Composition Of Particulate Organic Matter Research Articles

Analytical pyrolysis of suspended particulate organic matter from the Black Sea water column

Abstract The chemical composition of suspended particulate organic matter (SPOM) in the Black Sea water column has been characterized by pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS). The results are discussed in terms of the hydro-chemical properties of the water column. Phytoplankton cultures grown in the lab also were analyzed to provide reference information for the euphotic zone SPOM. The composition of SPOM in the Black Sea displays important vertical variations dependent on specific distinct biochemical processes in the water column. Identical pyrolysis markers indicative for proteins, lipids, carbohydrates and chlorophylls were obtained from SPOM in the euphotic zone of the Black Sea and the phytoplankton cultures. Nevertheless, the relative contribution of lipids and carbohydrates to Black Sea SPOM was higher than in phytoplankton cultures. The lipid to protein ratio throughout the water column seems to determine the C/N ratio of SPOM. Below the oxycline, the relative contribution of proteins to the SPOM pool increased while the lipid content of the SPOM decreased. The protein composition changed substantially in the suboxic/anoxic transition zone, as expressed by the increase in pyrrole/indole ratio. Elemental sulfur was observed in particulates suspended at the upper anoxic zone, and the relative intensity of the S8 peak varied regionally with more intense multi-peaks in SW shelf-break station, in agreement with the lateral flux of O2. Sulfur-containing organic compounds (e.g. thiophenes), including organic polysulphides (e.g. 1,2-dithiole-3-thiones) were detected in some deep anoxic samples, indicating that sulphurization of organic matter may be active in the water column.

Distribution and sources of particulate organic matter in the water column and sediments of the Fly River Delta, Gulf of Papua (Papua New Guinea)

Suspended particles from surface and bottom waters and surficial sediments from the seabed were collected throughout the Fly River subaqueous delta region during the monsoon season in January 2003. Because of the unusually low river discharge associated with a strong El Niño, water-column salinities were relatively high (10 to 32) throughout most of the delta, with brackish salinities (<10) only measured within the main distributary channel of the Fly River. The concentration and composition of particulate organic matter (POM) in these samples showed distinct spatial differences and marked contrasts between water column and seabed samples. Overall, relatively low concentrations of total suspended solids (TSS), particulate organic carbon (POC) and particulate nitrogen (PN) were measured in surface (27 ± 50, 0.83 ± 1.2, and 0.05 ± 0.05 mg/L, respectively) and bottom (400 ± 743, 7.1 ± 7.3, and 1.7 ± 7.2 mg/L, respectively) waters throughout the delta. Particles in both surface and bottom waters displayed elevated organic carbon contents (%OC > 4 wt.%), relatively high organic carbon:nitrogen molar ratios (OC:N > 20 mol:mol) and quite depleted stable isotopic compositions of organic carbon (δ 13C OC < −27‰). In contrast, surface sediments in the seabed displayed spatially uniform compositions that were characterized by markedly lower %OC contents (1.1 ± 0.8 wt.%), lower OC:N ratios (17 ± 9 mol:mol) and relatively enriched δ 13C OC compositions (−25.5 ± 1.1‰). The radioisotopic compositions of OC from a selected set of seabed samples (Δ 14C OC of −408 ± 82‰) indicate OM in surface sediments is old ( 14C ages of 2800 to over 6000 years before present). The ratios of organic carbon to mineral surface area exhibited by these sediments are within the typical (mono-layer equivalent) ranges characteristic of shelf sediments and do not reflect the preferential removal of terrigenous OM. Overall, these compositions indicate that while water-column POM appears to be derived mainly from terrigenous vascular plant debris and riverine/estuarine phytoplankton, the source of most of the sedimentary POM is aged, soil OC ultimately derived from C3 vegetation. We speculate that the concentrations and sources of suspended POM in the water column of the Fly River delta region reflect the conditions of low river discharge, low wave energy and neap tides encountered at the time of sampling. In contrast, POM compositions in surface sediments are consistent with the transport and deposition of old, mineral-bound OC most likely eroded from the upland regions of the Fly River watershed, which is characterized by steep slopes, high precipitation and C3 tropical forests and grasslands.

Use of stable isotopes to investigate dispersal of waste from fish farms as a function of hydrodynamics

Stable isotopes were used to examine differential effects of fish farm waste on the water column and sediments. To achieve this objective, we chose 3 marine fish farms located along the coast of Sicily (Mediterranean Sea) as point-source disturbances, and a control area. The hypothesis that carbon and nitrogen isotope composition of particulate (POM) and sedimentary (SOM) organic matter varied with increasing distance (from cages to 1000 m) was tested at 3 levels of hydrodynam- ics: low (mean velocity of current (MVC) ~12 cm s -1 ), intermediate (MVC ~22 cm s -1 ), and high (MVC ~40 cm s -1 ). Different isotopic signals from allochthonous (fish waste) over natural (phytoplankton, terrigenous, and sand microflora) inputs allowed identification of the 'spatial effect regime' of fish farming. The increasing water current velocities seem to proportionally enlarge the relative area of influence of the cages, particularly on sediments. At low hydrodynamics, an increasing contribution of terrigenous signals was inferred: POM and SOM showing a depleted gradient of C (ranging from -22.0 to -24.0‰) and N (from 5.0 to 2.0‰). At an intermediate hydrodynamic level, C and N showed a slight increase in waste contribution, particularly in POM (δ 15 N from 2.6 to ~4.0‰). At high hydro- dynamics, an enriching isotopic gradient (δ 15 NPOM-SOM from 1.8 to 4.6‰) suggested a notable contribution of fish waste. Accordingly, the dispersal of waste from the cages seemed to be related to movements at the bottom of the water column, confirming the recently identified role played by resuspension movements.

Dissolved and particulate organic matter source-age characterization in the upper and lower Chesapeake Bay: A combined isotope and biochemical approach

In order to characterize the sources and ages of organic matter contributing to river and estuarine outflow waters, the present study investigated δ14C and δ13C signatures of the major operationally defined biochemical classes of ultrafiltered dissolved organic matter (UDOM) in conjunction with lipid biomarker and elemental compositions of UDOM and suspended particulate organic matter (POM) in the Chesapeake Bay. Freshwater (Susquehanna River) UDOM was dominated by a molecularly uncharacterized (MUC) fraction, followed by total carbohydrate (TCHO), total hydrolysable amino acid (THAA) and total lipid (TLE) components. In contrast, UDOM at the bay mouth (salinity ~22–24) was comprised mainly of TCHO, followed by MUC, THAA, and TLE. The δ14C and δ13C signatures of both UDOM and its major biochemical classes indicate that Susquehanna DOM is derived in part from old allochthonous terrestrial sources, whereas young marine sources dominate at the bay mouth. In contrast to the other biochemical classes, lipophilic DOM at both sites was very old (~5,000–7,000 years B.P.). In addition, factor analysis of lipid biomarker compounds revealed unique signatures for the UDOM and POM pools that imply disparate source and/or recycling properties as well as potential influences due to physical partitioning. Lipid biomarker compounds showed that although autochthonous riverine/estuarine sources dominated both the UDOM and POM pools, terrigenous lipids were elevated in the Susquehanna during high flow conditions. The presence of lipid biomarkers diagnostic of “fresh” algal material in UDOM further suggested its greater reactivity than POM. The observed biochemical and lipid biomarker compositions and isotopic signatures of UDOM and POM are consistent with previous findings suggesting that these two major organic matter pools have dissimilar reactivities and cycling times, and they derive from comparatively unique source-age materials in rivers and estuaries.

Temporal variability of stable carbon and nitrogen isotopic composition of size-fractionated particulate organic matter in the hypertrophic Sumida River Estuary of Tokyo Bay, Japan

To clarify the major factors that control stable carbon and nitrogen isotopic compositions (δ 13C and δ 15N) of suspended particulate organic matter in a hypertrophic estuary, seasonal variation in concentrations and stable isotopic compositions of particulate organic carbon and nitrogen (POC, PN), chlorophyll a content (Chl) and dry weight of suspended solid (SS) was studied in the Sumida River estuary of Tokyo Bay. Particulate material was fractioned into four size-fractions (3–20, 20–53, 53–250, 250–1000 μm). At the lower-estuarine (LE) site, the finer size fraction (<20 μm) was predominant (>80% Chl) during the autumnal bloom (Aug–Sep 2003). The middle size fractions (20–53 and 53–250 μm) however increased during the spring bloom (50–80% Chl; Feb–Apr 2004), reflecting difference in taxonomic composition of phytoplankton. At the upper-estuarine and the riverine sites, the <20 μm fraction was most abundant throughout the survey period. δ 13C POC and δ 15N PN varied between −32 and −16‰ and between −8 and +12‰, respectively. Relationships of δ 13C POC with POC/Chl and POC/PN ratios could be explained by assuming two end members: allochthonous (terrestrial) POC with a δ 13C close to −26.5‰, and autochthonous (planktonic) POC whose δ 13C was variable and apparently correlated with δ 13C of ambient DIC (−10 to 0‰) and phytoplankton abundance (Chl). δ 15N PN was apparently controlled by water temperature and NH 4 + concentration. The variability of δ 13C and δ 15N was different between size fractions: the apparent dependence of δ 13C on Chl as well as that of δ 15N on water temperature was significantly stronger for the middle size fractions (mainly diatoms) than the finer fraction (mainly microflagellates), suggesting different physiological response between taxa. The second objective of this study was to evaluate contribution of terrestrial organic carbon (TPOC) to the POC size fractions by using δ 13C of isolated chlorophyll a at LE site as surrogate for the autochthonous δ 13C end member. During the autumnal bloom, TPOC was 23–36% on average for the middle size fractions (20–53 and 53–250 μm), and ≥59% for the finest (3–20 μm) and coarsest (250–1000 μm) fractions. During the spring bloom, the average contribution of TPOC was generally low (≤21%) except for the finest fraction (36%). Negative correlation between salinity and calculated TPOC suggested that transport of terrestrial organic carbon to this estuary was controlled principally by the river discharge.

Bloom dynamics in early opening waters of the Arctic Ocean

We measured the isotopic composition and accumulation of particulate organic matter (POM) and the uptake of carbon (C) and nitrogen (N) in an early bloom of the most productive recurring polynya of the Arctic Ocean. The estimated compensation irradiance at the onset of the bloom was similar to the average for the North Atlantic Ocean, implying that shallow mixing was of critical importance for the bloom’s early initiation. Planktonic POM had a much lower d 13 C than ice POM, suggesting that ice-algae contributed little to the pelagic biomass. The overall isotopic fractionation of pelagic N during bloom development was consistent with in situ diatom growth under saturating irradiance and limiting NO . Soon after the ice cleared, rapid physiological changes induced an order of 2 3 magnitude increase in the C and NO uptake capacity of diatoms, leading to very high f ratios (NO uptake : total 2 2 3 3 N uptake). Most of the NO taken up appeared in the POM, so that little net release of reduced N occurred during 2 3 the period of active growth. Given the tight coupling between photosynthesis and NO uptake under N limitation, 2 3 the magnitude of primary production in the Arctic Ocean is expected to respond to changes in N supply.

Sources of estuarine dissolved and particulate organic matter: A multi-tracer approach

Assessing the fate of organic matter (OM) in estuaries is challenging due to its numerous sources, diagenetic processing, and physical and biogeochemical sinks. In order to evaluate differences in the sources and potential transformations of OM, particulate organic matter (POM) and high molecular weight dissolved organic matter (HMW DOM (⩾3 kDa)) were characterized along the York River estuary (Virginia, USA) using lipid biomarker, elemental ratios, and stable isotope ( δ 13C and δ 15N) analyses. Isotopic signatures of HMW DOM from the freshwater site were distinct from both its POM counterpart and HMW DOM from higher salinity regions of the estuary. Lipid compound classes were depleted in HMW DOM relative to POM. Saturated fatty acids (FA) dominated the dissolved organic pool while monounsaturated and polyunsaturated FA were the primary FA classes in POM. Concentrations of sterols diagnostic of allochthonous (i.e., plant/freshwater algae) sources were enriched in HMW DOM relative to the POM at higher salinities. Ternary plots based on FA biomarkers show that POM is characterized by contributions from polyunsaturated FA, labile compounds representing contributions from “fresh” phytoplankton/zooplankton sources. In contrast, FA in HMW DOM reflect bacterial and vascular plant signatures. Disparities in the composition of HMW DOM and POM likely reflect differences in the dominant biological sources, different susceptibilities to transformation and/or differential influences from abiotic processes (sorption–desorption, flocculation, etc.). Thus, the physical associations of OM (particulate vs. dissolved) may be a fundamental control on both the distribution and biogeochemical processing of OM whereby terrigenous DOM may be selectively exported while terrigenous POM is retained within the estuary. These observations have implications for the selective processing of different sources and forms of OM in rivers and estuaries prior to export to the coastal ocean.

Fractionation of stable isotopes in the Arctic marine copepod Calanus glacialis: Effects on the isotopic composition of marine particulate organic matter

Fractionation of δ 13C and δ 15N between food, consumer, and faecal pellets was studied in the Arctic marine copepod Calanus glacialis Jaschnov, fed with isotopically distinct algal monocultures. Temporal variations in δ 13C and δ 15N of copepods that were fed ice algae and phytoplankton followed those of a control group consisting of starved animals. There were no significant trends in the δ 13C and δ 15N values of copepods that were starved for 42 days, suggesting that the isotopic composition of non-lipid body tissues is unaffected by the metabolic processes during prolonged periods of starvation. The stable isotopic composition of starved copepods therefore seems to reflect food consumed during the previous period of feeding and growth. Faecal pellets produced by feeding copepods were depleted in 13C and 15N by 6.3–11.2‰ and 0.7–9.1‰, respectively, relative to the food ingested. These results indicate that faecal pellet production is an important pathway for the trophic fractionation of δ 13C, whereas other fractionation pathways, such as excretion of ammonia, may be relatively more important for δ 15N. The strong depletion of 13C in faecal pellets compared to the food suggests that grazing by herbivorous copepods on primary production adds to the variability of δ 13C in marine particulate organic matter.

Short-term variation in behavior of allochthonous particulate organic matter accompanying changes of river discharge in Ise Bay, Japan

The composition and behavior of allochthonous particulate organic matter (POM) in the northern part of Ise Bay, Japan were investigated to elucidate the short-term variation in POM accompanying changes in river discharge. The behavior of POM was significantly regulated by hydrographic conditions, but behavior was different in the upper layer versus the middle and lower layers. The former showed simple dynamics controlled by the river plume, while the latter showed complex dynamics because of changes in river discharge and subsequent variation in estuarine circulation. During normal discharge, the contribution of riverine materials to POC in the surface water within the bay was negligible because most riverine organic matter is deposited before flowing into the bay. During high discharge, on the other hand, the contribution of riverine organic matter to total POM increased to 50% at ∼10 km from the river mouth. Though riverine organic matter loads increased, the total amount of POC decreased around the river mouth due to flushing of phytoplankton. After river discharge, the contribution decreased rapidly. The behavior of POM in the middle and lower layers differed from that in the surface layer. At normal discharge, the influence of riverine organic matter was weak. During high discharge, high flooding temporarily weakened the bottom inflow, resulting in heavier riverine organic matter distributed from the river mouth to mid regions within the bay in the lower layer. The maximum contribution of riverine organic matter to total POM was estimated to be ∼60% around 25 km from the river mouth. After high discharge, riverine POM in the lower layer was pushed to the bay head by enhanced estuarine circulation and was uplifted to the middle layer. The behavior of riverine POM dynamically changed in relation to river discharge, and exerted a significant influence on bottom water conditions in the bay.

TRACING RIVER INFLUENCES ON PHYTOPLANKTON DYNAMICS IN TWO LOUISIANA ESTUARIES

To evaluate effects of river inputs on estuarine phytoplankton, we compared particulate organic matter (POM) dynamics in two neighboring estuaries from the Missis- sippi River delta. The two estuaries, Barataria Basin and Breton Sound, have been isolated from the Mississippi River at their upstream ends for most of the last century due to levee construction, but for the last 13 years, Breton Sound has been reconnected to the Mississippi River via a gated diversion at Caernarvon, Louisiana. Thus, Barataria Basin was the ref- erence estuary and Breton Sound was the river-impacted estuary. We focused on POM because it was relatively easy to collect and analyze, and because POM dynamics gave a good ecosystem-level overview of phytoplankton responses to riverine nutrient forcing and hydrologic forcing. A combination of C:N and PO13C analyses indicated that most POM was phytoplankton with smaller contributions of sediment. Estuarine concentrations of particulate organic carbon (POC) and nitrogen (PON) were typically higher than end- member freshwater or marine inputs, indicating the importance of within-system phyto- plankton growth likely fueled by rapid nutrient regeneration in these warm, shallow and well-mixed estuarine systems. Isotopic analyses generally showed typical estuarine gra- dients in the reference Barataria Basin estuary, but the river-influenced upper Breton Sound had elevated PO'3C and PO'5N values useful for following the spatial extent of river impacts. Rapid river-assisted flushing in Breton Sound probably prevented widespread development of algal blooms in response to introductions of the nutrient-rich river water, but study of some isolated lakes suggested that when longer residence times apply, introduction of river water to coastal estuaries can lead to eutrophic conditions and in some cases, harmful algal blooms. Overall, routinely monitoring amounts and isotopic compositions of POM proved an effective way to trace linkages between river inputs and downstream estuarine phyto- plankton dynamics.

Impact of natural (storm) and anthropogenic (trawling) sediment resuspension on particulate organic matter in coastal environments

In order to assess the impact of natural and anthropogenic sediment resuspension on quantity, biochemical composition and bioavailability of particulate organic matter (POM), two field investigations were carried out in two shallow coastal areas of the Mediterranean Sea. In the Gulf of Lions, we investigated the impact of a storm resuspension of sediment, whereas in the Thermaikos Gulf we investigated the impact of bottom trawling. Resuspension in the Gulf of Lions determined the increase of sedimentation rates, modified the composition of the organic fraction of settling particles and decreased the labile fraction of POM, as indicated by a drop in the enzymatically hydrolysable amino acid fraction. The increase in the refractory fraction, following short-term storm-induced resuspension, increased also the contribution of glycine and decreased the contribution of aspartic acid contents to the total amino acid pools. Trawling activities in Thermaikos Gulf determined a significant increase in suspended POM concentrations and important changes in its biochemical composition. After trawling, the protein to carbohydrate ratio decreased (as a result of a major input of sedimentary carbohydrates at the water–sediment interface) and the fraction of enzymatically hydrolysable biopolymeric C decreased by ≈30%, thus reducing the bioavailability of resuspended organic particles. Results of the present study indicate that changes in suspended POM, induced by storms and trawling activities, can have similar consequences on benthic systems and on food webs. In fact, the potential benefit of increased organic particle concentration for suspension feeders, is depressed by the shift of suspended food particles towards a more refractory composition.

Origin, composition and nutritional quality of particulate matter at deep-sea hydrothermal vents on Axial Volcano, NE Pacific

Deep-sea hydrothermal vent communities are supported by local microbial chemolitho- autotrophic production. While nutritional symbiotic associations between microbial primary producers and their metazoan hosts are well characterised, food sources used by the diverse and abundant non symbiont-containing vent species remain poorly known. Vent suspension- and deposit-feeders are usually considered as primary consumers directly relying on free-living microbial primary produc- tion, but other sources of particulate organic matter (POM) may also be part of their diet. We investi- gated the origin, composition and nutritional quality of POM at Axial Volcano (NE Pacific) vents, using microscopic observations, stable isotopic and biochemical analyses. A positive correlation between the stable carbon isotopic composition of POM and that of vent fluid dissolved inorganic car- bon (DIC) indicates that the bulk of vent particulate organic carbon is derived from the local chemolithoautotrophic fixation of vent DIC. Low estimates of bacterial and total microbial carbon might reflect a rapid turnover of free-living bacterial biomass in the vent ecosystem. The low micro- bial fraction, together with the presence of abundant debris, point to the existence of a large detrital fraction in the POM pool. This implies that the previously largely overlooked detrital compartment may be a significant part of the diet of consumers, and that organic matter recycling may be a major process in these ecosystems.

Lipids and their oxidation products as biomarkers for carbon cycling in the northwestern Mediterranean Sea: results from a sediment trap study

Classical lipid biomarkers (sterols, fatty acids and pigments) and their photo-oxidation and autoxidation products were measured in sediment trap samples collected during the summer of 1998 at the DYFAMED time-series station in the northwestern Mediterranean Sea to investigate changes in phytoplanktonic composition and alteration of particulate organic matter (POM). The study period covered a transitional situation from bloom event to oligotrophic conditions. At the beginning of the study, the phytoplankton community appeared to be mainly composed of diatoms, prymnesiophytes, pelagophytes and dinoflagellates. During the transition to the oligotrophic period, the contribution of small cells (pico- and nanoplankton) increased. Quantification of a “pool” of oxidation products resulting from photo-oxidation and autoxidation of phytoplanktonic sterols, monounsaturated fatty acids and chlorophyll phytyl side-chain provided very useful indications concerning the alteration state of POM during the investigated period. This “pool,” composed of some oxidation products which are very sensitive (but labile) indicators and of others which are less sensitive (but more refractory), allowed us to obtain a large range of diagnostic information about organic matter behaviour. Thanks to these indicators, visible light-induced and autoxidative degradative processes, which have been virtually ignored to date owing to the lack of suitable tracers, may be easily distinguished and appear to act intensively in the northwestern Mediterranean Sea.

Carbon, nitrogen, and sulfur isotope changes and hydro-geological processes in a saline lake chain

The ionic concentrations,conductivity and pH of water in the Lake Chany complex in West Siberian Russia change from the mouth to the interior of the lake. This difference is indicative of marked evaporation of lake water from the closed water body system in the dry climate of Western Siberia. The carbon isotope composition of particulate organic matter (POM, composed mainly of phytoplankton) clearly changes, along with the pH of the water, reflecting the concentration of dissolved CO2. Carbon and nitrogen isotope signatures of Chironomus plumosus larvae, a benthic invertebrate that may feed on bulk lake sediment, systematically increase, along with those of POM and sediment organic matter (SOM), through the lake chain. Both sulfate-sulfur and nitrogen isotope compositions of the POM and SOM increase with distance from the estuary into the Lake Chany complex. Heavier sulfur and nitrogen isotope recycling from the sediment, caused by microbial sulfate reduction and denitrification, respectively, may have led to the increased sulfate-sulfur and nitrogen isotope compositions of the POM and SOM.

Green urchin as a significant source of fecal particulate organic matter within nearshore benthic ecosystems

The role of green sea urchin Strongylocentrotus droebachiensis as a source of fecal particulate organic matter (POM) for the benthic nearshore ecosystems has been studied over a 3.5-month period. Three macroalgae were tested as food sources: Alaria esculenta, Laminaria longicruris and Ulvaria obscura. Urchins were fed ad libitum with either a single alga species or a mixture of all three algae. Consumption and defecation rates were determined as well as the feces/alga ratio in term of biomass and biochemical composition. Consumption rate increased exponentially with urchin size and also varied with alga species. In the single alga trial, consumption rate was higher for both brown algae ( Laminaria and Alaria) compared to Ulvaria. Urchins feeding on the mixture of algae maintained their total ingestion rate (sum of the three algae) at the same level to those feeding on a single alga diet. The mixed algae trial showed that urchins clearly preferred Laminaria (72% of total ingestion) over Alaria (22%) and Ulvaria (6%). The defecation rate was tightly correlated with the food consumption rate and thus increased with urchin size. On average, 75% of the ingested algal biomass was released as fecal POM. The percentage of food defecated changed with alga species, with the highest value for Alaria (81%) and the lowest for Laminaria (67%). The percentage of food defecated by urchins feeding on the mixture of algae was generally comparable to those feeding on single alga diet. Biochemical composition (in soluble carbohydrates, proteins and lipids) of urchin fecal POM reflected that of the algae content. From 40% to 80% of macronutrients in algal food persisted in fecal matter. This proportion varied with the alga species and macronutrient considered. This study shows that the green sea urchin plays a significant role in the production of POM within nearshore benthic ecosystems, and it is a potentially nutritious food source for detritivores.

“Cellular” vs. “detrital” POM: a preliminary study using fluorescent stains, flow cytometry, and mass spectrometry

The mass of nonliving particulate organic matter (POM) in marine waters can be significantly greater than that in living cells, yet very little is known about the composition and ultimate fate of this “detrital” POM. In recent marine chemistry studies, flow cytometry has been used to provide further insights into the composition, source, and fate of chlorophyll-fluorescing POM (which includes living algal cells) vs. detrital POM within natural waters. In this study, we extend such morphological characterization of POM beyond the use of chlorophyll autofluorescence and particle size or light scatter. Instead, nucleic acid staining with SYTO-13 and flow cytometry sorting were used to identify and isolate “cellular” and “detrital” subclasses of POM from samples collected in the Lafayette River and the Elizabeth River (subestuaries in the lower Chesapeake Bay). The resulting subclasses, as well as aliquots of unsorted POM sample, were then analyzed by direct temperature-resolved mass spectrometry. We show that “cellular” and “detrital” subclasses differ significantly from each other, and that detrital POM appears chemically distinct from the bulk POM. We then suggest further applications for flow cytometry approaches in aquatic organic biogeochemistry.

Distributions, speciation and stable isotope composition of organic matter in the southeastern Bering Sea

Seawater samples were collected from the southeastern Bering Sea for determinations of dissolved (DOC, <0.7 μm), colloidal (COC, 1 kDa–0.7 μm) and particulate organic carbon (POC, >0.7 μm), dissolved organic nitrogen (DON), particulate nitrogen (PN), and dissolved carbohydrate (DCHO) species, as well as stable isotopic composition ( δ 13C and δ 15N) of suspended particulate organic matter (POM). DOC concentrations ranged from 61 to 92 μM (average 76±9 μM) with increasing concentrations from outer shelf to inner shelf along 166°W. DOC concentrations were significantly inversely correlated with salinity, indicating that the DOC distribution was predominantly controlled by water mixing although primary production is the ultimate source of organic matter. DON concentrations averaged 6.4±0.9 μM, ranging from 4.3 to 8.4 μM, resulting in an average DOC/DON ratio of 12±2 (ranging from 8 to 15). Total dissolved carbohydrates (TDCHO) comprised 13±3% of bulk DOC, with 35±15% of TDCHO in the polysaccharide and 65±15% in the monosaccharide fractions. Suspended POM had an average C/N ratio of 7.4±1.1, significantly lower than the average DOC/DON ratio (12±2). There was a significant correlation between chlorophyll- a and POC concentrations. However, Chl- a concentration was not closely coupled to DOC abundance. On average, POC comprised 14±5% of the total organic carbon (TOC) pool in the water column, and COC comprised 34±8% of the TOC, leaving 52±5% of the TOC in the <1 kDa low-molecular-weight dissolved phase. Values of δ 13C and δ 15N in the suspended POM ranged from −27.1‰ to −23.4‰ (average −24.5±0.9‰) and from 2.8‰ to 9.9‰ (average 6.0±1.9‰), respectively. Interestingly, particulate δ 15N values were negatively correlated with nitrate (NO 3) concentrations at low NO 3 concentrations (≤5 μM) but positively correlated with NO 3 concentrations when NO 3 concentrations were >5 μM, indicating the effects of nutrient drawdown in surface waters and diagenesis below the euphotic zone on the isotope fractionation. POM containing lighter C and N isotopes appeared to be decomposed preferentially during their transport from upper water column to bottom sediments.

Stable Carbon and Nitrogen Isotopic

The perennially ice-covered lakes in the McMurdo Dry Valleys, Antarctica, are part of the coldest and driest ecosystem on earth. To understand lacustrine carbon and nitrogen cycling in this end-member ecosystem, and to define paleolimnological proxies for ice-covered lakes, we measured the stable carbon and nitrogen isotopic composition of particulate organic matter (POM) and benthic organic matter (BOM) within the lakes of Taylor Valley. The δ13C compositions of seasonally ice-free edges of the lakes (moats) are enriched relative to under-ice organic matter. Thus, the organic carbon isotopic composition of buried sediments may be a proxy for sample position within the lake. In the moats, δ13C values are governed by limited CO2diffusion across benthic cyanobacterial cell membranes. During a high glacial melt (2001–2002) season, both δ13CPOM and δ13CBOM in the moats were more depleted than during previous low melt years. We propose that this occurred in response to higher [CO2](aq) and/or reduced growth rates resulting from turbidity-induced light limitation. Though moats and under-ice environments are usually poorly connected, during the 2001–2002 season, the enrichment of the δ13CPOM values at 6 m depth in the stream-proximal sites relative to deep-profile sites implies enhanced connectivity between these environments. The δ13C compositions of BOM and POM profiles in Lake Hoare and Lake Fryxell indicate that these lakes are dominated by benthic productivity. In contrast, in Lake Bonney, the similarity of the δ13C values of BOM and POM indicates the pelagic component dominance in the carbon cycle.

Soil organic matter fractions as early indicators for carbon stock changes under different land-use?

With respect to carbon sequestration in soil, attempts have been made to identify soil organic matter (SOM) fractions that respond more rapidly to changes in land-use than bulk SOM, which could thus serve as early indicators for the overall stock change. We used a combination of physical fractionation (size and density separation) and chemical characterisation (C-to-N ratios, CuO lignin signature, 13C NMR spectroscopy) to identify sensitive SOM fractions in an agricultural system with sandy dystric cambisols in Bavaria, Germany, 7 years after a land-use change. Land-use types included long-term arable land and grassland, and conversion from one system to the other. Soil carbon and nitrogen contents in 0–3 cm increased from 14 to 39 mg organic carbon g −1 soil, and from 1.7 to 3.9 mg nitrogen g −1 soil in the following order: permanent arable, conversion grassland to arable, conversion arable to grassland, and permanent grassland. Wet sieving and ultrasonic dispersion with 22 J ml −1 released <5% and 60% to 80%, respectively, of the amount of particles >20 μm relative to complete dispersion. The most sensitive fraction, with respect to land-use, was SOM in the fraction >20 μm not released after sequential wet sieving and ultrasonic dispersion. In contrast, the proportion of free light (wet sieving, density <1.8 g cm −3) and occluded light (ultrasonic dispersion with 22 J ml −1, <1.8 g cm −3) particulate organic matter (POM) showed no clear response to land-use. The structural composition of POM indicated its vegetation origin with a selective enrichment of lignin and a loss of O-alkyl C relative to its plant precursors. Decomposition of the occluded light POM was only slightly advanced relative to the free light POM. In mineral fractions <20 μm, SOM was significantly more transformed than in the coarse fractions, as shown by NMR spectroscopy; however, it revealed no specific land-use pattern. An exception to this was the proportion of O-alkyl C in the clay fraction, which increased with SOC content. Ratios of alkyl to O-alkyl C in mineral fractions <20 μm differentiated samples gave a better differentiation of samples than the C-to-N ratios. We conclude that neither free nor occluded light POM are appropriate early indicators for changes in land-use at the investigated sites; however, total SOM, its distribution with depth, and SOM allocated in stable aggregates >20 μm were more sensitive.

Content and composition of free and occluded particulate organic matter in a differently textured arable Cambisol as revealed by solid‐state 13C NMR spectroscopy

AbstractThe composition of functional light soil organic matter pools of arable Cambisols with a gradient in clay content was investigated. Soil texture differences originate from increasing loess admixture to the parent material (coarse‐grained tertiary sediments). Using density fractionation in combination with ultrasonic dispersion, two types of particulate organic matter (POM) were obtained: (1) free POM and (2) POM occluded in soil aggregates. Both POM fractions were analyzed by elemental analysis (C, N) and CPMAS 13C NMR spectroscopy. With increasing clay content the amount of organic carbon stored in the occluded POM fraction increased considerably, whereas the amounts of free POM were not related to the soil clay content. With increasing soil clay contents increasing proportions of O‐alkyl C and decreasing proportions of aryl C were found for both POM fractions. The occluded POM fraction showed a higher degree of degradation as indicated by lower amounts in O‐alkyl carbon. A lower degree of POM degradation was associated with higher clay contents. Higher soil clay contents promoted the conservation of POM with a low degree of alteration. This effect of soil texture was found to be highly significant when the aryl C : O‐alkyl C ratio was used as indicator for POM decomposition rather than the alkyl C : O‐alkyl C ratio.