Particulate Organic Matter Samples Research Articles

Drifting along: using diatoms to track the contribution of microbial mats to particulate organic matter transport in a glacial meltwater stream in the McMurdo Dry Valleys, Antarctica.

Flow pulses mobilize particulate organic matter (POM) in streams from the surrounding landscape and streambed. This POM serves as a source of energy and nutrients, as well as a means for organismal dispersal, to downstream communities. In the barren terrestrial landscape of the McMurdo Dry Valleys (MDV) of Antarctica, benthic microbial mats occupying different in-stream habitat types are the dominant POM source in the many glacier-fed streams. Many of these streams experience daily flow peaks that mobilize POM, and diatoms recovered from underlying stream sediments suggest that mat-derived diatoms in the POM are retained there through hyporheic exchange. Yet, 'how much' and 'when' different in-stream habitat types contribute to POM diatom assemblages is unknown. To quantify the contribution of different in-stream habitat types to POM diatom assemblages, we collected time-integrated POM samples over four diel experiments, which spanned a gradient of flow conditions over three summers. Diatoms from POM samples were identified, quantified, and compared with dominant habitat types (i.e., benthic 'orange' mats, marginal 'black' mats, and bare sediments). Like bulk POM, diatom cell concentrations followed a clockwise hysteresis pattern with stream discharge over the daily flow cycles, indicating supply limitation. Diatom community analyses showed that different habitat types harbor distinct diatom communities, and mixing models revealed that a substantial proportion of POM diatoms originated from bare sediments during baseflow conditions. Meanwhile, orange and black mats contribute diatoms to POM primarily during daily flow peaks when both cell concentrations and discharge are highest, making mats the most important contributors to POM diatom assemblages at high flows. These observations may help explain the presence of mat-derived diatoms in hyporheic sediments. Our results thus indicate a varying importance of different in-stream habitats to POM generation and export on daily to seasonal timescales, with implications for biogeochemical cycling and the local diatom metacommunity.

Source apportionment of suspended particulate organic matter in a shallow eutrophic lake of Southwest China using MixSIAR model

Eutrophication has become prominent in many lakes of the world, resulting in a continuous rise of suspended particulate organic matter (POM) that can be of vital consequence for water quality and carbon cycling. While the accumulation and decomposition of POM can enhance nutrient cycling at a rate depending on the property of lake-water organic carbon, it is important to identify the source of suspended POM for evaluating eutrophication and carbon burial. Here we analyzed the distribution of stable isotopic ratios (δ13C and δ15N) and the carbon/nitrogen ratio (C/N) in suspended POM in a shallow eutrophic lake of subtropical China, for apportioning the POM sources using Bayesian mixing model. Specifically, we collected seasonal samples of lake-water POM (>0.7 μm) from 20 sites in Yilong Lake from September 2020 to June 2022, as well as six types of modern samples covering the main carbon sources in the catchment. The spatial surveys showed that in the dry year (low water level) the δ13C of POM was more enriched in the western basin than those in the central and eastern basins, while the seasonal surveys showed the most enriched δ13C of POM in September. However, this spatio-temporal heterogeneity disappeared in the wet year (high water level). It was also found that the δ13C, δ15N and C/N values of POM varied significantly with those of phytoplankton across sites and seasons. Application of the MixSIAR model further showed that phytoplankton was the predominant source of POM in Yilong Lake (79.8 ± 13.4 %), but with large annual difference, 68.9 ± 10.3 % in the dry year and 90.7 ± 3.7 % in the wet year. In comparison, the contribution of the other five endmembers was much more moderate. In particular, the proportion of allochthonous organic carbon in POM was relatively low, with terrestrial plants and soils contributing 6.1 ± 4.8 % and 4.3 ± 2.5 % of POM, respectively. Our quantitative evidence for the dominance of the autochthonous source (i.e., phytoplankton) in lake-water POM over time and space suggested a determining role of eutrophication in the composition of lake organic carbon. Therefore, the coupling of algal blooms with organic carbon cycling in inland waters can be enhanced with continuous catchment development and regional warming.

Thermal-gradient analysis of soil organic matter using an elemental analyser – A tool for qualitative characterization?

Analysing the thermal stability of soil organic matter (SOM) by heating soil material up to 900 °C and recording the time/temperature-dependent evolution of CO2, using an elemental analyser, may be an easily available tool to obtain information on the composition and stabilization of SOM. We studied four sample sets, including size fractions of mineral horizons, particulate organic matter (POM), soil aggregates, and thermally treated materials. All analyses were conducted with a thermal gradient of 70 K min−1, with i) stages of constant temperature at 400, 600 and 900 °C (‘DIN 19359′), ii) stages at 450, 600 and 900 °C (modified DIN method), or iii) continual heating. With both stage methods, the large majority of organic carbon (OC) was present in the least thermally stable fraction, thermally decomposing at T ≤400 or 450 °C, respectively. However, neither did these methods result in the same relative proportions of fractions of similar thermal stability, nor did they result in time/temperature-dependent CO2-evolution patterns characteristically reflecting the different sample sets. Thus, results of the stage-heating methods were operationally defined, as different SOM fractions in terms of composition or stabilization mechanism were combusted, but showing similar patterns. Nonetheless, most POM samples revealed less thermal stability than the other fractions with any method. Evaluation of CO2 development during continual heating provided more options, including T of maximum CO2 evolution, T50, that is, T at which 50% of CO2 has evolved, and peak deconvolution using Gauss functions. The mineral samples showed at least two maxima and variable T50 values up to 550 °C. Most POM fractions had lower T50, but not in general, possibly caused by the presence of black carbon or strongly complexing cations. The aggregate samples were mostly in between the mineral and POM samples. Irrespective of the method applied, direct unambiguous identification of samples in terms of composition or stabilization mechanisms was not possible. Nonetheless, we consider an elemental analyser a valuable tool to evaluate the thermal stability of large samples sets at relatively low cost and time effort. Particularly peak deconvolution from continual heating offers the option of quantifying OC fractions, differing in thermal stability.

Redox Properties of Peat Particulate Organic Matter: Quantification of Electron Accepting Capacities and Assessment of Electron Transfer Reversibility

AbstractPeat particulate organic matter (POM) has been hypothesized to act as a terminal electron acceptor (TEA) for anaerobic microbial respiration in northern peatlands, thereby lowering dissolved methane (CH4) concentrations in these systems. However, the redox properties of peat POM and an assessment of its redox state in situ remain missing, mainly due to the lack of an accurate analytical approach to quantify the number of electrons that are transferred to/from POM. Here, we first developed a new spectrophotometric method to quantify the number of electrons that are transferable to peat POM—denoted hereafter as electron accepting capacity (EAC)—by reacting POM with the reduced species of the probe compound 4,4’‐bipyridinium‐1,1'bis(2‐ethylsulfonate). Second, we used this analytical approach to quantify the EAC of an oxidized reference POM material (0.38 ± 0.039 mmol e−/g POM) and of redox‐preserved POM from three ombrotrophic bogs in Värmland, Sweden (0.17–0.24 mmol e−/g POM). These EAC values are substantially higher than previous estimates, likely reflecting kinetic artifacts in previous analytical methods. We further established reversible electron transfer to the oxic reference POM over a cycle of electrochemical reduction and O2 re‐oxidation. Finally, we determined that the EAC of two of the three bog POM samples that were collected anoxically (i.e., redox state‐preserved POM) increased by ∼0.09 mmol e−/g POM upon exposure to air for 8 days, demonstrating that the POM was indeed reduced in situ. Collectively, our results strongly support that POM acts as a sustainable TEA in northern peatlands, which may lower methane release from these systems.

Mercury biomagnification in a Southern Ocean food web.

Biomagnification of mercury (Hg) in the Scotia Sea food web of the Southern Ocean was examined using the stable isotope ratios of nitrogen (δ15N) and carbon (δ13C) as proxies for trophic level and feeding habitat, respectively. Total Hg and stable isotopes were measured in samples of particulate organic matter (POM), zooplankton, squid, myctophid fish, notothenioid fish and seabird tissues collected in two years (austral summers 2007/08 and 2016/17). Overall, there was extensive overlap in δ13C values across taxonomic groups suggesting similarities in habitats, with the exception of the seabirds, which showed some differences, possibly due to the type of tissue analysed (feathers instead of muscle). δ15N showed increasing enrichment across groups in the order POM to zooplankton to squid to myctophid fish to notothenioid fish to seabirds. There were significant differences in δ15N and δ13C values among species within taxonomic groups, reflecting inter-specific variation in diet. Hg concentrations increased with trophic level, with the lowest values in POM (0.0005±0.0002μgg-1 dw) and highest values in seabirds (3.88±2.41μgg-1 in chicks of brown skuas Stercorarius antarcticus). Hg concentrations tended to be lower in 2016/17 than in 2007/08 for mid-trophic level species (squid and fish), but the opposite was found for top predators (i.e. seabirds), which had higher levels in the 2016/17 samples. This may reflect an interannual shift in the Scotia Sea marine food web, caused by the reduced availability of a key prey species, Antarctic krill Euphausia superba. In 2016/17, seabirds would have been forced to feed on higher trophic-level prey, such as myctophids, that have higher Hg burdens. These results suggest that changes in the food web are likely to affect the pathway of mercury to Southern Ocean top predators.

Metagenomic Insights Into the Microbial Assemblage Capable of Quorum Sensing and Quorum Quenching in Particulate Organic Matter in the Yellow Sea.

Quorum sensing (QS) is a density-dependent communicating mechanism that allows bacteria to regulate a wide range of biogeochemical important processes and could be inhibited by quorum quenching (QQ). Increasing researches have demonstrated that QS can affect the degradation of particulate organic matter (POM) in the photic zone. However, knowledge of the diversity and variation of microbial QS and QQ systems in sinking POM is scarce. Here, POM samples were collected from surface seawater (SW), bottom seawater (BW), and surficial sediment (SS) in the Yellow Sea of China. 16S rRNA gene amplicon and metagenome sequencing were performed to analyze the community structure of particle-associated microorganisms and distribution of QS genes [acylated homoserine lactone (AHL) synthesizing gene luxI and AHL sensing gene luxR] and QQ genes (genes encoding for AHL lactonase and acylase) in POM. Shifting community structures were observed at different sampling depths, with an increase of microbial abundance and diversity from SW to BW. Along with the variation of microbial communities, the abundances of luxI and luxR decreased slightly but were restored or even exceeded when POM arrived at SS. Comparatively, abundances of AHL lactonase and acylase remained constant during the transportation process from SW to BW but increased dramatically in SS. Correlation tests indicated that abundances of luxI and luxR were positively correlated with temperature, while those of AHL acylase were positively correlated with depth, SiO42–, PO43–, and NO3–, but negatively correlated with temperature and pH. According to phylogenetic analyses, the retrieved QS and QQ genes are more diverse and distinctive than ever experimentally identified. Besides, the vertical transmission of QS and QQ genes along with POM sinking was observed, which could be one of the key factors leading to the prevalence of QS and QQ genes in marine ecosystems. Overall, our results increase the current knowledge of QS and QQ metabolic pathways in marine environment and shed light on the intertwined interspecies relationships to better investigate their dynamics and ecological roles in POM cycling.

The Lability and Source of Particulate Organic Matter in the Northern Gulf of Mexico Hypoxic Zone

AbstractDecomposition of particulate organic matter (POM) plays a key role in the formation of hypoxia in subsurface waters of coastal ocean, yet little is known about the lability and transformation of POM in the hypoxic zone. Suspended particles were collected from surface waters to overlying waters (~30 cm above the sediment‐water interface) along the shelf of northern Gulf of Mexico (nGOM) in late spring/early summer of 2010–2013. Total hydrolyzable amino acids (THAA) and pigments were measured in these particulate samples to trace organic matter lability. The degradation indices, derived from the THAA and chloropigments, were positively correlated with dissolved oxygen (DO) concentrations in the shelf region, suggesting that decomposition of POM contributed greatly to DO utilization. Bacterial degradation appears to be the major pathway for POM decomposition on both inner and mid shelves, while zooplankton grazing played a minor role. POM samples in the overlying water on the inner shelf were the most degraded from the THAA and pigment results, and they also had high C/N ratios (9–14) and depleted δ 13C values (−29‰ to −24‰), pointing to a source of terrestrial C3 plant material. This distinct terrestrial signal of POM in the overlying water suggests strong selective degradation of marine‐sourced organic matter, but how the terrestrial organic matter is settled to this layer and its ultimate fate remain unclear. Taken together, these data offer new angles looking into the lability and degradation pathways of POM, and mechanisms of hypoxia formation in coastal waters.

Linking regional shifts in microbial genome adaptation with surface ocean biogeochemistry.

Linking 'omics measurements with biogeochemical cycles is a widespread challenge in microbial community ecology. Here, we propose applying genomic adaptation as 'biosensors' for microbial investments to overcome nutrient stress. We then integrate this genomic information with a trait-based model to predict regional shifts in the elemental composition of marine plankton communities. We evaluated this approach using metagenomic and particulate organic matter samples from the Atlantic, Indian and Pacific Oceans. We find that our genome-based trait model significantly improves our prediction of particulate C : P (carbon : phosphorus) across ocean regions. Furthermore, we detect previously unrecognized ocean areas of iron, nitrogen and phosphorus stress. In many ecosystems, it can be very challenging to quantify microbial stress. Thus, a carefully calibrated genomic approach could become a widespread tool for understanding microbial responses to environmental changes and the biogeochemical outcomes. This article is part of the theme issue 'Conceptual challenges in microbial community ecology'.

Sample acidification has a predictable effect on isotopic ratios of particulate organic matter along the Chilean coast.

Stable isotopes of carbon and nitrogen have proved to be valuable tools for researchers working across the different subfields of ecology. However, the chemical pretreatment of samples prior to analytical determination of stable isotope ratios can influence the results, and therefore conclusions regarding the ecology of the taxon or system under study. Here, we determined the effect of vapor acidification with concentrated HCl on the δ13 C and δ15 N values of particulate organic matter (POM), which are commonly used as baselines for studies of trophic ecology, or to understand oceanographic patterns. Samples of marine POM were obtained along a large-scale latitudinal gradient (ca 3000 km) along the Chilean coast, along with a range of oceanographic variables thought to potentially influence inorganic carbon at each sampling location. A random subset of 50 samples was divided into two parts: one acidified by HCl fumigation treatment, and the other acting as a control. We compared paired differences in δ13 C and δ15 N values measured by continuous flow isotope ratio mass spectrometry and used a model selection approach to examine which oceanographic factor best explained shifts in values following acid treatment. Acidification resulted in statistically significant reductions in both δ13 C and δ15 N values, but the effect was relatively small. The model that best explained the differences between acidified and non-acidified δ13 C values included depth, salinity and sea-surface temperature at the sampling point. A regression of acidified on non-acidified δ13 C values shows that the treatment effect was strongest on samples more depleted in 13 C. The differences between δ13 C and δ15 N values in acidified and non-acidified samples are linear and predictable. This implies that the nature of the POM and its possible alteration during the acid treatment are important factors that support the reliable determination of the values of δ13 C and δ15 N of POM.

Hydrographic and Biological Impacts of a Glacial Lake Outburst Flood (GLOF) in a Patagonian Fjord

Glacial lake outburst floods (GLOFs) in Northern Patagonian Ice Field affecting the Baker River basin have increased their frequency in recent years. To evaluate the impact of a GLOF in the hydrography and biological components of the plankton in the Baker Fjord, we assessed the relative contributions of terrigenous versus marine plankton carbon sources to the particulate organic matter (POM) in the fjord before and after a GLOF in the austral summer 2014. We also evaluated whether terrestrial carbon brought into the fjord by the river may reach higher trophic levels via a deposit-feeding organism the juvenile pelagic Munida gregaria. Over a 10-day period, hydrographic profiles, water samples for POM, and zooplankton samples were collected daily from three stations and two depths along the fjord’s inner section. Samples of suspended POM and tissue from M. gregaria were analyzed for stable-isotope composition of carbon (δ13C). The GLOF arrival produced a thermal front in the fjord, followed by an oscillation of the pycnocline; an abrupt increase in the total organic carbon content of POM, which was attributed to terrestrial input; and a concurrent peak in the abundance of M. gregaria, suggesting an aggregation response to the GLOF. Understanding GLOF effects on local hydrography, productivity, and food web structure provides valuable insight on the potential responses of fjord ecosystems in general to climate change-induced variability. Given present climatic trends in high-latitude zones, more frequent GLOFs might be expected in Patagonian fjords and channels as well as in other high-latitude basins.

Carbon geochemistry of plankton-dominated samples in the Laptev and East Siberian shelves: contrasts in suspended particle composition

Abstract. Recent Arctic studies suggest that sea ice decline and permafrost thawing will affect phytoplankton dynamics and stimulate heterotrophic communities. However, in what way the plankton composition will change as the warming proceeds remains elusive. Here we investigate the chemical signature of the plankton-dominated fraction of particulate organic matter (POM) collected along the Siberian Shelf. POM (> 10 µm) samples were analysed using molecular biomarkers (CuO oxidation and IP25) and dual-carbon isotopes (δ13C and Δ14C). In addition, surface water chemical properties were integrated with the POM (> 10 µm) dataset to understand the link between plankton composition and environmental conditions. δ13C and Δ14C exhibited a large variability in the POM (> 10 µm) distribution while the content of terrestrial biomarkers in the POM was negligible. In the Laptev Sea (LS), δ13C and Δ14C of POM (> 10 µm) suggested a heterotrophic environment in which dissolved organic carbon (DOC) from the Lena River was the primary source of metabolisable carbon. Within the Lena plume, terrestrial DOC probably became part of the food web via bacteria uptake and subsequently transferred to relatively other heterotrophic communities (e.g. dinoflagellates). Moving eastwards toward the sea-ice-dominated East Siberian Sea (ESS), the system became progressively more autotrophic. Comparison between δ13C of POM (> 10 µm) samples and CO2aq concentrations revealed that the carbon isotope fractionation increased moving towards the easternmost and most productive stations. In a warming scenario characterised by enhanced terrestrial DOC release (thawing permafrost) and progressive sea ice decline, heterotrophic conditions might persist in the LS while the nutrient-rich Pacific inflow will likely stimulate greater primary productivity in the ESS. The contrasting trophic conditions will result in a sharp gradient in δ13C between the LS and ESS, similar to what is documented in our semi-synoptic study.

Influence of Major Storm Events on the Quantity and Composition of Particulate Organic Matter and the Phytoplankton Community in a Subtropical Estuary, Texas

Variations in the freshwater inflow regimes of estuaries due to perturbations, such as storm events, alter the source and composition of particulate organic matter (POM) and the phytoplankton community, which are key links in estuarine carbon and nitrogen cycling. To evaluate the impact of varying freshwater discharge on POM quantity and composition and the phytoplankton community, monthly samples of surface water were collected at four long-term monitoring stations from 2012-2015 in a subtropical estuarine system, the Mission-Aransas Estuary, Texas (USA). This system is characterized by a semiarid climate, drought, and sporadic precipitation events. We analyzed organic carbon and nitrogen content, carbon and nitrogen stable isotopes, and chlorophyll a and accessory pigments of the POM samples. Following a prolonged dry spell (2012-2015), consecutive major storm events in spring 2015 led to a dramatic freshening of the entire estuary. Large increases in particulate organic carbon (POC) concentrations and decreases in δ13C values of POC over several weeks following the storms suggest an increase in in-situ production at lower salinities as a result of increased freshwater inflow. These changes in bulk POM coincided with an increase in chlorophyll a concentration, further indicating a significant contribution by phytoplankton to the elevated POM. Concurrently, pigment biomarkers revealed a significant (10-fold) increase in the cyanobacterial pigment, zeaxanthin, and further 16S rRNA analysis showed that Cyanobium spp. was responsible for the observed bloom. The combination of environment conditions, including freshening, high temperature, and high nutrients, likely contributed to the cyanobacteria bloom. These results show that episodic rain events can substantially affect estuarine phytoplankton community composition, which impacts the available energy resources for secondary production and thus may have ecosystem-wide implications on productivity.

Identifying the drivers of the pelagic ecosystem of an oligotrophic bight (KwaZulu–Natal, South Africa) using stable isotopes (δ13C, δ15N) and C : N ratio analysis

The international literature highlights the importance of understanding the role and fate of terrestrial organic matter (OM) in the marine system, yet little is known about this on the oligotrophic east coast of South Africa. Within the Bight, three important processes make OM biologically available: (1) a topographically induced upwelling cell (north), (2) a Cyclonic Eddy (south), and (3) riverine input. We hypothesise that: (i) these OM sources differ substantially in their isotope ratios, and (ii) zooplankton at the inshore region has an isotope signature linked to localised fluvial fluxes. Particulate OM samples were collected at five locations along the Bight and in three main estuaries during austral summer and winter seasons in 2010 and subjected to isotopic and mixing model analysis. Riverine input played an important role in this ecosystem inshore during the wet season. Only marine OM played a role as a biological driver offshore and at all sites during the dry season. Although there were indications that the upwelling cell did occur, it was not confirmed as active in either season. The current study provides valuable insights into the Bight's biological functioning and principle insights into the fate of terrestrial organic carbon on the east coast of southern Africa.

Dinosterol δD values in stratified tropical lakes (Cameroon) are affected by eutrophication

In freshwater settings, dinosterol (4α,23,24-trimethyl-5α-cholest-22E-en-3β-ol) is produced primarily by dinoflagellates, which encompass various species including autotrophs, mixotrophs and heterotrophs. Due to its source specificity and occurrence in lake and marine sediments, its presence and hydrogen isotopic composition (δD) should be valuable proxies for paleohydrological reconstruction. However, because the purity required for hydrogen isotope measurements is difficult to achieve using standard wet chemical purification methods, their potential as a paleohydrological proxy is rarely exploited. In this study, we tested δD values of dinosterol in both particulate organic matter (POM) and sediments of stratified tropical freshwater lakes (from Cameroon) as a paleohydrological proxy, the lakes being characterized by variable degrees of eutrophication. In POM and sediment samples, the δD values of dinosterol correlated with lake water δD values, confirming a first order influence of source water δD values. However, we observed that sedimentary dinosterol was D enriched from ca. 19 to 54‰ compared with POM dinosterol. The enrichment correlated with lake water column conditions, mainly the redox potential at the oxic–anoxic interface (Eh OAI). The observations suggest that paleohydrologic reconstruction from δD values of dinosterol in the sediments of stratified tropical lakes ought to be sensitive to the depositional environment, in addition to lake water δD values, with more positive dinosterol δD values potentially reflecting increasing lake eutrophication. Furthermore, in lake sediments, the concentration of partially reduced vs. non-reduced C34 botryococcenes, stanols vs. stenols, and bacterial (diploptene, diplopterol and ββ-bishomohopanol) vs. planktonic/terrestrial lipids (cholesterol, campesterol and dinosterol) correlated with Eh OAI. We suggest using such molecular proxies for lake redox conditions in combination with dinosterol δD values to evaluate the effect of lake trophic status on sedimentary dinosterol δD values, as a basis for accurately reconstructing tropical lake water δD values.

Isotope and fatty acid trends along continental shelf depth gradients: Inshore versus offshore hydrological influences on benthic trophic functioning

Anthropogenic activities and land-based inputs into the sea may influence the trophic structure and functioning of coastal and continental shelf ecosystems, despite the numerous opportunities and services the latter offer to humans and wildlife. In addition, hydrological structures and physical dynamics potentially influence the sources of organic matter (e.g., terrestrial versus marine, or fresh material versus detrital material) entering marine food webs. Understanding the significance of the processes that influence marine food webs and ecosystems (e.g., terrestrial inputs, physical dynamics) is crucially important because trophic dynamics are a vital part of ecosystem integrity. This can be achieved by identifying organic matter sources that enter food webs along inshore–offshore transects. We hypothesised that regional hydrological structures over wide continental shelves directly control the benthic trophic functioning across the shelf. We investigated this issue along two transects in the northern ecosystem of the Bay of Biscay (north-eastern Atlantic). Carbon and nitrogen stable isotope analysis (SIA) and fatty acid analysis (FAA) were conducted on different complementary ecosystem compartments that include suspended particulate organic matter (POM), sedimentary organic matter (SOM), and benthic consumers such as bivalves, large crustaceans and demersal fish. Samples were collected from inshore shallow waters (at ∼1m in depth) to more than 200m in depth on the offshore shelf break. Results indicated strong discrepancies in stable isotope (SI) and fatty acid (FA) compositions in the sampled compartments between inshore and offshore areas, although nitrogen SI (δ15N) and FA trends were similar along both transects. Offshore the influence of a permanently stratified area (described previously as a “cold pool”) was evident in both transects. The influence of this hydrological structure on benthic trophic functioning (i.e., on the food sources available for consumers) was especially apparent across the northern transect, due to unusual carbon isotope compositions (δ13C) in the compartments. At stations under the cold pool, SI and FA organism compositions indicated benthic trophic functioning based on a microbial food web, including a significant contribution of heterotrophic planktonic organisms and/or of SOM, notably in stations under the cold pool. On the contrary, inshore and shelf break areas were characterised by a microalgae-based food web (at least in part for the shelf break area, due to slope current and upwelling that can favour fresh primary production sinking on site). SIA and FAA were relevant and complementary tools, and consumers better medium- to long-term system integrators than POM samples, for depicting the trophic functioning and dynamics along inshore–offshore transects over continental shelves.

Photochemical production of singlet oxygen from particulate organic matter.

Dissolved organic matter is established as one of the most relevant photosensitizers in aquatic environments, producing singlet oxygen (1O2) alongside other photochemically produced reactive intermediates. While the production of 1O2 from DOM has been well studied, the relative importance of particulate organic matter (POM) to the overall 1O2 production is less well understood. POM is known to play an important role in pollutant fate through the sorption and transport of hydrophobic pollutants. If POM is directly involved in 1O2 production, sorbed molecules would be expected to undergo enhanced photodegradation. In this work, synthetic POM was prepared by coating silica particles with commercial humic acid. The photochemical behavior of these POM samples was compared to dissolved commercial humic acids (DOM). Suspended natural sediment was also studied to test the environmental relevance of the synthetic POM model. Synthetic POM particles appear to simulate well the 1O2-production of suspended sediment. The 1O2 concentrations experienced by POM-sorbed probe molecules was up to 30% higher than experienced by DOM-sorbed ones, even though the aqueous concentration of 1O2 in irradiated POM suspensions was much lower than the analogous DOM solutions. These results were interpreted with a reaction-diffusion model, which suggested that the production rate of 1O2 by POM is lower than DOM, but the loss of 1O2 from the POM-phase is also lower than DOM. Based on the experimental results of this study, calculations were conducted to estimate the impact of removing POM on 1O2-mediated processes. These calculations indicate that compounds with a log Koc value near 4 will be most affected by removal of POM and that the magnitude of the effect is proportional to the fraction of the total organic matter represented by POM. This study demonstrates that particles can play an important role in the degradation of organic compounds via aquatic photochemistry.

Effect of reservoir‐derived plankton released from dams on particulate organic matter composition in a tailwater river (Uji River, Japan): source partitioning using stable isotopes of carbon and nitrogen

ABSTRACTWe estimated the contribution of reservoir‐derived plankton released from an upstream dam to particulate organic matter (POM) relative to terrestrial allochthonous (fallen leaves) and instream autochthonous (downstream epilithic algae) sources in the Uji River, Japan, to investigate the influence of the reservoir plankton on downstream POM composition. Four types of POM such as suspended fine, suspended coarse, benthic fine and benthic coarse POM were collected and then analyzed using two types of mixing models combining δ13C–δ15N: a standard linear model (SLM) and a concentration‐weighted model (CWM), which are compared with the microscopic examination. Results demonstrate that the three trophic sources were isotopically distinct, and all POM samples are plotted inside the mixing triangle defined by the three end members in the δ13C–δ15N biplot. SLM underestimated the terrestrial source contribution and overestimated that of the reservoir plankton, suggesting that despite popular application of the SLM method, CWM was more appropriate for the source partitioning of riverine POM particularly in the case where large differences in source concentrations of C and N are present. Reservoir plankton contribution was highest in suspended fine POM (S‐FPOM), accounting for 47%. Nitrogen in S‐FPOM was found to be supplied mostly by the reservoir plankton (68%). These results collectively suggest that reservoir plankton from dams can greatly influence downstream S‐FPOM composition and play a role in supplying nitrogen to tailwater ecosystems. Our findings on the riverine POM source partitioning in a tailwater channel should be useful in assessing downstream heterotrophic food webs and nutrient transport in response to the plankton‐derived POM originating from upstream reservoirs. Copyright © 2013 John Wiley & Sons, Ltd.

Compound-specific stable carbon isotope ratios of phenols and nitrophenols derivatized with N,O-bis(trimethylsilyl)trifluoroacetamide

We developed an analytical method for measuring compound-specific stable carbon isotope ratios (δ13C) of phenols and nitrophenols in filter samples of particulate organic matter. The method was tested on 13 phenols derivatized with N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA), together with four nonphenolic compounds. The data obtained by our method required two specific corrections for the determination of valid δ13C values: (1) for nitro compounds, the routine correction with use of m/z 46 for the contribution of 12C17O16O molecules) to m/z 45 was modified due to impact of NO2 on the m/z 46 trace, and (2) for the derivatized phenols, measured δ13C values were corrected for the shift in δ13C due to the addition of carbon atoms from the BSTFA moiety. Analysis of standard-spiked filters showed that overall there was a small compound-dependent bias in the δ13C values: the average bias±the standard error of the mean of −0.21±0.1‰ for the standard compounds tested, except 3-methylcatechol, methylhydroquinone, 4-methyl-2-nitrophenol, and 2,6-dimethyl-4-nitrophenol, whereas the average biases±the standard errors of the mean for those were +1.2±0.3‰, +1.2±0.2‰, −1.2±0.2‰, and −1.4±0.5‰, respectively, when the injected mass of a derivatized compound exceeded 15ngC. In situations where such small biases and uncertainties are acceptable, the method described here could be used to obtain valuable information about δ13C values. We also analyzed a real filter sample to demonstrate the practical applicability of the method.

Stable isotope composition of suspended particulate organic matter in twenty reservoirs from Guangdong, southern China: Implications for pelagic carbon and nitrogen cycling

We studied seasonal variations in the carbon and nitrogen stable isotopes (δ13C and δ15N) of particulate organic matter (POM) in the surface water of 20 man-made reservoirs in southern China during March, August and December 2010. These reservoirs are located from subtropical to tropical region, varied in trophic states and were influenced by several types of human activities. The geomorphometric and biogeochemical gradients in tropical/subtropical regions are complicated and poorly understood because of low variation in temperature and high variation in hydrological processes. The POM samples were collected from all the reservoirs to assess the seasonal variation patterns of δ13CPOM and δ15NPOM. Variation in δ13CPOM followed seasonal thermal and hydrological cycles. By contrast, δ15NPOM did not seasonally differ, which may have been complicated by phytoplankton assimilation of N originating in agricultural waste entering the reservoirs and of atmospheric nitrogen during the wet season as well as an increased relative contribution of animal waste in the reservoirs in the dry season. Within each sampling period, nutrient availability is more important than thermal and solar radiation inputs in the summer, and these physical drivers are more important during the dry and cold winter month in controlling δ13CPOM and δ15NPOM. On an annual basis, trophic states (total P, total N and chlorophyll a) are the primary drivers for the changes in both δ13CPOM and δ15NPOM across reservoirs. When the seasonal effect is removed using annual averages from each reservoir, we found that latitude, trophic states, pH, rainfall, water temperature, reservoir age, catchment area to reservoir area (CA:RA) ratio and together explained about 80% of the variance in both δ13CPOM and δ15NPOM. Our findings also suggest that the trend of δ15NPOM is less predictable than δ13CPOM. The consistent 15N depletion and enrichment of POM in different reservoirs point to the importance of different anthropogenic sources of nitrogen in the reservoirs.

Characterization of particulate organic matter in the water column of the South China Sea using a shotgun proteomic approach

We characterized particulate organic matter (POM) collected from both the surface (41 m and 200 m) and mesopelagic layers (500 m and 1000 m) in the western South China Sea. By using a shotgun proteomic approach, a total of 3035 proteins matching one or more peptides were detected from four POM samples, 505 of which were identified as high‐confidence proteins matching two or more peptides. Cyanobacteria was the largest contributor throughout the water column, while crustaceans and dinophytes were the two major groups contributing to the particulate proteins in the POM collected from 200 m. Subcellular locations and biological processes of particulate proteins varied significantly between the 41‐m and 200‐m layers: photosynthesis‐associated proteins were highly abundant in the 41‐m layer while tubulins and actins accumulated in the midwaters, especially at the 200‐m layer. Porins, adenosine triphosphate synthases, nutrient transporters, molecular chaperones, and ectoenzymes were frequently detected in the POM samples and presented different distribution patterns within the water column, revealing complex biological processes at the different water layers and/or during the sinking of POM. The sources of surface and midwater particulate proteins are different, and the cellular metabolism, generation of energy, and transport processes in POM are attenuated rapidly down ocean water column. Zooplankton fecal pellet packages and membrane encapsulation might play important roles in protecting particulate proteins from degradation.