Fate Of Particulate Organic Matter Research Articles

Source, composition, and reactivity of particulate organic matter along the Changjiang Estuary salinity gradient and adjacent sea

The sources, distribution, and fate of particulate organic matter (POM) in estuaries are dynamic and complex, influenced by highly intensive human activities and high productivity. In this study, water samples were collected along the Changjiang Estuary salinity gradient and adjacent sea (CEAS) in February and May 2017. Particulate organic carbon (POC), particulate nitrogen (PN), the δ13C isotope values and major biochemical constituent (total particulate amino acids, TPAA) were measured. The concentrations of POC, PN, and TPAA showed an overall decreasing trend from the river end-member to the open sea; however, their maximum always occurred around the turbidity maximum zone (TMZ). Concentrations of POC and TPAA showed a negative correlation with salinity and a positive correlation with chlorophyll a, indicating that the variation in POM concentrations and composition was mainly controlled by both terrigenous input and in situ phytoplankton production. The δ13C values gradually increased from the river mouth to the open sea in both winter and spring, in contrast to the molar C/N, reflecting the transition from terrestrial POC to phytoplankton-derived fresh POC with increasing salinity. Major biochemical indicators of TPAA/POC (%) and the degradation index (DI), showed a gradual shift towards more bioactive POM with increasing salinity in spring, although low TPAA/POC (%) values appeared within the TMZ. In spring, POC reactivity was higher than in winter. The proportions of glycine (Gly) and serine (Ser) were higher in winter, indicating that POM had suffered extensive degradation. Based on a two end-member mixing model, the contribution of marine POC in spring (53 ± 14%) was significantly greater than in winter (39 ± 19%), indicating that phytoplankton-derived POM was dominant in spring, associated with the increase in phytoplankton biomass from winter to spring. Based on mass balance, a box model showed evidence of a net POC sink over the Changjiang estuary and its adjacent East China Sea shelf in both winter and spring, with a net POC budget of 20.49 ± 7.01 and 15.87 ± 6.57 kmol s−1, respectively. Results illustrate that the spatio-temporal distribution of POM varies distinctively and will further affect the variability in its composition and reactivity in the CEAS.

Interactions between particulate organic matter and dissolved organic matter in a weak dynamic bay revealed by stable isotopes and optical properties

Few studies have incorporated the tools of stable isotopes and optical properties to study the biogeochemical process of organic matter (OM), including particulate organic matter (POM) and dissolved organic matter (DOM), which prevents our comprehension of the interactions between POM and DOM in the marine environment. In this study, the origin, distribution, and fate of POM and DOM in Tieshangang Bay, a weak dynamic bay were investigated by measuring δ13C and δ15N of POM, dissolved organic carbon (DOC), and absorption and fluorescence of DOM (CDOM and FDOM). In the upper bay, POM source was mainly originated from terrestrial discharge, whereas the high fluorescence index (FI) indicated that external aquatic organism activity dominated DOM sources. In contrast, in the outer bay, the slightly increased δ13C and enriched δ15N of POM and enhanced fluorescence intensities of protein-like DOM components indicated the considerable contributions of the in situ biological activities to OM. A net addition of DOC and optical components of DOM occurred in both the upper and outer bays. The decomposition of terrestrial POM was responsible for the addition of DOM due to the weak dynamics in the upper bay, whereas the enhanced primary production and the strong decomposition of freshly produced POM jointly contributed to the addition of DOM in the outer bay. Our study suggests that hydrodynamics regulate the biogeochemistry and interactions of POM and DOM in the weak dynamic bay.

Subarctic soil carbon losses after deforestation for agriculture depend on permafrost abundance

The northern circumpolar permafrost region is experiencing considerable warming due to climate change, which is allowing agricultural production to expand into regions of discontinuous and continuous permafrost. The conversion of forests to arable land might further enhance permafrost thaw and affect soil organic carbon (SOC) that had previously been protected by frozen ground. The interactive effect of permafrost abundance and deforestation on SOC stocks has hardly been studied. In this study, soils were sampled on 18 farms across the Yukon on permafrost and non-permafrost soils to quantify the impact of land-use change from forest to cropland and grassland on SOC stocks. Furthermore, the soils were physically and chemically fractionated to assess the impact of land-use change on different functional pools of SOC. On average, permafrost-affected forest soils lost 15.6 ± 21.3% of SOC when converted to cropland and 23.0 ± 13.0% when converted to grassland. No permafrost was detected in the deforested soils, indicating that land-use change strongly enhanced warming and subsequent thawing. In contrast, the change in SOC at sites without permafrost was not significant but had a slight tendency to be positive. SOC stocks were generally lower at sites without permafrost under forest. Furthermore, land-use change increased mineral-associated SOC, while the fate of particulate organic matter (POM) after land-use change depended on permafrost occurrence. Permafrost soils showed significant POM losses after land-use change, while grassland sites without permafrost gained POM in the topsoil. The results showed that the fate of SOC after land-use change greatly depended on the abundance of permafrost in the pristine forest, which was driven by climatic conditions more than by soil properties. It can be concluded that in regions of discontinuous permafrost in particular, initial conditions in forest soils should be considered before deforestation to minimize its climate impact.

Sources and features of particulate organic matter in tropical small mountainous rivers (SW China) under the effects of anthropogenic activities

Small mountainous rivers (SMRs), particularly those in the tropics, play a disproportionate role in global carbon cycling and have been heavily disturbed by anthropogenic activities during the Anthropocene. Identifying the sources of particulate organic matter (POM) in SMRs and their influencing factors have crucial scientific significance in terms of filling a cognitional gap concerning the fate of POM in tropical land–ocean ecosystems and practical values regarding environmental management and ecological evaluation. In this study, total suspended solids concentration, particulate organic carbon and particulate nitrogen contents, stable carbon isotope (δ13C), and C/N ratio of suspended particulates were investigated from five SMRs (Fangcheng, Maoling, Qin, Dafeng, and Nanliu) in Guangxi Province, southwest China. The soil (OMsoil), C3 plants (OMC3), and autochthonous production (OMauto) were identified as potential POM sources, and their relative contributions were quantified via the use of end-member mixing models that were based on the classic indicators of δ13C and C/N. The results showed that soil was the biggest POM contributor in Nanliu, Qin, and Maoling rivers, whereas OMC3 was more important in Fangcheng River. Compared to other watersheds, the OMauto accounted for a very minor proportion of the POM sources in Fangcheng River. Nanliu and Fangcheng, as typical representatives of agricultural and forest watersheds, respectively, exhibited remarkable differences in terms of POM sources. Land use patterns that were dominated by agricultural expansion, the soil erosion that resulted from the destruction of vegetation and engineering construction, and hydrological characteristics such as flow velocity and nutrient concentration were all found to vastly impact the sources of POM reaching the different types of SMR. These influencing factors are mainly derived from anthropogenic activities, whose effects on the tropical SMRs ecosystems and even the global biogeochemical cycle, may exceed our expectation.

Role of deposit feeders in integrated multi-trophic aquaculture — A model analysis

The performance of deposit feeders in integrated multi-trophic aquaculture (IMTA) was analysed through the application of mathematical models. Loading of organic particulates to the benthos as a result of finfish cage culture and shellfish suspended culture was analysed by means of a deposition model (ORGANIX), and an individual model for growth and environmental effects was developed for the California sea cucumber Parastichopus californicus. Following validation, the model was combined with existing models for finfish, shellfish, and macroalgae into a framework for simulation of IMTA interactions at the local scale. Several scenarios for different culture combinations and densities were simulated using the Farm Aquaculture Resource Management (FARM) model, using a layout which reflects typical stocking densities and spatial occupation in Europe and North America. The model allows an analysis of the different sources and fate of particulate organic matter associated with distinct culture groups. Our results illustrate the production enhancement for deposit feeders cultivated below both finfish (600%) and shellfish (150%). Furthermore, sea cucumbers are responsible for a significant removal of the particulate organic carbon loading to the bottom, reducing the gross load by up to 86% for finfish culture and 99% for shellfish culture. The role of cultivated seaweeds in reducing the dissolved nitrogen concentration in the farm area was also examined—no significant reduction in ambient nutrient concentration was observed, but the added nitrogen provides a clear stimulus (22% increase) to kelp production. By contrast, shellfish grown in suspended culture in the vicinity of finfish cages show very little change in individual growth or harvestable biomass. This work helps to analyse the ecological and economic potential of various forms of IMTA, and the role of co-cultivation in direct extraction and re-use of materials and energy at both the local (farm) and system (bay, estuary) scales. Statement of relevanceAnalysis of the ecological and economic potential of various forms of IMTA, and the relevance of co-cultivation in direct extraction and re-use of materials and energy at both the local (farm) and system (bay, estuary) scales. FARM model framework applied to understand the roles of suspended bivalve culture and benthic deposit-feeder culture in mitigating the negative externalities of finfish culture.

A new device to follow temporal variations of oxygen demand in deltaic sediments: the LSCE benthic station

A new benthic station equipped with oxygen microelectrodes and environmental sensors was developed by Laboratoire des Sciences du Climat et de L'Environnement (LSCE) and Division Technique of the Institut National des Sciences de L'Univers (DT‐ INSU) to perform in situ time series monitoring of sediment oxygen demand, linked to the mineralization of organic matter. The time series typically cover periods of 2‐ 3 months, with a base frequency of 1 set of oxygen profiles per day. The profiling head assessed the lateral heterogeneity of the sediment oxygen demand at the beginning of the time series over a 0.8‐ m long rectangle to discriminate spatial and temporal variability. A continuous recalibration is performed using a moored oxygen optode anchored to the benthic station together with a set of environmental sensors. These sensors (turbidity, temperature, salinity, and oxygen) can trigger a high‐ frequency profiling mode to investigate the fate of particulate organic matter delivered during floods, resuspension, and deposition events. Deployments of the benthic station were performed in the Rhone River subaqueous delta (Mediterranean Sea). We show that “stable” periods (when neither floods nor storms occur) were characterized by a stable oxygen demand. In the case of resuspension events, an increase of the sediment oxygen demand by a factor of 2‐ 3 with a relaxation time of 1 day was observed, indicating that the new benthic station can adequately capture the impact of resuspension events on the oxygen demand in deltaic sediments.

The isotopic signature of particulate organic C and N in bottom ice: Key influencing factors and applications for tracing the fate of ice‐algae in the Arctic Ocean

AbstractThe isotopic, taxonomic, and nutrient dynamics of protist communities at the bottom of first‐year sea ice were studied for a wide range of ice conditions during the spring‐summer transition in the southeast Beaufort Sea. In bottom ice, total protist abundance ranged from 0.07 to 9.94 × 109 cells m–2 and chlorophyll a biomass ranged from 0.33 to 110 mg m–2. Both variables were positively related to the concentrations of nitrate, silicic acid, and phosphate at the ice‐water interface, with r2 values between 0.63 and 0.79. Results showed that the δ13C of sympagic (ice‐associated) particulate organic matter (POM) (–27.1 to –11.4‰) was mainly influenced by ice protist stocks and dissolved inorganic carbon availability. Flagellated cells maintained a low δ13C relative to diatoms. The δ15N of POM (3.9 to 9.4‰) could not be explained by any of the variables considered and showed considerable overlap with the pelagic δ15N‐POM (0.0 to 10.3‰). Detailed analyses of δ13C‐POM through size fractionation and settling experiments along with a short temporal study of the interaction between sympagic and pelagic communities confirmed or demonstrated that (1) the analysis of stable C isotopes is a valuable tool to trace the flow of ice‐derived matter, (2) sympagic POM contributes substantially to suspended pelagic biomass during release episodes, and (3) ice‐grown autotrophic flagellated cells have a strong potential to interact with the pelagic biota in the coastal Arctic Ocean. This study highlights the need to carefully evaluate stable isotope dynamics before assessing food web interactions or the fate of POM in seasonally ice‐covered waters.

Density fractions versus size separates: does physical fractionation isolate functional soil compartments?

Abstract. Physical fractionation is a widely used methodology to study soil organic matter (SOM) dynamics, but concerns have been raised that the available fractionation methods do not well describe functional SOM pools. In this study we explore whether physical fractionation techniques isolate soil compartments in a meaningful and functionally relevant way for the investigation of litter-derived nitrogen dynamics at the decadal timescale. We do so by performing aggregate density fractionation (ADF) and particle size-density fractionation (PSDF) on mineral soil samples from two European beech forests a decade after application of 15N labelled litter. Both density and size-based fractionation methods suggested that litter-derived nitrogen became increasingly associated with the mineral phase as decomposition progressed, within aggregates and onto mineral surfaces. However, scientists investigating specific aspects of litter-derived nitrogen dynamics are pointed towards ADF when adsorption and aggregation processes are of interest, whereas PSDF is the superior tool to research the fate of particulate organic matter (POM). Some methodological caveats were observed mainly for the PSDF procedure, the most important one being that fine fractions isolated after sonication can not be linked to any defined decomposition pathway or protective mechanism. This also implies that historical assumptions about the "adsorbed" state of carbon associated with fine fractions need to be re-evaluated. Finally, this work demonstrates that establishing a comprehensive picture of whole soil OM dynamics requires a combination of both methodologies and we offer a suggestion for an efficient combination of the density and size-based approaches.

Macrobenthic assemblage structure and organismal stoichiometry control faunal processing of particulate organic carbon and nitrogen in oxygen minimum zone sediments

Abstract. The Arabian Sea oxygen minimum zone (OMZ) impinges on the western Indian continental margin between 150 and 1500 m, causing gradients in oxygen availability and sediment geochemistry at the sea floor. Oxygen availability and sediment geochemistry are important factors structuring macrofaunal assemblages in marine sediments. However, relationships between macrofaunal assemblage structure and sea-floor carbon and nitrogen cycling are poorly understood. We conducted in situ 13C:15N tracer experiments in the OMZ core (540 m [O2] = 0.35 μmol l–1) and lower OMZ boundary (800–1100 m, [O2] = 2.2–15.0 μmol l–1) to investigate how macrofaunal assemblage structure, affected by different oxygen levels, and C:N coupling influence the fate of particulate organic matter. No macrofauna were present in the OMZ core. Within the OMZ boundary, relatively high abundance and biomass resulted in the highest macrofaunal assimilation of particulate organic carbon (POC) and nitrogen (PON) at the lower oxygen 800 m stations ([O2] = 2.2–2.36 μmol l–1). At these stations the numerically dominant cirratulid polychaetes exhibited greatest POC and PON uptake. By contrast, at the higher oxygen 1100 m station ([O2] = 15.0 μmol l–1) macrofaunal C and N assimilation was lower, with POC assimilation dominated by one large solitary ascidian. Macrofaunal POC and PON assimilation were influenced by changes in oxygen availability, and significantly correlated to differences in macrofaunal assemblage structure between stations. However, macrofaunal feeding responses were ultimately characterised by preferential organic nitrogen assimilation, relative to their internal C:N budgets.

Origin and fate of particulate organic matter in the southern Beaufort Sea – Amundsen Gulf region, Canadian Arctic

To establish the relative importance of terrigenous and marine organic matter in the southern Beaufort Sea, we measured the concentrations and the stable isotopic compositions of organic carbon and total nitrogen in sediments and in settling particles intercepted by sediment traps. The organic carbon content of surface sediment in the Chukchi and southern Beaufort Seas ranged from 0.6 to 1.6% dry wt., without a clear geographical pattern. The C ORG:N TOT ratio ranged from 7.0 to 10.4 and did not vary significantly downcore at any one station. Values of δ 13C ORG and δ 15N TOT in the sediment samples were strongly correlated, with the highest values, indicative of a more marine contribution, in the Amundsen Gulf. In contrast, the organic matter content, elemental (C ORG:N TOT ratio) and isotopic ( δ 13C ORG and δ 15N TOT) composition of the settling particles was different from and much more variable than in the bottom sediments. The isotopic signature of organic matter in the Beaufort Sea is well constrained by three distinct end-members: a labile marine component produced in situ by planktonic organisms, a refractory marine component, the end product of respiration and diagenesis, and a refractory terrigenous component. A three-component mixing model explains the scatter observed in the stable isotope signatures of the sediment trap samples and accommodates an apparent two-component mixing model of the organic matter in sediments. The suspended matter in the water column contains organic matter varying from essentially labile and marine to mostly refractory and terrigenous. As it settles through the water column, the labile marine organic matter is degraded, and its original stable isotope signature changes towards the signature of the marine refractory component. This process continues in the bottom sediment with the result that the sedimentary organic matter becomes dominated by the refractory terrigenous and marine components.

2-n-Pentyl-4-quinolinol produced by a marine Alteromonas sp. and its potential ecological and biogeochemical roles.

Bacterium-bacterium interactions occur at intimate spatial scales on the order of micrometers, but our knowledge of interactions at this level is rudimentary. Antagonism is a potential interaction in such microenvironments. To study the ecological role of antibiosis, we developed a model system involving an antibiotic-producing isolate (SWAT5) derived from a marine particle and its dominant antibiotic product, 2-n-pentyl-4-quinolinol (PQ). This system was used to address questions about the significance of this antibiotic for microbial ecology and carbon cycling on particles. We characterized the chemical and inhibitory properties of PQ in relation to the mechanisms used by particle-associated bacteria in interacting with particles and with other attached bacteria. PQ was produced by SWAT5 only on surfaces. When SWAT5 was grown in polysaccharide matrices, PQ diffused within the matrices but not into the surrounding seawater. SWAT5 might thus be able to generate a localized zone of high antibiotic concentration on particles suspended or sinking through seawater. Target bacterial respiration was most sensitive to PQ (75 nM), while inhibition of DNA synthesis, protein synthesis, and bacterial motility required higher (micromolar) PQ levels. The presence of PQ altered the composition of the bacterial community that colonized and developed in a model particle system. PQ also inhibited Synechococcus and phytoplankton growth. Our results suggest that antibiosis may significantly influence community composition and activities of attached bacterial and thus regulate the biogeochemical fate of particulate organic matter in the ocean.

Relationships between microbial extracellular enzymatic activity and suspended and sinking particulate organic matter: seasonal transformations in the North Water

Abstract Despite the importance of hydrolytic activities by bacterial extracellular enzymes (EE) in the temperate ocean, little is known about the role of extracellular enzymatic activity (EEA) in determining the fate of particulate organic matter (POM) in polar seas. To explore the issue further, we measured various chemical and bacterial parameters in the near-0°C waters of the North Water during the months of May and July of 1998. Seawater (SW) samples were collected by Niskin bottle at the depth of the chlorophyll fluorescence maximum (8–90 m), while samples of sinking particles and aggregates were collected in short-term (0.5–1.2 d), unpoisoned, floating sediment traps deployed at depths typically below the mixed layer (50–136 m). Samples were analyzed for POC, PON, and abundance of total and actively respiring bacteria. They were also incubated with fluorescently tagged substrate analogs to measure potential maximal rates of three classes of EE (leucine-aminopeptidase, chitobiase, and β -glucosidase) at –1°C. The percentage of actively respiring bacteria was always higher in sediment trap samples than in SW (medians of 38% and 24% versus 10% and 12% in May and July, respectively). Cell-specific rates of EEA were also higher in the trap samples and, for both sample types, similar to published rates from temperate waters. Rates of EEA when scaled to the abundance of actively respiring bacteria, however, did not differ between sample types, suggesting that the elevated EEA associated with sinking material is due to the greater abundance of metabolically active cells supported by such material and not due to enhanced enzyme expression in general, as suggested by previous studies. In this study, leucine-aminopeptidase activity was always much higher than the other classes of EEA, becoming even more dominant later in the season; it always correlated positively with the abundance of both total and actively respiring bacteria. Enzyme ratios indicating protease dominance corresponded with the seasonal increases in C/N ratios of both suspended and sinking POM. Overall the results suggest bacterial responses to seasonal changes in POM quality through differential expression of EE and an important role for proteases in influencing the nitrogen content of organic matter even at near-0° temperatures in this polynya.

Fate of particulate organic matter in soil aggregates during cultivation

SummaryParticulate organic matter (POM) is a labile fraction of soil organic matter which is thought to be physically protected from biodegradation when within soil aggregates. We have developed a fractionation method to separate POM located outside stable soil macroaggregates (> 200 μm) and microaggregates (50–200 μm) from that within them, and applied it to a cultivation sequence of humic loamy soils. The natural abundance of 13C was used to determine the amounts of POM derived from forest and that derived from crop in the free and occluded fractions. In the forest soil the free and occluded POM fractions had the same composition, morphology and isotopic signature. On cultivation the amounts of POM decreased sharply. The loss of C in the POM from forest was mainly from POM outside the aggregates. The POM occluded within microaggregates was found to turnover slowly. This may be due either to its recalcitrant chemical nature or to its physical protection within microaggregates

Lipid class and alkane distributon in settling particles of the upper Laurentian Great Lakes

Sediment traps were deployed at up to 6 depths at seven stations in Lakes Superior, Michigan, and Huron for 2 months during the summer of 1984. Total organic carbon, total lipid, lipid classes, and alkanes were measured in the collected material to gain information on the composition, sources, and fate of particulate organic matter in the Great Lakes. Total aliphatic hydrocarbon was a large component of all samples, accounting for more than 15% of the extractable lipids. Particles collected at 10-m depth in Lake Huron had especially high relative contributions(more than 65% of total lipids) of total aliphatic hydrocarbons at the three stations eamined. The high proportion of aliphatic hydrocarbon appears not to have originated from customary pollution sources. The alkane distribution was strongly dominated by even-numbered carbon chains of unknown origin. Algae are a possible source for the unsaturated hydrocarbons that were present at relatively elevated levels in settiing particles. Botryococcus braunti, sampled from Lake Michigan had as much as 80% of its lipids in the form of hydrocarbons.Wax and sterol estersconstituted less than 10% of the lipids in trap material except in samples from mid-depths in Lake Huron where high levels of these compounds may have resulted from elevated amounts of calanoid capepod debris or swimmers entering the traps.