Composition Of Sedimentary Organic Matter Research Articles

Fish-farming effects on benthic community structure in coastal sediments: analysis of meiofaunal recovery

the Farmocean cage (2000 m 3 ), while the other (the control) was located at about 1 km from an area not impacted by fish farming. The fish farm contained about 120 000 Dicentrarchus labrax (density 18 kg m 3 ). The most evident changes in the benthic habitat under the cage were a large accumulation of primary organic material (phytopigment concentrations up to 44 gg 1 ), changes in sedimentary organic matter composition (increased lipid levels related to the composition of the fish diet), and a strong reduction of redox potential values, which resulted in a significant reduction of meiofaunal penetration depth into the sediments. Organic loads had a clear impact on meiofaunal densities, which were 50% lower under the cage than at the control site (1112118 and 2160339 ind. 10 cm 2 , respectively). Compared with the control, farm sediments also showed an increased importance of copepods, which dominated the meiofauna together with nematodes (both 39% of the total density) and polychaetes (17%). The removal of the fish farm from the site on July 1997 allowed an analysis to be made of the initial short and medium-term recovery of the assemblages. After two months, meiofaunal densities were closer to the control (about 30% recovery). Community structure recovered only partially to the characteristics typical of the study area, with increased importance of nematodes (about 70% of total density) and reduced copepod contribution, but it was still characterized by a lower number of taxa after four months. These data suggest that meiofaunal recovery after fish farm disturbance is rapid, but far from complete after four months. 2000 International Council for the Exploration of the Sea

Distribution, partitioning and fluxes of dissolved and particulate organic C, N and P in the eastern North Pacific and Southern Oceans

Concurrent distributions of dissolved and suspended particulate organic carbon (DOC and POC susp), nitrogen (DON and PON susp) and phosphorus (DOP and POP susp), and of suspended particulate inorganic phosphorus (PIP susp), are presented for the open ocean water column. Samples were collected along a three-station transect from the upper continental slope to the abyssal plain in the eastern North Pacific and from a single station in the Southern Ocean. The elemental composition of surface sedimentary organic matter (SOM) was also measured at each location, and sinking particulate organic matter (POM sink) was measured with moored sediment traps over a 110-d period at the abyssal site in the eastern North Pacific only. In addition to elemental compositions, C : N, C : P and N : P ratios were also calculated. Surface and deep ocean concentrations of dissolved organic matter (DOM) and inorganic nutrients between the two sites displayed distinct differences, although suspended POM (POM susp) concentrations were similar. Concentrations of DOM and POM susp displayed unique C, N and P distributions, with POM susp concentrations generally about 1–2 orders of magnitude less than the corresponding DOM concentrations. These differences were likely influenced by different biogeochemical factors: whereas the dissolved constituents may have been influenced more by the physical regime of the study site, suspended particulate matter may have been controlled to a greater extent by biological and chemical alteration. Up to 80% of total particulate P in POM susp, POM sink and SOM consisted of PIP. For all organic matter pools measured, elemental ratios reveal that organic P is preferentially remineralized over organic C and organic N at both sites. Increases in C : P and N : P ratios with depth were also observed for DOM at both sites, suggesting that DOP is also preferentially degraded over C and N as a function of depth. A simple one-dimensional vertical eddy diffusion model was applied to estimate the contributions of dissolved and suspended particulate organic C, N and P fluxes from the upper mixed layer into the permanent thermocline. Estimated vertical DOM fluxes were 28–63% of the total organic matter fluxes; POM susp and POM sink fluxes were 8–20 and 28–52% of the total.

Meiofaunal assemblages associated with scallop beds (Adamussium colbecki) in the coastal sediments of Terra Nova Bay (Ross Sea, Antarctica)

Meiofaunal community structure in coastal sediments of Terra Nova Bay (Ross Sea) was related to quantity and biochemical composition of sedimentary organic matter. The sediments were generally characterized by large amounts of chloropigments and labile compounds (dominated by proteins), indicating high inputs of primary organic matter. Meiofaunal densities were very high and comparable to those from the most productive areas worldwide. Sediments with high densities of the scallop had low meiofaunal densities especially in the top 2 cm, suggesting that scallop clapping contributed to meiofauna resuspension. However, it is not possible to exclude the probability that meiofauna are part of the scallop beds apparently have an important role in structuring meiofaunal communities with nematodes dominant where Adamussium colbecki is absent, and gastrotrichs dominant (44–51%) in A. colbecki beds, reaching the highest density reported so far.

Biochemical composition of sedimentary organic matter and bacterial distribution in the Aegean Sea: trophic state and pelagic–benthic coupling

Biochemical composition of sedimentary organic matter (OM), vertical fluxes and bacterial distribution were studied at 15 stations (95–2270 m depth) in the Aegean Sea during spring and summer. Downward fluxes of labile OM were significantly higher in the northern than in the southern part and were higher in summer than in spring. Primary inputs of OM were not related to sedimentary OM concentrations, which had highest values in summer. Sedimentary chlorophyll-a concentrations were similar in the northern and southern parts. Carbohydrates, the main component of sedimentary OM, were about 1.2 times higher in the southern part than in the northern, without significant temporal changes. Total proteins were higher in summer and about double in the northern part. Sedimentary proteins appeared more dependent upon the downward flux of phytopigment than of proteins. Sedimentary OM was characterised by a relatively large fraction of soluble compounds and showed better quality in the northern part. The lack of a depth-related pattern in sedimentary OM and the similar concentrations in the two areas suggest that differences in sedimentary OM quality in the Aegean basin are dependent on system productivity; the bulk of sedimentary OM is largely conservative. Sedimentary bacterial density was about double in the northern part and higher in spring than in summer, but bacterial size was about three times higher in summer, resulting in a larger bacterial biomass in summer. Bacterial density was coupled with total and protein fluxes, indicating a rapid bacterial response to pelagic production. Bacterial biomass was significantly correlated with sedimentary protein and phytopigment concentrations, indicating a clear response to accumulation of labile OM in the sediments. In all cases bacteria accounted for <5% of the organic C and N pools. The efficiency of benthic bacteria in exploiting protein pools, estimated as amounts of protein available per unit bacterial biomass, indicates a constant ratio of about 70 μg proteins/μg C. This suggests a similar bacterial efficiency all over the area studied, unaffected by different trophic conditions.

Differential responses of bacteria, meiofauna and macrofauna in a shelf area (Ligurian Sea, NW Mediterranean): role of food availability

Density and biomass of bacteria, meio- and macrofauna were examined along a transect of eight stations (5–135 m depth) facing the estuary of the river Entella (Ligurian Sea) during summer 1990. Sediment samples were collected to determine organic detritus composition (total organic matter, lipid, protein and carbohydrate concentrations) and microphytobenthic biomass (as chlorophyll-a). Synoptic water samples were collected to determine the quantity and the quality of suspended matter (total suspended matter, particulate lipids, proteins, carbohydrates and chloroplastic pigments). Particulate organic matter in the surface water decreased from the coast towards the open sea both in quality and quantity. By contrast, the organic-matter concentration in the sediment increased with water depth. Quantity and biochemical composition of suspended and sedimentary organic matter affected the distribution of all the benthic assemblages. Bacteria appeared to be controlled by different parameters at different depths: generally they appeared to depend upon sediment particle surface and the quantity of organic matter, but when metazoan (particularly meiofauna) densities were high, grazing pressure might also exert a control on their abundance. The distribution of meio- and macrofauna along the continental shelf of the oligotrophic Ligurian Sea appears to depend more upon the quality of organic matter than on its quantity.

Early diagenetic alteration of organic matter by sulfate reduction in Quaternary sediments from the northeastern Arabian Sea

Hemipelagic sediments from the northeastern Arabian Sea off Pakistan were analyzed by geochemical and microscopical methods to study the importance of bacterial degradation processes on the quantity and composition of sedimentary organic matter (OM). On the basis of elemental analysis of total organic carbon (TOC) and total sulfur (TS), the decay of OM by sulfate-reducing bacteria were calculated. Concentrations of total iron, reactive iron, total nitrogen (TN), total phosphorus (TP) and hydrogen indices (HI) were used to assess organic matter reactivity and redox conditions. Stable sulfur isotopic compositions provided further information about pathway, timing and mechanisms of sulfate reduction. Microscopical observations indicate that throughout the study area, OM is amorphous (75–95%) in almost all samples with only a small contribution of terrigenous OM. Bacterially mediated sulfate reduction leads to continuously decreasing TOC/TS ratios from 0.4 m to a burial depth of 2–3 m. At this depth, the anaerobic oxidation of particulate organic matter ceases. Sulfate reduction in this narrow interval accounts for the decay of up to 70% of the OM which has primarily entered the anoxic domain. During the bacterially mediated diagenesis preferential consumption of nitrogen and phosphorus containing organic compounds can be observed and quantified. The regeneration of up to 60% of TN and more than 50% of TP can lead to a feedback of nutrients to the seawater. This recycling process may contribute to the high productivity in the northeastern Arabian Sea.

Seasonal and spatial changes in the sediment organic matter of a semi-enclosed marine system (W-Mediterranean Sea)

The composition of sediment organic matter and the related role of microphytobenthic biomass have been investigated during one-year in a semi-enclosed marine system (Marsala lagoon, Mediterranean Sea). Monthly samples from June 1993 to May 1994 were analysed for carbohydrate, protein, lipid, photosynthetic pigment and total organic matter. The three main biochemical classes of organic compounds (i.e. carbohydrates, proteins and lipids) showed higher concentrations than reported in the literature. However, photosynthetic pigment was quite low, compared to other enclosed marine basins or Mediterranean coastal lagoons. As a result, the contribution of primary organic carbon to the sedimentary biopolymeric fraction of organic matter was low (on average 2.2%), indicating that most of the sedimentary organic matter in the study area originated from sources other than micro-algae. The sedimentary organic matter, dominated by carbohydrates (on average 51.2%) followed by proteins (39.0%) and lipids (9.8%), as well as the low protein to carbohydrate ratio, indicate the presence of large amounts of non-living and/or aged organic matter. Comparing data on spatial distribution of sedimentary and suspended organic matter, the dynamic balance of resuspension vs. sedimentation along a north-south axis is invoked as one major factor affecting the distribution and composition of the main classes of organic compounds. The macroalgal and vascular plant coverage is suggested to be another major factor affecting both amounts and composition of sedimentary organic matter. The northern area, characterised by partially unvegetated sediments, showed higher amounts of proteins, whilst moving southward and approaching a luxuriant Posidonia oceanica reef, carbohydrates became more important relative to proteins. As only the biopolymeric fraction of sediment organic matter showed significant seasonal changes, the quantity of sediment OM behaves as an emerging property. By contrast, OM quality is strictly connected to algal coverage as well as to episodic inputs of primary organic matter from deposited phytoplankton and/or microphytobenthos. The uncoupling between large amounts and relatively low nutritional value of sedimentary OM suggests that this particular environment behaves as a detrital ‘trap’.

Stable Sulfur Isotopic Evidence for Historical Changes of Sulfur Cycling in Estuarine Sediments from Northern Florida

Data on abundance and isotopic composition of porewater and sedimentary sulfur species are reported for relatively uncontaminated and highly contaminated fine-grained anoxic sediments of St. Andrew Bay, Florida. A strong contrast in amount and composition of sedimentary organic matter at the two sites allows a comparative study of the historical effects of increased organic loading on sulfur cycling and sulfur isotopic fractionation. In the contaminated sediments, an increase in organic loading caused increased sedimentary carbon/sulfur ratios and resulted in higher rates of bacterial sulfate reduction, but a lower efficiency of sulfide oxidation. These differences are well reflected in the isotopic composition of dissolved sulfate, sulfide, and sedimentary pyrite. Concentration and isotopic profiles of dissolved sulfate, organic carbon, and total sulfur suggest that the anaerobic decomposition of organic matter is most active in the upper 8cm but proceeds at very slow rates below this depth. The rapid formation of more than 90% of pyrite in the uppermost 2 cm which corresponds to about 3 years of sediment deposition allows the use of pyrite isotopic composition for tracing changing diagenetic conditions. Sediment profiles of the sulfur isotopic composition of pyrite reflect present-day higher rates of bacterial sulfate reduction and lower rates of sulfide oxidation, and record a profound change in the diagenetic cycling of sulfur in the contaminated sediments coincident with urban and industrial development of the St. Andrew Bay area.

Lacustrine sedimentary organic matter records of Late Quaternary paleoclimates

Identification of the sources of organic matter in sedimentary records provides important paleolimnologic information. As the types and abundances of plant life in and around lakes change, the composition and amount of organic matter delivered to lake sediments changes. Despite the extensive early diagenetic losses of organic matter in general and of some of its important biomarker compounds in particular, bulk identifiers of organic matter sources appear to undergo minimal alteration after sedimentation. Age-related changes in the elemental, isotopic, and petrographic compositions of bulk sedimentary organic matter therefore preserve evidence of past environmental changes.

Lipid biogeochemistry of surface sediments in the Lower Saxonian Wadden Sea, northwest Germany, and the effect of the strong winter 1995–1996

Ice rafting in the Lower Saxonian Wadden Sea, northwest Germany, in winter 1995–1996 had a dramatic effect on surface sediments. At the beginning of a longer-term biogeochemical study, the investigated area (six sampling sites) comprised beds of the mussel Mytilus edulis which disappeared under the severe weather conditions. Large amounts of sediment were eroded and redeposited. Evidence for emplaced biodeposits was given by TOC measurements and molecular distribution profiles of n- and iso/anteiso-fatty acids and of sterols. Absolute concentrations of fatty acids and sterols in 1996 were lower than in 1995, despite a higher organic carbon content in 1996 (>1.4% compared to 0.8%). This indicates that redeposition was accompanied by a decrease in the labile organic matter fraction by oxidation. After removal of the mussels, the sedimentary organic matter composition was nearly uniform at all sampling sites. This was manifested in n-fatty acid distribution patterns, relative proportions of algal to terrigenous and bacterial acids, cholestanol/cholesterol ratios and sterol distributions. An enormous diatom bloom of Coscinodiscus concinnus in spring 1996 caused an increase in the amounts of n-C 14 and n-C 16 fatty acids in the surface sediments. These fatty acids then dominated the distribution patterns of almost all samples investigated.

Enzymatic activity, bacterial distribution, and organic matter composition in sediments of the ross sea (Antarctica)

Enzymatic activities of aminopeptidase and beta-glucosidase were investigated in Antarctic Ross Sea sediments at two sites (sites B and C, 567 and 439 m deep, respectively). The sites differed in trophic conditions related to organic matter (OM) composition and bacterial distribution. Carbohydrate concentrations at site B were about double those at site C, while protein and lipid levels were 10 times higher. Proteins were mainly found in a soluble fraction (>90%). Chloropigment content was generally low and phaeopigments were almost absent, indicating the presence of reduced inputs of primary organic matter. ATP concentrations (as a measure of the living microbial biomass) were significantly higher at site B. By contrast, benthic bacterial densities at site C were about double those at site B. Bacterial parameters do not appear to be "bottom-up controlled" by the amount of available food but rather "top-down controlled" by meiofauna predatory pressure, which was significantly higher at site B. Aminopeptidase and beta-glucosidase extracellular enzyme activities (EEA) in Antarctic sediments appear to be high and comparable to those reported for temperate or Arctic sediments and characterized by low aminopeptidase/beta-glucosidase ratios (about 10). Activity profiles showed decreasing patterns with increasing sediment depth, indicating vertical shifts in both availability and nutritional quality of degradable OM. Vertical profiles of aminopeptidase activity were related to a decrease in protein concentration and/or to an increase in the insoluble refractory proteinaceous fraction. The highest aminopeptidase activity rates were observed at site C, characterized by much lower protein concentrations. Differences in EEA between sites do not seem to be explained by differences in the in situ temperature (-1.6 and -0.8 degreesC at sites B and C, respectively). Aminopeptidase activity profiles are consistent with the bacterial biomass and frequency of dividing cells. Enzyme substrate affinity was generally dependent upon substrate concentrations. EEA, normalized to bacterial numbers, indicated specific activities comparable to those reported for equally deep sediments at temperate latitudes. Vertical patterns of specific enzymatic activity appeared to be controlled by chloroplastic pigment concentrations that accumulate in the deeper sediment layers. The overall conclusion from the analysis of EEA in Antarctic sediments is that enzyme-dependent transformations of OM proceed at rates similar to those measured in temperate environments. Protein carbon potentially liberated by aminopeptidase activities (12.597 to 26.190 mg of C m-2 day-1) indicates that the whole protein pool could be mobilized within 1.3 to 17 h. Carbohydrate carbon mobilization (773 to 2,552 mg of C m-2 day-1) is sufficient to turn over the carbohydrate pool within 16 to 20 h. Such rates are 6 to 45 times higher than fluxes of particulate organic proteins and carbohydrates, indicating an "uncoupled hydrolysis" by the Antarctic benthic assemblages, in which bacteria appear to be able to rapidly exploit episodic OM pulses.

The origin and cycling of particulate and sedimentary organic matter and nitrate in Lake Superior

The elemental and isotopic composition of water column particulate and sedimentary organic matter and nitrate in Lake Superior was determined to assess the origin and cycling of these materials. The δ 15 N and δ 13 C of sedimentary organic matter and suspended particles at three stations were consistent with an origin primarily from autochthonous production. The δ 15 N of seston was controlled by a balance between the isotope effects associated with nitrate uptake and microbial degradation. The ratio of chlorophyll fluorescence to light hindrance (100-transmittance) was used in this study as an indication of the relative composition of recently produced photosynthetic vs. refractory and non-photosynthetic materials (such as bacteria or microzooplankton). Chlorophyll fluorescence to light hindrance (CF:LH) ratios were greatest within the region of the thermocline at the shallow station and lowest in the near bottom waters of the unstratified deep station. These changes in CF:LH indicated a predominance of recently produced photosynthetic material in surface stratified waters and an increase in refractory or non-photosynthetic material at the deepest station relative to the shallow station. Seston at the deepest station was characterized by the highest δ 15 N value of the three stations suggesting that degradation, bacterial growth, and/or an enrichment in microzooplankton resulted in an increase in the 15 N content of seston. Seston at the shallowest station was characterized by high CF:LH ratios and low δ 15 N values suggesting a greater relative contribution of labile material and an influence of an isotope effect during nutrient assimilation. Suspended particles in the benthic nepheloid layer were characterized by marked depletions in 13 C and 15 N relative to seston and sedimentary organic matter and indicated a unique origin for this material, possibly from recent primary production. The δ 15 N and δ 13 C of particles within the sediment boundary layer were intermediate those of sediments and nepheloid layer particles and were suggestive of an origin from the mixing of these two materials. Nitrate in Lake Superior was characterized by the lowest δ 15 N reported for an aquatic environment (average of −4.1‰). These low δ 15 N values and large input of water from precipitation directly to the lake surface suggests that much of the nitrate in Lake Superior is derived from atmospheric deposition.

The role of the southern Indian Ocean in the glacial to interglacial atmospheric CO 2 change: organic carbon isotope evidences

We use the carbon isotopic composition of sedimentary organic matter from marine cores as a proxy for paleoproductivity and paleo-dissolved carbon dioxide concentrations in surface waters of the Southern Ocean during the last 50,000 years. For the Holocene period, paleo-pCO 2 reconstructions between 44° and 55°S reflect the preindustrial values (270 μatm) and are in the variation range of modern ones. During the glaciation and deglaciation periods, pCO 2 was 50 to 100 μatm higher in surface water than in the atmosphere as recorded in the Vostok ice core. This suggests that the Southern Indian Ocean could have been a potential source of CO 2 for the glacial and deglacial atmosphere. These results, plus those of sedimentary organic carbon content suggest that the biological pump was off and unable to lower atmospheric CO 2 concentration. This indicates that a winter ice covered ocean and stratification of summer surface water caused a reduction of gas exchange with the atmosphere during that period.

Heterotrophic Nanoflagellates, Bacteria, and Labile Organic Compounds in Continental Shelf and Deep-Sea Sediments of the Eastern Mediterranean

The abundance and biomass of heterotrophic nanoflagellates were examined in continental and deep-sea sediments of the Cretan Sea (Eastern Mediterranean); at depths of 40 to 1540 m. Nanoflagellate distribution was compared to the composition of sedimentary organic matter and bacterial standing stocks to investigate trophic interactions and factors potentially affecting distribution. Quantitative estimates were obtained using different samplers for testing whether the box corer is as effective as the multiplecorer for bacterial and protozoan population estimates. The sediments of the deep Cretan Sea appeared extremely deficient in organic nutrients, and were composed mostly (more than 90%) of detritus. Labile organic compounds (such as lipids, proteins, and soluble carbohydrates) were present at extremely low concentrations, decreasing with water depth. Refractory and structural carbohydrates were the dominant biochemical class. The decrease in food quality with depth was associated with a strong decline of the RNA:DNA ratio. Benthic bacteria were constrained by food availability, and reacted to different organic matter inputs (especially total carbohydrates) at different depths. Large size bacteria were significantly correlated with the amounts of proteins and chloroplastic pigments. Heterotrophic nanoflagellate distribution in the continental shelf and deep-sea sediments of the Cretan Sea was controlled by available food sources (i.e., labile organic compounds and bacteria). Flagellate density was significantly correlated with the concentration of food indicators (chlorophyll a, soluble carbohydrates, and lipids), and to bacterial number and biomass. Despite the oligotrophy of the system, flagellate densities were high (40–119 × 103 g−1) and dominated by small cells (3 to 6 μm in length). These results, coupled with the high nanoflagellate to bacterial biomass ratio (up to 0.27 at 40 m depth), suggest that benthic nanoflagellates may contribute significantly to the direct transfer of detrital carbon and bacterial biomass to the metazoan component of the food web in the Cretan Sea.

Organic carbon δ13C variations in sedimentary rocks as chemostratigraphic and paleoenvironmental tools

Abstract Carbon isotopic variations in marine carbonate rocks and organic matter can provide important information on stratigraphic correlations and paleoenvironments. In particular, carbon isotopic variations in marine organic materials may yield information about the concentration of oceanic dissolved CO 2 and/or the physiology of the microalgal community. This paper reviews evidence from laboratory experiments and oceanographic studies aimed at understanding controls on carbon isotopic fractionation by marine phytoplankton. We also review factors affecting carbon isotopic compositions of bulk sedimentary organic matter and individual biomarker compounds. Guidelines for analysis of isotopic compositions of organic materials and interpretations of these results for use as chemostratigraphic and paleoenvironmental tools are presented. We recommend caution in interpreting carbon isotopic variations of sedimentary organic matter because no environmental factor explains adequately variations in the carbon isotopic composition of modern phytoplankton. Studies of ancient materials should include compound-specific isotopic analyses of known phytoplankton biomarkers or organism-specific isotopic analyses with bulk isotopic results to assess the proportions of allochthonous organic materials, the degree of reworking and the extent of thermal degradation to confirm any trends present in δ-values of bulk sedimentary organic matter.

Is there evidence for a substantial contribution of prokaryotic biomass to organic carbon in Phanerozoic carbonaceous sediments?

The role of prokaryotes (bacteria and archaea) in producing biomass and reworking settling and buried organic matter is of paramount importance since it exerts a major impact on the composition of sedimentary organic matter and, ultimately, petroleum. This key role of prokaryotes, in combination with a biomarker fingerprint often dominated by prokaryotic molecular fossils (e.g. hopanoids), has led to the assumption that prokaryotic biomass is a substantial source for organic carbon in carbonaceous sediments. However, biosynthesising and reworking organic matter does not necessarily imply sourcing organic matter by the prokaryotes, and the abundance of biomarkers does not always reflect relative contributions to sedimentary organic matter. In this paper the existing evidence for a substantial prokaryotic biomass contribution to sedimentary organic carbon (OC) is critically evaluated. This shows that despite the often prominent presence of prokaryotic biomarkers in sediments, evidence for a substantial prokaryotic contribution to sedimentary OC is generally lacking. This is demonstrated by: (i) the application of the carbon isotopic mass balance principle in various sedimentary settings; (ii) an alternative explanation for the proposed dominant role of heterotrophic reworking in the Oxford Clay; (iii) a summary of the effects of bacterial oxidation in organic matter-rich turbidites; and (iv) recent data showing that the evidence for recalcitrant carbon in prokaryotes is scant. Although these observations do not rule out substantial contributions of prokaryotic carbon to OC in Phanerozoic carbonaceous sediments, this remains to be demonstrated and the final answer awaits the development of new technologies to full characterise sedimentary OC.

Differential contribution of bacteria to sedimentary organic matter in oxic and anoxic environments, Santa Monica Basin, California

Two sediment box-cores in the Santa Monica Basin (SMB), one from the depocenter (900 m water depth) with anoxic bottom water and one from the periphery (840 m water depth) with oxic bottom water, were comparatively studied to assess the role of bottom water oxygen conditions to organic matter preservation and the differential contribution of bacteria to sedimentary organic matter accumulation and composition. Fatty acids from the sediment extracts were analyzed for their abundance and carbon isotopic compositions. Molecular and isotopic evidence indicates that most of the short-chain FAs from the SMB sediment extracts are sourced by bacteria as a result of intense microbial recycling and resyntheses of organic matter, especially in the aerobic environment at the periphery site. Long-chain fatty acids are probably derived from terrestrial sources and partially sourced by microbial processes. Very depleted δ 13C values (−30 to −45‰) of bishomohopanoic acids and triglycerides suggest the presence of chemoautotrophic bacteria in the anoxic water column and in the sediments. There is a much greater contribution from anaerobic microbial biomass and chemoautotrophic bacteria in the depocenter sediments relative to the periphery sediments. In addition, organic matter exhibits bulk and molecular characteristics of enhanced preservation under the condition of anaerobic decomposition at the depocenter site, especially at the sediment/water interface and the first couple of centimeters. Together, enhanced preservation with the addition of microbial biomass could explain the higher organic accumulation rates and H/C ratios in the basin depocenter and the periphery sediments and the trend of decreasing δ 13C values of bulk organic matter in the depocenter sediments. This study has implications for understanding the role of anoxia and associated bacterial processes in controlling the geochemical characteristics ancient organic carbon-rich sediments deposited under oxic and anoxic conditions.

Deposition of Organic Matter in the Norwegian-Greenland Sea during the Past 2.7 Million Years

Variations in the amount and composition of sedimentary organic matter in glacial and interglacial deposits of the last 2.7 myr correlate with late Cenozoic climatic and oceanographic changes in the Norwegian-Greenland Sea. These variations are predominantly caused by the changing supply of terrestrial and reworked organic matter. The highest amounts of terrestrial organic particles (macerals) and of reworked coal clasts in glacial and early deglacial diamictons are closely related to glacial erosion of Mesozoic strata that crop out along the Scandinavian Shelf. The first occurrence of coal clasts at 2.53 myr demonstrates an initial advance of continental ice margins to these source areas. The establishment of anoxic conditions at the sea floor during diamicton deposition probably reflects an increased vertical flux of labile organic matter due to lithogenic adsorbtion followed by an almost complete mineralization at the water/sediment interface. The content of marine organic matter was continuously low over the past 2.7 myr except for past interglacial highstands (i.e., isotopic event 5.5.1), suggesting persistent diagenetic degradation of the labile organic fraction. The marine organic matter exclusively consists of residual dinoflagellate cysts and fragments of them.

A three-year time series of elemental and biochemical composition of organic matter in subtidal sandy sediments of the Ligurian Sea (northwestern Mediterranean)

Variations in organic matter composition and microphytobenthic biomass were examined in the surface sandy sediments at a water depth of 10 m in the Gulf of Marconi (NW Mediterranean Sea) over a three year period. Seasonal changes in elemental (organic C and total N) and biochemical (lipids, proteins, carbohydrates) composition of sediment organic matter as well as Chla were assessed in order to provide information about the origin and fate of sedimentary organic matter, the contribution of microphytobenthic biomass, seasonal and interannual variations of food quantity and quality, and factors related to food availability. Data obtained in this three-year study revealed that organic matter determined with a muffle furnace is clearly an overestimate of the organic content of the sediment and is thus of little significance for benthic ecologists studying community dynamics in relation to food availability. Labile organic matter, utilized to estimate the food potentially available for benthic consumers, accounted for only a small percentage (on average less than 10%) of total organic C. The highest labile fraction was observed in spring, whereas minima were recorded in winter. Analysis of elemental and biochemical composition of organic matter showed an inverse relationship between amount of organic matter and its potential availability to consumers; small quantities of high-quality organic matter were replaced by large quantities of refractory material. The labile portion was mostly microphytobenthic (65% of the labile carbon). Protein: carbohydrate ratios were low and confirmed the role of proteins as a potentially limiting factor for consumers. Significant differences in nutritional quality of the sediment organic matter were observed from year to year, changes due to the increase in specific labile compound content.

Seasonal and inter-annual variation of bacteria in a seagrass bed of the Mediterranean Sea: relationship with labile organic compounds and other environmental factors

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 09:17-26 (1995) - DOI: https://doi.org/10.3354/ame009017 Seasonal and inter-annual variation of bacteria in a seagrass bed of the Mediterranean Sea: relationship with labile organic compounds and other environmental factors Danovaro R, Fabiano M Seasonal and inter-annual variations in density and biomass of benthic bacteria were examined in the surface sediments of a Mediterranean seagrass bed [Posidonia oceanica (L.) Delile] in the Gulf of Marconi (northwestern Mediterranean Sea) from January 1990 to January 1992. Bacterial parameters were compared to changes in elemental (organic C and total N) and biochemical (lipids, proteins, carbohydrates) composition of sediment organic matter, as well as to photosynthetic pigments. Bacterial density and biomass were high and exhibited marked seasonal variations with highest values in late spring. Bacterial biomass was positively correlated with temperature and negatively correlated with chlorophyll a content. By contrast no correlations were found with the bulk of organic carbon or nitrogen. In the year-to-year comparison the seasonal patterns of bacterial density were similar. However, bacterial parameters showed significant inter-annual variations. Such changes may be related to different organic matter composition and availability or to competition with microphytobenthos for inorganic nutrient uptake. These data evidence that, whereas temperature might be the most important factors controlling bacterial seasonality, the quality of organic matter (sedimentary protein content coupled to C/N values) may be an important factor for explaining the inter-annual variations. Benthic bacteria . Posidonia oceanica . Labile organic compounds Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 09, No. 1. Publication date: April 28, 1995 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 1995 Inter-Research.