bluuebunny/crossref_metadata_2025 huggingface dataset:
Review: the effects of secular variation in seawater Mg/Ca on marine biocalcification
Authors: Ries J. B.
Published: April 2010

Abstract. Synchronized transitions in the polymorph mineralogy of the major reef-building and sediment-producing calcareous marine organisms and abiotic CaCO3 precipitates (ooids, marine cements) throughout Phanerozoic time is believed to have been caused by tectonically-induced variations in seawater molar Mg/Ca (>2="aragonite seas"; <2="calcite seas"). Here, I review a series of experiments in which extant calcifying taxa were reared in experimental seawater formulated over the range of mMg/Ca ratios (1.0 to 5.2) that occurred throughout their geologic history. Aragonite-secreting bryopsidalean algae and scleractinian corals and calcite-secreting coccolithophores exhibited higher rates of calcification and growth in the experimental seawaters that favored their skeletal mineral. These results support the assertion that seawater Mg/Ca played an important role in determining which hypercalcifying marine organisms were the major reef-builders and sediment-producers throughout Earth history. The observation that primary production increased along with calcification in mineralogically-favorable seawater is consistent with the hypothesis that calcification promotes photosynthesis within autotrophs through the liberation of CO2. The Mg/Ca ratio of calcite secreted by the coccolithophores, coralline algae and reef-dwelling animals (crustacea, urchins, calcareous tube worms) declined with reductions in seawater Mg/Ca. Calcifying microbial biofilms varied their mineral polymorph with seawater Mg/Ca (mMg/Ca<2=low Mg calc; mMg/Ca>2=arag+high Mg calc), suggesting a nearly abiotic mode of calcification. These results indicate that biomineralogical control can be partially overridden by ambient seawater Mg/Ca and suggests that modern high Mg calcite organisms probably secreted low Mg calcite in calcite seas of the past. Notably, Mg fractionation in autotrophic organisms was more strongly influenced by changes in seawater Mg/Ca, a probable consequence of them inducing a less controlled mode of calcification simply through the removal of CO2 via photosynthesis. This body of work also has implications for thermal-chemical reconstructions of seawater that are based upon skeletal Mg/Ca. And by identifying how marine calcifiers respond to changes in seawater Mg/Ca and absolute Ca2+ concentration, this work should enhance our interpretation of the parallel studies investigating the effects of CO2-induced ocean acidification on marine calcification.
Natural iron enrichment around the Antarctic Peninsula in the Southern Ocean
Authors: Ardelan M. V. , Holm-Hansen O. , Hewes C. D. , Reiss C. S. , Silva N. S. , Dulaiova H. , Steinnes E. , Sakshaug E.
Published: April 2010

Abstract. As part of the US-AMLR program that occupied 99 hydrographic stations in the South Shetland Islands-Antarctic Peninsula region in January–February of 2006, concentrations of dissolved iron (DFe) and total acid-leachable iron (TaLFe) were measured in the upper 150 m at 16 stations (both coastal and pelagic waters). The concentrations in the upper mixed layer (UML) of DFe and TaLFe were relatively high in Weddell Sea Shelf Waters (~0.6 nM and 15 nM, respectively) and lowest in Drake Passage waters (~0.2 nM and 0.9 nM, respectively). In the Bransfield Strait, representing a mixture of waters from the Weddell Sea and the Antarctic Circumpolar Current (ACC), concentrations of DFe were ~0.4 nM and of TaLFe ~1.7 nM. The highest concentrations of DFe and TaLFe in the UML were found at shallow coastal stations close to Livingston Island (~1.6 nM and 100 nM, respectively). The ratio of TaLFe:DFe varied with the distance to land: ~45 at the shallow coastal stations, ~15 in the high-salinity waters of Bransfield Strait, and ~4 in ACC waters. Concentrations of DFe increased slightly with depth in the water column, while that of TaLFe did not show any consistent trend with depth. Our data are consistent with the hypothesis that the relatively high rates of primary production known from the central regions of the Scotia Sea are partially sustained by natural iron enrichment resulting from a northeasterly flow of iron-rich coastal waters originating in the South Shetland Islands-Antarctic Peninsula region.
Regional impacts of iron-light colimitation in a global biogeochemical model
Authors: Galbraith E. D. , Gnanadesikan A. , Dunne J. P. , Hiscock M. R.
Published: April 2010

Abstract. Laboratory and field studies have revealed that iron has multiple roles in phytoplankton physiology, with particular importance for light-harvesting cellular machinery. However, although iron-limitation is explicitly included in numerous biogeochemical/ecosystem models, its implementation varies, and its effect on the efficiency of light harvesting is often ignored. Given the complexity of the ocean environment, it is difficult to predict the consequences of applying different iron limitation schemes. Here we explore the interaction of iron and nutrient cycles using a new, streamlined model of ocean biogeochemistry. Building on previously published parameterizations of photoadaptation and export production, the Biogeochemistry with Light Iron Nutrients and Gasses (BLING) model is constructed with only three explicit tracers but including macronutrient and micronutrient limitation, light limitation, and an implicit treatment of community structure. The structural simplicity of this computationally inexpensive model allows us to clearly isolate the global effects of iron availability on maximum light-saturated photosynthesis rates from those of photosynthetic efficiency. We find that the effect on light-saturated photosynthesis rates is dominant, negating the importance of photosynthetic efficiency in most regions, especially the cold waters of the Southern Ocean. The primary exceptions to this occur in iron-rich regions of the Northern Hemisphere, where high light-saturated photosynthesis rates cause photosynthetic efficiency to play a more important role. Additionally, we speculate that the small phytoplankton dominating iron-limited regions tend to have relatively high photosynthetic efficiency, such that iron-limitation has less of a deleterious effect on growth rates than would be expected from short-term iron addition experiments.
Effects of ultraviolet B radiation on (not so) transparent exopolymer particles
Authors: Ortega-Retuerta E. , Passow U. , Duarte C. M. , Reche I.
Published: April 2010

Abstract. Transparent exopolymer particles (TEP) are the most ubiquitous gel particles in the ocean and form abiotically from dissolved precursors. Although these particles can accumulate at the ocean surface, being thus exposed to intense sunlight, the role of solar radiation on the assembly and degradation of TEP is unknown. In this study, we experimentally determined the effects of visible and ultraviolet B (UVB) radiation on (1) TEP degradation (photolysis experiments), (2) TEP assembly from dissolved polymers (photoinhibition experiments) and (3) TEP release by microorganisms. Solar radiation, particularly in the UVB range, caused significant TEP photolysis, with loss rates from 27 to 34% per day. Dissolved polysaccharides did not increase in parallel. No TEP were formed under UVB, visible or dark conditions, indicating that light does not promote TEP assembly. UVB radiation enhanced TEP release by microorganisms, possibly due to cell deaths, or as a protective measure. Increases in UVB may lead to enhanced TEP photolysis in the ocean, with further consequences for TEP dynamics and, ultimately, sea-air gas exchange.
The interaction of ocean acidification and carbonate chemistry on coral reef calcification: evaluating the carbonate chemistry Coral Reef Ecosystem Feedback (CREF) hypothesis on the Bermuda coral reef
Authors: Bates N. R. , Amat A. , Andersson A. J.
Published: April 2010

Abstract. Despite the potential impact of ocean acidification on ecosystems such as coral reefs, surprisingly, there is very limited field data on the relationships between calcification and carbonate chemistry. In this study, contemporaneous in situ datasets of carbonate chemistry and calcification rates from the high-latitude coral reef of Bermuda over annual timescales provide a framework for investigating the present and future potential impact of rising pCO2 and ocean acidification on coral reef ecosystems in their natural environment. A strong correlation was found between the in situ rates of calcification for the major framework building coral species Diploria labyrinthiformis and the seasonal variability of [CO32-] and Ωaragonite, rather than other environmental factors such as light and temperature. These field observations also provide sufficient data to hypothesize that there is a seasonal "Carbonate Chemistry Coral Reef Ecosystem Feedback" (CREF hypothesis) between the primary components of the reef ecosystem (i.e. scleractinian hard corals and macroalgae) and carbonate chemistry. In early summer, strong net autotrophy from benthic components of the reef system enhance [CO32-] and Ωaragonite conditions, and rates of coral calcification due to the photosynthetic uptake of CO2. In late summer, rates of coral calcification are suppressed by release of CO2 from reef metabolism during a period of strong net heterotrophy. It is likely that this seasonal CREF mechanism is present in other tropical reefs although attenuated compared to high-latitude reefs such as Bermuda. Due to lower annual mean surface seawater [CO32-] and Ωaragonite in Bermuda compared to tropical regions, we anticipate that Bermuda corals will experiences seasonal periods of zero net calcification within the next decade at [CO32-] and Ωaragonite thresholds of ~184 mmoles kg−1 and 2.65. The Bermuda coral reef is one of the first responders to the negative impacts of ocean acidification, and we estimate that calcification rates for D. labyrinthiformis have declined by >50% compared to pre-industrial times.
A spatial resolution threshold of land cover in estimating regional terrestrial carbon sequestration
Authors: Zhao S. , Liu S. , Li Z. , Sohl T. L.
Published: April 2010

Abstract. Changes in carbon density (i.e., carbon stock per unit area) and land cover greatly affect carbon sequestration. Previous studies have shown that land cover change detection strongly depends on spatial scale. However, the influence of the spatial resolution of land cover change information on the estimated terrestrial carbon sequestration is not known. Here, we quantified and evaluated the impact of land cover change databases at various spatial resolutions (250 m, 500 m, 1 km, 2 km, and 4 km) on the magnitude and spatial patterns of regional carbon sequestration in the southeastern United States using the General Ensemble biogeochemical Modeling System (GEMS). Results indicated a threshold of 1 km in the land cover change databases and in the estimated regional terrestrial carbon sequestration. Beyond this threshold, significant biases occurred in the estimation of terrestrial carbon sequestration, its interannual variability, and spatial patterns. In addition, the overriding impact of interannual climate variability on the temporal change of regional carbon sequestration was unrealistically overshadowed by the impact of land cover change beyond the threshold. The implications of these findings directly challenge current continental- to global-scale carbon modeling efforts relying on information at coarse spatial resolution without incorporating fine-scale land cover dynamics.
Spring molybdenum enrichment in scallop shells: a potential tracer of diatom productivity in coastal temperate environments (Brittany, NW France)?
Authors: Barats A. , Amouroux D. , Pécheyran C. , Chauvaud L. , Thébault J. , Donard O. F. X.
Published: April 2010

Abstract. Skeletal molybdenum/calcium ([Mo]/[Ca])shell ratios were recently examined in bivalves. These ratios were determined by quantitative LA-ICP-MS analyses every third daily striae (i.e. a temporal resolution of 3 days) in 36 flat valves of the Great Scallop shells Pecten maximus (2-year old; 3 shells/year) collected in temperate coastal environments of Western Europe (42 to 49° N). Variations of ([Mo]/[Ca])shell ratio were significant and reproducible for scallops from a same population, from different years (1998–2004) and from different coastal temperate locations. ([Mo]/[Ca])shell exhibits typical profiles characterized by a background content, below the method detection limit (<0.003 μmol/mol) for most of the shell growth period, which is punctuated by a significant transient enrichment (0.031−2.1 μmol/mol) mainly occurring from May to June. The Bay of Brest (France) was especially investigated because of long term observations on scallop communities, environmental variables, and high resolution analyses of dissolved Mo in bottom seawater in 2000. In 2000, dissolved Mo exhibited significant increasing concentration just preceding a maximum of ([Mo]/[Ca])shell ratio. The environmental conditions preceding ([Mo]/[Ca])shell maximum events, both in 2000 and over the 7-year survey indicates a direct influence of the scallop environmental conditions at the sediment water interface subsequent to the intense and periodic spring bloom event. Spring maxima of ([Mo]/[Ca])shell ratio were found to be specifically related to the dynamic of spring diatom blooms and to the extent of the subsequent silicate depletion. ([Mo]/[Ca])shell records reveal thus unexpected biogeochemical routes of Mo, potentially related to coastal spring productivity.
Optical characterization of an eddy-induced diatom bloom west of the island of Hawaii
Authors: Nencioli F. , Chang G. , Twardowski M. , Dickey T. D.
Published: April 2010

Abstract. Optical properties are used to characterize the biogeochemistry of cyclonic eddy Opal in the lee of Hawaii. The eddy featured an intense diatom bloom. Our results show that the ratio of chlorophyll concentration to particulate beam attenuation coefficient, [chl]/cp, is not a good indicator of the changes in particle composition through the water column. The ratio is controlled primarily by the variation in chlorophyll concentration per cell with depth (photoadaptation), so that its values increase throughout the Deep Chlorophyll Maximum Layer (DCML). Below the DCML, high values of [chl]/cp suggest that remineralization might be another important controlling factor. On the other hand, the backscattering ratio (particle backscattering to particle scattering ratio, b~bp) clearly indicates a shift from a small phytoplankton to a diatom dominated community. Below an upper layer characterized by constant values, the b~bp ratio showed a rapid decrease to a broad minimum within the DCML. The higher values below the DCML are consistent with enhanced remineralization below the eddy-induced bloom. The DCML was characterized by a layer of "healthy" diatoms underlying a layer of "senescent" diatoms. These two layers are characterized by similar optical properties, indicating some possible limitations in using optical measurements to fully characterize the composition of suspended material in the water column. An inverse relationship between b~bp and [chl]/cp, also reported by others, is observed as deep as the DCML. There, [chl]/cp increases whereas b~bp remains similar to values found in the empty frustule layer. This is a further indication that [chl]/cp might not be a good alternative to the backscattering ratio for investigating changes in particle composition with depth in Case I waters.
Estimation of NH3 emissions from a naturally ventilated livestock farm using local-scale atmospheric dispersion modelling
Authors: Hensen A. , Loubet B. , Mosquera J. , van den Bulk W. C. M. , Erisman J. W. , Dämmgen U. , Milford C. , Löpmeier F. J. , Cellier P. , Mikuška P. , Sutton M. A.
Published: April 2010

Abstract. Agricultural livestock represents the main source of ammonia (NH3) in Europe. In recent years, reduction policies have been applied to reduce NH3 emissions. In order to estimate the impacts of these policies, robust estimates of the emissions from the main sources, i.e. livestock farms are needed. In this paper, the NH3 emissions were estimated from a naturally ventilated livestock farm in Braunschweig, Germany during a joint field experiment of the GRAMINAE European project. An inference method was used with a Gaussian-3-D plume model and a local-scale dispersion and deposition model (FIDES-2-D). NH3 concentrations downwind of the source were used together with micrometeorological data to estimate the source strength over time. Mobile NH3 concentration measurements provided information on the spatial distribution of source strength. The estimated emission strength ranged between 6.0±0.17 kg NH3 d−1 (FIDES-2-D model) and 9.2±0.7 kg NH3 d−1 (Gaussian model). These estimates were 94% and 63% of what was obtained using emission factors from the German national inventory (9.6 kg d−1 NH3. However, the FIDES-2-D approach was shown to be very sensitive to the source size, the roughness height and to whether deposition was taken into account downwind of the source. Accounting for deposition in FIDES-2-D gives a potential emission estimate of 11.7 kg NH3 d−1, showing that deposition can explain the observed difference. The daily pattern of the source was correlated with net radiation and with the temperature inside the animal houses. The daily pattern resulted from a combination of a temperature effect on the source concentration together with an effect of variations in free and forced convection of the building ventilation rate. Further development of the plume technique is especially relevant for naturally ventilated farms, since the variable ventilation rate makes other emission measurements difficult.
Multi-model analysis of terrestrial carbon cycles in Japan: reducing uncertainties in model outputs among different terrestrial biosphere models using flux observations
Authors: Ichii K. , Suzuki T. , Kato T. , Ito A. , Hajima T. , Ueyama M. , Sasai T. , Hirata R. , Saigusa N. , Ohtani Y. , Takagi K.
Published: April 2010

Abstract. Terrestrial biosphere models show large uncertainties when simulating carbon and water cycles, and reducing these uncertainties is a priority for developing more accurate estimates of both terrestrial ecosystem statuses and future climate changes. To reduce uncertainties and improve the understanding of these carbon budgets, we investigated the ability of flux datasets to improve model simulations and reduce variabilities among multi-model outputs of terrestrial biosphere models in Japan. Using 9 terrestrial biosphere models (Support Vector Machine-based regressions, TOPS, CASA, VISIT, Biome-BGC, DAYCENT, SEIB, LPJ, and TRIFFID), we conducted two simulations: (1) point simulations at four flux sites in Japan and (2) spatial simulations for Japan with a default model (based on original settings) and an improved model (based on calibration using flux observations). Generally, models using default model settings showed large deviations in model outputs from observation with large model-by-model variability. However, after we calibrated the model parameters using flux observations (GPP, RE and NEP), most models successfully simulated seasonal variations in the carbon cycle, with less variability among models. We also found that interannual variations in the carbon cycle are mostly consistent among models and observations. Spatial analysis also showed a large reduction in the variability among model outputs, and model calibration using flux observations significantly improved the model outputs. These results show that to reduce uncertainties among terrestrial biosphere models, we need to conduct careful validation and calibration with available flux observations. Flux observation data significantly improved terrestrial biosphere models, not only on a point scale but also on spatial scales.
Yellow substance and the shades of blue in the Mediterranean Sea
Authors: Morel A. , Gentili B.
Published: April 2010

Abstract. When the nominal algorithms commonly in use in Space Agencies are applied to satellite Ocean Color data, the retrieved chlorophyll concentrations in the Mediterranean Sea are recurrently notable overestimates of the field values. Accordingly, several regionally tuned algorithms have been proposed in the past to correct for this deviation; actually, the blueness of the Mediterranean waters is not as deep as expected from the actual (low) chlorophyll content, and the modified algorithm account for this peculiarity. Among the possible causes for such a deviation, an excessive amount of yellow substance, or of colored detrital matter (CDM), has been frequently cited. This conjecture is presently tested, by using a new technique simply based on the simultaneous consideration of marine reflectance determined at four spectral bands (namely 412, 443, 490, and 555 nm, available on the NASA-SeaWiFS satellite). It results from this test that the concentration in yellow colored material (quantified as ay, the absorption coefficient at 443 nm) is about twice that one observed in the nearby Atlantic Ocean at the same latitude. There is a strong seasonal signal, with maximal ay values in late fall and winter, an abrupt decrease beginning in spring, and then a deep minimum during the summer months, which plausibly results from the intense photo-bleaching process favored by the high level of sunshine in these areas. Systematically, the ay values, reproducible from year to year, are higher in the western basin compared with those in the eastern basin (by about 50%). The relative importance of the river discharges into this semi-enclosed sea, as well as the winter deep vertical mixing occurring in the northern parts of the basins may explain the high yellow substance background. The regionally tuned [Chl] algorithms, actually reflect the presence of an excess of CDM with respect to its standard (Chl- related) values. When corrected for the presence of the actual CDM content, the [Chl] values as derived via the nominal algorithms are restored to more realistic values, i.e., approximately divided by about two; the strong autumnal increase is smoothed whereas the spring bloom remains as an isolated feature.
Heat storage in forest biomass significantly improves energy balance closure particularly during stable conditions
Authors: Lindroth A. , Mölder M. , Lagergren F.
Published: April 2010

Abstract. Temperature measurements in trunks and branches in a mature ca. 100 years-old mixed pine and spruce forest in central Sweden were used to estimate the heat storage in the tree biomass. The estimated heat flux in the sample trees and data on biomass distributions were used to scale up to stand level biomass heat fluxes. The rate of change of sensible and latent heat storage in the air layer below the level of the flux measurements was estimated from air temperature and humidity profile measurements and soil heat flux was estimated from heat flux plates and soil temperature measurements. The fluxes of sensible and latent heat from the forest were measured with an eddy covariance system in a tower. The analysis was made for a two-month period in summer of 1995. The tree biomass heat flux was the largest of the estimated storage components and varied between 40 and −35 W m−2 on summer days with nice weather. Averaged over two months the diurnal maximum of total heat storage was 45 W m−2 and the minimum was −35 W m−2. The soil heat flux and the sensible heat storage in air were out of phase with the biomass flux and they reached maximum values that were about 75% of the maximum of the tree biomass heat storage. The energy balance closure improved significantly when the total heat storage was added to the turbulent fluxes. The slope of a regression line with sum of fluxes and storage as independent and net radiation as dependent variable, increased from 0.86 to 0.95 for half-hourly data and the scatter was also reduced. The most significant finding was, however, that during nights with strongly stable conditions when the sensible heat flux dropped to nearly zero, the total storage matched the net radiation nearly perfectly. Another interesting result was that the mean energy imbalance started to increase when the Richardson number became more negative than ca. −0.1. In fact, the largest energy deficit occurred at maximum instability. Our conclusion is that eddy covariance measurements can function well during stable conditions but that the functioning under strong instabilities might be a so far unforeseen problem.
Patterns in planktonic metabolism in the Mediterranean Sea
Authors: Regaudie-de-Gioux A. , Vaquer-Sunyer R. , Duarte C. M.
Published: April 2010

Abstract. Planktonic gross community production (GPP), net community production (NCP) and community respiration (CR) across the Mediterranean Sea was examined in two cruises, THRESHOLDS 2006 and 2007, each crossing the Mediterranean from West to East to test for consistent variation along this longitudinal gradient. GPP averaged 2.4±0.4 mmol O2m−3 d−1, CR averaged 3.8±0.5 mmol O2m−3 d−1, and NCP averaged -0.8±0.6 mmol O2m−3 d−1across the studied sections, indicative of a tendency for a net heterotrophic metabolism, prevalent across studied sections of the Mediterranean Sea as reflected in 70% of negative NCP estimates. The median P/R ratio was 0.58, also indicating a strong prevalence of heterotrophic communities (P/R<1) along the studied sections of the Mediterranean Sea. The communities tended to be net heterotrophic (i.e. P/R<1) at GPP less than 3.5 mmol O2m−3 d−1. Although the Western Mediterranean supports a higher gross primary production than the Eastern basin does, it also supported a higher community respiration. The net heterotrophy nature of the studied sections of the Mediterranean Sea indicates that allochthonous carbon should be important to subsidise planktonic metabolism, and that the planktonic communities in the Mediterranean Sea acted as CO2 sources to the atmosphere during the study.
Chronic nitrogen addition causes a reduction in soil carbon dioxide efflux during the high stem-growth period in a tropical montane forest but no response from a tropical lowland forest in decadal scale
Authors: Koehler B. , Corre M. D. , Veldkamp E. , Sueta J. P.
Published: April 2010

Abstract. Atmospheric nitrogen (N) deposition is rapidly increasing in tropical regions. We studied the response of soil carbon dioxide CO2 efflux to long-term experimental N-addition (125 kg N ha−1 yr-1) in mature lowland and montane forests in Panamá. In the lowland forest, on soils with high nutrient-supplying and buffering capacity, fine litterfall and stem-growth were neither N- nor phosphorus-limited. In the montane forest, on soils with low nutrient supplying capacity and an organic layer, fine litterfall and stem-growth were N-limited. Our objectives were to 1) explore the influence of soil temperature and moisture on the dynamics of soil CO2 efflux and 2) determine the responses of soil CO2 efflux from an N-rich and N-limited forest to elevated N input. Annual soil CO2-C efflux was larger from the lowland (15.20±1.25 Mg C ha−1) than the montane forest (9.36±0.29 Mg C ha−1). In the lowland forest, soil moisture explained the largest fraction of the variance in soil CO2 efflux while soil temperature was the main explanatory variable in the montane forest. Soil CO2 efflux in the lowland forest did not differ between the control and 9–11 yr N-addition plots, suggesting that chronic N input to nutrient-rich tropical lowland forests on well-buffered soils may not change their C balance in decadal scale. In the montane forest, first year N addition did not affect soil CO2 efflux but annual CO2 efflux was reduced by 14% and 8% in the 2- and 3 yr N-addition plots, respectively, compared to the control. This reduction was caused by a decrease in soil CO2 efflux during the high stem-growth period of the year, suggesting a shift in carbon partitioning from below- to aboveground in the N-addition plots where stem diameter growth was promoted.
Soil respiration in a fire scar chronosequence of Canadian boreal jack pine forest
Authors: Smith D. R. , Kaduk J. D. , Balzter H. , Wooster M. J. , Mottram G. N. , Lynham T. J. , Studens J. , Curry J. , Hartley G. , Stocks B. J.
Published: April 2010

Abstract. To fully understand the carbon (C) cycle impacts of forest fires, both C emissions during the fire and post-disturbance fluxes need to be considered. The latter are dominated by soil respiration (Rs), which is still subject to large uncertainties. This research investigates Rs in a boreal jack pine fire scar chronosequence at Sharpsand Creek, Ontario, Canada. During two field campaigns in 2006 and 2007, Rs was measured in a chronosequence of fire scars aged between 0 and 59 years since the last fire. Mean Rs per fire scar was adjusted for soil temperature (Ts) and soil moisture (Ms) (denoted RST,M). RST,M ranged from 0.56 μmol CO2/m2/s (32 years post fire) to 8.18 μmol CO2/m2/s (58 years post fire). The coefficient of variation (CV) of RST,M ranged from 20% (16 years post fire) to 56% (58 years post fire). Across the field site, there was a statistically highly significant exponential relationship between Rs adjusted for soil organic carbon (Cs) and Ts (P<0.00001; Q10=2.21) but no effect of Ms on Rs adjusted for Cs and Ts for the range 0.21 to 0.77 volumetric Ms (P=0.702). RST,M decreased significantly (P=0.030) after fire (4 to 8 days post fire) in mature forest, though no significant (P>0.1) difference could be detected between recently burned (4 to 8 days post fire) and unburned young forest. There were significant differences in RST,M between recently burned (4 to 8 days post fire) scar age categories that differed in their burn history, with between-fire intervals of 32 vs. 16 years (P<0.001) and 32 vs 59 years (P=0.044). There was a highly significant exponential increase in RST,M with time since fire (r2=0.999; P=0.006) for the chronosequence 0, 16 and 59 years post fire, and for all these age categories, RST,M was significantly different from one another (P<0.05). The results of this study contribute to a better quantitative understanding of Rs in boreal jack pine fire scars and will facilitate improvements in C cycle modelling. Further work is needed in quantifying autotrophic and heterotrophic contributions to Rs in jack pine systems; in monitoring Rs for extended time periods after fire; and in measuring different fire-prone forest types.
Effect of peat quality on microbial greenhouse gas formation in an acidic fen
Authors: Reiche M. , Gleixner G. , Küsel K.
Published: April 2010

Abstract. Peatlands play an important role in the global carbon cycle and represent both an important stock of soil carbon and a substantial natural source of relevant greenhouse gases like CO2 and CH4. While it is known that the microbial availability of organic matter affects degradation and mineralization processes in peatlands, the manner in which peat organic matter affects the formation of CO2 and CH4 remains unclear. In this study we developed a fast and simple peat quality index in order to estimate its greenhouse gas potential by linking the thermo-degradability of peat with anaerobic CO2 and CH4 formation rates. Peat samples were obtained at several depths (0–40 cm) at four sampling locations from an acidic fen (pH∼4.7). CO2 and CH4 formation rates were highly spatially variable and depended on depth, sampling location, and the composition of pyrolysable organic matter. Peat samples active in CO2 and CH4 formation had a quality index above 1.35, and the fraction of thermally labile pyrolyzable organic matter (comparable to easily available carbon substrates for microbial activity) obtained by thermogravimetry was above 35%. Curie-point pyrolysis-gas chromatography/mass spectrometry mainly identified carbohydrates and lignin as pyrolysis products in these samples, indicating that undecomposed organic matter was found in this fraction. In contrast, lipids and unspecific pyrolysis products, which indicate recalcitrant and highly decomposed organic matter, correlated significantly with lower CO2 formation and reduced methanogenesis. Our results suggest that undecomposed organic matter is a prerequisite for CH4 and CO2 development in acidic fens. Furthermore, the new peat quality index should aide the estimation of greenhouse gas formation potential resulting from peatland restoration and permafrost thawing and help yield more robust models of trace gas fluxes from peatlands for climate change research.
Process-based simulation of seasonality and drought stress in monoterpene emission models
Authors: Grote R. , Keenan T. , Lavoir A.-V. , Staudt M.
Published: April 2010

Abstract. Canopy emissions of volatile hydrocarbons such as isoprene and monoterpenes play an important role in air chemistry. They depend on various environmental conditions, are highly species-specific and are expected to be affected by global change. In order to estimate future emissions of these isoprenoids, differently complex models are available. However, seasonal dynamics driven by phenology, enzymatic activity, or drought stress strongly modify annual ecosystem emissions. Although these impacts depend themselves on environmental conditions, they have yet received little attention in mechanistic modelling. In this paper we propose the application of a mechanistic method for considering the seasonal dynamics of emission potential using the ''Seasonal Isoprenoid synthase Model'' (Lehning et al., 2001). We test this approach with three different models (GUENTHER, Guenther et al., 1993; NIINEMETS, Niinemets et al., 2002a; BIM2, Grote et al., 2006) that are developed for simulating light-dependent monoterpene emission. We also suggest specific drought stress representations for each model. Additionally, the proposed model developments are compared with the approach realized in the MEGAN (Guenther et al., 2006) emission model. Models are applied to a Mediterranean Holm oak (Quercus ilex) site with measured weather data. The simulation results demonstrate that the consideration of a dynamic emission potential has a strong effect on annual monoterpene emission estimates. The investigated models, however, show different sensitivities to the procedure for determining this seasonality impact. Considering a drought impact reduced the differences between the applied models and decreased emissions at the investigation site by approximately 33% on average over a 10 year period. Although this overall reduction was similar in all models, the sensitivity to weather conditions in specific years was different. We conclude that the proposed implementations of drought stress and internal seasonality strongly reduce estimated emissions and indicate measurements are needed to further evaluate the models.
Measuring and modelling continuous quality distributions of soil organic matter
Authors: Bruun S. , Ågren G. I. , Christensen B. T. , Jensen L. S.
Published: April 2010

Abstract. An understanding of the dynamics of soil organic matter (SOM) is important for our ability to develop management practices that preserve soil quality and sequester carbon. Most SOM decomposition models represent the heterogeneity of organic matter by a few discrete compartments with different turnover rates, while other models employ a continuous quality distribution. To make the multi-compartment models more mechanistic in nature, it has been argued that the compartments should be related to soil fractions actually occurring and having a functional role in the soil. In this paper, we make the case that fractionation methods that can measure continuous quality distributions should be developed, and that the temporal development of these distributions should be incorporated into SOM models. The measured continuous SOM quality distributions should hold valuable information not only for model development, but also for direct interpretation. Measuring continuous distributions requires that the measurements along the quality variable are so frequent that the distribution is approaching the underlying continuum. Continuous distributions leads to possible simplifications of the model formulations, which considerably reduce the number of parameters needed to describe SOM turnover. A general framework for SOM models representing SOM across measurable quality distributions is presented and simplifications for specific situations are discussed. Finally, methods that have been used or have the potential to be used to measure continuous quality SOM distributions are reviewed. Generally, existing fractionation methods have to be modified to allow measurement of distributions or new fractionation techniques will have to be developed. Developing the distributional models in concert with the fractionation methods to measure the distributions will be a major task. We hope the current paper will help spawning the interest needed to accommodate this.
Evidence for greater oxygen decline rates in the coastal ocean than in the open ocean
Authors: Gilbert D. , Rabalais N. N. , Diaz R. J. , Zhang J.
Published: April 2010

Abstract. In the global ocean, the number of reported hypoxic sites (oxygen <30% saturation) is on the rise both near the coast and in the open ocean. But unfortunately, most of the papers on hypoxia only present oxygen data from one or two years, so that we often lack a long-term perspective on whether oxygen levels at these locations are decreasing, steady or increasing. Consequently, we cannot rule out the possibility that many of the newly reported hypoxic areas were hypoxic in the past, and that the increasing number of hypoxic areas partly reflects increased research and monitoring efforts. Here we address this shortcoming by computing oxygen concentration trends in the global ocean from published time series and from time series that we calculated using a global oxygen database. Our results show that oxygen concentrations are generally decreasing in the ocean. They also reveal a greater percentage of negative trends for published oxygen time series than for oxygen time series computed from the global oxygen database, particularly in the open ocean. Finally, the oxygen decline rates computed from the global oxygen database are more severe in a 30 km band near the coast than in the open ocean, probably in response to human-caused eutrophication.
Surface energy exchanges above two grassland ecosystems on the Qinghai-Tibetan Plateau
Authors: Liu S. , Li S.-G. , Yu G.-R. , Sun X.-M. , Zhang L.-M. , Hu Z.-M. , Li Y.-N. , Zhang X.-Z.
Published: April 2010

Abstract. This paper explores the seasonal and inter-annual variations in surface energy exchanges at an alpine shrub-meadow ecosystem (AM) and an alpine meadow-steppe (AS) on the Qinghai-Tibetan Plateau using the 4–5-year continuous flux measurements by the eddy covariance technique. Although ground solar radiation (Kd) was lower at AM than at AS, net radiation (Rn) was higher at AM than at AS due to the lower surface reflectivity for short-wave radiation (albedo) and the lower ratio of upward long-wave radiation to downward long-wave radiation (Lu/Ld) at AM. The ratio of Rn/Kd was lower (about 0.4) at AS than at AM (about 0.6). In the annual scale, annual precipitation was the most important factor controlling energy partitioning at AS, while temperature, especially during the spring, controlled the inter-annual variations in energy partitioning at AM characterized by low temperature. Vegetation growth played an important role in controlling energy partitioning at seasonal scale at both sites. However, soil water content was also one of major factors affecting Bowen ratio (β) due to the lower soil water-holding capacity at AS as compared with AM. Under non-drought conditions, the influence of vapor pressure deficit (VPD) on energy partitioning at half-hourly scale was stronger at AM than at AS due to higher vegetation cover thereof.
The subtle effects of sea water acidification on the amphipod Gammarus locusta
Authors: Hauton C. , Tyrrell T. , Williams J.
Published: April 2010

Abstract. We report an investigation of the effects of increases in pCO2 on the growth and molecular physiology of the neritic amphipod Gammarus locusta, which has a cosmopolitan distribution in estuaries. Amphipods were reared from juvenile to mature adult in laboratory microcosms at three different levels of pH in nominal range 8.1–7.6. Growth rate was estimated from weekly measures of body length. At sexual maturity the amphipods were sacrificed and assayed for changes in the expression of genes coding for a heat shock protein (hsp70 gene) and the metabolic enzyme glyceraldehyde-3-phosphate dehydrogenase (gapdh gene). The data show that the growth and survival rate of this species is not significantly impacted by a decrease in sea water pH of up to 0.5 units. Quantitative real-time PCR analysis indicated that there was no significant effect of growth in acidified sea water on the expression of the hsp70 gene. However, there was a consistent and significant increase in the expression of the gapdh gene at a pH of ~7.5 which indicated a possible disruption to oxidative metabolic processes. It was concluded that future predicted changes in sea water pH may have subtle effects on the physiology and metabolism of coastal and marine species which may be overlooked in studies of whole organism response.
Coastal hypoxia/anoxia as a source of CH4 and N2O
Authors: Naqvi S. W. A. , Bange H. W. , Farías L. , Monteiro P. M. S. , Scranton M. I. , Zhang J.
Published: April 2010

Abstract. We review here available information on distributions of methane (CH4) and nitrous oxide (N2O) from major, mostly coastal, oxygen (O2)-deficient zones produced due to both natural processes and human activities (mainly eutrophication). Concentrations of both gases in subsurface waters are affected by ambient O2 levels. In the case of CH4, bottom-water O2 content probably affects emission from sediments, believed to be the main source of water-column CH4, as well as its oxidative loss in water itself. Highest CH4 accumulation (several μM) occurs in silled basins having anoxic deep waters such as the Black Sea and the Cariaco Basin. One to two orders of magnitude smaller, but still significant, accumulation also occurs in bottom waters of open margins experiencing anoxia and in silled basins containing suboxic/severely hypoxic waters. In highly eutrophic waters over open continental shelves (such as the upwelling zone off Namibia and the "dead zone" in the Gulf of Mexico) high CH4 concentrations (several hundred nM) may occur in non-sulphidic waters as well, but in these regions it is difficult to differentiate the hypoxia-induced enhancement from in situ production of CH4 in the water column and, sometimes, large inputs of CH4 associated with freshwater runoff or seepage from sediments. Despite the observed CH4 build-up in low-O2 bottom waters, methanotrophic activity severely restricts its emission from the ocean. As a result, an intensification or expansion of coastal hypoxic zones will probably not drastically change the present status where emission from the ocean as a whole forms an insignificant term in the atmospheric CH4 budget. The situation is different for N2O, the production of which is greatly enhanced in severely hypoxic waters, and although it is lost through denitrification in most suboxic and anoxic environments, the peripheries of such environments offer most suitable conditions for its production, with the exception of semi-enclosed/land-locked anoxic basins such as the Black Sea. Most O2-deficient systems serve as strong net sources of N2O to the atmosphere. This is especially true for regions of coastal upwelling with shallow oxygen minimum zones where a dramatic increase in N2O production often occurs in rapidly denitrifying waters. Nitrous oxide emissions from these zones are globally significant, and so their ongoing intensification and expansion is likely to lead to a significant increase in N2O emission from the ocean. However, a meaningful quantitative prediction of this increase is not possible at present because of continuing uncertainties concerning the formative pathways to N2O as well as insufficient data from some key coastal regions.
Summer microplankton community structure across the Scotia Sea: implications for biological carbon export
Authors: Korb R. E. , Whitehouse M. J. , Gordon M. , Ward P. , Poulton A. J.
Published: April 2010

Abstract. During the austral summer of 2008, we carried out a high resolution survey of the microplankton communities along a south to north transect covering a range of environments across the Scotia Sea, Southern Ocean; high and low productivity, sea-ice to open water conditions, and over a number of oceanographic fronts and bathymetric features. Cluster analysis revealed five distinct communities that were geographically constrained by physical features of bathymetry and fronts. From south to north the communities were: (1) The South Orkney group, a mixed community of naked dinoflagellates and heavily silicified diatoms, (2) Southern Scotia Sea, a mixed community of cyptophytes and naked dinoflagellates, (3) Central Scotia Sea, dominated by naked dinoflagellates, (4) southwest of the island of South Georgia, lightly silicified diatoms and naked dinoflagellates (5) northwest of South Georgia, dominated by diatoms. Data from a previous summer cruise (2003) to the Scotia Sea followed a similar pattern of community distribution. MODIS images, chlorophyll-a and macronutrient deficits revealed dense phytoplankton blooms occurred around the island of South Georgia, were absent near the ice edge and in the central Scotia Sea and were moderate in the southern Scotia Sea. Using these environmental factors, together with community composition, we propose that south of the Southern Antarctic Circumpolar Current Front, biogenic silica is preferentially exported and north of the front, in the vicinity of South Georgia, carbon is exported to depth.
Small scale spatial heterogeneity of soil respiration in an old growth temperate deciduous forest
Authors: Jordan A. , Jurasinski G. , Glatzel S.
Published: April 2010

Abstract. The large scale spatial heterogeneity of soil respiration caused by differences in site conditions is quite well understood. However, comparably little is known about the micro scale heterogeneity within forest ecosystems on homogeneous soils. Forest age, soil texture, topographic position, micro topography and stand structure may influence soil respiration considerably within short distance. In the present study within site spatial heterogeneity of soil respiration has been evaluated. To do so, an improvement of available techniques for interpolating soil respiration data via kriging was undertaken. Soil respiration was measured with closed chambers biweekly from April 2005 to April 2006 using a nested design (a set of stratified random plots, supplemented by 2 small and 2 large nested groupings) in an unmanaged, beech dominated old growth forest in Central Germany (Hainich, Thuringia). A second exclusive randomized design was established in August 2005 and continually sampled biweekly until July 2007. The average soil respiration values from the random plots were standardized by modeling soil respiration data at defined soil temperature and soil moisture values. By comparing sampling points as well as by comparing kriging results based on various sampling point densities, we found that the exclusion of local outliers was of great importance for the reliability of the estimated fluxes. Most of this information would have been missed without the nested groupings. The extrapolation results slightly improved when additional parameters like soil temperature and soil moisture were included in the extrapolation procedure. Semivariograms solely calculated from soil respiration data show a broad variety of autocorrelation distances (ranges) from a few centimeters up to a few tens of meters. The combination of randomly distributed plots with nested groupings plus the inclusion of additional relevant parameters like soil temperature and soil moisture data permits an improved estimation of the range of soil respiration, which is a prerequisite for reliable interpolated maps of soil respiration.