Composition Of Suspended Organic Matter Research Articles

Sources and export of particle-borne organic matter during a monsoon flood in a catchment of northern Laos

Abstract. The yields of the tropical rivers of Southeast Asia supply large quantities of carbon to the ocean. The origin and dynamics of particulate organic matter were studied in the Houay Xon River catchment located in northern Laos during the first erosive flood of the rainy season in May 2012. This cultivated catchment is equipped with three successive gauging stations draining areas ranging between 0.2 and 11.6 km2 on the main stem of the permanent stream, and two additional stations draining 0.6 ha hillslopes. In addition, the sequential monitoring of rainwater, overland flow and suspended organic matter compositions was conducted at the 1 m2 plot scale during a storm. The composition of particulate organic matter (total organic carbon and total nitrogen concentrations, δ13C and δ15N) was determined for suspended sediment, soil surface (top 2 cm) and soil subsurface (gullies and riverbanks) samples collected in the catchment (n = 57, 65 and 11, respectively). Hydrograph separation of event water was achieved using water electric conductivity and δ18O measurements for rainfall, overland flow and river water base flow (n = 9, 30 and 57, respectively). The composition of particulate organic matter indicates that upstream suspended sediments mainly originated from cultivated soils labelled by their C3 vegetation cover (upland rice, fallow vegetation and teak plantations). In contrast, channel banks characterized by C4 vegetation (Napier grass) supplied significant quantities of sediment to the river during the flood rising stage at the upstream station as well as in downstream river sections. The highest runoff coefficient (11.7%), sediment specific yield (433 kg ha−1), total organic carbon specific yield (8.3 kg C ha−1) and overland flow contribution (78–100%) were found downstream of reforested areas planted with teaks. Swamps located along the main stream acted as sediment filters and controlled the composition of suspended organic matter. Total organic carbon specific yields were particularly high because they occurred during the first erosive storm of the rainy season, just after the period of slash-and-burn operations in the catchment.

Seasonal and Spatial Variation of Carbon and Nitrogen Stable Isotopes in Mangrove Oysters (Crassostrea corteziensis) from the Northwest Coast of Mexico

The aim of this study was to evaluate how Crassostrea corteziensis interacts with its habitat (abiotic factors) and to examine the isotopic variations that may exist over different scales of space and time at a subtropical gradient. This information will contribute to the work required to understand the ecology of this oyster species. We analyzed stable isotopes (δ13C and δ15N) in oyster soft tissue and the concentrations of chlorophyll a, NH4 , and NO3 - in the waters of 6 lagoons during the dry and rainy seasons of 2008 and 2009. The mean C:N ratios in soft tissue were generally greater than 3.5 in the different lagoons and during both seasons, indicating a relatively high amount of lipids in the soft tissue related to reproductive activity. The isotopic variability between sites in this study (-25.5 to — 17.1‰ for δ13C and 6.5 to 13.7‰ for δ15N) suggested differences in the composition of suspended organic matter in the diet of this species at different locations, showing an opportunistic behavior that allowed us to detect differences on temporal and spatial scales. On a temporal scale, however, there was no difference in δ13C, which was most likely related to the high food availability and the observation that individuals tend to feed on and share the same overabundant prey. In the case of δ15N, significant temporal differences were evident in only a few lagoons. These differences were related to a differential contribution of NH4 and NO3 - as primary sources to the suspended organic matter pool during the 2 seasons. On a spatial scale, some lagoons showed significant differences (P < 0.05) and high δ15N values (>10‰) in addition to high concentrations of NH4 . Enriched δ15N values are often associated with an anthropogenic impact, as human and animal waste nitrate have δ15N values of approximately 10–20‰.

Abundance and activity of microorganisms at the water-sediment interface and their effect on the carbon isotopic composition of suspended organic matter and sediments of the Kara Sea

At ten stations of the meridian profile in the eastern Kara Sea from the Yenisei estuary through the shallow shelf and further through the St. Anna trough, total microbial numbers (TMN) determined by direct counting, total activity of the microbial community determined by dark CO2 assimilation (DCA), and the carbon isotopic composition of organic matter in suspension and upper sediment horizons (δ13C, per thousand) were investigated. Three horizons were studied in detail: (1) the near-bottom water layer (20-30 cm above the sediment); (2) the uppermost, strongly hydrated sediment horizon, further termed warp (5-10 mm); and (3) the upper sediment horizon (1-5 cm). Due to decrease in the amount of isotopically light carbon of terrigenous origin with increasing distance from the Yenisei estuary, the TMN and DCA values decreased, and the δ13C changed gradually from -29.7 to -23.9 per thousand. At most stations, a noticeable decrease in TMN and DCA values with depth was observed in the water column, while the carbon isotopic composition of suspended organic matter did not change significantly. Considerable changes of all parameters were detected in the interface zone: TMN and DCA increased in the sediments compared to their values in near-bottom water, while the 13C content increased significantly, with δ13C of organic matter in the sediments being at some stations 3.5- 4.0 per thousand higher than in the near-bottom water. Due to insufficient illumination in the near-bottom zone, newly formed isotopically heavy organic matter (δ13C(-) -20 per thousand) could not be formed by photosynthesis, active growth of chemoautotrophic microorganisms in this zone is suggested, which may use reduced sulfur, nitrogen, and carbon compounds diffusing from anaerobic sediments. High DCA values for the interface zone samples confirm this hypothesis. Moreover, neutrophilic sulfur-oxidizing bacteria were retrieved from the samples of this zone.

Effect of phytoplankton and microorganisms on the isotopic composition of organic carbon in the Russian Arctic seas

Carbon isotope composition of suspended organic matter (CICSOM) and of organic carbon of the bottom sediments (CICBS) was studied in a series of expeditions (starting in 1993) to the White, Kara, Chukchi, and Barents seas in the Russian Arctic. For each sea, CICSOM and CICBS was found to depend primarily on the ratio of OM produced in the water and OM of terrigenous origin. While in the White Sea, where the primary production (PP) is 5.3 times higher than the yearly inflow of terrigenous OM, δ13C of SOM carbon is −29.1‰, in the Chukchi Sea, where PP is more than 300 times higher than the inflow of terrigenous OM, δ13C of SOM carbon is −21.8‰. In the Barents and Chukchi seas, a considerable effect of suspended material arriving with the currents from the neighboring seas on formation of the CICSOM was demonstrated. The difference between CIC OM of the bottom sediments form CICSOM, the main component of organic matter in the sediments of all shelf seas, was demonstrated for the first time for all the seas studied. This results from production of additional microbial OM due to CO2 assimilation at the water-sediment redox boundary or in near-bottom water.

Variations in some environmental characteristics including C and N stable isotopic composition of suspended organic matter in the Mandovi estuary

Chemical and isotopic (δ13C and δ15N) investigation of the Mandovi estuary along the Indian west coast affected strongly by the seasonal monsoon cycle was carried out. The Mandovi estuary is a major waterway for Goa and extensively used for transportation of iron and manganese ore. In addition, with large population centers as well as agricultural fields located on its shores, the estuary is assumed to have been influenced by human activities. Measurements of chemical and isotopic parameters made in the lower part of the estuary during the southwest (SW) monsoon and post-monsoon seasons reveal distinct changes, and it is observed that despite considerable enrichment of macronutrients during the SW monsoon, productivity of the estuary (phytoplankton biomass), as inferred from the chlorophyll-a content, is not as high as expected. This is due to occurrences of high turbidity and cloud cover that limits photosynthetic productivity. The isotopic characterization (C and N isotopes) of suspended organic matter produced/transported during the monsoon and post-monsoon seasons of year 2007 provides a baseline dataset for future isotopic studies in such type of tropical estuaries.

Biological fractionation of stable carbon isotopes at the aerobic/anaerobic water interface of meromictic water bodies

Mass-spectrometric investigation of carbon isotope composition (delta13C) was carried out for suspended organic matter and dissolved mineral compounds for the water column of some meromictic water bodies differing in salinity and trophic state. As a rule, a more pronounced carbon isotope fractionation (resulting from the metabolism of phytoplankton and anoxygenic phototrophic bacteria) was revealed in the zones of enhanced oxygenic and anoxygenic photosynthesis. Carbon isotope fractionation at the border between oxidized and reduced waters depends both on the activity of microbial communities and on the dominant species of phototrophic microorganisms. Analysis of the distribution profiles of the isotopic composition of suspended organic matter and dissolved mineral carbon revealed active mineralization of the organic matter newly formed via anoxygenic photosynthesis in the monimolimnion by microbial communities, resulting in the release of isotopically light carbon dioxide. Mineral carbon in the anaerobic zones of highly productive meromictic water bodies is therefore enriched with the light 12C isotope.

Microbiological and biogeochemical processes in a pockmark of the Gdansk depression, Baltic Sea

Comprehensive microbiological and biogeochemical investigation of a pockmark within one of the sites of gas-saturated sediments in the Gdansk depression, Baltic Sea was carried out during the 87th voyage of the Professor Shtokman research vessel. Methane content in the near-bottom water and in the underlying sediments indicates stable methane flow from the sediment into the water. In the 10-m water layer above the pockmark, apart from methane anomalies, elevated numbers of microorganisms and enhanced rates of dark CO2 fixation (up to 1.15 micromol C/(1 day)) and methane oxidation (up to 2.14 nmol CH4/(1 day)) were revealed. Lightened isotopic composition of suspended organic matter also indicates high activity of the near-bottom microbial community. Compared to the background stations, methane content in pockmark sediments increased sharply from the surface to 40-60 ml/dm3 in the 20-30cm horizon. High rates of bacterial sulfate reduction (SR) were detected throughout the core (0-40 cm); the maximum of 74 micromol/(dm3 day) was located in subsurface horizons (15-20 cm). The highest rates of anaerobic methane oxidation (AMO), up to 80 micromol/(dm3 day), were detected in the same horizon. Good coincidence of the AMO and SR profiles with stoichiometry close to 1:1 is evidence in favor of a close relation between these processes performed by a consortium of methanotrophic archaea and sulfate-reducing bacteria. Methane isotopic composition in subsurface sediments of the pockmark (from -53.0 to -56.5% per hundred) does not rule out the presence of methane other than the biogenic methane from the deep horizons of the sedimentary cover.

Oxic–anoxic conditions in the water column of a tropical freshwater reservoir (Peña-Larga dam, NW Venezuela)

A freshwater reservoir (area: 122 km 2, volume: 2850 × 10 6 m 3, maximum depth: 72 m) located at the outlet of a tropical Andean watershed (Rio Boconó, 1620 km 2, 08°57 ′–09°31 ′N, 70°02 ′–70°34 ′W, NW Venezuela) was studied combining: (1) high resolution monitoring of the major environmental parameters using a CTD probe; (2) dissolved (major and trace) element concentration measurements; (3) water (δ 18O, δ 2H) isotope measurements and; (4) organic carbon (C) concentration – δ 13C measurements on suspended matter and on sediments. Composed of two sub-reservoirs with contrasted water inputs, the Peña Larga dam provides the opportunity to better constrain C budgets and source – composition of suspended organic matter within two parts of a unique freshwater system with contrasted redox conditions. The redox status is reflected by drastic differences between the two parts of the reservoir, pictured by water residence time, water column ventilation, dissolved trace element concentration and suspended organic matter composition. Seasonal renewal of water near the main inlet supports higher surface productivity, oxic water column conditions, homogenous water chemical composition and, below the thermocline, the transport of suspended organic matter mainly derived from soil erosion on the watershed. Calculated sediment and organic C storages amount to 5.6 × 10 6 t a −1 and 21 ± 5 × 10 3 t C a −1, respectively. Despite a markedly lower suspended sediment load, outflow waters exhibit POC-δ 13C values similar to those in inflow waters, due to a short water residence time and a rapid recycling of primary products. In the more isolated parts of the reservoir, where water discharge is thoroughly reduced, hypoxic to anoxic conditions are found immediately below the thermocline (ca. 16 m, [O 2]<30 μmol L −1). They induce a correlative increase of trace element concentrations with water depth. Suspended organic matter display 13C-depleted compositions, contrasting sharply with that of photosynthetic organisms and land-derived detritus, that either reflect the contribution of microbial biomass (chemoautotrophic and/or methanotrophic bacteria) or the selective degradation of more labile organic compounds in the water column. The hypoxic to anoxic conditions displayed in the major part of the water column extend to the first cm of lake bottom sediments. A simplified budget based on the main redox processes active in the hypolimnion and on average water residence times, supports drastic differences in mineralization rate: 83–444 μmol C L −1 a −1 for the oxic reservoir and only 43 μmol C L −1 a −1 for the hypoxic to anoxic reservoir. This study shows that, if water renewal is not sufficient, tropical freshwater lakes may be subject to severe dissolved O 2 depletion conditions at shallow depths, comparable to those observed in deep sections of the water column of temperate eutrophic lakes.

Carbon isotope and C/N ratios of suspended matter in rivers: an indicator of seasonal change in C 4/C 3 vegetation

A combination of δ 13C values with C/N ratios in suspended matter has been used to examine the seasonal relationship between C 4 and C 3 vegetation along the Loess Plateau, NW China. The C isotopic composition of suspended organic matter in rivers, together with C/N ratios can differentiate between soil and plant material, and can be used to estimate the relative contributions of soil organic C and plant litter to the suspended matter. The relationship between C isotopic composition and C/N ratios indicates that the samples are a mixture of two end members: (1) modern soils with relatively constant δ 13C values, low C content and low C/N ratios; (2) plant litter with varying δ 13C values, high C content and high C/N ratios. The results reflect the seasonal distribution of C 4/C 3 vegetation within the area studied, as part of the Loess Plateau. The abundance of C 4 grasses is about 20% for the current summer vegetation ecosystem in the eastern part of the Loess Plateau. Hence, the use of δ 13C values and C/N ratios of suspended matter in rivers and modern soil may be useful for reflecting seasonal distribution of C 4/C 3 vegetation in catchments. This could be a useful tool for distinguishing between catchments for GIS studies, and long term planning for ecological management of catchment areas.

Estimating Spatial and Seasonal Phytoplankton δ13C Variations in an Estuarine Mangrove Ecosystem

In estuarine ecosystems, large spatial and seasonal variations in δ13C values of primary producers can occur, and knowledge of these variations may be crucial when interpreting stable isotope data of higher trophic levels. Obtaining clean phytoplankton samples for isotope analysis is usually impossible in such systems, and analysis of total suspended matter is not a simple proxy for phytoplankton δ13C variations. Based on a few simple assumptions regarding the C and N content of the two end-members (terrestrial detritus and phytoplankton) and the δ13C of the terrestrial component, we here present a simple model to estimate the phytoplankton δ13C variations using an existing dataset on the δ13C and elemental (C:N) composition of suspended organic matter from an estuarine mangrove ecosystem in southeast India. These variations are related to the monthly rainfall pattern during the sampling period. It is stressed that this method estimates approximate phytoplankton δ13C values, which should not be used in e.g., mixing models. However, we propose that in cases where sufficiently large datasets are available, the described procedure could provide a valuable method to semi-quantitatively estimate the seasonal or spatial variations of the phytoplankton δ13C signal.

The application of pyrolysis-mass spectrometry and pyrolysis-gas chromatography-mass spectrometry to the chemical characterization of suspended matter in the ocean

Suspended matter from surface microlayers and various water depths, plankton and surface sediments have been collected at various Mediterranean sites and have been subjected to flash pyrolysis techniques to study the scope of pyrolysis methods in the analysis of the chemical composition of suspended organic matter in the ocean. Samples have been analysed by both pyrolysis-mass spectrometry (Py-MS) and pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS). Discriminant analysis of the Py-MS spectra of 26 samples indicated the amplitude and nature of the variance between the samples. By means of Py-MS fingerprints, the samples are separated into well-defined clusters, reflecting the nature, the geographic origin and the sampling depth. The differences observed are due to varying amounts of peptides, polysaccharides and lipids. The plankton samples are clearly separated due to their high content in recently biosynthesized material, such as proteins. Other samples collected in coastal areas, especially in the Northern Adriatic Sea, are characterized by relatively high contents of polysaccharides probably due to high riverine nutrient inputs and consequently a high plankton productivity. The chemical interpretations of the Py-MS data are based on Py-GC-MS data of a few selected samples. Py-GC-MS traces allow the detailed identification of pyrolysis products derived from peptides, polysaccharides, lipids, lignin and others. Flash pyrolysis techniques appear to be a useful tool to study the chemical nature of suspended matter, allowing the determination of major building blocks of complex mixtures of biopolymers, degraded biopolymers and condensation products, present in the suspended matter of the ocean.