Daniel J. Repeta

Woods Hole Oceanographic Institution, FMH, Massachusetts, United States

Are you Daniel J. Repeta?

Claim your profile

Publications (90)432.91 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Picophytoplankton, including photosynthetic picoeukaryotes (PPE) and unicellular cyanobacteria, are important contributors to plankton biomass and primary productivity. In this study, phytoplankton composition and rates of carbon fixation were examined across a large trophic gradient in the South East Pacific Ocean (SEP) using a suite of approaches: photosynthetic pigments, rates of 14C-primary productivity, and phylogenetic analyses of partial 18S rRNA genes PCR amplified and sequenced from flow cytometrically sorted cells. While phytoplankton >10 μm (diatoms and dinoflagellates) were prevalent in the upwelling region off the Chilean coast, picophytoplankton consistently accounted for 55–92% of the total chlorophyll a inventories and >60% of 14C-primary productivity throughout the sampling region. Estimates of rates of 14C-primary productivity derived from flow cytometric sorting of radiolabeled cells revealed that the contributions of PPE and Prochlorococcus to euphotic zone depth-integrated picoplankton productivity were nearly equivalent (ranging 36–57%) along the transect, with PPE comprising a larger share of picoplankton productivity than cyanobacteria in the well-lit (>15% surface irradiance) region compared with in the lower regions (1–7% surface irradiance) of the euphotic zone. 18S rRNA gene sequence analyses revealed the taxonomic identities of PPE; e.g., Mamiellophyceae (Ostreococcus) were the dominant PPE in the upwelling-influenced waters, while members of the Chrysophyceae, Prymnesiophyceae, Pelagophyceae, and Prasinophyceae Clades VII and IX flourished in the oligotrophic South Pacific Subtropical Gyre. Our results suggest that, despite low numerical abundance in comparison to cyanobacteria, diverse members of PPE are significant contributors to carbon cycling across biogeochemically distinct regions of the SEP.
    Full-text · Article · Jan 2016
  • Daniel J. Repeta
    [Show abstract] [Hide abstract]
    ABSTRACT: This chapter summarizes advances in our knowledge of dissolved organic matter (DOM) composition, with a particular emphasis on studies completed over the last decade that utilize high-field nuclear magnetic resonance, high-resolution mass spectrometry, proteomics, and related immunochemical assays. Up to 75% of marine DOM can now be recovered for analysis using solid-phase extraction, ultrafiltration, or electrodialysis/reverse osmosis. Spectral and chemical analyses show carbohydrates, proteins, and structurally complex carboxyl-rich aliphatic matter (CRAM) contribute the majority of characterized material. The chemical composition of DOM has a large impact on its accumulation and residence time in the ocean. Carbohydrates and proteins are cycled much more quickly, have much shorter residence times and lower global inventories than CRAM. Processes that lead to the accumulation of carbohydrates and proteins are highly selective, and only a specific fraction of these biopolymers escape degradation to accumulate as DOM. The origin and fate of CRAM are largely unknown, although recent studies suggest a portion of this material has been subjected to elevated temperatures, either in hydrothermal systems or during biomass burning of terrestrial organic matter before transport to the ocean. Radiocarbon measurements and global surveys of deep-sea DOM concentration are providing new insights into the location and timescale of refractory DOM removal processes, significantly enhancing our understanding of CRAM cycling.
    No preview · Chapter · Dec 2015
  • [Show abstract] [Hide abstract]
    ABSTRACT: Abstract One of the most intriguing aspects of dissolved organic carbon (DOC) dynamics in the Mediterranean Sea (Med Sea) is that in the intermediate and deep waters, DOC concentrations are equal to the lowest values found in the deep Atlantic and Pacific (36–42 μM). The very low DOC values in the deep Med Sea were unexpected since the renewal time of deep waters in the basin is only 20–126 years. Over this short timescale, we expected just a very small, nearly undetectable, fraction of refractory DOC (RDOC) to be removed. The first DOC isotope data show that DOC in Med Sea deep water is more depleted in both Δ14C and δ13C than in the deep Atlantic Ocean, with an estimated age of 4500–5100 years. These data suggest that at least 10%, and up to 45%, of the Atlantic RDOC entering the Med Sea is removed and replaced by isotopically lighter DOC in less than 126 years. Potential allocthonous sources include: fossil methane and methane derived DOC seeps from sediments, anthropogenic combustion products and terrestrial organic matter delivered by the atmosphere, rivers, and groundwaters. Using current information, it is not possible to quantify the relative contribution of these potential sources. Based on estimated flux and isotopic value, atmospheric input of soluble organic carbon from soils or combustion products as well as DOC from groundwater are the most likely sources of allochthonous DOC. Our results suggest that DOC cycling in the deep Med Sea is dynamic and support the idea that a substantial fraction (up to 45%) of what has traditionally been defined as “refractory” DOC imported from the Atlantic Ocean, can be removed on temporal scales of < 126 y, thereby opening intriguing questions about deep sea DOC cycling.
    No preview · Article · Jun 2015 · Marine Chemistry
  • Source
    Oscar A Sosa · Scott M Gifford · Daniel J Repeta · Edward F DeLong
    [Show abstract] [Hide abstract]
    ABSTRACT: The role of bacterioplankton in the cycling of marine dissolved organic matter (DOM) is central to the carbon and energy balance in the ocean, yet there are few model organisms available to investigate the genes, metabolic pathways, and biochemical mechanisms involved in the degradation of this globally important carbon pool. To obtain microbial isolates capable of degrading semi-labile DOM for growth, we conducted dilution to extinction cultivation experiments using seawater enriched with high molecular weight (HMW) DOM. In total, 93 isolates were obtained. Amendments using HMW DOM to increase the dissolved organic carbon concentration 4x (280 μM) or 10x (700 μM) the ocean surface water concentrations yielded positive growth in 4-6% of replicate dilutions, whereas <1% scored positive for growth in non-DOM-amended controls. The majority (71%) of isolates displayed a distinct increase in cell yields when grown in increasing concentrations of HMW DOM. Whole-genome sequencing was used to screen the culture collection for purity and to determine the phylogenetic identity of the isolates. Eleven percent of the isolates belonged to the gammaproteobacteria including Alteromonadales (the SAR92 clade) and Vibrio. Surprisingly, 85% of isolates belonged to the methylotrophic OM43 clade of betaproteobacteria, bacteria thought to metabolically specialize in degrading C1 compounds. Growth of these isolates on methanol confirmed their methylotrophic phenotype. Our results indicate that dilution to extinction cultivation enriched with natural sources of organic substrates has a potential to reveal the previously unsuspected relationships between naturally occurring organic nutrients and the microorganisms that consume them.The ISME Journal advance online publication, 15 May 2015; doi:10.1038/ismej.2015.68.
    Preview · Article · May 2015 · The ISME Journal
  • Source
    Rene M Boiteau · Daniel J Repeta
    [Show abstract] [Hide abstract]
    ABSTRACT: Siderophores are thought to play an important role in iron cycling in the ocean, but relatively few marine siderophores have been identified. Sensitive, high throughput methods hold promise for expediting the discovery and characterization of new siderophores produced by marine microbes. We developed a methodology for siderophore characterization that combines liquid chromatography (LC) inductively coupled plasma mass spectrometry (ICPMS) with high resolution electrospray ionization mass spectrometry (ESIMS). To demonstrate this approach, we investigated siderophore production by the marine cyanobacteria Synechococcus sp. PCC 7002. Three hydroxamate siderophores, synechobactin A-C, have been previously isolated and characterized from this strain. These compounds consist of an iron binding head group attached to a fatty acid side chain of variable length (C12, C10, and C8 respectively). In this study, we detected six iron-containing compounds in Synechococcus sp. PCC 7002 media by LC-ICPMS. To identify the molecular ions of these siderophores, we aligned the chromatographic retention times of peaks from the LC-ICPMS chromatogram with features detected from LC-ESIMS spectra using an algorithm designed to recognize metal isotope patterns. Three of these compounds corresponded to synechobactins A (614 m/z), B (586m/z), and C (558m/z). The MS2 spectra of these compounds revealed diagnostic synechobactin fragmentation patterns which were used to confirm the identity of the three unknown compounds (600, 628, and 642 m/z) as new members of the synechobactin suite with side chain lengths of 11, 13, and 14 carbons. These results demonstrate the potential of combined LCMS techniques for the identification of novel iron-organic complexes.
    Preview · Article · Mar 2015 · Metallomics
  • [Show abstract] [Hide abstract]
    ABSTRACT: Marine dissolved organic carbon (DOC) is a large (660 Pg C) reactive carbon reservoir that mediates the oceanic microbial food web and interacts with climate on both short and long timescales. Carbon isotopic content provides information on the DOC source via [Formula: see text]C and age via [Formula: see text]C. Bulk isotope measurements suggest a microbially sourced DOC reservoir with two distinct components of differing radiocarbon age. However, such measurements cannot determine internal dynamics and fluxes. Here we analyze serial oxidation experiments to quantify the isotopic diversity of DOC at an oligotrophic site in the central Pacific Ocean. Our results show diversity in both stable and radio isotopes at all depths, confirming DOC cycling hidden within bulk analyses. We confirm the presence of isotopically enriched, modern DOC cocycling with an isotopically depleted older fraction in the upper ocean. However, our results show that up to 30% of the deep DOC reservoir is modern and supported by a 1 Pg/y carbon flux, which is 10 times higher than inferred from bulk isotope measurements. Isotopically depleted material turns over at an apparent time scale of 30,000 y, which is far slower than indicated by bulk isotope measurements. These results are consistent with global DOC measurements and explain both the fluctuations in deep DOC concentration and the anomalous radiocarbon values of DOC in the Southern Ocean. Collectively these results provide an unprecedented view of the ways in which DOC moves through the marine carbon cycle.
    No preview · Article · Nov 2014 · Proceedings of the National Academy of Sciences
  • Source

    Full-text · Dataset · Apr 2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Production of dissolved organic matter (DOM) by marine phytoplankton supplies the majority of organic substrate consumed by heterotrophic bacterioplankton in the sea. This production and subsequent consumption converts a vast quantity of carbon, nitrogen, and phosphorus between organic and inorganic forms, directly impacting global cycles of these biologically important elements. Details regarding the chemical composition of DOM produced by marine phytoplankton are sparse, and while often assumed, it is not currently known if phylogenetically distinct groups of marine phytoplankton release characteristic suites of DOM. To investigate the relationship between specific phytoplankton groups and the DOM they release, hydrophobic phytoplankton-derived dissolved organic matter (DOMP) from eight axenic strains was analyzed using high-performance liquid chromatography coupled to mass spectrometry (HPLC-MS). Identification of DOM features derived from Prochlorococcus, Synechococcus, Thalassiosira, and Phaeodactylum revealed DOMP to be complex and highly strain dependent. Connections between DOMP features and the phylogenetic relatedness of these strains were identified on multiple levels of phylogenetic distance, suggesting that marine phytoplankton produce DOM that in part reflects its phylogenetic origin. Chemical information regarding the size and polarity ranges of features from defined biological sources was also obtained. Our findings reveal DOMP composition to be partially conserved among related phytoplankton species, and implicate marine DOM as a potential factor influencing microbial diversity in the sea by acting as a link between autotrophic and heterotrophic microbial community structures.
    Full-text · Article · Mar 2014 · Frontiers in Microbiology
  • Source

    Full-text · Dataset · Dec 2013
  • Source

    Full-text · Dataset · Dec 2013
  • Source
    Christos Panagiotopoulos · Daniel J. Repeta · Carl G. Johnson

    Full-text · Dataset · Dec 2013
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The aim of this study was to investigate the chemical composition and cycling of dissolved organic matter (DOM), focusing on the influence of thermal stratification and mixing. Samples were collected at the surface, 500 m and 1500 m, in April, July and October 2004 at the DYFAMED time-series site in the Northwestern Mediterranean. High molecular weight (HMW) DOM was concentrated using ultrafiltration. The HMW DOM fraction was characterised by 1H NMR, amino acid and neutral sugar analysis. Results were interpreted in the context of the wealth of information available on site DYFAMED. Three key observations were made. Firstly, the carbohydrate component of DOM decreased with depth, in agreement with previous studies, indicating degradation of this labile material. Secondly, the July surface water sample was particularly carbohydrate rich; it is proposed this may be the result of increased carbohydrate production by phytoplankton under low nutrient conditions, and accumulation in the surface layers due to stratification of the water column. Finally, the October samples showed a distinctly different chemical signature to the April and July samples, potentially indicating a shift from a net production system in the spring and summer to a net re-mineralisation system in autumn. The results of this study offer an insight into the dynamic nature of DOM at station DYFAMED.
    Full-text · Article · Dec 2013 · Progress In Oceanography
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Dissolved organic matter (DOM) is the largest active organic carbon reservoir in the ocean (662 GT C), a major fraction (> 95%) of which remains chemically uncharacterized. The concentration and isolation of DOM from seawater by ultrafiltration facilitates its chemical characterization by spectroscopic techniques. Using ultrafiltration, silver cation preparative chromatography and gas chromatography coupled with mass spectrometry (GC–MS), we identified 50 novel sugar compounds after hydrolysis of the high molecular weight dissolved organic matter fraction (HMWDOM; the fraction of DOM isolated after ultrafiltration). Sugars were identified by comparison of their mass spectra with those of chemically synthetized standards and with spectra previously described in the literature. Our results showed that mono- and di-methylated hexoses; mono- and di-methylated pentoses; mono- and di-methylated 6-deoxysugars, as well as heptoses, methylated heptoses, 3,6-dideoxysugars and 1,6 anhydrosugars (levoglucosan, mannosan, and galactosan) are components of HMWDOM, which may explain the low apparent yields of sugars recovered by molecular level (HPLC) analyses of HMWDOM after hydrolysis.From three depths spanning the surface (15 m) to bathypelagic (1800 m) ocean in the North Pacific near Hawaii our results showed that mono- and di-methylated hexoses were most abundant in the surface sample (64% of the total identified methylated sugar compounds), while at 1800 m monomethylated 6-deoxysugars were the dominant sugars (42% of the total identified methylated sugar compounds). The high diversity of mono- and di-methylated hexoses in the surface sample most likely suggests an algal and/or bacterial source, while the high abundance of methylated 6-deoxy hexoses in the deep sample points toward an important bacterial contribution because the latter sugars are mostly found in bacterial lipopolysaccharides as well as highly degraded organic material.
    Full-text · Article · Nov 2013 · Marine Chemistry
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A considerable fraction of the Earth's organic carbon exists in dissolved form in seawater. To investigate the roles of planktonic marine microbes in the biogeochemical cycling of this dissolved organic matter (DOM), we performed controlled seawater incubation experiments and followed the responses of an oligotrophic surface water microbial assemblage to perturbations with DOM derived from an axenic culture of Prochlorococcus, or high-molecular weight DOM concentrated from nearby surface waters. The rapid transcriptional responses of both Prochlorococcus and Pelagibacter populations suggested the utilization of organic nitrogen compounds common to both DOM treatments. Along with these responses, both populations demonstrated decreases in gene transcripts associated with nitrogen stress, including those involved in ammonium acquisition. In contrast, responses from low abundance organisms of the NOR5/OM60 gammaproteobacteria were observed later in the experiment, and included elevated levels of gene transcripts associated with polysaccharide uptake and oxidation. In total, these results suggest that numerically dominant oligotrophic microbes rapidly acquire nitrogen from commonly available organic sources, and also point to an important role for carbohydrates found within the DOM pool for sustaining the less abundant microorganisms in these oligotrophic systems.
    Full-text · Article · Aug 2013 · Environmental Microbiology
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Plant wax lipids and lignin phenols are the two most common classes of molecular markers that are used to trace vascular plant-derived OM in the marine environment. However, their 13C and 14C compositions have not been directly compared, which can be used to constrain the flux and attenuation of terrestrial carbon in marine environment. In this study, we describe a revised method of isolating individual lignin phenols from complex sedimentary matrices for 14C analysis using high pressure liquid chromatography (HPLC) and compare this approach to a method utilizing preparative capillary gas chromatography (PCGC). We then examine in detail the 13C and 14C compositions of plant wax lipids and lignin phenols in sediments from the inner and mid shelf of the Washington margin that are influenced by discharge of the Columbia River. Plant wax lipids (including n-alkanes, n-alkanoic (fatty) acids, n-alkanols, and n-aldehydes) displayed significant variability in both δ13C (‒28.3 to ‒37.5 ‰) and ∆14C values (‒204 to +2 ‰), suggesting varied inputs and/or continental storage and transport histories. In contrast, lignin phenols exhibited similar δ13C values (between ‒30 to ‒34 ‰) and a relatively narrow range of ∆14C values (‒45 to ‒150 ‰; HPLC-based mesurement) that were similar to, or younger than, bulk OM (‒195 to ‒137 ‰). Moreover, lignin phenol 14C age correlated with the degradation characteristics of this terrestrial biopolymer in that vanillyl phenols were on average ~500 years older than syringyl and cinnamyl phenols that degrade faster in soils and sediments. The isotopic characteristics, abundance, and distribution of lignin phenols in sediments suggest that they serve as promising tracers of recently biosynthesized terrestrial OM during supply to, and dispersal within the marine environment. Lignin phenol 14C measurements may also provide useful constraints on the vascular plant end member in isotopic mixing models for carbon source apportionment, and for interpretation of sedimentary records of past vegetation dynamics.
    Full-text · Article · Apr 2013 · Geochimica et Cosmochimica Acta
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Organic ligands dominate the speciation of iron in the ocean. Little is known, however, about the chemical composition and distribution of these compounds. Here we describe a method to detect low concentrations of organic Fe ligands using reverse phase high-performance liquid chromatography (HPLC) tandem multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). This technique can be used to screen seawater and marine cultures for target compounds that can be isolated and structurally characterized. Sensitive detection (<1 picomole Fe) is achieved by using an iron-free HPLC system to reduce background Fe levels, minimizing 40Ar16O+ interferences on 56Fe with a hexapole collision cell, and introducing oxygen into the sample carrier gas to prevent the formation of reduced carbon deposits that decrease sensitivity. This method was tested with a chromatographic separation of five trace metal complexes that represent the polarity range likely found in seawater. Good separation was achieved with a 20 min water/methanol gradient, although sensitivity decreased by a factor of two at high organic solvent concentrations. Finally, Fe ligand complexes were detected from the organic extract of surface South Pacific seawater and from culture media of the siderophore producing cyanobacteria Synechococcus sp. PCC 7002.
    Full-text · Article · Apr 2013 · Analytical Chemistry
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Dissolved organic carbon (DOC) is divided into three reservoirs of different reactivities (labile, semi-labile and non-reactive). The contribution of the semi-labile DOC to the global prokaryotic production has been assessed in very few previous studies. Interestingly enough some experiments show rapid utilization of semi-reactive DOC by prokaryotes, while other experiments show almost no utilization at all (1,2). However, all these studies did not take into account the role of hydrostatic pressure for the degradation of organic matter (3). In this study, we investigate the degradation of HMW-DOM incubated with deep-sea water samples (2000 m-depth, NW Mediterranean Sea) collected under in situ pressure conditions with their own prokaryotic assemblages during stratified water conditions (summer) and mixed water conditions (winter). 1 1 1 2 1 1 1 1 1 2 In the framework of MERMEX (Marine Ecosystems Response in the Mediterranean Experiment, Chantier MISTRALS), we attempt to better understand the role of prokaryotes into the degradation of dissolved organic matter in the deep-sea Mediterranean waters (4). Deep-sea water samples, recovered under in situ pressure conditions, were enriched with natural occurring organic matter [high molecular weight dissolved organic matter or HMW-DOM (corresponding to the organic matter > 1000 Da)], and incubated for 10 days under atmospheric (ATM) and in situ hydrostatic pressure (HP) conditions. Total organic carbon (TOC), dissolved sugars (DCHO), and bacterial abundance were monitored overtime. Using these parameters we estimated TOC and DCHO decay rates (k) as well as prokaryotic growth rates (µ). Our results indicated that during HP incubations TOC and DCHO exhibited the highest degradation rates (k = 0.82 d ; k = 0.98 d) compared to the ATM conditions were no degradation was observed (k = 0.007 d , k = 0.002 d). Similarly prokaryotic growth rate was higher in HP than in ATM conditions (µ = 0.47 d vs µ = 0.39 d). These results suggest that deep-sea prokaryotic communities are autochthonous to the deep-sea realm and therefore more adapted to degrade HMW-DOC under in situ hydrostatic pressure conditions. An opposite trend was observed for the HP incubations from mixed deep water masses (MWM). HP incubation measurements displayed the lowest TOC degradation and prokaryotic growth rates (k =0.031 d ; k =0.35 d ; µ = 0.25 d) compared to the ATM conditions (k =0.62 d ; (k =0.60 d ; µ = 0.46 d). These results imply the presence of allochthonous prokaryotic cells in deep-sea samples after a winter water mass convection. Apparently, these prokaryotic communities are more adapted at atmospheric pressure conditions, pointing to a surface origin. This study demonstrates that remineralization rates of semi-labile DOC in deep NW Med. Sea are controlled by the prokaryotic communities, which are influenced by the hydrological conditions of the water column.
    Full-text · Conference Paper · Jan 2013
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Phytoplankton and bacterial pigment composi-tions were determined by high performance liquid chro-matography (HPLC) and liquid chromatography-mass spec-trometry (LC-MS) in two freshwater reservoirs (Tillari Dam and Selaulim Dam), which are located at the foothills of the Western Ghats in India. These reservoirs experience anoxia in the hypolimnion during summer. Water samples were collected from both reservoirs during anoxic periods while one of them (Tillari Reservoir) was also sampled in winter, when convective mixing results in well-oxygenated conditions throughout the water column. During the pe-riod of anoxia (summer), bacteriochlorophyll (BChl) e iso-mers and isorenieratene, characteristic of brown sulfur bac-teria, were dominant in the anoxic (sulfidic) layer of the Tillari Reservoir under low light intensities. The winter ob-servations showed the dominance of small cells of Chloro-phyll b-containing green algae and cyanobacteria, with minor presence of fucoxanthin-containing diatoms and peridinin-containing dinoflagellates. Using total BChl e concentration observed in June, the standing stock of brown sulfur bacteria carbon in the anoxic compartment of Tillari Reservoir was estimated to be 2.27 gC m −2 , which is much higher than the similar estimate for carbon derived from oxygenic photosyn-thesis (0.82 gC m −2). The Selaulim Reservoir also displayed similar characteristics with the presence of BChl e isomers and isorenieratene in the anoxic hypolimnion during summer. Although sulfidic conditions prevailed in the water column below the thermocline, the occurrence of photo-autotrophic bacteria was restricted only to mid-depths (maximal concen-tration of BChl e isomers was detected at 0.2 % of the sur-face incident light). This shows that the vertical distribution of photo-autotrophic sulfur bacteria is primarily controlled by light penetration in the water column where the presence of H 2 S provides a suitable biogeochemical environment for them to flourish.
    Full-text · Article · Jul 2012 · Biogeosciences
  • [Show abstract] [Hide abstract]
    ABSTRACT: Phytoplankton and bacterial pigment compositions were determined by high performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS) in two freshwater reservoirs (Tillari Dam and Selaulim Dam), which are located at the foothills of the Western Ghats in India. These reservoirs experience anoxia in the hypolimnion during summer. Water samples were collected from both reservoirs during anoxic periods while one of them (Tillari Reservoir) was also sampled in winter, when convective mixing results in well-oxygenated conditions throughout the water column. During the period of anoxia (summer), bacteriochlorophyll (BChl) e isomers and isorenieratene, characteristic of brown sulfur bacteria, were dominant in the anoxic (sulfidic) layer of the Tillari Reservoir under low light intensities. The winter observations showed the dominance of small cells of Chlorophyll b-containing green algae and cyanobacteria, with minor presence of fucoxanthin-containing diatoms and peridinin-containing dinoflagellates. Using total BChl e concentration observed in June, the standing stock of brown sulfur bacteria carbon in the anoxic compartment of Tillari Reservoir was estimated to be 2.27 gC m−2, which is much higher than the similar estimate for carbon derived from oxygenic photosynthesis (0.82 gC m−2. The Selaulim Reservoir also displayed similar characteristics with the presence of BChl e isomers and isorenieratene in the anoxic hypolimnion during summer. Although sulfidic conditions prevailed in the water column below the thermocline, the occurrence of photo-autotrophic bacteria was restricted only to mid-depths (maximal concentration of BChl e isomers was detected at 0.2% of the surface incident light). This shows that the vertical distribution of photo-autotrophic sulfur bacteria is primarily controlled by light penetration in the water column where the presence of H2S provides a suitable biogeochemical environment for them to flourish.
    No preview · Article · Jul 2012 · Biogeosciences
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: River inputs of nutrients and organic matter impact the biogeochemistry of arctic estuaries and the Arctic Ocean as a whole, yet there is considerable uncertainty about the magnitude of fluvial fluxes at the pan-Arctic scale. Samples from the six largest arctic rivers, with a combined watershed area of 11.3 × 106km2, have revealed strong seasonal variations in constituent concentrations and fluxes within rivers as well as large differences among the rivers. Specifically, we investigate fluxes of dissolved organic carbon, dissolved organic nitrogen, total dissolved phosphorus, dissolved inorganic nitrogen, nitrate, and silica. This is the first time that seasonal and annual constituent fluxes have been determined using consistent sampling and analytical methods at the pan-Arctic scale and consequently provide the best available estimates for constituent flux from land to the Arctic Ocean and surrounding seas. Given the large inputs of river water to the relatively small Arctic Ocean and the dramatic impacts that climate change is having in the Arctic, it is particularly urgent that we establish the contemporary river fluxes so that we will be able to detect future changes and evaluate the impact of the changes on the biogeochemistry of the receiving coastal and ocean systems. KeywordsArctic–Rivers–Arctic rivers–Siberia–Land–ocean linkage–Climate change–Permafrost–Dissolved organic carbon–DOC
    Full-text · Article · Mar 2012 · Estuaries and Coasts

Publication Stats

4k Citations
432.91 Total Impact Points

Institutions

  • 1984-2015
    • Woods Hole Oceanographic Institution
      • • Department of Marine Chemistry and Geochemistry
      • • Department of Biology
      FMH, Massachusetts, United States
  • 2007
    • Boston University
      Boston, Massachusetts, United States
  • 2004
    • University of Cambridge
      Cambridge, England, United Kingdom
  • 2003
    • Christian-Albrechts-Universität zu Kiel
      Kiel, Schleswig-Holstein, Germany
  • 1997
    • University of Massachusetts Boston
      Boston, Massachusetts, United States