Changes In Carbon Fluxes Research Articles

The Past, Present, and Future of the Hemlock Woolly Adelgid (Adelges tsugae) and Its Ecological Interactions with Eastern Hemlock (Tsuga canadensis) Forests.

The nonnative hemlock woolly adelgid is steadily killing eastern hemlock trees in many parts of eastern North America. We summarize impacts of the adelgid on these forest foundation species; review previous models and analyses of adelgid spread dynamics; and examine how previous forecasts of adelgid spread and ecosystem dynamics compare with current conditions. The adelgid has reset successional sequences, homogenized biological diversity at landscape scales, altered hydrological dynamics, and changed forest stands from carbon sinks into carbon sources. A new model better predicts spread of the adelgid in the south and west of the range of hemlock, but still under-predicts its spread in the north and east. Whether these underpredictions result from inadequately modeling accelerating climate change or accounting for people inadvertently moving the adelgid into new locales needs further study. Ecosystem models of adelgid-driven hemlock dynamics have consistently forecast that forest carbon stocks will be little affected by the shift from hemlock to early-successional mixed hardwood stands, but these forecasts have assumed that the intermediate stages will remain carbon sinks. New forecasting models of adelgid-driven hemlock decline should account for observed abrupt changes in carbon flux and ongoing and accelerating human-driven land-use and climatic changes.

Impact of K deficiency on leaves and siliques photosynthesis via metabolomics in Brassica napus

Potassium (K) deficiency is known for its impact on photosynthesis via the synthetic constraints of CO2 diffusion and assimilation. However, insights in the different responses of net photosynthetic rate (A) to K depletion inside primary metabolites, produced specifically by each organ, remain to be elucidated. Leaves and siliques of Brassica napus were investigated to clarify the responses of A, the CO2 conductance and that of main metabolism pathways under K deficiency. Leaves were far more sensitive to K deficiency, displaying simultaneous decreases in A and CO2 diffusion reflected in a broader range of metabolites. To compensate the carbon starvation under K deficiency, leaf metabolic profile was adaptively regulated thorough downregulation of the carbon flux into carbohydrates and organic acids and upregulation of nitrogen-rich amino acids. Significant relationships between leaf A and sugars, organic acids and amino acids were observed. In comparison with leaves, the carbon shortage observed in siliques was negligible or even absent, with no obvious change in carbon flux through the glycolysis and tricarboxylic acid cycle pathways. Proline represented the only metabolite that correlated positively with silique A. These results provide new insights into differences in photosynthetic responses to K deficiency between leaf and non-leaf organs based on the potential metabolic processes involved.

Development of the DayCent-Photo model and integration of variable photosynthetic capacity

We integrated a photosynthetic sub-model into the daily Century model (DayCent) to improve the estimations of seasonal changes in carbon fluxes at the Niwot Ridge LTER site and the Harvard forest LTER site (DayCent-Photo). The photosynthetic sub-model, adapted from the SIPNET/PnET family of models, includes solar radiation and vapor pressure deficit controls on production, as well as temperature and water stress terms. A key feature we added to the base photosynthetic equations is the addition of a variable maximum net photosynthetic rate (Amax). We optimized the parameters controlling photosynthesis using a variation of the Metropolis-Hastings algorithm along with data-assimilation techniques. The model was optimized and validated against observed net ecosystem exchange (NEE) and estimated gross primary production (GPP) and ecosystem respiration (RESP) values for AmeriFlux sites at Niwot Ridge and Harvard forest. The inclusion of a variable Amax rate greatly improved model performance (NEE RMSE = 0.63 gC·m–2, AIC = 2099) versus a version with a single Amax parameter (NEE RMSE = 0.74 gC·m–2, AIC = 3724). DayCent-Photo was able to capture the inter-annual and seasonal flux patterns for NEE, GPP, ecosystem respiration (RESP), and daily actual evapotranspiration (AET), but tended to overestimate yearly NEE uptake. The DayCent-Photo model has been successfully set up to simulate daily NEE, GPP, RESP, and AET for deciduous forest, conifer forests, and grassland systems in the US using AmeriFlux data sets and has recently been improved to include the impact of UV radiation surface litter decay (DayCent-UV). The simulated influence of a variable Amax rate suggests a need for further studies on the process controls affecting the seasonal photosynthetic rates. The results for all of the forest and grassland sites show that maximum Amax values occurs early during the growing period and taper off toward the end of the growing season.

Dynamics of soil respiration and microbial communities: Interactive controls of temperature and substrate quality

Soil microbial communities mediate soil feedbacks to climate; a thorough understanding of their response to increasing temperatures is therefore central to predict climate-induced changes in carbon (C) fluxes. However, it is unclear how microbial communities will change in structure and function in response to temperature change and to the availability of organic C which varies in complexity. Here we present results from a laboratory incubation study in which soil microbial communities were exposed to different temperatures and organic C complexity. Soil samples were collected from two land-use types differing in climatic and edaphic conditions and located in two regions in southwest Germany. Soils amended with cellobiose (CB), xylan, or coniferyl alcohol (CA, lignin precursor) were incubated at 5, 15 or 25 °C. We found that temperature predominantly controlled microbial respiration rates. Increasing temperature stimulated cumulative respiration rates but decreased total microbial biomass (total phospholipid fatty acids, PLFAs) in all substrate amendments. Temperature increase affected fungal biomass more adversely than bacterial biomass and the temperature response of fungal biomass (fungal PLFAs, ergosterol and ITS fragment) depended upon substrate quality. With the addition of CB, temperature response of fungal biomass did not differ from un-amended control soils, whereas addition of xylan and CA shifted the fungal temperature optima from 5 °C to 15 °C. These results provide first evidence that fungi which decompose complex C substrates (CA and xylan) may have different life strategies and temperature optima than fungal communities which decompose labile C substrate (CB). Gram-positive and gram-negative bacteria differed strongly in their capacity to decompose CB under different temperature regimes: gram-positive bacteria had highest PLFA abundance at 5 °C, while gram-negative bacteria were most abundant at 25 °C. Bacterial community composition, as measured by 16S rRNA gene abundance, and PLFAs showed opposite temperature and substrate decomposition trends. Using multivariate statistics, we found a general association of microbial life strategies and key members of the microbial community: oligotrophic Alphaproteobacteria and Acidobacteria were associated with complex substrates and copiotrophic Actinobacteria with labile substrates. Our study provides evidence that the response of C cycling to warming will be mediated by shifts in the structure and function of soil microbial communities.

Change in carbon flux (1960–2015) of the Red River (Vietnam)

Global riverine carbon concentrations and fluxes have been impacted by climate and human-induced changes for many decades. This paper aims to reconstruct the longterm carbon concentrations and carbon fluxes of the Red River, a system under the coupled pressures of environmental change and human activity. Based on (1) the relationships between particulate and dissolved organic carbon (POC, DOC) or dissolved inorganic carbon (DIC), and suspended sediments (TSS) or river water discharge and on (2) the available detailed historical records of river discharge and TSS concentration, the variations of the Red River carbon concentration and flux were estimated for the period 1960–2015. The results show that total carbon flux of the Red River averaged 2555 ± 639 kton C year−1. DIC fluxes dominated total carbon fluxes, representing 64% of total, reflecting a strong weathering process from carbonate rocks in the upstream basin. Total carbon fluxes significantly decreased from 2816 kton C year−1 during the 1960s to 1372 kton C year−1 during the 2010s and showed clear seasonal and spatial variations. Organic carbon flux decreased in both quantity and proportion of the total carbon flux from 40.9% in 1960s to 14.9% in 2010s, reflecting the important impact of dam impoundment. DIC flux was also reduced over this period potentially as a consequence of carbonate precipitation in the irrigated, agricultural land and the reduction of the Red River water discharge toward the sea. These decreases in TSS and carbon fluxes are probably partially responsible for different negatives impacts observed in the coastal zone.

Quantitative proteomics using SWATH-MS identifies mechanisms of chloride tolerance in the halophilic acidophile Acidihalobacter prosperus DSM 14174

In this study, the differential protein expression of the acidophilic halophile, Acidihalobacter prosperus DSM 14174 (strain V6) was studied with the aim of understanding its mechanisms of tolerance to high chloride ion stress in the presence of low pH, using Sequential Window Acquisition of all Theoretical Mass Spectra (SWATH-MS). In acidophiles, chloride stress results in both osmotic stress as well as acidification of the cytoplasm due to the ability of chloride to permeate the cell membrane and disrupt the reversed transmembrane potential which normally extrudes protons. The proteomic response of A. prosperus DSM 14174 to elevated chloride concentrations included the production of osmotic stress regulators that potentially induced the production of compatibles solutes, of which the most significant increase was in the synthesis of ectoine. Other responses directly related to the increased chloride and acid stress, included the increased synthesis of glutathione, changes in carbon flux, the increased production of amino acids, the decreased production of ribosomal proteins, the efflux of metals and protons, and the increase in proteins involved in DNA repair and membrane biosynthesis. Energy generation through iron oxidation and sulphur oxidation were decreased, and energy was probably obtained from the metabolism of glycogen stores. Overall, these studies have helped to create a model of tolerance to elevated chloride under acidic conditions by A. prosperus DSM 14174 that differs from the previous model developed for the type strain, A. prosperus DSM 5130T.

Biological uptake and reversible scavenging of zinc in the global ocean.

Zinc (Zn) is a key micronutrient for marine phytoplankton, with a global distribution that is similar to silicic acid. The processes that govern this relationship, despite the very different biological cycling of Zn and silica, remain poorly understood. Here, we use diagnostic and mechanistic models to show that only a combination of Southern Ocean biological uptake and reversible scavenging of Zn onto sinking particles can explain the observations. The distinction between organic and adsorbed Zn can also reconcile the vertical distribution and mass balance of Zn isotopes, which previously appeared at odds. This holistic understanding explains the Zn deficiencies observed throughout the low-latitude ocean and implies a greater sensitivity of the marine Zn cycle to climate-driven changes in organic matter cycling than previously recognized.

Diurnal Change of Soil Carbon Flux of Binhai New District

In order to investigate the factors influencing diurnal change of soil carbon flux of Binhai New District. Field observation experiments were carried out by using LC pro-SD photosynthetic apparatus. The diurnal changes of soil carbon flux and its environmental factors such as atmosphere temperature and soil temperature were analysed. The results indicated that soil carbon flux appeared single diurnal pattern. The diurnal average of soil carbon flux ranked from 0.2761 to 2.3367μmo1/m2/s. Soil carbon flux varied significantly among different land use regimes(P<0.001). Significant relationships were found between soil respiration rate and atmosphere temperature, which could he best described by exponential equations (P<0.05). The Q10 value was based on the exponential correlations. Its value of Tian Keyuan, ECO-city, Dagu-Outlet and Yongding-River was 8.331, 6.049, 2.651 and 1.391, respectively. There were quadratic correlations between soil carbon flux and soil temperature (10cm). And soil temperature could account for more than 32.27% of the soil carbon flux changes (P<0.05, R2=0.3227-0.7465).

Treated Wastewater Changes the Export of Dissolved Inorganic Carbon and Its Isotopic Composition and Leads to Acidification in Coastal Oceans.

Human-induced changes in carbon fluxes across the land-ocean interface can influence the global carbon cycle, yet the impacts of rapid urbanization and establishment of wastewater treatment plants (WWTPs) on coastal ocean carbon cycles are poorly known. This is unacceptable as at present ∼64% of global municipal wastewater is treated before discharge. Here, we report surface water dissolved inorganic carbon (DIC) and sedimentary organic carbon concentrations and their isotopic compositions in the rapidly urbanized Jiaozhou Bay in northeast China as well as carbonate parameters in effluents of three large WWTPs around the bay. Using DIC, δ13CDIC and total alkalinity (TA) data and a tracer model, we determine the contributions to DIC from wastewater DIC input, net ecosystem production, calcium carbonate precipitation, and CO2 outgassing. Our study shows that high-DIC and low-pH wastewater effluent represents an important source of DIC and acidification in coastal waters. In contrast to the traditional view of anthropogenic organic carbon export and degradation, we suggest that with the increase of wastewater discharge and treatment rates, wastewater DIC input may play an increasingly more important role in the coastal ocean carbon cycle.

A Systematic Algorithm for Metabolically Based Sensitization to Cisplatin in Head and Neck Squamous Cell Carcinoma

Cisplatin is the most commonly utilized systemic agent in the setting of head and neck squamous cell carcinoma (HNSCC). Despite nearly half a century of clinical use, our understanding of cisplatin activity in HNSCC and the primary drivers of its effectiveness or lack thereof, remains limited. Here we undertook a systematic evaluation of cisplatin processing and effects on HNSCC cell death in order to develop a broadly applicable approach for chemo-sensitization. Established HNSCC cell lines were chosen to represent the relative frequency of oncogenic events common to HNSCC (ie, TP53 mutation, HPV activation, NOTCH mutation). In vivo experiments were carried out using a xenograft murine model. Induction coupled plasma mass spectrometry was used to measure cisplatin transport and DNA-binding. DNA damage secondary to oxidative stress was measured using quantitative gammaH2AX phosphorylation and foci formation. Cisplatin effects on cell death were measured using individual assays for senescence, apoptosis, mitotic catastrophe, as well as the clonogenic survival assay. Cisplatin-generated metabolic shifts were measured using both steady-state metabolomics interrogation and 13C kinetic experiments. HNSCC cells import cisplatin in a dose- and time-dependent fashion but only a small fraction of cisplatin is bound to DNA following acute exposure. Cisplatin generated oxidative stress is a critical driver of cisplatin toxicity in HNSCC across molecular backgrounds. Mechanisms of cell death vary by genomic background and are not predictive of relative cisplatin sensitivity. Cisplatin transport, neutralization by thiol moieties, and DNA-damage repair are dependent on energy (ATP) and reducing equivalents (NADH, NADPH). Cisplatin exposure triggers measurable changes in central carbon flux, particularly in the conversion of pyruvate into lactate due to NADH depletion; compensatory changes in the citric acid cycle and pentose phosphate pathway are insufficient to restore reducing equivalent levels and are not consistent across molecular backgrounds. Direct measurements of intratumoral pyruvate and lactate correlate with relative cisplatin toxicity under in vivo conditions. Targeted inhibition of glycolytic flux potentiates cisplatin toxicity through (1) altered transport, (2) prolonged oxidative stress, and (3) decreased DNA repair (via PARP). Cisplatin toxicity in HNSCC is largely driven by non-specific oxidative stress across molecular backgrounds. Lactate dehydrogenase (LDH) activity appears to be both a marker of cisplatin toxicity and a potentially suitable target for metabolic targeting in HNSCC resistant to cisplatin. Conversely, metabolic targeting of non-glycolytic metabolic pathways is unlikely to be successful in sensitizing HNSCC cells to cisplatin across molecular backgrounds.

Cisplatin generates oxidative stress which is accompanied by rapid shifts in central carbon metabolism

Cisplatin is commonly utilized in the treatment of solid tumors. Its mechanism of action is complex and multiple mechanisms of resistance have been described. We sought to determine the impact of cisplatin-generated oxidative stress on head and neck squamous cell carcinoma (HNSCC) proliferation, survival and metabolic activity in order to identify a potential metabolic signature associated with cisplatin response. DNA-bound cisplatin represents a small fraction of total intra-cellular cisplatin but generates a robust oxidative stress response. Neutralization of oxidative stress reverses cisplatin toxicity independent of the mechanism of cell death and TP53 mutational status. Cisplatin-induced oxidative stress triggers rapid shifts in carbon flux in 3 commonly utilized catabolic pathways: glycolysis, pentose phosphate pathway and citric acid cycle. Among these metabolic shifts, decreased flux from pyruvate into lactate is the only metabolic effect consistently observed across multiple HNSCC cell lines of varying genomic backgrounds and may reflect differential cisplatin sensitivity. Oxidative stress is a critical component of cisplatin cytotoxicity in HNSCC and is reflected in acute changes in carbon flux from pyruvate into lactate. This suggests that lactate may contribute to a metabolic signature of acute cisplatin toxicity, and could prove useful in optimizing cisplatin-based treatment regimens in HNSCC.

Carbon Mass Change and Its Drivers in a Boreal Coniferous Forest in the Qilian Mountains, China from 1964 to 2013

Carbon storage of mountain forests is vulnerable to climate change but the changes in carbon flux through time are poorly understood. Moreover, the relative contributions to carbon flux of drivers such as climate and atmospheric CO2 still have significant uncertainties. We used the dynamic model LPJ-GUESS with climate data from twelve meteorological stations in the Qilian Mountains, China to simulate changes in carbon mass of a montane boreal forest, and the influence of temperature, precipitation, and CO2 concentration from 1964 to 2013 on carbon flux. The results showed that the carbon mass has increased 1.202 kg/m2 from 1964 to 2013, and net primary productivity (NPP) ranged from 0.997 to 1.122 kg/m2/year. We concluded that the highest carbon mass proportion for this montane boreal forest was at altitudes 2700–3100 m (proportion of ecosystem carbon was between 93–97%), with maximum carbon density observed at 2700–2900 m. In the last 50 years, the increase in precipitation and in CO2 concentration is expected to increase carbon mass and NPP of Picea crassifolia Kom. (Pinaceae) (Qinghai spruce). The effect of temperature on NPP was positive but that on carbon mass was not clear. The increase in CO2 concentration over the past 50 years was a major contributor to the increase in carbon storage, and drought was the foremost limiting factor in carbon storage capacity of this montane boreal forest. Picea crassifolia forest was vulnerable to climate change. Further studies need to focus on the impact of extreme weather, especially drought, on carbon storage in Picea crassifolia forests.

Tree Stress and Mortality from Emerald Ash Borer Does Not Systematically Alter Short-Term Soil Carbon Flux in a Mixed Northeastern U.S. Forest

Invasive insect pests are a common disturbance in temperate forests, but their effects on belowground processes in these ecosystems are poorly understood. This study examined how aboveground disturbance might impact short-term soil carbon flux in a forest impacted by emerald ash borer (Agrilus planipennis Fairmaire) in central New Hampshire, USA. We anticipated changes to soil moisture and temperature resulting from tree mortality caused by emerald ash borer, with subsequent effects on rates of soil respiration and methane oxidation. We measured carbon dioxide emissions and methane uptake beneath trees before, during, and after infestation by emerald ash borer. In our study, emerald ash borer damage to nearby trees did not alter soil microclimate nor soil carbon fluxes. While surprising, the lack of change in soil microclimate conditions may have been a result of the sandy, well-drained soil in our study area and the diffuse spatial distribution of canopy ash trees and subsequent canopy light gaps after tree mortality. Overall, our results indicate that short-term changes in soil carbon flux following insect disturbances may be minimal, particularly in forests with well-drained soils and a mixed-species canopy.

Genetic engineering to alter carbon flux for various higher alcohol productions by Saccharomyces cerevisiae for Chinese Baijiu fermentation.

Higher alcohols significantly influence the quality and flavor profiles of Chinese Baijiu. ILV1-encoded threonine deaminase, LEU1-encoded α-isopropylmalate dehydrogenase, and LEU2-encoded β-isopropylmalate dehydrogenase are involved in the production of higher alcohols. In this work, ILV1, LEU1, and LEU2 deletions in α-type haploid, a-type haploid, and diploid Saccharomyces cerevisiae strains and ILV1, LEU1, and LEU2 single-allele deletions in diploid strains were constructed to examine the effects of these alterations on the metabolism of higher alcohols. Results showed that different genetic engineering strategies influence carbon flux and higher alcohol metabolism in different manners. Compared with the parental diploid strain, the ILV1 double-allele-deletion diploid mutant produced lower concentrations of n-propanol, active amyl alcohol, and 2-phenylethanol by 30.33, 35.58, and 11.71%, respectively. Moreover, the production of isobutanol and isoamyl alcohol increased by 326.39 and 57.6%, respectively. The LEU1 double-allele-deletion diploid mutant exhibited 14.09% increased n-propanol, 33.74% decreased isoamyl alcohol, and 13.21% decreased 2-phenylethanol production, which were similar to those of the LEU2 mutant. Furthermore, the LEU1 and LEU2 double-allele-deletion diploid mutants exhibited 41.72 and 52.18% increased isobutanol production, respectively. The effects of ILV1, LEU1, and LEU2 deletions on the production of higher alcohols by α-type and a-type haploid strains were similar to those of double-allele deletion in diploid strains. Moreover, the isobutanol production of the ILV1 single-allele-deletion diploid strain increased by 27.76%. Variations in higher alcohol production by the mutants are due to the carbon flux changes in yeast metabolism. This study could provide a valuable reference for further research on higher alcohol metabolism and future optimization of yeast strains for alcoholic beverages.

Impact of land use changes on the storage of soil organic carbon in active and recalcitrant pools in a humid tropical region of India

Quantifying soil organic carbon (SOC) dynamics is important in understanding changes in soil properties and carbon (C) fluxes. However, SOC measures all C fractions and it is not adequate to distinguish between the active C (AC) and recalcitrant or passive C (PC) fractions. It has been suggested that PC pools are the main drivers of long term soil C sink management. Therefore, the present study was undertaken with the objective of determining whether or not SOC fractions vary with land use changes under a humid tropical climate in the North East India. A chronosequence study was established consisting of natural forest, Imperata cylindrica grassland and 6, 15, 27 and 34yr old rubber (Hevea brasiliensis) plantations to determine changes in the different fractions of SOC and total SOC stock. SOC stocks significantly varied with soil depth in each land use practice. SOC stocks increased from 106Mgha−1 under 6yr to 130Mgha−1 under 34yr old rubber plantations. The SOC stocks under 34yr old plantations were 20% higher than that under I. cylindrica grassland, but 34% lower than SOC stocks recorded under natural forest soil. The proportion of AC pools decreased with increase in plantation age, AC pools being 59% of SOC stock in 6yr old stands and 33% of SOC stocks in 34yr old plantations. In contrast, the proportion of PC pools increased from 41% of SOC stock in 6yr old plantation to 67% of SOC in 34yr old plantation. In the 50–100cm soil depth, the PC pool under 27–34yr old plantations was comparable with that under natural forest but much higher than in I. cylindrica grassland. Therefore, it is concluded that old rubber plantations can play a significant role in long term soil C sink management.

The influence of increasing temperature and CO2 concentration on recent growth of old-growth larch: contrasting responses at leaf and stem processes derived from tree-ring width and stable isotopes.

Time series of tree-ring growth show significant increases since the early 1970s at the alpine tree line, with simultaneously increasing temperatures and atmospheric CO2 concentration. For a comprehensive understanding of this growth change, the physiological response patterns at both the leaf and stem level need to be separately analyzed and identified, and can be retrieved from tree-ring growth and isotope (δ13C, δ18O) series. In this study, we assessed the relative contribution of environmental factors to interannual tree-ring variability by multivariate linear mixed-effects models and the dual isotope approach on a dataset of tree-ring records of ~400-year-old larch (Larix decidua Mill.) from a non-water-limited high-elevation site in the Swiss Alps. The models suggest that summer temperatures and the recent lack of larch budmoth outbreaks were most important for explaining growth variations and trends, while a significant direct effect of the continuously increasing CO2 concentration could not be confirmed. In contrast, δ13C and δ18O, which are strongly influenced by fractionation changes in the leaf, clearly reflected the impact of air humidity (precipitation and vapor pressure deficit) and CO2 concentration: the increase in (δ13C-derived) intrinsic water-use efficiency over the second half of the 20th century suggests an increase in carbon assimilation as a result of enhanced CO2 concentration. The tree-ring δ18O largely reflected recent precipitation as source water, thus indicating a low variability in stomatal conductance, which was confirmed by the dual isotope approach. These leaf-level effects were not reflected in stem growth as they may have been masked by the temperature-caused growth limitation controlling the allocation of increased amounts of photosynthates into wood growth. Our approach demonstrates that the identification of different roles of environmental factors on leaf and stem processes helps to improve the assessment of site-specific changes of carbon fluxes and growth performance under future environmental conditions.

Influence of top-down control in the plankton food web on vertical carbon flux: A case study in the Chesapeake Bay

Carnivorous gelatinous zooplankton dominate the zooplankton community in Chesapeake Bay during summer months, exerting considerable top-down control on the planktonic food web. To examine the cascading effects of gelatinous zooplankton blooms on the plankton food web and particulate organic carbon (POC) flux, multiple 2-day mesocosm experiments were conducted in the York River tributary of Chesapeake Bay in July–August 2015. Mesocosms contained a natural assemblage of phytoplankton, microzooplankton, and copepods, and each treatment received additions of the ctenophore (Mnemiopsis leidyi), the scyphozoan medusae (Chrysaora chesapeakei), or both gelatinous zooplankton. There was no significant difference between treatments in total POC or PON flux. However, presence of M. leidyi reduced the abundance of copepods, in turn significantly decreasing copepod fecal pellet carbon flux compared to treatments without M. leidyi by 50% (from 36 to 18μgCd−1m−3, or 6% to 3% of total POC flux). Total POC export fluxes were small in all treatments (<1%) compared to previously measured sedimentation rates in the Chesapeake Bay. Top-down changes in copepod fecal pellet carbon flux are equivalent to a modest portion (~10%) of previously calculated C. chesapeakei carcass flux. Future experiments and models of zooplankton contributions to vertical carbon flux should include top-down processes and cascading effects.

Improving models of photosynthetic thermal acclimation: Which parameters are most important and how many should be modified?

Photosynthetic temperature acclimation could strongly affect coupled vegetation-atmosphere feedbacks in the global carbon cycle, especially as the climate warms. Thermal acclimation of photosynthesis can be modelled as changes in the parameters describing the direct effect of temperature on photosynthetic capacity (i.e., activation energy, Ea ; deactivation energy, Hd ; entropy parameter, ΔS) or the basal value of photosynthetic capacity (i.e., photosynthetic capacity measured at 25°C). However, the impact of acclimating these parameters (individually or in combination) on vegetative carbon gain is relatively unexplored. Here we compare the ability of 66 photosynthetic temperature acclimation scenarios to improve the ability of a spatially explicit canopy carbon flux model, MAESTRA, to predict eddy covariance data from a loblolly pine forest. We show that: (1) incorporating seasonal temperature acclimation of basal photosynthetic capacity improves the model's ability to capture seasonal changes in carbon fluxes and outperforms acclimation of other single factors (i.e., Ea or ΔS alone); (2) multifactor scenarios of photosynthetic temperature acclimation provide minimal (if any) improvement in model performance over single factor acclimation scenarios; (3) acclimation of Ea should be restricted to the temperature ranges of the data from which the equations are derived; and (4) model performance is strongly affected by the Hd parameter. We suggest that a renewed effort be made into understanding whether basal photosynthetic capacity, Ea , Hd and ΔS co-acclimate across broad temperature ranges to determine whether and how multifactor thermal acclimation of photosynthesis occurs.

Climate drivers of diatom distribution in shallow subarctic lakes

Abstract Global warming can induce profound changes to the functioning of northern freshwater ecosystems. Diatom (Bacillariophyceae) communities often provide early warning signs of associated ecological regime shifts, responding sensitively to alterations in underwater light climate, nutrient regimes, habitat availability and lake water acid–base balance. The underlying mechanisms are manifold and may be mediated via direct climate impact on the physical and chemical properties of lakes or via changes in the terrestrial environment and catchment‐lake coupling. To address catchment‐mediated climate effects on diatom community composition, spatial diatom distribution in the surface sediments of 31 subarctic treeline lakes displaying a broad gradient in terrestrial dissolved organic matter (tDOM) was contrasted with limnological indices of light climate, nutrient availability and lake water pH. To evaluate direct and indirect climate impacts on the long‐term development of benthic phototrophic communities at the subarctic treeline, fossil diatom assemblages in the sediments of a shallow oligotrophic lake were examined against established temperature variability and inferences of terrestrial influence over the past 600 years. The regional lake set was used to test local calibration models for reconstructing dissolved organic carbon as well as lake water pH that is a fundamental environmental determinant for diatom distribution and may echo temperature variability in dilute lakes. Across the treeline, lake water pH imposed primary control over the benthic‐dominated surface sediment diatom communities. The pH influence was connected to catchment geomorphology, soils and vegetation cover and, together with habitat controls, largely superseded tDOM impact on underwater light attenuation and nutrient levels. Similarly, temporal changes in diatom distribution in the sediment core appeared to be relatively little affected by tDOM variability. The species shifts were subtle yet occurred in distinct synchrony with centennial temperature fluctuations, attributed to changing length of the ice cover period and associated effects on lake water chemistry, nutrient regimes and physical habitats. Our results suggest that diatom flora in shallow lakes at the subarctic Fennoscandian treeline may be comparatively resilient towards climate‐driven changes in terrestrial carbon and nutrient fluxes. Diatom communities in poorly buffered lakes may, however, be susceptible to catchment greening and changes in hydrology through effects on lake water acid–base balance. While diatom responses in the sediment sequence were subtle, the palaeolimnological record indicates that periphytic diatom communities in shallow oligotrophic subarctic lakes may be sensitive to the effects of global warming.

Determination of the life cycle climate change impacts of land use and albedo change in algal biofuel production

Life cycle assessments (LCAs) of algal biofuels have typically ignore direct land use change impacts (DLUC), although a few papers have recently shown that the carbon lost from removing the original biomass and disrupting soil organic carbon (SOC) during pond construction can significantly contribute to the climate change impacts. In this study, LCA methods were developed to integrate climate change impacts of DLUC associated with cultivation of microalgae in open ponds and the effects of temporal and geographic variables on algal growth. In addition to direct land use change impacts due to loss of original biomass and the disruption of SOC, this study presents DLUC impacts caused by changing the surface albedo of an area and changing the carbon flux on the land. The methodology is presented with two LCA cases for the production of renewable gasoline from microalgae in climatically similar regions: the Everglades ecoregion and the Tamaulipas-Texas Semi-Arid Plain ecoregion. These were chosen to isolate the differences in DLUC impacts that are created by different land cover types, equalizing the life cycle impacts caused by temperature and solar radiation on algal growth. The DLUC impacts arising from albedo change and carbon flux change contributed significantly to the life cycle climate change impacts and differed between the two climatically similar regions. The baseline life cycle climate change impacts with DLUC impacts in the Everglades are 46.7% higher than those of conventional gasoline, while production in the Tamaulipas-Texas Semi-Arid Plain leads to a 3.0% reduction. The inclusion of albedo change impacts into the DLUC impacts contributed 10.2% of the life cycle climate change impacts in the Everglades and 36.4% in the Tamaulipas. This methodology can help assess the geographically specific sustainability of algal biofuels on a life cycle basis and can guide siting decisions for algal biofuel feedstock production.