/Corg ratios suggested that diatoms also play an important role in OM sequestration in surface sediments of the CRES and adjacent shelf. The correlations of the δ13C-org and δ15N isotopic values vs. C-org/N ratios resulted in scatter plots, indicating that the distributions of sediment OM in the CRES and adjacent shelf are influenced by post depositional processes, fixed inorganic N adsorbed on fine-grained sediments, microbial degradation, as well as sediment grain size.

Discriminating sources and preservation of organic matter in surface sediments from five Antarctic lakes in the Fildes Peninsula (King George Island) by lipid biomarkers and compound-specific isotopic analysis

Lakes are important paleoenvironmental archives retaining abundant information due to their typical high sedimentation rates and susceptibility to environmental changes. Here, we scrutinize the organic matter (OM) composition, origin and preservation state in surface sediments from five lakes in a remote, warming-sensitive, and poorly explored region partially covered by the retreating Collins Glacier in King George Island (Antarctica), the Fildes Peninsula. Lipid biomarkers of terrestrial origin (i.e. high-molecular weight n-alkanes, n-alkanoic acids, and n-alkanols; β-sitosterol, campesterol, and stigmasterol) were detected in the five Fildes Lakes, with the smallest basin (i.e., Meltwater) showing a particularly strong moss imprint. Aquatic source indicators such as low C/N and terrestrial over aquatic ratios (TAR), or less negative δ13C values were preferentially found in the mid-sized lakes (i.e., Drake and Ionospheric). Sedimentary carbon in the larger lakes (i.e., Uruguay and Kitezh) displayed a largely biogenic origin (i.e., values of carbon preference index, CPI, ≫1), whereas the three lakes close to Collins Glacier (i.e., Drake, Meltwater, and Ionospheric) showed certain contribution from petrogenic sources (CPI ~ 1). The results suggest that the geochemical signature of the surface sediments in the five Fildes lakes is determined by factors such as the distance to the retreating Collins Glacier, the proximity to the coast, or the lake depth. This study illustrates the forensic interest of combining lipid biomarkers, compound-specific isotopic analysis, and bulk geochemistry to reconstruct paleoenvironments and study climate-sensitive regions.

Sources of organic matter in Chilika lagoon, India inferred from stable C and N isotopic compositions of particulates and sediments

Abstract Stable isotopic compositions (δ13C and δ15N) and C/N ratios of suspended particulate organic matter (POM) and surface sediment organic matter (SOM) in the Asia's largest lagoon (Chilika, India) were analyzed to identify spatial and seasonal variabilities in sources of organic matter. The variability of POM composition (δ13C: −23.5‰ to −27.9‰, δ15N: 2.1‰ to 7.5‰, C/N: 9.5 ± 0.9) collected during monsoon, a period of highest river discharge, suggests dominant input of terrestrial organic matter, whereas wintertime POM (δ13C: −22.3‰ to −27.7‰, δ15N: −0.2‰ to 4.8‰, C/N: 9.1 ± 1.0) exhibits a mixed source of agricultural runoff, and lagoon phytoplankton and cyanobacteria. The composition of POM collected during summer/dry season (δ13C: −21.2‰ to −26.2‰, δ15N: 2.0‰ to 6.0‰, C/N: 8.9 ± 1.1) indicates enhanced lagoon phytoplankton and bacterial productivity. Spatial variability of POM isotopic composition clearly shows more contribution of terrestrial sources in the northern sector- influenced by perennial Mahanadi River distributaries- compared to the central and southern sectors. The isotopic compositions (δ13C: −20.9‰ to −22.9‰, δ15N: 1.9‰ to 6.6‰) and C/N ratios (10.1 ± 1.3) of SOM indicate major contribution from terrestrial and macrophyte sources and minor contribution from phytoplankton and/or cyanobacteria. The northern sector is also heavily influenced by urban/industrial wastewater input, whereas the outer channel with higher salinity remains dominated by marine organic matter. Western part of the central sector is significantly influenced by untreated domestic sewage discharged from nearby townships and villages, which endangers the lagoon ecosystem.

Exploring the composition of macromolecular organic matter in Arctic Ocean sediments under a changing sea ice gradient

Arctic sea ice has been affected by climate change, leading to reductions in summer sea ice extent over the past few decades, impacting nutrient dynamics, ocean temperature fluxes and the biological communities present in the ocean. Marine organic matter is a complex mixture of differentially degraded terrestrial and marine organisms from a range of sources and time periods. In this study pyrolysis-gas chromatography-mass spectrometry (Py-GC–MS) has been used to quantify the solvent-insoluble component of marine organic matter in surface sediments to gain a more holistic understanding of the macromolecular composition at five stations along a south to north transect in the Arctic Barents Sea, east of Svalbard (depths 288–334 m.b.s.l). Two methods were compared to identify the effectiveness of rapid screening, in contrast to grouping similar pyrolysis products. There were changes in macromolecular composition of marine surface sediments across the S-N transect using both methods, highlighting the varying benthic and pelagic communities north of the Polar Front and across the variable sea ice margin, corresponding to differing biological communities (e.g. fish, phytoplankton, ice algae, zooplankton). All five stations across the changing sea ice transect were interpreted as having primarily marine surface sediment macromolecular signatures, given their locations far from major terrestrial inputs and the more subtle changes when compared with previous investigations on the East Siberian Artic shelf. Fluctuations in macromolecular compositions across the transect included increasing N-containing compounds (including pyridines) and n-alkene/n-alkane doublet pyrolysis products from sediments collected in stations with the greatest average ice cover. If the future position of the Polar Front moves northwards then deposition of labile organic matter which appears to be efficiently processed will move further north, meaning greater deposition of organic carbon under areas of open ocean. Future research needs to understand how this OC will be buried and if it is regionally significant, given anticipated weakening stratification and a more Atlantic influenced northern Barents Sea.

Environmental controls on benthic food web functions and carbon resource use in subarctic lakes

Abstract Climate warming and consequent greening of subarctic landscapes increase the availability of organic carbon to the detrital food webs in aquatic ecosystems. This may cause important shifts in ecosystem functioning through the functional feeding patterns of benthic organisms that rely differently on climatically altered carbon resources. Twenty‐five subarctic lakes in Finnish Lapland across a tree line ecotone were analysed for limnological and optical variables, carbon (δ13C) and nitrogen (δ15N) stable isotope (SI) composition of surface sediment organic matter (OM) and fossil Chironomidae (Diptera) remains to examine environmental controls behind chironomid functional feeding group (FFG) structure and their isotopic associations for assessing ecosystem functioning and carbon utilisation. We hypothesise that the chironomid SI signatures reflect increased allochthony with increasing allochthonous input, but the resource use may be altered by the functional characteristics of the assemblage. Multivariate analyses indicated that carbon geochemistry in the sediments (δ13C, δ15N, C/N), nutrients, indices of productivity (chlorophyll‐a) and lake water optical properties, related to increasing presence of OM, played a key role in defining the chironomid FFG composition and isotopic signatures. Response modelling was used to examine how individual FFGs respond to environmental gradients. They showed divergent responses for OM quantity, dissolved organic carbon and nutrients between feeding strategies, suggesting that detritivores and filter feeders prefer contrasting carbon and nutrient conditions, and may thus hold paleoecological indicator potential to identify changes between different carbon fluxes. Benthic production was the primary carbon source for the chironomid assemblages according to a three‐source SI mixing model, whereas pelagic and terrestrial components contributed less. Between‐lake variability in source utilisation was high and controlled primarily by allochthonous OM inputs. Combination of biogeochemical modelling and functional classification is useful to widen our understanding of subarctic lake ecosystem functions and responses to climate‐driven changes in limnology and catchment characteristics for long‐term environmental change assessments and functional paleoecology.

Interplay between sediment properties and stream flow conditions influences surface sediment organic matter and microbial biomass in a Mediterranean river

River sediment physical properties are linked to flow and are important for the attachment of microorganisms. The objective of this study was to assess the relationship between physical characteristics of surface sediments in a Mediterranean river and their organic matter content and microbial biomass. To do this, we analyzed particle-size distribution, organic matter content, chlorophyll-a, and bacterial density in sediments collected along a 54 km reach under three flow conditions (i.e., drought, low-flow, and base-flow). Multiple regression analysis revealed that during the drought condition, sediment heterogeneity and porosity regulated bacterial density and organic matter content and that bacterial density tended to be lower as the proportion of mud increased. However, under the low-flow and base-flow conditions, bacterial density was related to percent mud, which may provide more surface area for colonization than cobbles. Algal biomass was affected by sediment particle-size distribution only under the base-flow condition, when chlorophyll-a content was enhanced by sediment heterogeneity and a higher relative abundance of sand, suggesting that when biomass declines due to increased shear stress, sediment particle-size distribution becomes more determinant for algal colonization. Our results highlight the importance of considering the interplay of sediment particle-size distribution and flow regime when studying microbial communities in river sediments.

Shallow ponds are biogeochemically distinct habitats in salt marsh ecosystems

AbstractRunaway expansion of shallow ponds can catalyze the conversion of vegetated marshes into open water environments. Predicting how this transition affects ecosystem functioning is difficult because little is known about pond biogeochemistry. We characterized sediment organic matter sources and transformations in three ponds with different plant communities, over alternating periods of tidal isolation and flushing, during summer and fall, using a combination of stable isotopes, lipid biomarkers, and benthic fluxes. Sediment respiration rates (1.66 ± 0.09 mmol C m−2 d−1 to 28.53 ± 7.76 mmol C m−2 d−1) were comparable to shallow estuaries and driven by sulfate reduction. Rates varied across ponds, reflecting differences in summertime Ruppia maritima and macroalgae abundances, but were similar between seasons. Interactions between aboveground plant and sediment bacterial communities translated into distinct biogeochemical processes across the three ponds. Tidal isolation and summer weather intensified plant and bacterial community effects on pond carbon dynamics, resulting in algal biomass and lipid δ13C values that were 3–12‰ enriched, compared to nearby habitats. Surface sediment organic matter mainly derived from pond microalgae and was compositionally distinct from tidal creeks and marshes. Surprisingly, sediment bacteria were not tightly coupled to benthic microalgae but decomposed multiple carbon sources in surface sediments and became increasingly reliant on buried peat at deeper horizons. Pond development over time could largely be explained by sediment respiration and the simultaneous accretion of the surrounding marsh platform. The role of decomposition in pond expansion is consistent with previous assessments based on whole‐pond metabolism rates. Consequently, future pond expansion could alter ecosystem biogeochemistry and reduce carbon storage.

Food web of a subtropical tidal flat, Atlantic Southwestern: Temporal and spatial variability of the primary organic sources

Trophodynamic parameters have been used as input data for trophic models and are used as indicators to support ecosystem management decisions. To understand the ecosystems' trophic structure and functioning, it is important to identify the major primary carbon sources within a food web. This study aimed to investigate the spatial and temporal variability in isotopic composition of the basal sources of the food web of the Araçá Bay (23°49′S; 45°24′W) ecosystem. This site is a sheltered tidal flat and presents a sharp seasonal change in water temperature and salinity. Samples of 15 basal sources were analysed for δ13C (n = 187) and δ15N (n = 180) values and also for the C:N ratio, in January–March (austral summer) and June–July (austral winter) 2013, in the continental, intertidal (mangrove patches and soft and rocky bottoms) and sublittoral regions. The isotope values and C:N ratios were, in general, in the range of those available in previously published studies. C3 and C4 plants, green macroalgae and surface sediment organic matter (SSOM) did not present significant changes in the δ13C and δ15N values with time. For suspended particulate matter (SPM), microphytobenthos (MPB) and turf-forming algae, δ13C values were higher in the summer than in the winter; also in the summer, red macroalgae showed higher δ15N values. Spatial variability was investigated for SPM, SSOM and MPB. δ13C values were significantly higher for SSOM and MPB in the intertidal region than in the sublittoral but for SPM there was no difference among regions. Values of δ15N were higher for MPB in the intertidal region than in the sublittoral, and were higher for SPM in the sublittoral region than in the intertidal and continental regions. The δ13C associated to C:N values of SSOM indicated a coupling of the marine sublittoral region and the land organic matter of an adjacent island and the SPM δ15N variability pointed out a continental land-sea connection. Some mechanisms could explain the isotopic temporal and spatial shifts and sources of organic C in the study area: the seasonality of hydrodynamics and of phytoplankton size structures and biomass; the steady low concentration of nitrate and the episodic ammonia inputs; terrestrial runoff. Our results can contribute to the construction and understanding of a trophic model of the Araçá Bay ecosystem, and also for its management, considering the seasonal hydrodynamics, the coupling of marine sublittoral region in the São Sebastião channel and terrestrial organic matter from São Sebastião Island, as well as the continental land-sea connection. We suggest a monitoring program based on SPM, bottom detritus and Ulva fasciata, important primary organic sources for the Araçá Bay food web.

Modification of benthic food web structure by recovering seagrass meadows, as revealed by trophic markers and mixing models

Seagrass meadows are among the most diverse and productive coastal ecosystems in the world. Currently, the accelerating loss of these habitats is recognized worldwide. In the southern Baltic Sea, a natural recovery of Zostera marina meadows has occurred after a dramatic reduction within the last century. The aim of this study is to understand if and how the recovering eelgrass meadows affect the functioning of benthic ecosystems. The trophic links within the benthic food webs in the seagrass meadows and bare sandy bottoms were depicted and compared. The trophic connections were examined by combining stable isotope (SI) composition (δ13C, δ15N) and fatty acid (FA) profiles of meio- and macrofauna consumers and of potential food sources (particulate organic matter, surface sediment organic matter, epiphytes, microphytobenthos/bacteria and macrophytes) in a Bayesian mixing model framework (MixSIAR). Significantly higher amounts of the FA bacterial marker (C18:1ɷ7) were observed in meiofauna (approximately 40%) than in the macrofauna (1% on average), suggesting that bacteria are an important part of the meiofauna diet. The mixing model results indicated that the benthic consumers in the vegetated habitat utilized more food sources (e.g., epiphytes in the diets of meiofauna and macrofaunal grazers) and thus had a more diverse diet. Macrofaunal omnivores relied to a larger degree on animal-derived organic matter in vegetated habitat, which could be linked to higher invertebrate prey availability. The results highlight the importance of recovering seagrass meadows in driving the mechanisms responsible for food web organization. Any type of change to the state of seagrass meadows is crucial to the functioning and stability of marine ecosystems.

Riverine input of organic carbon and nitrogen in water-sediment system from the Yellow River estuary reach to the coastal zone of Bohai Sea, China

The temporal-spatial distribution of the carbon and nitrogen contents and their isotopic compositions of suspended matter and sediments from the Yellow River estuary reach (YRER), the estuary to the offshore area were measured to identify the source of organic matter. The higher relative abundances of suspended and sedimentary carbon and nitrogen (POC, TOC, PN and TN) in the offshore marine area compared to those of the riverine and estuarine areas may be due to the cumulative and biological activity impact. The organic matter in surface sediments of YRER, the estuary and offshore area of Bohai Sea is basically the mixture of continental derived material and marine material. The values of δ13Csed fluctuate from values indicative of a land source (− 22.50‰ ± 0.31) to those indicative of a sea source (− 22.80‰ ± 0.38), which can be attributed to the fine particle size and decrease in terrigenous inputs to the offshore marine area. Contrary to the slight increase of POC and PN during the dry season, TOC and TN contents of the surface sediments during the flood season (October) were higher than those during the dry season (April). The seasonal differences in water discharge and suspended sediment discharge of the Yellow River Estuary may result in seasonal variability in TOC, POC, TN and PN concentrations in some degree. Overall, the surface sediments in the offshore area of Bohai Sea are dominated by marine derived organic carbon, which on average, accounts for 58–82% of TOC when a two end-member mixing model is applied to the isotopic data.

Using Isotopes to Identify the Sources of Organic Carbon and Nitrogen in Surface Sediment in Shallow Lakes Alongside Poyang Lake

Water exchange in shallow lakes alongside Poyang Lake is weak because of sluggish water flow, and thus these lakes are susceptible to the effects of human activities and nutrient oversupply. Therefore, it is meaningful to study the sources of organic matter in these lakes, which is supposed to provide theoretical basis for the control of eutrophication of the lakes. The sources of organic matter in surface sediment in shallow lakes (Bang, Sha, Dahuchi, and Zhu lakes) alongside Poyang Lake and urban lakes (Qingshan and Xiang lakes) in Nanchang City were investigated by measuring the δ15N and δ13C values, total organic carbon and total organic nitrogen contents, and C/N ratios of sedimented organic matter (SOM). The δ15N, δ13C, and C/N ratios indicated that SOM in Sha and Dahuchi lakes mostly originated in phytoplankton, SOM in Bang and Zhu lakes was supplied by phytoplankton and soil organic matter, SOM in Qingshan Lake was strongly affected by sewage organic matter, and SOM in Xiang Lake mainly came from aquatic macrophytes. The results demonstrate that stable isotopes in SOM in lakes can be used to discriminate between different sources of organic material.

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.