Particulate Organic Matter Pools Research Articles

Role of eddies on organic matter production and f-ratios in the Bay of Bengal

In order to examine the sources of nutrients and f-ratios in the cyclonic (CE) and anticyclonic eddy (ACE), the content and isotopic composition of carbon and nitrogen in suspended particulate organic matter (POM) were analyzed in one CE and two ACEs in the western Bay of Bengal (BoB). Relatively higher concentrations of Chlorophyll-a (Chl-a) and nutrients were observed in the CE and no-eddy (NE) regions than ACE region. Higher concentrations of particulate organic carbon (POC) and nitrogen (PN) were observed in the CE and NE than ACE in the BoB. Isotope mixing model (SIAR) suggested that POM pool mainly consists of marine plankton biomass (~80%) and contribution of terrestrial organic matter is insignificant (<20%). The depth of nitracline was shallower in the CE due to divergence while it was deeper in the ACEs due to convergence of upper ocean. The isotopic composition of nitrogen in the POM varied between 2.4% and 8.5‰ and relatively lower values were associated with deeper nitracline depth in the ACE and vice versa in the CE. This suggests that upward mixing of subsurface nitrate supported primary production in the CE and regenerated nitrogen in the ACE region. The estimated f-ratios using δ15NPOM and depth of nitracline were almost double in the CE (0.31 ± 0.04) and NE (0.28 ± 0.05) than ACEs (0.19 ± 0.09) in the BoB. Though upward mixing of subsurface low DO water intensified oxygen minimum zone (OMZ) below 75 m in the CE region, enhanced phytoplankton biomass and its export may further intensify. In contrast, convergence of DO rich surface water and low f-ratios in the ACE resulted in weaker OMZ. This study suggests that eddies significantly influence primary and export productions that may lead to modifications in intensity of OMZ in the BoB. >50 eddies form every year in the BoB and its impact on primary and export production leading to intensification/weakening of OMZ should be evaluated using both observations and models.

On the separation between inorganic and organic fractions of suspended matter in a marine coastal environment

A central aspect of coastal biogeochemistry is to determine how nutrients, lithogenic and organic matter are distributed and transformed within coastal and estuarine environments. Analyses of the spatio-temporal changes of total suspended matter (TSM) concentration indicate strong and variable linkages between intertidal fringes and pelagic regions. In particular, knowledge about the organic fraction of TSM provides insight to how biogenic and lithogenic particulate matter are distributed in suspension. In our study we take advantage of a set of over 3000 in situ Loss on Ignition (LoI) data from the Southern North Sea that represent fractions of particulate organic matter (POM) relative to TSM (LoI ≡ POM:TSM). We introduce a parameterization (POM-TSM model) that distinguishes between two POM fractions incorporated in TSM. One fraction is described in association with mineral particles. The other represents a seasonally varying fresh pool of POM. The performance of the POM-TSM model is tested against data derived from MERIS/ENVISAT-TSM products of the German Bight. Our analysis of remote sensing data exhibits specific qualitative features of TSM that can be attributed to distinct coastal zones. Most interestingly, a transition zone between the Wadden Sea and seasonally stratified regions of the Southern North Sea is identified where mineral associated POM appears in concentrations comparable to those of freshly produced POM. We will discuss how this transition is indicative for a zone of effective particle interaction and sedimentation.The dimension of this transition zone varies between seasons and with location. Our proposed POM-TSM model is generic and can be calibrated against in situ data of other coastal regions.

Carbon and Nitrogen Dynamics of Soil Organic Matter Fractions in Thickets and Intergrowth Areas of Sudanian Savannah Grasslands, Bondoukuy, Western Burkina Faso

Soil organic carbon (SOC) and total nitrogen (TN) play a central role in physico-chemical fertility of a soil, and thus promoting agricultural productivity. Yet little is known about SOC and TN dynamics in tropical ferruginous soils of Sub-Saharan Africa. In this study, thicket and intergrowth soil samples, under both cultivation and perennial grass fallow (Andropogon gayanus), were collected in Bondoukuy, western Burkina Faso. The samples were fractionated and their SOC and TN contents in six organo-mineral fractions were analyzed. Because of the high labile organic matter pools in coarse sand fractions, SOC (~630 &amp;micro;g C g &amp;ndash;1 soil) associated with these fractions appeared to be more accessible to soil microbes than recalcitrant and occluded pools (~440 &amp;micro;g C g &amp;ndash;1 soil) within the fine fractions of the fallow soils. The results also indicated that clay fractions are likely to represent a source of the available nitrogen to crop following long fallow periods (~20 years). In contrast, the differences in TN contents were not significant (p&amp;gt;0.05) between ploughed plots and young fallow lands (~10 years). The substantial decrease in C/N ratios from coarse particulate organic matter pools (C/N=68) to fine pools (C/N=10) suggested an increase in the SOC decomposition rate in the fine fractions. This indicates a substantial decrease in microbial activities following a reduction in particulate organic matter sizes. The SOC contents were relatively high in coarse (~930 &amp;micro;g C g &amp;ndash;1 soil) fractions of the thicket soils compared to those of the adjacent intergrowth soils (~620 &amp;micro;g C g &amp;ndash;1 soil). A similar SOC distribution pattern was also observed in fine fractions of the thicket and the intergrowth soils. Total nitrogen also exhibited a high distribution pattern in fine sand and very fine sand fractions. The findings of this study demonstrated that SOC and TN restoration in semi-arid tropical savannah soils is a function of particulate organic matter sizes, vegetation type and soil management practices.

Eutrophication Leads to Accumulation of Recalcitrant Autochthonous Organic Matter in Coastal Environment

AbstractAnthropogenic nutrient enrichment is changing the structure and the function of coastal ecosystems. These coastal zones are transitions between freshwater and marine systems where multiple biogeochemical processes remove, produce, and transform organic matter. The extent to which the coastal zone is merely a conduit for terrestrial (allochthonous) organic matter versus a distinct source of autochthonous organic matter fueled by eutrophication is unclear. To address this issue, we characterized the freshwater and marine dissolved organic matter (DOM) pools in a eutrophic estuary with a long water residence time (Roskilde Fjord, Denmark) over an annual cycle. We combined elemental, optical (absorbance and fluorescence), and isotopic analyses to obtain insight about the bulk properties of the DOM pool during this period. We also used sediment traps to analyze the changes related to the exchange of organic matter between the particulate organic matter and DOM fractions. The results showed that labile autochthonous DOM from in situ primary production was rapidly transformed to more recalcitrant DOM that accumulated in the estuary despite continuous exchange with the open sea. Also, parts of the particulate organic matter pool were degraded rapidly (within 24 hr) and transformed into the DOM pool. Accumulated DOM was characterized by relatively low molecular size and stable carbon isotopic value and by high protein‐like fluorescence. These results indicate that autotrophic material can be a major source of specific recalcitrant DOM in eutrophic coastal waters, contributing significantly to the flux of organic carbon to the ocean.

Physicochemical Organic Matter Stabilization across a Restored Grassland Chronosequence

Core Ideas • Twenty‐one years after cropland‐to‐grassland conversion, soil organic C (SOC) was within 5% of a new equilibrium but remained 64% of a never‐cultivated remnant. • Mineral‐associated and free particulate organic matter pools increased with time but microaggregate pools did not. • Ammonium oxalate‐extractable Fe was highly concentrated on microaggregate silt and clay. • Polyvalent cations and ammonium oxalate‐extractable Fe were not correlated to total SOC or soil N. In reconstructed grasslands, soil organic matter (SOM) is the largest CO2 and reactive N sink but SOM gains after reconstruction rarely achieve precultivation levels. Through a chronosequence of reconstructed grasslands 1 to 21 yr after establishment, we explored which physicochemical mechanisms protect accumulated soil organic C (SOC) and N from mineralization. After 21 yr, total SOC and soil N concentrations increased by 32 and 23%. The SOC concentration was within 5% of a new equilibrium but was 64% of a never‐cultivated remnant. Chemically stabilized C and N pools on free silt and clay surfaces increased with time. Coarse particulate organic matter C increased with time but accounted for &lt;12% of SOC. Microaggregate‐stabilized SOM did not change. The positive linear relationship between total SOC and free silt and clay C indicates that 21 yr after establishment, reconstructions have unsatisfied capacity for further SOM storage, despite proximity to a new SOC equilibrium. The accumulated C and N associated with free silt and clay suggest that ammonium oxalate‐extractable Fe (AmOx‐Fe) and polyvalent cation concentrations could be correlated with total C and N stocks. These promote SOM stabilization and were possibly affected by human activity before reconstruction. However, AmOx‐Fe and polyvalent cation concentrations were not associated with total SOC or soil N and could not explain the slowing SOM accumulation. Regardless of time since reconstruction, AmOx‐Fe was highly concentrated on microaggregate surfaces compared with other fractions and was positively associated with microaggregate C and N, suggesting a link between Fe and microaggregate stabilization.

Spatio-temporal variability in the sources of particulate organic carbon and nitrogen in a tropical Godavari estuary

In order to evaluate the impact of seasonal river discharge on the spatio-temporal variability in the major sources of particulate organic matter (POM) in the tropical Godavari estuary, India, a comprehensive study was conducted for one year from head to mouth of the estuary. Higher concentrations of both particulate organic carbon (POC) and nitrogen (PN) were observed during the peak discharge period followed by moderate and lean discharge periods associated with magnitude of discharge. POC:Chl-a ratio and isotopic composition of particulate organic carbon (δ13C) and nitrogen (δ15N) suggest that predominant contribution of OM is from in-situ production during the moderate discharge period associating with higher phytoplankton biomass. Contribution from in-situ sources dominates over terrestrial sources during the lean discharge period also. However, both terrestrial and in-situ sources contributed during the peak discharge period. Discharge mobilised significant amounts of terrestrial OM such as the debris of land plants, agricultural crops, forests and soils in the catchment during peak and moderate discharge periods. Strong stratification during the moderate discharge period enhanced phytoplankton production, mainly by freshwater algae. Dual isotope mixing model analysis revealed that freshwater phytoplankton contributed significantly to the POM pool (20–90%) during the moderate discharge period whereas contribution from terrestrial C3 plants (20–40%) dominated during the peak discharge period. Our study demonstrates that river discharge modifies content and source of OM to the estuary that may have significant impact of heterotrophic activity.

Legacy effects of long-term nitrogen fertilizer application on the fate of nitrogen fertilizer inputs in continuous maize

Nitrogen fertilizer management can impact soil organic C (SOC) stocks in cereal-based cropping systems by regulating crop residue inputs and decomposition rates. However, the impact of long-term N fertilizer management, and associated changes in SOC quantity and quality, on the fate of N fertilizer inputs is uncertain. Using two 15-year N fertilizer rate experiments on continuous maize (Zea mays L.) in Iowa, which have generated gradients of SOC, we evaluated the legacy effects of N fertilizer inputs on the fate of added N. Across the historical N fertilizer rates, which ranged from 0 to 269 kg N ha−1 yr−1, we applied isotopically-labeled N fertilizer at the empirically-determined site-specific agronomic optimum rate (202 kg N ha−1 at the central location and 269 kg N ha−1 at the southern location) and measured fertilizer recovery in crop and soil pools, and, by difference, environmental losses. Crop fertilizer N recovery efficiency (NREcrop) at physiological maturity averaged 44% and 14% of applied N in central Iowa and southern Iowa, respectively (88 kg N ha−1 and 37 kg N ha−1, respectively). Despite these large differences in NREcrop, the response to historical N rate was remarkably similar across both locations: NREcrop was greatest at low and high historical N rates, and least at the intermediate rates. Decreasing NREcrop from low to intermediate historical N rates corresponded to a decline in early-season fertilizer N recovery in the relatively slow turnover topsoil mineral-associated organic matter pool (0–15 cm), while increasing NREcrop from intermediate to high historical N rates corresponded to an increase in early-season fertilizer N recovery in the relatively fast turnover topsoil particulate organic matter pool and an increase in crop yield potential. Despite the variation in NREcrop along the historical N rate gradient, we did not detect an effect of historical N rate on environmental losses during the growing season, which averaged 34% and 69% of fertilizer N inputs at the central and southern locations, respectively (69 kg N ha−1 and 185 kg N ha−1, respectively). Our results suggest that, while beneficial for SOC storage over the long term, fertilizing at the agronomic optimum N rate can lead to significant environmental N losses.

Influence of river discharge on zooplankton diet in the Godavari estuary (Bay of Bengal, Indian Ocean)

In estuaries, detrital (i.e., non-living) organic matter (OM) contributes significantly to the particulate organic matter (POM) pool and we hypothesize that it may be a major source of estuarine zooplankton diet. To test this hypothesis, the isotopic composition of carbon (d13C) and nitrogen (d15N) of phytoplankton, zooplankton, and POM was assessed in the Godavari estuary (Bay of Bengal, Indian Ocean) during wet (November) and dry periods (January). As a result of higher riverine discharge, POM concentrations and values of the C/Chl-a ratio during the wet period were higher than those measured during the dry one. Relatively lower δ13CPOM values were observed during wet than dry period and contrasting to that was found for δ15NPOM. Detritus from fresh water algae and C3 plants contributed significantly to the POM pool during the wet and dry period, respectively. Based on isotopic mixing model, detrital OM and phytoplankton mostly characterized the POM pools during the wet and dry periods, respectively. Accordingly, our results suggest also that the zooplankton diet was mostly supported by detrital OM during the wet period and by both phytoplankton and detrital OM during the dry one. The zooplankton trophic level (TL, 2.7) during the wet period was relatively higher than that (1.9) during the dry one, suggesting a relative higher preference for detritus than phytoplankton during the wet period. The results of this study allowed us confirming that detrital OM can significantly support zooplankton production in the Godavari estuary.

Tree Mortality From Insect Infestation Enhances Carbon Stabilization in Southern Appalachian Forest Soils

AbstractForest insect and pathogen outbreaks may exacerbate anthropogenic climate change if they accelerate soil carbon loss to the atmosphere. We quantified soil respiration and carbon content for nearly a decade after girdling or natural infestation of hemlock (Tsuga canadensis L. Carr., a codominant species in southern Appalachian forests) by hemlock woolly adelgid (Adelges tsugae) to improve understanding of soil carbon response to disturbance from forest insect and pathogens. From 2005 to 2013, net soil respiration was similar among hemlock mortality (~50% basal area reduction) and reference hardwood plots, but both girdled and hemlock woolly adelgid‐infested plots showed greater activities of β‐glucosidase (a cellulose‐hydrolyzing extracellular enzyme), decreased O‐horizon, and decreased fine root biomass. During this period, mineral soil carbon accumulated at a higher rate in hemlock mortality plots than in reference plots in both surface (0–10 cm) and subsurface (10–30 cm) soils, driven by increases in the mineral‐associated fraction of the soil organic matter. In contrast, particulate organic matter (POM) carbon accrued slowly in surface soils and declined in the subsurface of girdled plots. δ13C values of the POM fraction demonstrate increased microbial processing of surface soil organic matter over time, suggesting enhanced decomposition of organic matter in this pool. These findings indicate that hemlock mortality in this system has led to enhanced soil carbon stabilization through the transformation and translocation of carbon from detrital and POM pools to the mineral‐associated organic matter pool. Accelerated responses in the girdled versus naturally infested treatments highlight limitations associated with using girdling to simulate natural mortality.

The efficiency of Cd phytoextraction by S. plumbizincicola increased with the addition of rice straw to polluted soils: the role of particulate organic matter

The addition of organic amendments to soils can significantly increase soil organic matter (SOM) content as well as the mass of large (e.g. 2000–250 μm) particulate organic matter (POM) fractions. Here we address whether the addition of organic amendments might affect both POM pools and the phytoextraction efficiency of Sedum plumbizincicola (S. plumbizincicola) in cadmium (Cd) polluted soils. We carried out a 442-days long pot experiment and measured Cd uptake by S. plumbizincicola for four successive plant crops in two polluted paddy soils subjected to rice straw (RS) amendment. We also measured soil pH, nutrient availability, pore water dissolved organic carbon (DOC) and Cd concentrations across different soil particle-size fractions. We found that RS amendments had no significant effects on soil pH, DOC or Cd concentrations in bulk soils during repeated phytoextraction. However, RS amendments were associated with significant (1) increases (i.e. 14.3 to 20.7%) in Cd uptake by S. plumbizincicola plants from the first crop harvest, (2) increases in available soil nutrients content, (3) increases in the percentage of oxidizable Cd fractions and the mass of soil POM fractions, and (4) decreases in POM Cd concentrations after repeated phytoextraction. These findings suggest that POM fractions may represent important accessible pools of Cd for uptake by S. plumbizincicola plants. RS amendments enhance both Cd bioavailability within soil POM pools and Cd uptake by the hyperaccumulator S. plumbizincicola thus providing an effective management practice to improve Cd removal from polluted soils through phytoextraction.

Particulate sulfur-containing lipids: Production and cycling from the epipelagic to the abyssopelagic zone

There are major gaps in our understanding of the distribution and role of lipids in the open ocean especially with regard to sulfur-containing lipids (S-lipids). Here, we employ a powerful analytical approach based on high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to elucidate depth-related S-lipid production and molecular transformations in suspended particulate matter from the Northeast Atlantic Ocean in this depth range. We show that within the open-ocean environment S-lipids contribute up to 4.2% of the particulate organic carbon, and that up to 95% of these compounds have elemental compositions that do not match those found in the Nature Lipidomics Gateway database (termed “novel”). Among the remaining 5% of lipids that match the database, we find that sulphoquinovosyldiacylglycerol (SQDG) are efficiently removed while sinking through the mesopelagic zone. The relative abundance of other assigned lipids (sulphoquinovosylmonoacylglycerol (SQMG), sulfite and sulfate lipids, Vitamin D2 and D3 derivatives, and sphingolipids) did not change substantially with depth. The novel S-lipids, represented by hundreds of distinct elemental compositions (160–300 molecules at any one depth), contribute increasingly to the lipid and particulate organic matter pools with increased depth. Depth-related transformations cause (i) incomplete degradation/transformation of unsaturated S-lipids which leads to the depth-related accumulation of the refractory saturated compounds with reduced molecular weight (average 455 Da) and (ii) formation of highly unsaturated S-lipids (average abyssopelagic molecular double bond equivalents, DBE=7.8) with lower molecular weight (average 567 Da) than surface S-lipids (average 592 Da). A depth-related increase in molecular oxygen content is observed for all novel S-lipids and indicates that oxidation has a significant role in their transformation while (bio)hydrogenation possibly impacts the formation of saturated compounds. The instrumentation approach applied here represents a step change in our comprehension of marine S-lipid diversity and the potential role of these compounds in the oceanic carbon cycle. We describe a very much higher number of compounds than previously reported, albeit at the level of elemental composition and fold-change quantitation with depth, rather than isomeric confirmation and absolute quantitation of individual lipids. We emphasize that saturated S-lipids have the potential to transfer carbon from the upper ocean to depth and hence are significant vectors for carbon sequestration.

Dynamics of particulate organic matter composition in coastal systems: Forcing of spatio-temporal variability at multi-systems scale

In costal systems, particulate organic matter (POM) results from a multiplicity of sources having their respective dynamics in terms of production, decomposition, transport and burial. The POM pool experiences thus considerable spatial and temporal variability. In order to better understand this variability, the present study employs statistical multivariate analyses to investigate links between POM composition and environmental forcings for a panel of twelve coastal systems distributed along the three maritime regions of France and monitored weekly to monthly for 1 to 8 years.At multi-system scale, two main gradients of POC composition have been identified: a ‘Continent-Ocean’ gradient associated with hydrodynamics, sedimentary dynamics and depth of the water column, and a gradient of trophic status related to nutrient availability. At local scale, seasonality of POC composition appears to be station-specific but still related to part of the above-mentioned forcings. A typology of systems was established by coupling spatial and temporal variability of POC composition. Four groups were highlighted: (1) the estuarine stations where POC composition is dominated by terrestrial POM and driven by hydrodynamics and sedimentary processes, (2) the oligotrophic systems, characterized by the contribution of diazotrophs due to low nutrient availability, and the marine meso/eutroph systems whose POC composition is (3) either deeply dominated by phytoplankton or (4) dominated by phytoplankton but where the contribution of continental and benthic POC is not negligible and is driven by hydrodynamics, sedimentary processes and the height of the water column.Finally, the present study provides several insights into the different forcings to POM composition and dynamics in temperate coastal systems at local and multi-system scales. This work also presents a methodological approach that establishes statistical links between forcings and POM composition, helping to gain more objectively insight of forcings.

Organic matter bioavailability in tropical coastal waters: The Great Barrier Reef

AbstractThe bioavailability of organic matter (OM) was assessed at three locations during the dry and wet seasons in the Great Barrier Reef (GBR), by measuring changes in particulate organic matter (POM) and dissolved organic matter (DOM) concentrations during laboratory incubations over 50 d. The sites did not show any difference in salinity and, therefore, observed changes could be related to factors such as disparities in the biological activity and/or the impact of sediment resuspension rather than to location. Our results demonstrate that the POM pool has a higher bioavailability than the DOM pool, with the C, N, and P‐containing compounds being more bioavailable than the C and N‐containing molecules, which in turn are more labile than compounds containing just C. The addition of labile POM to the DOM pool did not impact the bioavailability of DOM, suggesting that priming had no major impact on the degradation of this pool in these experiments. Furthermore, our results demonstrate that 94% and 75% of the bioavailable N and P are contained in the organic fraction delivering enough nutrients to sustain phytoplankton productivity in the GBR. Using the obtained degradation rate constants and an average water residence time of 2 weeks, we show that most bioavailable POM (&gt; 96%) and DOM (&gt; 83%) is degraded before reaching the outer shelf. Our results emphasize that OM is a key and mostly unaccounted part of the C, N, and P cycles in tropical coastal waters of the GBR.

Effects of human land use on the terrestrial and aquatic sources of fluvial organic matter in a temperate river basin (The Meuse River, Belgium)

The impact of human activities on the concentrations and composition of dissolved organic matter (DOM) and particulate organic matter (POM) was investigated in the Walloon Region of the Meuse River basin (Belgium). Water samples were collected at different hydrological periods along a gradient of human disturbance (50 sampling sites ranging from 8.0 to 20,407 km2) and during a 1.5 year monitoring of the Meuse River at the city of Liège. This dataset was completed by the characterization of the DOM pool in groundwaters. The composition of DOM and POM was investigated through elemental (C:N ratios), isotopic (δ13C) and optical measurements including excitation emission matrix fluorescence with parallel factor analysis (EEM–PARAFAC). Land use was a major driver on fluvial OM composition at the regional scale of the Meuse Basin, the composition of both fluvial DOM and POM pools showing a shift toward a more microbial/algal and less plant/soil-derived character as human disturbance increased. The comparison of DOM composition between surface and groundwaters demonstrated that this pattern can be attributed in part to the transformation of terrestrial sources by agricultural practices that promote the decomposition of soil organic matter in agricultural lands and subsequent microbial inputs in terrestrial sources. In parallel, human land had contrasting effects on the autochthonous production of DOM and POM. While the in-stream generation of fresh DOM through biological activity was promoted in urban areas, summer autochthonous POM production was not influenced by land use. Finally, soil erosion by agricultural management practices favored the transfer of terrestrial organic matter via the particulate phase. Stable isotope data suggest that the hydrological transfer of terrestrial DOM and POM in human-impacted catchment are not subject to the same controls, and that physical exchange between these two pools of organic matter is limited.

Dynamics of particulate organic matter composition in coastal systems: A spatio-temporal study at multi-systems scale

In coastal systems, the multiplicity of sources fueling the pool of particulate organic matter (POM) leads to divergent estimations of POM composition. Eleven systems (two littoral systems, eight embayments and semi-enclosed systems and one estuary) distributed along the three maritime façades of France were studied for two to eight years in order to quantify the relative contribution of organic matter sources to the surface-water POM pool in coastal systems. This study was based on carbon and nitrogen elemental and isotopic ratios, used for running mixing models.The POM of the estuary is dominated by terrestrial material (93% on average), whereas the POM of the other systems is dominated by phytoplankton (84% on average). Nevertheless, for the latter systems, the POM composition varies in space, with (1) systems where POM is highly composed of phytoplankton (≥93%), (2) systems characterized by a non-negligible contribution of benthic (8–19%) and/or river (7–19%) POM sources, and (3) the Mediterranean systems characterized by the contribution of diazotroph organisms (ca. 14%). A continent-to-ocean gradient of river and/or benthic POM contribution is observed. Finally, time series reveal (1) seasonal variations of POM composition, (2) differences in seasonality between systems, and (3) an inshore-offshore gradient of seasonality within each system that were sampled at several stations.Spatial and seasonal patterns of POM composition are mainly due to local to regional processes such as hydrodynamics and sedimentary hydrodynamic (e.g. resuspension processes, changes in river flows, wind patterns influencing along-shore currents) but also due to the geomorphology of the systems (depth of the water column, distance to the shore). Future studies investigating the link between these forcings and POM composition would help to better understand the dynamics of POM composition in coastal systems.

Mechanisms of P* Reduction in the Eastern Tropical South Pacific

Water masses influenced by oxygen minimum zones (OMZ) feature low inorganic nitrogen (N) to phosphorus (P) ratios. The surplus of P over N is thought to favor non-Redfield primary production by bloom-forming phytoplankton species. Additionally, excess phosphate (P*) is thought to provide a niche for nitrogen fixing organisms. In order to assess the effect of low inorganic nutrient ratios on the stoichiometry and composition of primary producers, biogeochemical measurements were carried out in 2012 during a research cruise in the eastern tropical South Pacific (ETSP). Based on pigment analyses, a succession of different phytoplankton functional groups was observed along onshore – offshore transects with diatoms dominating the productive upwelling region, and prymnesiophytes, cryptophytes and Synechococcus prevailing in the oligotrophic open ocean. Although inorganic nutrient supply ratios were below Redfield proportions throughout the sampling area, the stoichiometry of particulate organic nitrogen to phosphorus (PON:POP) generally exceeded ratios of 16:1. Despite PON:POP ≥ 16, high P* values in the surface layer (0-50 m) above the shelf rapidly decreased as water masses were advected offshore. There are three mechanisms which can explain these observations: (1) non-Redfield primary production, where the excess phosphorus in the biomass is directly released as dissolved organic phosphorus (DOP), (2) non-Redfield primary production, which is masked by a particulate organic matter pool mainly consisting of P-depleted detrital biomass, and/or (3) Redfield primary production combined with dinitrogen (N2) fixation. Our observations suggest that the three processes occur simultaneously in the study area; quantifying the relative importance of each of these mechanisms needs further investigation. Therefore, it remains uncertain whether the ETSP is a net sink for bioavailable N or whether the N-deficit in this area is replenished locally.

Fungal\u2013bacterial dynamics and their contribution to terrigenous carbon turnover in relation to organic matter quality

Ecological functions of fungal and bacterial decomposers vary with environmental conditions. However, the response of these decomposers to particulate organic matter (POM) quality, which varies widely in aquatic ecosystems, remains poorly understood. Here we investigated how POM pools of substrates of different qualities determine the relative contributions of aquatic fungi and bacteria to terrigenous carbon (C) turnover. To this end, surface sediments were incubated with different POM pools of algae and/or leaf litter. 13C stable-isotope measurements of C mineralization were combined with phospholipid analysis to link the metabolic activities and substrate preferences of fungal and bacterial heterotrophs to dynamics in their abundance. We found that the presence of labile POM greatly affected the dominance of bacteria over fungi within the degrader communities and stimulated the decomposition of beech litter primarily through an increase in metabolic activity. Our data indicated that fungi primarily contribute to terrigenous C turnover by providing litter C for the microbial loop, whereas bacteria determine whether the supplied C substrate is assimilated into biomass or recycled back into the atmosphere in relation to phosphate availability. Thus, this study provides a better understanding of the role of fungi and bacteria in terrestrial–aquatic C cycling in relation to environmental conditions.

Stable isotopes measurements reveal dual carbon pools contributing to organic matter enrichment in marine aerosol.

Stable carbon isotope ratios in marine aerosol collected over the Southern Indian Ocean revealed δ13C values ranging from −20.0‰ to −28.2‰. The isotope ratios exhibited a strong correlation with the fractional organic matter (OM) enrichment in sea spray aerosol. The base-level isotope ratio of −20.0‰ is characteristic of an aged Dissolved Organic Matter (DOM) pool contributing a relatively homogeneous background level of DOM to oceanic waters. The range of isotope ratios, extending down to −28.2‰, is characteristic of more variable, stronger, and fresher Particulate Organic Matter (POM) pool driven by trophic level interactions. We present a conceptual dual-pool POM-DOM model which comprises a ‘young’ and variable POM pool which dominates enrichment in sea-spray and an ‘aged’ but invariant DOM pool which is, ultimately, an aged end-product of processed ‘fresh’ POM. This model is harmonious with the preferential enrichment of fresh colloidal and nano-gel lipid-like particulate matter in sea spray particles and the observed depleted δ13C ratio resulting from isotope equilibrium fractionation coupled with enhanced plankton photosynthesis in cold water (−2 °C to +8 °C). These results re-assert the hypothesis that OM enrichment in sea-spray is directly linked to primary production and, consequently, can have implications for climate-aerosol-cloud feedback systems.

Annual cycle and spatial trends in fatty acid composition of suspended particulate organic matter across the Beaufort Sea shelf

Fatty acid profiles of suspended particulate organic matter (POM) were determined over an annual cycle (September 2003 to August 2004) on the Beaufort Sea shelf, Canadian Arctic. Special emphasis was placed on the nutritional quality of the fatty acid pool available to zooplankton by examining spatial and temporal patterns in the proportions of total polyunsaturated fatty acids (PUFA) and the essential fatty acids 22:6n−3 (DHA) and 20:5n−3 (EPA). EPA and DHA were the two most abundant PUFA throughout the study period. A log-ratio multivariate (LRA) analysis revealed strong structure in fatty acid profiles related to season and depth. Dominant fatty acids accounting for the observed trend included 18:5n−3, 18:4n−3, 16:1n−7, 20:5n-3, 18:0 and 20:3n−3. We observed a shift in fatty acid profiles from summer to autumn (e.g., from 16:1n−7 and EPA to 18:5n−3 and 18:4n−3) that likely corresponded to a shift in the relative importance of diatoms versus dinoflagellates, prymnesiophytes and/or prasinophytes to the POM pool. Fatty acid composition during winter was dominated by more refractory saturated fatty acids. A surprising finding was the depth and seasonal trend of 20:3n-3, which was higher in winter, aligned with 18:0 in the LRA, but behaved differently than other n−3 PUFA. We interpret fatty acid profiles during summer to be predominantly driven by phytoplankton inputs, whereas fatty acid profiles in winter were dominated by fatty acids that were left over after consumption and/or were generated by heterotrophs. The highest diatom inputs (EPA, the diatom fatty acid marker), n−3/n−6 ratios, and C16 PUFA index were located in an upwelling region off Cape Bathurst. This study is the first annual time series of fatty acid profiles of POM in Arctic seas, expanding our knowledge of the composition of POM throughout the dark season.

Distribution and behaviour of polycyclic aromatic hydrocarbons in subtropical river estuary, Okinawa, Japan

ABSTRACTSurface water samples were collected along the salinity gradient of Manko estuary in Okinawa, Japan, between January and November 2012, to examine spatial and temporal variability in the distribution, composition, behaviour, and sources of particulate polycyclic aromatic hydrocarbons (PAHs). Concentrations of total particulate PAHs ranged from 169.7 ng g−1 at riverine station (N1) in November 2012 to 922 ng g−1 at another riverine station (K3) in May 2012. Based on observed environmental behaviour, particularly relationships to organic matter, the PAHs appear divisible into two groups: group I (low molecular weight) PAHs were characterised by low particles affinity and strong correlations with a pool of autochthonous organic matter (e.g. high concentrations of polyunsaturated fatty acids), reflecting processes controlling their delivery to the estuary; and, in contrast, group II (high molecular weight) PAHs were characterised by high particles affinity and exhibited estuarine distribution primarily controlled by suspended solid load and inputs from terrigenous sources through river inflows. This relationship, coupled with isomer ratios indicative of combined mixed sources from petroleum and petroleum combustions, indicates that terrestrial run-off is the dominant mode of delivery of particulate PAHs in the estuary. The relationships between distinct groups of PAHs and lipid biomarkers demonstrated in this study suggest that very specific pools of particulate organic matter, as opposed to concentrations of particulate organic carbon alone, can be important in the transport and fate of hydrophobic contaminants.