ALBERT

Description: Characterizing Leaf Area Index (LAI) and Vertical Foliage Profile (VFP) over the United States Biogeosciences Discussions, 12, 13675-13710, 2015 Author(s): H. Tang, S. Ganguly, G. Zhang, M. A. Hofton, R. F. Nelson, and R. Dubayah Leaf area index (LAI) and vertical foliage profile (VFP) are among the important canopy structural variables. Recent advances in lidar remote sensing technology have demonstrated the capability of accurately mapping LAI and VFP over large areas. The primary objective of this study was to derive and validate a LAI and VFP product over the contiguous United States using spaceborne waveform lidar data. This product was derived at the footprint level from the Geoscience Laser Altimeter System (GLAS) using a biophysical model. We validated GLAS derived LAI and VFP across major forest biomes using airborne waveform lidar. The comparison results showed that GLAS retrievals of total LAI were generally accurate with little bias ( r 2 = 0.67, bias = −0.13, RMSE = 0.75). The derivations of GLAS retrievals of VFP within layers was not as accurate overall ( r 2 = 0.36, bias = −0.04, RMSE = 0.26), and these varied as a function of height, increasing from understory to overstory −0 to 5 m layer: r 2 = 0.04, bias = 0.09, RMSE = 0.31; 10 to 15 m layer: r 2 = 0.53, bias = −0.08, RMSE = 0.22; and 15 to 20 m layer: r 2 = 0.66, bias =−0.05, RMSE = 0.20. Significant relationships were also found between GLAS LAI products and different environmental factors, in particular elevation and annual precipitation. In summary, our results provide a unique insight into vertical canopy structure distribution across North American ecosystems. This data set is a first step towards a baseline of canopy structure needed for evaluating climate and land use induced forest changes at continental scale in the future and should help deepen our understanding of the role of vertical canopy structure on terrestrial ecosystem processes across varying scales.

Description: Application of the 15 N-Gas Flux method for measuring in situ N 2 and N 2 O fluxes due to denitrification in natural and semi-natural terrestrial ecosystems and comparison with the acetylene inhibition technique Biogeosciences Discussions, 12, 12653-12689, 2015 Author(s): F. Sgouridis, S. Ullah, and A. Stott Soil denitrification is considered the most un-constrained process in the global N cycle due to uncertain in situ N 2 flux measurements, particularly in natural and semi-natural terrestrial ecosystems. 15 N tracer approaches can provide in situ measurements of both N 2 and N 2 O simultaneously, but their use has been limited to fertilised agro-ecosystems due to the need for large 15 N additions in order to detect 15 N 2 production against the high atmospheric N 2 . For 15 N-N 2 analyses, we have used an "in house" laboratory designed and manufactured N 2 preparation instrument which can be interfaced to any commercial continuous flow isotope ratio mass spectrometer (CF-IRMS). The N 2 prep unit has gas purification steps, a copper based reduction furnace, and allows the analysis of small gas injection volumes (4 μL) for 15 N-N 2 analysis. For the analysis of N 2 O, an automated Tracegas Pre-concentrator (Isoprime Ltd) coupled to an IRMS was used to measure the 15 N-N 2 O (4 mL gas injection volume). Consequently, the coefficient of variation for the determination of isotope ratios for N 2 in air and in standard N 2 O (0.5 ppm) was better than 0.5 %. The 15 N Gas-Flux method was adapted for application in natural and semi-natural land use types (peatlands, forests and grasslands) by lowering the 15 N tracer application rate to 0.04–0.5 kg 15 N ha −1 . For our chamber design (volume / surface = 8:1) and a 20 h incubation period, the minimum detectable flux rates were 4 μg N m −2 h −1 and 0.2 ng N m −2 h −1 for the N 2 and N 2 O fluxes respectively. The N 2 flux ranged between 2.4 and 416.6 μg N m −2 h −1 , and the grassland soils showed on average 3 and 14 times higher denitrification rates than the woodland and organic soils respectively. The N 2 O flux was on average 20 to 200 times lower than the N 2 flux, while the denitrification product ratio (N 2 O/N 2 + N 2 O) was low, ranging between 0.03 and 13 %. Total denitrification rates measured by the acetylene inhibition technique under the same field conditions correlated ( r = 0.58) with the denitrification rates measured under the 15 N Gas-Flux method but were underestimated by a factor of 4 and this was attributed to the incomplete inhibition of N 2 O reduction to N 2 under relatively high soil moisture content. The results show that the 15 N Gas-Flux method can be used for quantifying N 2 and N 2 O production rates in natural terrestrial ecosystems, thus significantly improving our ability to constrain ecosystem N budgets.

Description: Natural variability in the surface ocean carbonate ion concentration Biogeosciences Discussions, 12, 13123-13157, 2015 Author(s): N. S. Lovenduski, M. C. Long, and K. Lindsay We investigate variability in the surface ocean carbonate ion concentration ([CO 3 2− ]) on the basis of a long control simulation with a fully-coupled Earth System Model. The simulation is run with a prescribed, pre-industrial atmospheric CO 2 concentration for 1000 years, permitting investigation of natural [CO 3 2− ] variability on interannual to multi-decadal timescales. We find high interannual variability in surface [CO 3 2− ] in the tropical Pacific and at the boundaries between the subtropical and subpolar gyres in the Northern Hemisphere, and relatively low interannual variability in the centers of the subtropical gyres and in the Southern Ocean. Statistical analysis of modeled [CO 3 2− ] variance and autocorrelation suggests that significant anthropogenic trends in the saturation state of aragonite (Ω aragonite ) are already or nearly detectable at the sustained, open-ocean timeseries sites, whereas several decades of observations are required to detect anthropogenic trends in Ω aragonite in the tropical Pacific, North Pacific, and North Atlantic. The detection timescale for anthropogenic trends in pH is shorter than that for Ω aragonite , due to smaller noise-to-signal ratios and lower autocorrelation in pH. In the tropical Pacific, the leading mode of surface [CO 3 2− ] variability is primarily driven by variations in the vertical advection of dissolved inorganic carbon (DIC) in association with El Niño–Southern Oscillation. In the North Pacific, surface [CO 3 2− ] variability is caused by circulation-driven variations in surface DIC and strongly correlated with the Pacific Decadal Oscillation, with peak spectral power at 20–30 year periods. North Atlantic [CO 3 2− ] variability is also driven by variations in surface DIC, and exhibits weak correlations with both the North Atlantic Oscillation and the Atlantic Multidecadal Oscillation. As the scientific community seeks to detect the anthropogenic influence on ocean carbonate chemistry, these results will aid the interpretation of trends calculated from spatially- and temporally-sparse observations.

Description: Spatial and temporal trends in summertime climate and water quality indicators in the coastal embayments of Buzzards Bay, Massachusetts Biogeosciences Discussions, 12, 13159-13192, 2015 Author(s): J. E. Rheuban, S. C. Williamson, J. E. Costa, D. M. Glover, R. W. Jakuba, D. C. McCorkle, C. Neill, T. Williams, and S. C. Doney Degradation of coastal ecosystems by eutrophication is largely defined by nitrogen loading from land via surface and groundwater flows. However, indicators of water quality are highly variable due to a myriad of other drivers, including temperature and precipitation. To evaluate these drivers, we examined spatial and temporal trends in a 22 year record of summer water quality data from 122 stations in 17 embayments within Buzzards Bay, MA (USA), collected through a citizen science monitoring program managed by Buzzards Bay Coalition. To identify spatial patterns across Buzzards Bay's embayments, we used a principle component and factor analysis and found that rotated factor loadings indicated little correlation between inorganic nutrients and organic matter and chlorophyll a (Chl a ) concentration. Factor scores showed that embayment geomorphology in addition to nutrient loading was a strong driver of water quality, where embayments with surface water inputs showed larger biological impacts than embayments dominated by groundwater influx. A linear regression analysis of annual summertime water quality indicators over time revealed that from 1992 to 2013, most embayments (15 of 17) exhibited an increase in temperature (mean rate of 0.082 ± 0.025 (SD) °C yr −1 ) and Chl a (mean rate of 0.0171 ± 0.0088 log 10 (Chl a ; mg m −3 ) yr −1 , equivalent to a 4.0 % increase per year). However, only 7 embayments exhibited an increase in total nitrogen (TN) concentration (mean rate 0.32 ± 0.47 (SD) μM yr −1 ). Average summertime log 10 (TN) and log 10 (Chl a ) were correlated with an indication that yield of Chl a per unit total nitrogen increased with time suggesting the estuarine response to TN may have changed because of other stressors such as warming, altered precipitation patterns, or changing light levels. These findings affirm that nitrogen loading and physical aspects of embayments are essential in explaining observed ecosystem response. However, climate-related stressors may also need to be considered by managers because increased temperature and precipitation may worsen water quality and partially offset benefits achieved by reducing nitrogen loading.

Description: Carbon dynamics in highly heterotrophic subarctic thaw ponds Biogeosciences Discussions, 12, 11707-11749, 2015 Author(s): T. Roiha, I. Laurion, and M. Rautio Global warming has accelerated the formation of permafrost thaw ponds in several subarctic and arctic regions. These ponds are net heterotrophic as evidenced by their greenhouse gas (GHG) supersaturation levels (CO 2 and CH 4 ), and generally receive large terrestrial carbon inputs from the thawing and eroding permafrost. We measured seasonal and vertical variations in the concentration and type of dissolved organic matter (DOM) in five subarctic thaw (thermokarst) ponds in northern Quebec, and explored how environmental gradients influenced heterotrophic and phototrophic biomass and productivity. Late winter DOM had low aromaticity indicating reduced inputs of terrestrial carbon, while the high concentration of dissolved organic carbon (DOC) suggests that some production of non-chromophoric dissolved compounds by the microbial food web took place under the ice cover. Summer DOM had a strong terrestrial signature, but was also characterized with significant inputs of algal-derived carbon, especially at the pond surface. During late winter, bacterial production was low (maximum of 0.8 mg C m −3 d −1 ) and was largely based on free-living bacterioplankton (58 %). Bacterial production in summer was high (up to 58 mg C m −3 d −1 ), dominated by particle-attached bacteria (67 %), and strongly correlated to the amount of terrestrial carbon. Primary production was restricted to summer surface waters due to strong light limitation deeper in the water column or in winter. The phototrophic biomass was equal to the heterotrophic biomass, but as the algae were mostly composed of mixotrophic species, most probably they used bacteria rather than solar energy in such shaded ponds. According to the δ 13 C analyses, non-algal carbon supported 51 % of winter and 37 % of summer biomass of the phantom midge larvae, Chaoborus sp., that are at the top of the trophic chain. Our results point to a strong heterotrophic energy pathway in these thaw pond ecosystems, where bacterioplankton dominates the production of new carbon in both summer and winter.

Description: Phototrophic pigment diversity and picophytoplankton abundance in permafrost thaw lakes Biogeosciences Discussions, 12, 12121-12156, 2015 Author(s): A. Przytulska, J. Comte, S. Crevecoeur, C. Lovejoy, I. Laurion, and W. F. Vincent Permafrost thaw lakes (thermokarst lakes) are widely distributed across the northern landscape, and are known to be biogeochemically active sites that emit large amounts of carbon to the atmosphere as CH 4 and CO 2 . However, the abundance and composition of the photosynthetic communities that consume CO 2 have been little explored in this ecosystem type. In order to identify the major groups of phototrophic organisms and their controlling variables, we sampled 12 permafrost thaw lakes along a permafrost degradation gradient in northern Québec, Canada. Additional samples were taken from 5 rock-basin reference lakes in the region to determine if the thaw waters differed in limnological properties and phototrophs. Phytoplankton community structure was determined by high performance liquid chromatography analysis of their photoprotective and photosynthetic pigments, and autotrophic picoplankton concentrations were assessed by flow cytometry. One of the black colored lakes located in a andscape of rapidly degrading palsas (permafrost mounds) was selected for high-throughput 18S rRNA sequencing to help interpret the pigment and cytometry data. The results showed that the limnological properties of the thaw lakes differed significantly from the reference lakes, and were more highly stratified. However, both waterbody types contained similarly diverse phytoplankton groups, with dominance of the pigment assemblages by fucoxanthin-containing taxa, as well as chlorophytes, cryptophytes and cyanobacteria. Chlorophyll a concentrations (Chl a ) were correlated with total phosphorus (TP), and both were significantly higher in the thaw lakes (overall means of 3.3 μg Chl a L −1 and 34 μg TP L −1 ) relative to the reference lakes (2.0 μg Chl a L −1 and 8.2 μg TP L −1 ). Stepwise multiple regression of Chl a against the other algal pigments showed that it was largely a function of lutein, fucoxanthin and peridinin ( R 2 = 0.78). The bottom waters of two of the thaw lakes also contained high concentrations of bacteriochlorophyll d , showing the presence of green photosynthetic sulphur bacteria. The molecular analyses indicated a relatively minor contribution of diatoms, while chrysophytes, dinoflagellates and chlorophytes were well represented; the heterotrophic eukaryote fraction was dominated by numerous ciliate taxa, and also included Heliozoa, Rhizaria, chytrids and flagellates. Autotrophic picoplankton occurred in cell concentrations up to 8.8 × 10 5 mL −1 (picocyanobacteria) and 4.6 × 10 5 mL −1 (picoeukaryotes). Both groups of picophytoplankton were positively correlated with total phytoplankton abundance, as measured by Chl a ; picocyanobacteria were inversely correlated with dissolved organic carbon, while picoeukaryotes were correlated with conductivity. Despite their net heterotrophic character, subarctic thaw lakes are rich habitats for diverse phototrophic communities.

Description: Skeletal mineralogy of coral recruits under high temperature and p CO 2 Biogeosciences Discussions, 12, 12485-12500, 2015 Author(s): T. Foster and P. L. Clode Aragonite, which is the polymorph of CaCO 3 precipitated by modern corals during skeletal formation, has a higher solubility than the more stable polymorph calcite. This higher solubility leaves animals that produce aragonitic skeletons more vulnerable to anthropogenic ocean acidification. It is therefore, important to determine whether scleractinian corals have the plasticity to adapt and produce calcite in their skeletons in response to changing environmental conditions. Both high p CO 2 and lower Mg / Ca ratios in seawater are thought to have driven changes in the skeletal mineralogy of major marine calcifiers in the past ∼540 myr. Experimentally reduced Mg / Ca ratios in ambient seawater have been shown to induce some calcite precipitation in both adult and newly settled modern corals, however, the impact of high p CO 2 on the mineralogy of recruits is unknown. Here we determined the skeletal mineralogy of one-month old Acropora spicifera coral recruits grown under high temperature (+3 °C) and p CO 2 (∼900 μatm) conditions, using X-ray diffraction and Raman spectroscopy. We found that newly settled coral recruits produced entirely aragonitic skeletons regardless of the treatment. Our results show that elevated p CO 2 alone is unlikely to drive changes in the skeletal mineralogy of young corals. Not having an ability to switch from aragonite to calcite precipitation may leave corals and ultimately coral reef ecosystems more susceptible to predicted ocean acidification. An important area for prospective research would be to investigate the combined impact of high p CO 2 and reduced Mg / Ca ratio on coral skeletal mineralogy.

Description: The impact of sedimentary alkalinity release on the water column CO 2 system in the North Sea Biogeosciences Discussions, 12, 12395-12453, 2015 Author(s): H. Brenner, U. Braeckman, M. Le Guitton, and F. J. R. Meysman Recently, it has been proposed that alkalinity release from sediments can play an important role in the carbonate dynamics on continental shelves, lowering the p CO 2 of seawater and hence increasing the CO 2 uptake from the atmosphere. To test this hypothesis, sedimentary alkalinity generation was quantified within permeable and muddy sediments across the North Sea during two cruises in September 2011 (basin-wide) and June 2012 (Dutch coastal zone). Benthic fluxes of alkalinity ( A T ) and dissolved inorganic carbon (DIC) were determined using shipboard closed sediment incubations. These results show that sediments can be an important source for alkalinity, particularly in the shallow southern North Sea, where high A T and DIC fluxes were recorded in near shore sediments of the Belgian, Dutch and German coastal zone. In contrast, fluxes of A T and DIC are substantially lower in the deeper, seasonally stratified, northern part of the North Sea. Overall, our results show that sedimentary alkalinity generation should be considered an important factor in the CO 2 dynamics of shallow coastal systems.

Description: Contribution of Marine Group II Euryarchaeota to cyclopentyl tetraethers in the Pearl River estuary and coastal South China Sea: impact on the TEX 86 paleothermometer Biogeosciences Discussions, 12, 12455-12484, 2015 Author(s): J. X. Wang, C. L. Zhang, W. Xie, Y. G. Zhang, and P. Wang TEX 86 (TetraEther indeX of glycerol dialkyl glycerol tetraethers (GDGTs) with 86 carbon atoms) has been widely applied to reconstruct (paleo-) sea surface temperature (SST). While Marine Group I (MG I) Thaumarchaeota have been commonly believed to be the source for GDGTs, Marine Group II (MG II Euryarchaeota ) have recently been suggested to contribute significantly to the GDGT pool in the ocean. However, little is known how the MG II Euryarchaeota -derived GDGTs may influence TEX 86 in marine sediment record. In this study, we characterize MG II Euryarchaeota -produced GDGTs and assess the likely effect of these tetraether lipids on TEX 86 . Analyses of core lipid (CL-) and intact polar lipid (IPL-) based GDGTs, 454 sequencing and quantitative polymerase chain reaction (qPCR) targeting MG II Euryarchaeota were performed on suspended particulate matter (SPM) and surface sediments collected along a salinity gradient from the lower Pearl River (river water) and its estuary (mixing water) to the coastal South China Sea (seawater). The results showed that the community composition varied along the salinity gradient with MG II Euryarchaeota as the second dominant group in the mixing water and seawater. qPCR data indicated that the abundance of MG II Euryarchaeota in the mixing water was three to four orders of magnitude higher than the river water and seawater. Significant linear correlations were observed between the gene abundance ratio of MG II Euryarchaeota vs. total archaea and the relative abundance of GDGTs-1, -2, -3, or -4 as well as the ring index based on these compounds, which collectively suggest that MG II Euryarchaeota may actively produce GDGTs in the water column. These results also show strong evidence that MG II Euryarchaeota synthesizing GDGTs with 1–4 cyclopentane moieties may bias TEX 86 in the water column and sediments. This study highlights that valid interpretation of TEX 86 in sediment record, particularly in coastal oceans, needs to consider the contribution from MG II Euryarchaeota .

Description: Fate of peat-derived carbon and associated CO 2 and CO emissions from two Southeast Asian estuaries Biogeosciences Discussions, 12, 8299-8340, 2015 Author(s): D. Müller, T. Warneke, T. Rixen, M. Müller, A. Mujahid, H. W. Bange, and J. Notholt Coastal peatlands in Southeast Asia release large amounts of organic carbon to rivers, which transport it further to the adjacent estuaries. However, little is known about the fate of this terrestrial material in the coastal ocean. Although Southeast Asia is, by area, considered a hotspot of estuarine CO 2 emissions, studies in this region are very scarce. We measured dissolved and particulate organic carbon, carbon dioxide (CO 2 ) partial pressure and carbon monoxide (CO) concentrations in two tropical estuaries in Sarawak, Malaysia, whose coastal area is covered by peatlands. We surveyed the estuaries of the rivers Lupar and Saribas during the wet and dry season, respectively. The spatial distribution and the carbon-to-nitrogen ratios of dissolved organic matter (DOM) suggest that peat-draining rivers convey terrestrial organic carbon to the estuaries. We found evidence that a large fraction of this carbon is respired. The median p CO 2 in the estuaries ranged between 618 and 5064 μatm with little seasonal variation. CO 2 fluxes were determined with a floating chamber and estimated to amount to 14–272 mol m −2 yr −1 , which is high compared to other studies from tropical and subtropical sites. In contrast, CO concentrations and fluxes were relatively moderate (0.3–1.4 nmol L −1 and 0.8–1.9 mmol m −2 yr −1 ) if compared to published data for oceanic or upwelling systems. We attributed this to the large amounts of suspended matter (4–5004 mg L −1 ), limiting the light penetration depth. However, the diurnal variation of CO suggests that it is photochemically produced, implying that photodegradation might play a role for the removal of DOM from the estuary as well. We concluded that unlike smaller peat-draining tributaries, which tend to transport most carbon downstream, estuaries in this region function as an efficient filter for organic carbon and release large amounts of CO 2 to the atmosphere. The Lupar and Saribas mid-estuaries release 0.4 ± 0.2 Tg C yr −1 , which corresponds to approximately 80% of the emissions from the aquatic systems in these two catchments.