bluuebunny/crossref_metadata_2025 huggingface dataset:
Particle-associated dissolved elemental fluxes: revising the stochiometry of mixed layer export
Authors: Antia A. N.
Published: April 2010

Abstract. Sinking particles carry substantial loads of dissolved elements in their interstitial spaces that contribute to the vertical transport of elements out of the euphotic zone. Elemental fluxes as traditionally measured by sediment traps underestimate total export when this particle-associated dissolved flux is not considered. The errors introduced are variable and alter both the absolute levels of flux as well as the stochiometry of export. Using samples from sediment traps in the North Atlantic and measuring excess dissolved carbon, nitrogen, phosphorus, silica and calcium in the supernatant of the collection cups, it is possible to quantitatively assess the total flux in the sample. At the base of the winter mixed layer, up to 90±6% of phosphorus fluxes are found as excess phosphate whereas for carbon and nitrogen dissolved concentrations account for between 30% and 47% of total fluxes respectively. Particle-associated dissolved silica fluxes are a mean of 61% of total flux. Little (<10%) of calcium fluxes are in dissolved form. The proportion of dissolved to total flux decreases with trap deployment depth. Calculations of the C:N:P ratios for particles only are well above the Redfield Ratio of 106:16:1 (Redfield et al., 1964), although the mid-water dissolved N:P and N:Si values as well as the C:N:P ratios of remineralisation along isopycnals conform to the Redfield Ratio at this site. Accounting for dissolved fluxes of all these elements brings the estimates in agreement with the Redfield Ratio and with other geochemical estimates of the stochiometry of winter mixed layer export. A factor of 3 to 4 higher ratios of organic: inorganic carbon export also implies that the net atmospheric CO2 sequestration by the biological pump is about 50% higher at this site when the particle-associated dissolved elemental fluxes are considered.
Pure stands of temperate forest tree species modify soil respiration and N turnover
Authors: Brüggemann N. , Rosenkranz P. , Papen H. , Pilegaard K. , Butterbach-Bahl K.
Published: April 2010

Abstract. The effects of five different tree species common in the temperate zone, i.e. beech (Fagus sylvatica L.), pedunculate oak (Quercus robur L.), Norway spruce (Picea abies [L.] Karst), Japanese larch (Larix leptolepis [Sichold and Zucc.] Gordon) and mountain pine (Pinus mugo Turra), on soil respiration, gross N mineralization and gross nitrification rates were investigated. Soils were sampled in spring and summer 2002 at a forest trial in Western Jutland, Denmark, where pure stands of the five tree species of the same age were growing on the same soil. Soil respiration, gross rates of N mineralization and nitrification were significantly higher in the organic layers than in the Ah horizons for all tree species and both sampling dates. In summer (July), the highest rates of soil respiration, gross N mineralization and gross nitrification were found in the organic layer under spruce, followed by beech > larch > oak > pine. In spring (April), these rates were also higher under spruce compared to the other tree species, but were significantly lower than in summer. For the Ah horizons no clear seasonal trend was observed for any of the processes examined. A linear relationship between soil respiration and gross N mineralization (r2=0.77), gross N mineralization and gross nitrification rates (r2=0.72), and between soil respiration and gross nitrification (r2=0.81) was found. The results obtained underline the importance of considering the effect of forest type on soil C and N transformations.
Efficiency of the silicate pump at a coastal oligotrophic site in the Mediterranean Sea
Authors: Leblanc K. , Quéguiner B. , Raimbault P. , Garcia N.
Published: April 2010

Abstract. A one-year survey of the sedimenting particulate material was conducted at the SOFi site located on the edge of the continental slope in the Gulf of Lions (Northwestern Mediterranean) between September 1999 and December 2000. The main focus of this paper was to investigate the seasonal pattern of particulate biogenic silica (BSi) sedimentation fluxes in comparison with the particulate carbon fluxes and establish annual budgets of Si and C export. The pattern of the downward flux of BSi was partly typical with increased daily sedimentation rates during spring followed by smaller peaks during summer. However, an unusual sedimentation event was recorded in the bottom trap in February before the spring thermal stratification occurred, and represented 37% of the annual BSi mass flux. The total annual BSi flux at the SOFi site was estimated from the bottom trap and amounted to 86.8 mmol Si m-2 y-1, while the organic carbon flux amounted to 154.9 mmol C m-2 y-1. The overall efficiency of C export to depth compared to surface primary production budgets was low (3%) and similar to values found for oligotrophic regions of the Eastern Mediterranean. On the other hand, we documented a very high efficiency of the Si export at the SOFi site, with 62% of the Si produced in the surface layer exported to the bottom trap. The excellent correlation between BSi and particulate organic C (POC) in the bottom trap further indicates that the bulk of the organic matter is transferred to depth in association with diatoms. However, the main fraction of the C produced by phytoplankton is lost in the water column through oxidation or metabolisation, while biogenic silica is more efficiently transferred to depth. This strong Si/C decoupling with depth is likely to be the cause of the spring depletion of silicic acid over the entire water column which was observed by mid-April in a companion study (Leblanc et al., 2003) and may cause a severe Si limitation of the diatom spring bloom in this oligotrophic region of the Gulf of Lions.
The role of N2-fixation to simulate the pCO2 observations from the Baltic Sea
Authors: Leinweber A. , Neumann T. , Schneider B.
Published: April 2010

Abstract. Measurements in the central Baltic Sea have shown that dissolved inorganic carbon (DIC) concentrations in the upper water column continue to decrease even after complete depletion of dissolved inorganic nitrogen (DIN). To explain this observation, a new external supply of nitrogen is required without a concomitant supply of inorganic carbon. The primary proposed candidate process is N2-fixation. In order to address this question in the eastern Gotland Sea, a biogeochemical model containing nine state variables including diatoms, flagellates, and nitrogen fixing cyanobacteria, was coupled to a 1D physical model. The results from the winter period until the onset of the spring bloom of 1997 gave reasonable surface water values for partial pressure of CO2 (pCO2) compared with measurements of the pCO2. However, the model failed to simulate the observed pCO2 drawdown for the period from the end of the spring bloom until late summer. Even after introducing a seasonal dissolved organic carbon (DOC) excess production and varying different process parameterization the simulated pCO2 values did not improve. Only the shift from a sharp to a moderate temperature dependency in addition to an increase in the C:P ratio of the nitrogen fixing cyanobacteria made it possible for the model to match the pCO2 observations. The resulting total nitrogen fixation (167 mmol m-2 a-1) exceeds previous measurement-based estimates but is in good agreement with recent rate measurement based estimates.
Variability of the surface water partial pressure of CO2 in the North Sea
Authors: Thomas H. , Bozec Y. , Elkalay K. , de Baar H. J. W. , Borges A. V. , Schiettecatte L.-S.
Published: April 2010

Abstract. The seasonal variability of the partial pressure of CO2 (pCO2) has been investigated in the North Sea, a northwest European shelf sea. Based on a seasonal, high resolution data set the main controlling factors - biological processes and temperature - have been identified and quantified. In the central and northern parts being a CO2-sink all year round, the biological control dominates the temperature control. In the southern part, the temperature control dominates the biological control at an annual scale, since the shallow water column prevents stronger net-CO2 removal from the surface layer due to the absence of seasonal stratification. The consequence is a reversal of the CO2 sea-to-air flux during the spring bloom period, the only time, when CO2 is taken up from the atmosphere in the southern region. Net community production in the mixed layer has been estimated to 4 mol C m-2yr-1 with higher values (4.3 mol C m-2yr-1) in the northern part and lower values in the southern part (2.6 mol C m-2yr-1).
The effects of biomanipulation on the biogeochemistry, carbon isotopic composition and pelagic food web relations of a shallow turf lake
Authors: Bontes B. M. , Pel R. , Ibelings B. W. , Boschker H. T. S. , Middelburg J. J. , van Donk E.
Published: April 2010

Abstract. The effects of fish removal on the biogeochemistry and lower-trophic level food web relations were studied in a shallow eutrophied turf lake. Biomanipulation led to an increase in transparency and macrophyte biomass and decrease in phytoplankton abundance, but zooplankton numbers did not increase. Moreover, fish removal resulted in high pH, high O2, low CO2, and more negative δ13CDIC values than expected, which is proposed to be the likely result of chemical enhanced diffusion with large negative fractionation (-13). By combining fluorescence activated cell sorting and isotope ratio mass spectrometry (IRMS) of fatty acids we were able to obtain group specific δ13C signatures and to trace possible shifts in δ13C resulting from fish removal. Fractionation values of green algae (20) and diatoms (22) were uniform and independent of treatment, while fractionation factors of filamentous cyanobacteria were variable between the treatments that differed in CO2 availability. 13C-labeling of the phytoplankton groups showed that biomanipulation led to increased growth rates of green algae and diatoms at the expense of cyanobacteria. Finally, the primary consumer Chydorus appeared to prefer cyanobacteria, whilst Asplanchna grazed predominantly upon eukaryotes.
Warming mobilises young and old soil carbon equally
Authors: Conen F. , Leifeld J. , Seth B. , Alewell C.
Published: April 2010

Abstract. The temperature sensitivity of soil organic carbon decomposition is critically important in determining the rate of future climate change because soils store 2–3 times the amount of atmospheric carbon. Of particular controversy is the question, whether temperature sensitivity differs between young or labile and old or more stable carbon pools. Contrasting hypotheses are currently supported. Depending on assumptions, identical data have been used to endorse contradicting hypotheses. Ambiguities in experimental methodology have so far limited corroboration of any particular hypothesis. Here, we show in a clear-cut approach that differences in temperature sensitivity between young and old carbon are negligible. Using the change in stable isotope composition in transitional systems from C3 to C4 vegetation, we were able to directly distinguish the temperature sensitivity of carbon differing several decades in age. Compared to previously followed approaches, we were able to identify release of much older carbon, un-natural conditions of long-term incubations were avoided and no arguable curve-fitting was employed. Our results demonstrate that feedbacks of the carbon cycle on climate change are driven equally by young and old soil organic carbon.
Consequences of respiration in the light on the determination of production in pelagic systems
Authors: Pringault O. , Tassas V. , Rochelle-Newall E.
Published: April 2010

Abstract. Oxygen microprobes were used to estimate Respiration (R), Net Production (NP) and Production (P) in coastal sea water samples. Using this highly stable and reproducible technique to measure oxygen change during alternating dark and light periods, we show that respiration in the light could represent up to 640% of respiration in the dark. This light enhanced dark respiration can remain elevated for several hours following a 12 h period of illumination. The non inclusion of Rlight into calculations of production leads to an underestimation of P, which can reach up to 650% in net heterotrophic systems. The P:R ratio is in turn affected by the higher respiration rates and by the underestimation of P. While the integration of Rlight in to the calculation of P:R ratio does not change the metabolic balance of the system, it decreases the observed tendency, thus net autotrophic systems become less autotrophic and net heterotrophic systems become less heterotrophic. As a consequence, we propose that efforts have to be focused on the estimation and the integration of Rlight into the determination of P and R for a better understanding of the aquatic carbon cycle.
Mn seasonal upwellings recorded in Lake Tanganyika mussels
Authors: Langlet D. , Alleman L. Y. , Plisnier P.-D. , Hughes H. , André L.
Published: April 2010

Abstract. Biogenic productivity of Lake Tanganyika is highly dependent on seasonal upwellings of anoxic deep waters. We investigated the shell of freshwater bivalve Pleiodon spekii as a geochemical archive of these periodic hydrological changes tuned by the monsoon regime. The results of a 2-years-long geochemical survey of the coastal waters performed on the dissolved and particulate fractions were put in perspective against laser ablation ICP-MS profiles of Mn in five aragonitic shells from the same lake location. Skeletal Mn profiles in 3 shells are very similar and dominated by episodic peaks that matched the Mn increase recorded in surface waters during the 2002 upwelling, while a shell collected during 2003 dry season detect both 2002 and 2003 upwelling events. Larger shells showing an extremely reduced growth display more than 8Mn peaks suggesting at least an 8 years record of seasonal changes in water composition.
Soils apart from equilibrium – consequences for soil carbon balance modelling
Authors: Wutzler T. , Reichstein M.
Published: April 2010

Abstract. Many projections of the soil carbon sink or source are based on kinetically defined carbon pool models. Parameters of these models are often determined in a way that the steady state of the model matches observed carbon stocks. The underlying simplifying assumption is that observed carbon stocks are near equilibrium. This assumption is challenged by observations of very old soils that do still accumulate carbon. In this modelling study we explored the consequences of the case where soils are apart from equilibrium. Calculation of equilibrium states of soils that are currently accumulating small amounts of carbon were performed using the Yasso model. It was found that already very small current accumulation rates cause big changes in theoretical equilibrium stocks, which can virtually approach infinity. We conclude that soils that have been disturbed several centuries ago are not in equilibrium but in a transient state because of the slowly ongoing accumulation of the slowest pool. A first consequence is that model calibrations to current carbon stocks that assume equilibrium state, overestimate the decay rate of the slowest pool. A second consequence is that spin-up runs (simulations until equilibrium) overestimate stocks of recently disturbed sites. In order to account for these consequences, we propose a transient correction. This correction prescribes a lower decay rate of the slowest pool and accounts for disturbances in the past by decreasing the spin-up-run predicted stocks to match an independent estimate of current soil carbon stocks. Application of this transient correction at a Central European beech forest site with a typical disturbance history resulted in an additional carbon fixation of 5.7±1.5 tC/ha within 100 years. Carbon storage capacity of forest soils is potentially much higher than currently assumed. Simulations that do not adequately account for the transient state of soil carbon stocks neglect a substantial amount of current carbon accumulation.
Spatial variation in N2-fixation rate and diazotroph activity in the Tropical Atlantic
Authors: Montoya J. P. , Voss M. , Capone D. G.
Published: April 2010

Abstract. A variety of N2-fixers occur in the tropical Atlantic and these diazotrophs make a substantial contribution to the nitrogen budget of the upper water column. A synthesis of previously published and novel rate measurements for the Tropical North Atlantic provides insight into the role of two different diazotroph groups in supporting N2 fixation in the tropical Atlantic. The overall rate of N2-fixation by the two groups of diazotrophs was similar in the eastern and western parts of the basin, but N2-fixation by \\textit{Trichodesmium} was strongly dominant in the western part of the basin while small diazotrophs played a much larger role to the east of 40° W. The reasons for this shift in dominance are unclear, as is the identity of the small organisms fixing N2 in the water column.
Direct and indirect metabolic CO2 release by humanity
Authors: Prairie Y. T. , Duarte C. M.
Published: April 2010

Abstract. The direct CO2 released by respiration of humans and domesticated animals, as well as the CO2 derived from the decomposition of their resulting wastes was calculated in order to ascertain the direct and indirect metabolic contribution of humanity to CO2 release. Human respiration was estimated to release 0.6 Gt C year−1 and that of their associated domestic animals was estimated to release 1.5 Gt C year−1, to which an indirect release of 1.0 Gt C year−1, derived from decomposition of the organic waste and garbage produced by humans and their domestic animals, must be added. These combined direct and indirect metabolic sources, estimated at 3.1 Gt C year−1, has increased 7 fold since pre-industrial times and is forecasted to continue to rise over the 21st century.
Quantifying biologically and physically induced flow and tracer dynamics in permeable sediments
Authors: Meysman F. J. R. , Galaktionov O. S. , Cook P. L. M. , Janssen F. , Huettel M. , Middelburg J. J.
Published: April 2010

Abstract. Insight in the biogeochemistry and ecology of sandy sediments crucially depends on a quantitative description of pore water flow and the associated transport of various solutes and particles. Here, we compare and analyse existing models of tracer dynamics in permeable sediments. We show that all models can be derived from a generic backbone, consisting of the same flow and tracer equations. The principal difference between model applications concerns the geometry of the sediment-water interface and the pressure conditions that are specified along this boundary. We illustrate this commonality with four different case studies. These include biologically and physically induced pore water flows, as well as simplified laboratory set-ups versus more complex field-like conditions: [1] lugworm bio-irrigation in laboratory set-up, [2] interaction of bio-irrigation and groundwater seepage on a tidal flat, [3] pore water flow induced by rotational stirring in benthic chambers, and [4] pore water flow induced by unidirectional flow over a ripple sequence. To illustrate the potential of the generic model approach, the same two example simulations are performed in all four cases: (a) the time-dependent spreading of an inert tracer in the pore water, and (b) the computation of the steady-state distribution of oxygen in the sediment. Overall, our model comparison indicates that model development is promising, but within an early stage. Clear challenges remain in terms of model development, model validation, and model implementation.
Microbial colonization and alteration of basaltic glass
Authors: Einen J. , Kruber C. , Øvreås L. , Thorseth I. H. , Torsvik T.
Published: April 2010

Abstract. Microorganisms have been reported to be associated with the alteration of the glassy margin of seafloor pillow basalts (Thorseth et al., 2001, 2003; Lysnes et al., 2004). The amount of iron and other biological important elements present in basalts and the vast abundance of basaltic glass in the earth's crust, make glass alteration an important process in global element cycling. To gain further insight into microbial communities associated with glass alteration, five microcosm experiments mimicking seafloor conditions were inoculated with seafloor basalt and incubated for one year. Mineral precipitations, microbial attachment to the glass and glass alteration were visualized by scanning electron microscopy (SEM), and the bacterial community composition was fingerprinted by PCR and denaturing gradient gel electrophoresis (DGGE) in combination with sequencing. SEM analysis revealed a microbial community with low morphological diversity of mainly biofilm associated and prosthecate microorganisms. Approximately 30 nm thick alteration rims developed on the glass in all microcosms after one year of incubation; this however was also seen in non inoculated controls. Calcium carbonate precipitates showed parallel, columnar and filamentous crystallization habits in the microcosms as well as in the sterile controls. DGGE analysis showed an alteration in bacterial community profiles in the five different microcosms, as a response to the different energy and redox regimes and time. In all microcosms a reduction in number of DGGE bands, in combination with an increase in cell abundance were recorded during the experiment. Sequence analysis showed that the microcosms were dominated by four groups of organisms with phylogenetic affiliation to four taxa: The Rhodospirillaceae, a family containing phototrophic marine organisms, in which some members are capable of heterotrophic growth in darkness and N2 fixation; the family Hyphomicrobiaceae, a group of prosthecate oligotrophic organisms; the genus Rhizobium, N2 fixating heterotrophs; and the genus Sphingomonas, which are known as bio-film producing oligotrophs. Although no bioalteration of glass could be confirmed from our experiments, oligotrophic surface adhering bacteria such as the Sphingomonas sp. and Hyphomicrobium sp. may nevertheless be important for bioalteration in nature, due to their firm attachment to glass surfaces, and their potential for biofilm production.
River biogeochemistry and source identification of nitrate by means of isotopic tracers in the Baltic Sea catchments
Authors: Voss M. , Deutsch B. , Elmgren R. , Humborg C. , Kuuppo P. , Pastuszak M. , Rolff C. , Schulte U.
Published: April 2010

Abstract. Nitrate input to a river is largely controlled by land use in its catchment. We compared the information carried by the isotopic signatures of nitrate in 12 Baltic rivers, in relation to the vegetation cover of their catchments. We found isotope values in nitrate ranging from −2 to 14 for δ15N and 8 to 25 per mil for δ18O. Seasonal signals were evident in all rivers. The annual variability of riverine isotope signatures is presented in detail for one Nordic, the Kemijoki, and two southern rivers, Vistula and Oder. Nordic rivers with relatively pristine vegetation in its catchments show not only low δ15N values and high δ18O-NO3− but also lower annual variability than rivers draining densely populated land. Seasonal signals could be found in all of the rivers. We used load weighted nitrate isotope data and data from the three major sources (farmland/sewage, atmospheric deposition and from runoff of pristine soils) to theoretically estimate the shares of nitrate from these sources. The results agree well with same estimates derived from a Global Land Cover data base. The comparison with an emission model (EM) reveals good agreements for intensively used catchments and rather bad ones for pristine catchments. Advantages and limitations of the tested model types are discussed.
Physiological constraints on the global distribution of Trichodesmium – effect of temperature on diazotrophy
Authors: Breitbarth E. , Oschlies A. , LaRoche J.
Published: April 2010

Abstract. The cyanobacterium Trichodesmium is an important link in the global nitrogen cycle due to its significant input of atmospheric nitrogen into the ocean. Incorporating Trichodesmium in ocean biogeochemical circulation models relies on field-based correlations between temperature and Trichodesmium abundance. The observed correlation of Trichodesmium abundance with temperature in the ocean may result in part from a direct effect on Trichodesmium growth rates through the control of cellular biochemical processes, or indirectly through its influence on mixed layer depth, light and nutrient regimes. Here we present results indicating that the observed correlation of Trichodesmium with temperature in the field reflects primarily the direct physiological effects of temperature on diazotrophic growth of Trichodesmium. Trichodesmium IMS-101 (an isolate of T. erythraeum) could acclimate and grow at temperatures ranging from 20 to 34°C. Maximum growth rates (μmax=0.25 day−1) and maximum nitrogen fixation rates (0.13 mmol N mol POC−1 h−1) were measured within 24 to 30°C. This empirical relationship and global warming scenarios derived from state-of-the-art climate models set a physiological constraint on the future distribution of Trichodesmium that could significantly affect nitrogen input into oligotrophic waters by this diazotroph.
Nitrous oxide net exchange in a beech dominated mixed forest in Switzerland measured with a quantum cascade laser spectrometer
Authors: Eugster W. , Zeyer K. , Zeeman M. , Michna P. , Zingg A. , Buchmann N. , Emmenegger L.
Published: April 2010

Abstract. Nitrous oxide fluxes were measured at the Lägeren CarboEurope IP flux site over the multi-species mixed forest dominated by European beech and Norway spruce. Measurements were carried out during a four-week period in October–November 2005 during leaf senescence. Fluxes were measured with a standard ultrasonic anemometer in combination with a quantum cascade laser absorption spectrometer that measured N2O, CO2, and H2O mixing ratios simultaneously at 5 Hz time resolution. To distinguish insignificant fluxes from significant ones it is proposed to use a new approach based on the significance of the correlation coefficient between vertical wind speed and mixing ratio fluctuations. This procedure eliminated roughly 56% of our half-hourly fluxes. Based on the remaining, quality checked N2O fluxes we quantified the mean efflux at 0.8 ± 0.4 μmol m−2 h−1 (mean ± standard error). Most of the contribution to the N2O flux occurred during a 6.5-h period starting 4.5 h before each precipitation event. No relation with precipitation amount could be found. Visibility data representing fog density and duration at the site indicate that wetting of the canopy may have as strong an effect on N2O effluxes as does below-ground microbial activity. It is speculated that above-ground N2O production from the senescing leaves at high moisture (fog, drizzle, onset of precipitation event) may be responsible for part of the measured flux. In comparison with the annual CO2 budget of –342 g C m−2 yr−1 it is estimated that concurrent N2O fluxes offset at least 5% of the greenhouse forcing reduction via net CO2 uptake.
Community shifts and carbon translocation within metabolically-active rhizosphere microorganisms in grasslands under elevated CO2
Authors: Denef K. , Bubenheim H. , Lenhart K. , Vermeulen J. , Van Cleemput O. , Boeckx P. , Müller C.
Published: April 2010

Abstract. The aim of this study was to identify the microbial communities that are actively involved in the assimilation of rhizosphere-C and are most sensitive in their activity to elevated atmospheric CO2 in grassland ecosystems. For this, we analyzed 13C signatures in microbial biomarker phospholipid fatty acids (PLFA) from an in situ 13CO2 pulse-labeling experiment in the Gießen Free-Air Carbon dioxide Enrichment grasslands (GiFACE, Germany) exposed to ambient and elevated (i.e. 50% above ambient) CO2 concentrations. Carbon-13 PLFA measurements at 3 h, 10 h and 11 months after the pulse-labeling indicated a much faster transfer of newly produced rhizosphere-C to fungal compared to bacterial PLFA. After 11 months, the proportion of 13C had decreased in fungal PLFA but had increased in bacterial PLFA compared to a few hours after the pulse-labeling. Nevertheless, a significant proportion of the rapidly assimilated rhizosphere-C was still present in fungal PLFA after 11 months. These results demonstrate the dominant role of fungi in the immediate assimilation of rhizodeposits in grassland ecosystems, while also suggesting a long-term retention of rhizosphere-C in the fungal mycelium as well as a possible translocation of the rhizosphere-C from the fungal to bacterial biomass. Elevated CO2 caused an increase in the relative abundance of root-derived PLFA-C in the saprotrophic fungal PLFA 18:2ω6,9 as well as arbuscular mycorrhizal fungal PLFA 16:1ω5, but a decrease in the saprotrophic fungal biomarker PLFA 18:1ω9. This suggests enhanced rhizodeposit-C assimilation only by selected fungal communities under elevated CO2.
Contribution of picoplankton to the total particulate organic carbon (POC) concentration in the eastern South Pacific
Authors: Grob C. , Ulloa O. , Claustre H. , Huot Y. , Alarcón G. , Marie D.
Published: April 2010

Abstract. Prochlorococcus, Synechococcus, picophytoeukaryotes and bacterioplankton abundances and contributions to the total particulate organic carbon concentration (POC), derived from the total particle beam attenuation coefficient (cp), were determined across the eastern South Pacific between the Marquesas Islands and the coast of Chile. All flow cytometrically derived abundances decreased towards the hyper-oligotrophic centre of the gyre and were highest at the coast, except for Prochlorococcus, which is not detected under eutrophic conditions. Temperature and nutrient availability appeared important in modulating picophytoplankton abundance, according to the prevailing trophic conditions. Although the non-vegetal particles tended to dominate the cp signal everywhere along the transect (50 to 83%), this dominance seemed to weaken from oligo- to eutrophic conditions, the contributions by vegetal and non-vegetal particles being about equal under mature upwelling conditions. Spatial variability in the vegetal compartment was more important than the non-vegetal one in shaping the water column particulate attenuation coefficient. Spatial variability in picophytoplankton biomass could be traced by changes in both Tchla and cp. Finally, picophytoeukaryotes contributed with ~38% on average to the total integrated phytoplankton carbon biomass or vegetal attenuation signal along the transect, as determined by direct size measurements on cells sorted by flow cytometry and optical theory. The role of picophytoeukaryotes in carbon and energy flow would therefore be very important, even under hyper-oligotrophic conditions.
The value of adding optics to ecosystem models: a case study
Authors: Fujii M. , Boss E. , Chai F.
Published: April 2010

Abstract. Many ecosystem models have been developed to study the ocean's biogeochemistry, but most of these models use simple formulations to describe light penetration and spectral quality. Given that processes such as photosynthesis and photo-oxidation are uniquely important for biogeochemical processes in the upper ocean, it is necessary to model light distribution accurately. In addition, the global scale observations of proxies of biogeochemical variables are based on the color of the ocean. The ability to simulate the color of the ocean provides the possibility of comparing model simulation with these observations. Here, an optical model is coupled with a previously published ecosystem model that explicitly represents two phytoplankton (picoplankton and diatoms) and two zooplankton functional groups, as well as multiple nutrients and detritus. Surface ocean color field and subsurface light field are calculated by coupling the ecosystem model with an optical model that relates biogeochemical standing stocks with inherent optical properties (absorption, scattering); this provides input to a commercially available radiative transfer model (Ecolight). We apply this bio-optical model to the equatorial Pacific upwelling region, and find the model to be capable of reproducing many measured optical properties and key biogeochemical processes in this region. Results include large contributions by non-algal particles to the total scattering or attenuation (>50% at 660 nm) and their small contribution to particulate absorption (<20% at 440 nm), and a remarkable contribution by picoplankton to total phytoplankton absorption (>95% at 440 nm). These results are consistent with the field observations. In order to achieve such good agreement between data and model results, however, key model parameters, for which no field data is available, have to be constrained. Sensitivity analysis of the model results to optical parameters reveals the significant role of colored dissolved organic matter to the modeled properties. Coupling explicit optics to an ecosystem model provides several advantages in generating: (1) a more accurate subsurface light-field, which is important for light sensitive biogeochemical processes such as photosynthesis and photo-oxidation, (2) added constraints on model parameters that help to reduce uncertainties in ecosystem model simulations, and (3) model output which is comparable to basic remotely-sensed properties. In addition, the coupling of biogeochemical models and optics paves the road for future assimilation of ocean color and in-situ measured optical properties into the models.
CO2 exchange and Carbon balance in two grassland sites on eutrophic drained peat soils
Authors: Veenendaal E. M. , Kolle O. , Leffelaar P. A. , Schrier-Uijl A. P. , Van Huissteden J. , Van Walsem J. , Möller F. , Berendse F.
Published: April 2010

Abstract. In this study we investigated the role of intensive and extensive dairy farm practices on CO2 exchange and the carbon balance of peatlands by means of eddy covariance (EC) measurements. Year long EC measurements were made in two adjacent farm sites on peat soil in the western part of the Netherlands. One site (Stein) is a new meadow bird reserve and is managed predominantly by mowing in June and August. The second site (Oukoop) is an intensive dairy farm. Minimum ecosystem flux of the grass sward (range −2 to −34 μmol CO2 m−2 s−1) showed a close and similar linear relationship with Leaf Area Index (LAI; range 1 to 5) except in maturing hay meadows, where minimum ecosystem flux did not decrease further. Apparent quantum yield varied between −0.02 and −0.08 (mean −0.045) μmol CO2 μmol−1 photons at both sites and was significantly correlated with LAI during the growth season. Ecosystem Respiration at 10°C (R10) calculated from the year round data set was 3.47 μmol CO2 m−2 s−1 at Stein and 3.64 μmol CO2 m−2 s−1 at Oukoop. Both sites were a source of carbon in winter and a sink during summer, with net ecosystem exchange varying between 50 to 100 mmol CO2m−2 d−1 in winter to below −400 mmol CO2 m−2 d−1 in summer. Periodically both sites became a source after mowing. Net annual ecosystem exchange (NEE) for Stein was −8.4 g C m−2 a−1 and for Oukoop 122.4 g C m−2 a−1, the difference between the sites was mainly due a difference in GEP (101 g C m−2 a−1). However when biomass removal, manure applications and estimates of methane emissions are taken into account, both eutrophic peat meadows are a strong source for C ((462 g C m−2 a−1 and 465 g C m−2 a−1 at Stein and Oukoop, respectively).
Effects of drought – altered seasonality and low rainfall – in net ecosystem carbon exchange of three contrasting Mediterranean ecosystems
Authors: Pereira J. S. , Mateus J. A. , Aires L. M. , Pita G. , Pio C. , Andrade V. , Banza J. , David T. S. , Rodrigues A. , David J. S.
Published: April 2010

Abstract. Droughts cause reductions in gross primary production (GPP) and also in net ecosystem exchange (NEE), contributing to most of the inter-annual variability in terrestrial carbon sequestration. In seasonally dry climates (Mediterranean) droughts result from reductions in annual rainfall and from changes in rain seasonality. In western Iberia, the hydrological-year (i.e., from October to September) of 2004–2005 was extremely dry, with precipitation 50% below the long-term mean (691 mm in 1961–1990), but 2005–2006 was normal. We compared the carbon fluxes measured by the eddy covariance technique from three contrasting ecosystems in southern Portugal: an evergreen oak woodland (savannah-like) with ca. 21% tree cover; a Mediterranean C3/C4 grassland; and a coppiced eucalyptus plantation. During the dry hydrological-year of 2004–2005, NEE was lowest, the highest sink strength was in the eucalypt plantation (NEE = –399 g C m −2 year−1) as compared to the oak woodland (NEE = –88 g C m −2 year−1), and the grassland (NEE = +49 g C m −2 year −1). The latter was a source of carbon dioxide. The NEE values of the dry year were, however, much lower than those for wetter years, e.g. NEE = –861 g C m−2 year −1 in 2002–2003 in the eucalypt plantation. The NEE of the grassland and the oak savannah in the 2005–2006 hydrological-year, with annual precipitation above the long term mean, were –190 and –120 g C m −2 year−1, respectively. All ecosystems studied increased their rain-use efficiency (GPP per unit of rain volume) increased in dry years. In the case of annual vegetation – grassland and low tree density woodland, however &ndash, rain-use efficiency decreased with severe drought. However, this was more pronounced in the eucalypt plantation due to greater GPP and the use of deep soil water resources. Although both calendar years of 2004 and 2005 had equally low rainfall, the effect of drought on the eucalypt plantation was delayed until the second dry year. This suggests that the effects of water deficits on Mediterranean forests are exacerbated by prolonged droughts when long-term soil water reserves are depleted. The grassland, however, was more vulnerable and responded faster to water deficits. This effect of drought was less pronounced in the oak woodland due to the sparse tree cover.
The sea surface CO2 fugacity and its relationship with environmental parameters in the subpolar North Atlantic 2005
Authors: Olsen A. , Brown K. R. , Chierici M. , Johannessen T. , Neill C.
Published: April 2010

Abstract. We present the first year-long subpolar transatlantic set of surface seawater CO2 fugacity (fCO2sw) data. The data were obtained aboard the MV Nuka Arctica in 2005 and provide a quasi-continuous picture of the fCO2sw variability between Denmark and Greenland. Complementary real-time high resolution data of surface chlorophyll a (chl a) concentrations and mixed layer depth (MLD) estimates have been collocated with the fCO2sw data. Off the shelves the fCO2sw goes through a pronounced seasonal cycle. Surface waters are saturated to slightly supersaturated over a wide range of temperatures in winter. Through spring and summer fCO2sw decreases by approximately 60 μatm due to biological carbon consumption which is not fully counteracted by the fCO2sw increase due to summer warming. The changes are synchronous with changes in chl a concentrations and MLD, and fCO2sw is correlated to both of these through exponential decay and growth curves, respectively. In particular, MLD appears to be able to predict open ocean subpolar fCO2sw to within 12 μatm using a single equation for the whole year. The predictive capability of chl a is around 20% less, with root mean square values of 14.5 μatm. As fCO2sw extrapolation parameters, both MLD and chl a outperform sea surface temperatures in the open subpolar North Atlantic. However, the situation on the shelves is much more heterogeneous and the fCO2sw–parameter relationships are not as strong as they are off the shelves.
Anthropogenic carbon in the eastern South Pacific Ocean
Authors: Azouzi L. , Gonçalves Ito R. , Touratier F. , Goyet C.
Published: April 2010

Abstract. We present results from the BIOSOPE cruise in the eastern South Pacific Ocean. In particular, we present estimates of the anthropogenic carbon CantTrOCA distribution in this area using the TrOCA method recently developed by Touratier and Goyet (2004a, b) and Touratier et al. (2007). We study the distribution of this anthropogenic carbon taking into account of the hydrodynamic characteristics of this region. We then compare these results with earlier estimates in nearby areas of the anthropogenic carbon as well as other anthropogenic tracer (CFC-11). The highest concentrations of CantTrOCA are located around 13° S 132° W and 32° S 91° W, and their concentrations are larger than 80 μmol kg−1 and 70 μmol kg−1, respectively. The lowest concentrations were observed below 800 m depths (≤2 μ mol kg−1) and at the Oxygen Minimum Zones (OMZ), mainly around 140° W (<11 μmol kg−1). The comparison with earlier work in nearby areas provides a general trend and indicates that the results presented here are in general agreement with previous knowledge. This work further improves our understanding on the penetration of anthropogenic carbon in the eastern Pacific Ocean.
The exchange of carbon dioxide between wet arctic tundra and the atmosphere at the Lena River Delta, Northern Siberia
Authors: Kutzbach L. , Wille C. , Pfeiffer E.-M.
Published: April 2010

Abstract. The exchange fluxes of carbon dioxide between wet arctic polygonal tundra and the atmosphere were investigated by the micrometeorological eddy covariance method. The investigation site was situated in the centre of the Lena River Delta in Northern Siberia (72°22' N, 126°30' E). The study region is characterized by a polar and distinctly continental climate, very cold and ice-rich permafrost and its position at the interface between the Eurasian continent and the Arctic Ocean. The soils at the site are characterized by high organic matter content, low nutrient availability and pronounced water logging. The vegetation is dominated by sedges and mosses. The micrometeorological campaigns were performed during the periods July–October 2003 and May–July 2004 which included the period of snow and soil thaw as well as the beginning of soil refreeze. The main CO2 exchange processes, the gross photosynthesis and the ecosystem respiration, were found to be of a generally low intensity. The gross photosynthesis accumulated to –432 g m−2 over the photosynthetically active period (June–September). The contribution of mosses to the gross photosynthesis was estimated to be about 40%. The diurnal trend of the gross photosynthesis was mainly controlled by the incoming photosynthetically active radiation. During midday the photosynthetic apparatus of the canopy was frequently near saturation and represented the limiting factor on gross photosynthesis. The synoptic weather conditions strongly affected the exchange fluxes of CO2 by changes in cloudiness, precipitation and pronounced changes of air temperature. The ecosystem respiration accumulated to +327 g m−2 over the photosynthetically active period, which corresponds to 76% of the CO2 uptake by photosynthesis. However, the ecosystem respiration continued at substantial rates during autumn when photosynthesis had ceased and the soils were still largely unfrozen. The temporal variability of the ecosystem respiration during summer was best explained by an exponential function with surface temperature, and not soil temperature, as the independent variable. This was explained by the major role of the plant respiration within the CO2 balance of the tundra ecosystem. The wet polygonal tundra of the Lena River Delta was observed to be a substantial CO2 sink with an accumulated net ecosystem CO2 exchange of –119 g m−2 over the summer and an estimated annual net ecosystem CO2 exchange of –71 g m−2.
Natural variability of bio-optical properties in Case 1 waters: attenuation and reflectance within the visible and near-UV spectral domains, as observed in South Pacific and Mediterranean waters
Authors: Morel A. , Claustre H. , Antoine D. , Gentili B.
Published: April 2010

Abstract. The optical properties of Case 1 waters have been empirically related to the chlorophyll concentration, [Chl], historically used as an index of the trophic state and of the abundance of the biological materials. The natural variability around the mean statistical relationships is here examined by comparing the apparent optical properties (spectral downward irradiance attenuation and reflectance as a function of [Chl]) which were determined in two environments, the Pacific and Mediterranean waters. These oceanic zones apparently form two extremes of the bio-optical variability range. The systematic deviations, in both directions with respect to the average laws, mainly result from the differing contents in non-algal detrital materials and dissolved colored substance for a given [Chl] level. These contents are higher and lower than the average, in the Mediterranean Sea and Pacific Ocean, respectively. The divergences between the two water bodies, detected in the visible spectral domain, are considerably accentuated in the UV domain. The bio-optical properties in this spectral domain (310–400 nm) are systematically explored. Their prediction based on the sole [Chl] index is problematic; although it is probably possible on a regional scale, an ubiquitous relationship does not seem to exist for the global scale.
Linking an economic model for European agriculture with a mechanistic model to estimate nitrogen losses from cropland soil in Europe
Authors: Leip A. , Marchi G. , Koeble R. , Kempen M. , Britz W. , Li C.
Published: April 2010

Abstract. For the comprehensive assessment of the policy impact on greenhouse gas emissions from agricultural soils both socio-economic aspects and the environmental heterogeneity of the landscape are important factors that must be considered. We developed a modelling framework that links the large-scale economic model for agriculture CAPRI with the bio-geochemistry model DNDC to simulate greenhouse gas fluxes, carbon stock changes and the nitrogen budget of agricultural soils in Europe. The framework allows the ex-ante simulation of agricultural or agri-environmental policy impacts on wide range of environmental problems such as climate change (greenhouse gas emissions), air pollution and groundwater pollution. Those environmental impacts can be analysed in the context of economic and social indicators as calculated by the economic model. The methodology consists in four steps (i) the definition of appropriate calculation units that can be considered as homogeneous in terms of economic behaviour and environmental response; (ii) downscaling of regional agricultural statistics and farm management information from a CAPRI simulation run into the spatial calculation units; (iii) setting up of environmental model scenarios and model runs; and finally (iv) aggregating results for interpretation. We show first results of the nitrogen budget in cropland for the area of fourteen countries of the European Union. These results, in terms of estimated nitrogen fluxes, must still be considered as illustrative as needs for improvements in input data (e.g. the soil map) and management data (yield estimates) have been identified and will be the focus of future work. Nevertheless, we highlight inter-dependencies between farmer's choices of land uses and the environmental impact of different cultivation systems.