Top Meter Research Articles

Seagrass meadows as a globally significant carbonate reservoir

Abstract. There has been growing interest in quantifying the capacity of seagrass ecosystems to act as carbon sinks as a natural way of offsetting anthropogenic carbon emissions to the atmosphere. However, most of the efforts have focused on the particulate organic carbon (POC) stocks and accumulation rates and ignored the particulate inorganic carbon (PIC) fraction, despite important carbonate pools associated with calcifying organisms inhabiting the meadows, such as epiphytes and benthic invertebrates, and despite the relevance that carbonate precipitation and dissolution processes have in the global carbon cycle. This study offers the first assessment of the global PIC stocks in seagrass sediments using a synthesis of published and unpublished data on sediment carbonate concentration from 403 vegetated and 34 adjacent un-vegetated sites. PIC stocks in the top 1 m of sediment ranged between 3 and 1660 Mg PIC ha−1, with an average of 654 ± 24 Mg PIC ha−1, exceeding those of POC reported in previous studies by about a factor of 5. Sedimentary carbonate stocks varied across seagrass communities, with meadows dominated by Halodule, Thalassia or Cymodocea supporting the highest PIC stocks, and tended to decrease polewards at a rate of −8 ± 2 Mg PIC ha−1 per degree of latitude (general linear model, GLM; p < 0.0003). Using PIC concentrations and estimates of sediment accretion in seagrass meadows, the mean PIC accumulation rate in seagrass sediments is found to be 126.3 ± 31.05 g PIC m−2 yr−1. Based on the global extent of seagrass meadows (177 000 to 600 000 km2), these ecosystems globally store between 11 and 39 Pg of PIC in the top metre of sediment and accumulate between 22 and 75 Tg PIC yr−1, representing a significant contribution to the carbonate dynamics of coastal areas. Despite the fact that these high rates of carbonate accumulation imply CO2 emissions from precipitation, seagrass meadows are still strong CO2 sinks as demonstrated by the comparison of carbon (PIC and POC) stocks between vegetated and adjacent un-vegetated sediments.

Geographic variability in organic carbon stock and accumulation rate in sediments of East and Southeast Asian seagrass meadows

AbstractOrganic carbon (OC) stored in the sediments of seagrass meadows has been considered a globally significant OC reservoir. However, the sparsity and regional bias of studies on long‐term OC accumulation in coastal sediments have limited reliable estimation of the capacity of seagrass meadows as a global OC sink. We evaluated the amount and accumulation rate of OC in sediment of seagrass meadows and adjacent areas in East and Southeast Asia. In temperate sites, the average OC concentration in the top 30 cm of sediment was higher in seagrass meadows (780–1080 μmol g−1) than in sediments without seagrass cover (52–430 μmol g−1). The average OC in the top 30 cm of subtropical and tropical seagrass meadow sediments ranged from 140 to 440 μmol g−1. Carbon isotope mass balancing suggested that the contribution of seagrass‐derived carbon to OC stored in sediments was often relatively minor (temperate: 10–40%; subtropical: 35–82%; tropical: 4–34%) and correlated to the habitat type, being particularly low in estuarine habitats. Stock of OC in the top meter of sediment of all the studied meadows ranged from 38 to 120 Mg ha−1. The sediment accumulation rates were estimated by radiocarbon dating of six selected cores (0.32–1.34 mm yr−1). The long‐term OC accumulation rates calculated from the sediment accumulation rate and the top 30 cm average OC concentration for the seagrass meadows (24–101 kg ha−1 yr−1) were considerably lower than the OC accumulation rates previously reported for Mediterranean Posidonia oceanica meadows (580 kg ha−1 yr−1 on average). Current estimates for the global carbon sink capacity of seagrass meadows, which rely largely on Mediterranean studies, may be considerable overestimations.

Variations in the Isotopic Composition of Dissolved Inorganic Carbon in the Unsaturated Zone of a Semi-Arid Region

A study of water and carbon isotopes was conducted in a bare plot in the unsaturated zone of the Yatir Forest in the northern Negev of Israel. Sediment cores were collected in three different seasons. Measurements include profiles of mineralogy, moisture and its δ18O and tritium content, dissolved inorganic carbon (DIC) and its δ13C (‰) and Δ14C (‰) content, and δ13C (‰) and Δ14C (‰) in the solid sediment. The profiles of moisture and δ18O in the cores show clearly the effect of evaporation. The tritium profile indicates infiltration of water (0.11 m yr−1). The source of carbon in the DIC is CO2 released by biotic activity through roots of trees and of seasonal plants, which show seasonal variations, and by decay of organic debris. The δ13C (‰) profiles show clearly the chemical transition from dissolved CO2 (δ13C = −22‰) to bicarbonate (δ13C = −14‰). At greater depth (–11.3‰), the δ13C becomes similar to the δ13C in the aquifer below (–12.5‰). The effect of secondary processes is evident in the profile of Δ14C in the DIC. It shows a clear decrease with depth due to exchange with the sediment at a rate of 10‰ yr−1. Precipitation of carbon from the DIC on the sediment is 1.1 mg C Lsed−1 yr−1, negligible compared to the 28 g C in 1 Lsed. In the solid sediment, there is a gradient in Δ14Ccarb at the top meter. The net precipitation of 14C from the DIC on the sediment (0.25 to 1.1‰ yr−1), corrected for decay, cannot be observed in the deeper sediment. The presence of 14C in the top 1 m of the sediment is explained by two possible processes: accumulation of 14C-tagged dust (∼0.05 mm yr−1) and/or long-term cumulative precipitation from the DIC.

Sediment accumulation, stratigraphic order, and the extent of time-averaging in lagoonal sediments: a comparison of 210Pb and 14C/amino acid racemization chronologies

Carbon-14 calibrated amino acid racemization (14C/AAR) data and lead-210 (210Pb) data are used to examine sediment accumulation rates, stratigraphic order, and the extent of time-averaging in sediments collected from the One Tree Reef lagoon (southern Great Barrier Reef, Australia). The top meter of lagoonal sediment preserves a stratigraphically ordered deposit spanning the last 600 yrs. Despite different assumptions, the 210Pb and 14C/AAR chronologies are remarkably similar indicating consistency in sedimentary processes across sediment grain sizes spanning more than three orders of magnitude (0.1–10 mm). Estimates of long-term sediment accumulation rates range from 2.2 to 1.2 mm yr−1. Molluscan time-averaging in the taphonomically active zone is 19 yrs, whereas below the depth of final burial (~15 cm), it is ~110 yrs/5 cm layer. While not a high-resolution paleontological record, this reef lagoon sediment is suitable for paleoecological studies spanning the period of Western colonization and development. This sedimentary deposit, and others like it, should be useful, albeit not ideal, for quantifying anthropogenic impacts on coral reef systems.

Vertical Distribution of Soil Organic Carbon Density in Relation to Land Use/Cover, Altitude and Slope Aspect in the Eastern Himalayas

In-depth understanding about the vertical distribution of soil organic carbon (SOC) density is crucial for carbon (C) accounting, C budgeting and designing appropriate C sequestration strategies. We examined the vertical distribution of SOC density under different land use/land cover (LULC) types, altitudinal zones and aspect directions in a montane ecosystem of Bhutan. Sampling sites were located using conditioned Latin hypercube sampling (cLHS) scheme. Soils were sampled based on genetic horizons. An equal-area spline function was fitted to interpolate the target values to predetermined depths. Linear mixed model was fitted followed by mean separation tests. The results show some significant effects of LULC, altitudinal zone and slope aspect on the vertical distribution of SOC density in the profiles. Based on the proportion of mean SOC density in the first 20 cm relative to the cumulative mean SOC density in the top meter, the SOC density under agricultural lands (34%) was more homogeneously distributed down the profiles than forests (39%), grasslands (59%) and shrublands (43%). Similarly, the SOC density under 3500–4000 m zone (35%) was more uniformly distributed compared to 3000–3500 m zone (43%) and 1769–2500 m and 2500–3000 m zones (41% each). Under different aspect directions, the north and east-facing slopes (38% each) had more uniform distribution of SOC density than south (40%) and west-facing slopes (49%).

Influence of water depth on the carbon sequestration capacity of seagrasses

AbstractThe actual estimates of carbon stocks beneath seagrass meadows worldwide are derived from few data, resulting in a tendency to generalize global carbon stocks from a very limited number of seagrass habitats. We surveyed Posidonia oceanica and Posidonia sinuosa meadows along depth‐induced gradients of light availability to assess the variability in their sedimentary organic carbon (Corg) stocks and accretion rates. This study showed a fourfold decrease in Corg stocks from 2–4 m to 6–8 m depth P. sinuosa meadows (averaging 7.0 and 1.8 kg m−2, respectively; top meter of sediment) and a fourteenfold to sixteenfold decrease from shallow (2 m) to deep (32 m) P. oceanica meadows (200 and 19 kg m−2 average, respectively; top 2.7 m of sediment). The average Corg accretion rates in shallow P. sinuosa meadows were higher (10.5 g m−2 yr−1) than in deeper meadows (2.1 g m−2 yr−1). The reduction of sedimentary Corg stocks and accretion rates along depth‐related gradients of light reduction suggests that irradiance, controlling plant productivity, meadow density, and sediment accretion rates, is a key environmental factor affecting Corg storage potential of seagrasses. The results obtained highlighted the exceptional carbon storage capacity of P. oceanica meadows at Balearic Islands (Spain), containing the highest areal Corg stocks of all seagrasses (estimated in up to 691–770 kg m−2 in 8–13 m thick deposits). Seagrass communities are experiencing worldwide decline, and reduced irradiance (following e.g., eutrophication or sediment regime alterations) will lead to photoacclimation responses (i.e., reduced plant productivity and shoot density), which may impact the carbon sequestration capacity of seagrasses.

Nutrient leaching under zero tension in a subtropical clonal eucalypt plantation on a sandy soil in South Africa

Little is known about the effects of residue burning or retention on nutrient leaching during the inter-rotation of clonal Eucalyptus grown on the sandy soils of subtropical Zululand, South Africa. A study compared zero-tension nutrient leaching through the top metre of soil at depths of 0.15, 0.5 and 1.0 m in an undisturbed crop with adjacent clearfelled areas subjected to residue burning and residue retention. Leaching at 1.0 m in the undisturbed crop was 80% less than at 0.15 m leaching due to high water use of the mature trees. Loss of nutrients past 1.0 m in the undisturbed crop amounted to 7.0, 13.1, 6.6, 15.1 and 60.7 kg ha−1 of nitrogen (N), potassium (K), calcium (Ca), magnesium (Mg) and sodium (Na) over the period between felling and new crop canopy closure (22 months). Annualised 1.0 m leaching amounted to 4.2, 7.8, 4.0, 9.0 and 36.3 kg ha−1 of N, K, Ca, Mg and Na, respectively. Clearfelling induced an increase in N and cation leaching that was apparent five months after clearfelling and persisted for nine months. Leaching loss declined rapidly in the new crop after planting to levels similar to the undisturbed crop by six months of age. Leaching past 1.0 m soil depth under residue retention amounted to 30.6, 132.0, 82.5, 108.7 and 299.1 kg ha−1 of N, K, Ca, Mg and Na, respectively, between felling and canopy closure. Although some weakly significantly differences were found between residue burning and retention, residue burning did not substantially alter leaching past 1 m soil depth. Burning rather induced a large loss of N (121 kg ha−1) through oxidisation, around half the residue N content. Residue retention or burning followed by rapid re-establishment can therefore be practiced to retain most nutrients on this site. Burning of residues should be practiced conservatively on low N soils or be followed by N fertilisation.

Alteration of the carbon and nitrogen isotopic composition in the Martian surface rocks due to cosmic ray exposure

Abstract13C/12C and 15N/14N isotopic ratios are pivotal for our understanding of the Martian carbon cycle, history of the Martian atmospheric escape, and origin of the organic compounds on Mars. Here we demonstrate that the carbon and nitrogen isotopic composition of the surface rocks on Mars can be significantly altered by the continuous exposure of Martian surface to cosmic rays. Cosmic rays can effectively produce 13C and 15N isotopes via spallation nuclear reactions on oxygen atoms in various Martian rocks. We calculate that in the top meter of the Martian rocks, the rates of production of both 13C and 15N due to galactic cosmic rays (GCRs) exposure can vary within 1.5–6 atoms/cm3/s depending on rocks' depth and chemical composition. We also find that the average solar cosmic rays can produce carbon and nitrogen isotopes at a rate comparable to GCRs in the top 5–10 cm of the Martian rocks. We demonstrate that if the total carbon content in a surface Martian rock is <10 ppm, then the “light,” potentially “biological” 13C/12C ratio would be effectively erased by cosmic rays over 3.5 billion years of exposure. We found that for the rocks with relatively short exposure ages (e.g., 100 million years), cosmogenic changes in 15N/14N ratio are still very significant. We also show that a short exposure to cosmic rays of Allan Hills 84001 while on Mars can explain its high‐temperature heavy nitrogen isotopic composition (15N/14N). Applications to Martian meteorites and the current Mars Science Laboratory mission are discussed.

Carbon Storage in Seagrass Beds of Abu Dhabi, United Arab Emirates

“Blue Carbon” initiatives have highlighted the significant role of seagrasses in organic carbon (C org) burial and sequestration. However, global databases on the extent of C org stocks in seagrass ecosystems are largely comprised of studies conducted in monospecific beds from a limited number of regions, thus potentially biasing global estimates. To better characterize carbon stocks in seagrass beds of varying structure and composition, and to further expand the current “Blue Carbon” database to under-represented regions, we evaluate the extent of C org stocks in the relatively undocumented seagrass meadows of the Arabian Gulf. Surveys were conducted along the coast of Abu Dhabi (UAE) and encompassed sites ranging from sheltered embayments to offshore islands. Seagrass beds consisted of Halodule uninervis, Halophila ovalis and Halophila stipulacea. While seagrasses were widely distributed along the coast, both living and soil C org stores were relatively modest on an areal basis. Total seagrass biomass ranged from 0.03 to 1.13 Mg C ha−1, with a mean of 0.4 ± 0.1 (±SEM), and soil C org stocks (as estimated over the top meter) ranged from 1.9 to 109 Mg C ha−1, with a mean of 49.1 ± 7.0 (±SEM). However, owing to the expansive distribution of seagrasses in the Arabian Gulf, seagrass “Blue Carbon” stocks were large, with 400 Gg C stored in living seagrass biomass and 49.1 Tg C stored in soils. Thus, despite low C org stores for any given location, the overall contribution of seagrass beds to carbon storage are relatively large given their extensive coverage. This research adds to a growing global dataset on carbon stocks and further demonstrates that even seagrass beds dominated by small-bodied species function to store carbon in coastal environments.

Mid-Paleocene event at Gabal Nezzazat, Sinai, Egypt: planktonic foraminiferal biostratigraphy, mineralogy and geochemistry

The Qreiya Beds that record the ‘mid-Paleocene event’ at Gabal Nezzazat occur within the Igorina albeari (P3b) Zone and constitute part of a 14-m thick shale succession that ranges in age from Early to Late Paleocene. They are composed of four alternating dark grey and brown shale beds, which are thinly laminated, phosphatic, organic-rich and extremely sulphidic. They are characterized by distinct enrichment and high peak anomalies in chalcophiles (Zn, Co, Ni, Cu and Pb) and organic association elements (V and Cr), especially within the brown organic-rich beds. It is concluded that these elements are incorporated into the phosphatic debris, sulphides and organic matter. In contrast, the grey beds are enriched in clay minerals and quartz. Clay mineral assemblages indicate alternating periods of warm/humid climate (high kaolinite) and dry climate (low kaolinite) during the formation of the grey and brown beds, respectively. The sediments of the Qreiya Beds yield lithological, biotic, geochemical and mineralogical data indicative of suboxic/anoxic marine environments as a result of high productivity and/or upwelling. The top metre of the succession below the Qreiya Beds is characterized by a progressive change from faunas dominated by praemurcurids to faunas dominated by Morozovilids, and by a progressive upward decrease in δ13Ccarb and δ18Ocarb values. The foraminiferal faunal change may reflect shallowing and warming preceding deposition of the Qreiya Beds. The change in isotopic values is inferred to be the result of surface weathering, fluvial input and diagenesis with no evidence of any primary change that could support presence of a hyperthermal event.

A field trial of nutrient stimulation of methanotrophs to reduce greenhouse gas emissions from landfill cover soils

Landfills are among the major sources of anthropogenic methane (CH4) estimated to reach 40 × 109kg per year worldwide by 2015 (IPCC, 2007). A 2½-year field experiment was conducted at a closed landfill in western Michigan where methanotrophs, methane-consuming bacteria, were stimulated by nutrient addition to the soil without significantly increasing biogenic nitrous oxide (N2O) production. The effects of the nitrogen amendments (KNO3 and NH4Cl), phenylacetylene (a selective inhibitor of nitrifying bacteria that contribute to N2O production), and a canopy (to reduce direct water infiltration) on the vertical soil gas profiles of CH4, CO2, and O2 were measured in the top meter of the soil. Methane and nitrous oxide fluxes were calculated from the corresponding soil gas concentration gradients with respect to depth and a Millington–Quirk diffusivity coefficient in soil derived empirically from soil porosity, water content, and diffusivity coefficients in air from the literature. Methane flux estimates were as high as 218.4 g m−2 day−1 in the fall and 12.8 g/m−2 day−1 in the summer. During the spring and summer, CH4 fluxes were reduced by more than half by adding KNO3 and NH4Cl into the soil as compared to control plots, while N2O fluxes increased substantially. The concurrent addition of phenylacetylene to the amendment decreased peak N2O production by half and the rate of peak methane oxidation by about one-third. The seasonal average methane and N2O flux data were extrapolated to estimate the reduction of CH4 and N2O fluxes into the atmosphere by nitrogen and inhibitor addition to the cover soils. The results suggest that such additions coupled with soil moisture management may provide a potential strategy to significantly reduce greenhouse gas emissions from landfills. Implications The results of a 2½-year study of effects of nutrient stimulation on methane oxidation in landfill cover soils demonstrates that nutrient addition does decrease methane emissions. The work further underscores the control which soil moisture exerts on methane oxidation. Water management is critical to the success of methane oxidation strategies. Supplemental Materials: Supplemental materials are available for this paper. Go to the publisher's online edition of the Journal of the Air & Waste Management Association.

Shockproof Experimental Study of Automated Stocker System in the High-Tech Factory

This study, with the shake table experiments in the National Earthquake Engineering Research Center, investigated the seismic behavior of automation stocker system. The automation stocker system provided a fast and effective method for stocking products in the high-tech facilities. Firstly, the original machine tests for testing common stocker system were to investigate the seismic capacity and failing modes. Secondly, the team completed 2 kinds of reinforcement tests to investigate the seismic behavior: 1) the installation of bracing to improve overall stiffness; 2) the installation of the viscous dampers to improve the overall damping ratio. In comparing the results and performance of the three experiments, we learned from the results of the top-layer acceleration: the installation of bracing had the largest acceleration value, the original machine the second acceleration value and the damper the lowest acceleration value; the best effect was the installation of the damper. The result of the comparison of the top floor displacement meter showed that the highest data was the original machine; the second data was with the damper, the lowest data was the installation of the bracing. Based on the preliminary assessment on the best seismic retrofit ways of the storage system, we further examined the feasibility and applicability of automatic storage seismic retrofit in the high-tech factory.

Assessing sub‐seafloor microbial activity by combined stable isotope probing with deuterated water and 13C‐bicarbonate

Sub-seafloor sediments are populated by large numbers of microbial cells but not much is known about their metabolic activities, growth rates and carbon assimilation pathways. Here we introduce a new method enabling the sensitive detection of microbial lipid production and the distinction of auto- and heterotrophic carbon assimilation. Application of this approach to anoxic sediments from a Swedish fjord allowed to compare the activity of different functional groups, the growth and turnover times of the bacterial and archaeal communities. The assay involves dual stable isotope probing (SIP) with deuterated water (D(2) O) and (13) C(DIC) (dissolved inorganic carbon). Culture experiments confirmed that the D content in newly synthesized lipids is in equilibrium with the D content in labelled water, independent of whether the culture grew hetero- or autotrophically. The ratio of (13) C(DIC) to D(2) O incorporation enables distinction between these two carbon pathways in studies of microbial cultures and in environmental communities. Furthermore, D(2) O-SIP is sufficiently sensitive to detect the formation of few hundred cells per day in a gram of sediment. In anoxic sediments from a Swedish fjord, we found that > 99% of newly formed lipids were attributed to predominantly heterotrophic bacteria. The production rate of bacterial lipids was highest in the top 5 cm and decreased 60-fold below this depth while the production rate of archaeal lipids was rather low throughout the top meter of seabed. The contrasting patterns in the rates of archaeal and bacterial lipid formation indicate that the factors controlling the presence of these two lipid groups must differ fundamentally.

Early diagenesis of organic-matter-rich sediments in a ría environment: Organic matter sources, pyrites morphology and limitation of pyritization at depth

The subtidal sediments of the inner Ría de Vigo (NW Spain) are fine-grained, rich in organic matter, and accumulate at rates of around 3 mm/yr. Redoxomorphic pyrites formation was studied in four sediment cores. Four operationally-defined iron fractions were determined (HCl-extracted, silicate-bound, organic and pyritic), and the dimensions and morphology of pyrites (which mainly occurred as isolated framboids 3–5 μm in diameter) were studied by SEM. Sediment appearance and degree of pyritization (DOP) identified an oxic zone limited to the top few millimetres, a suboxic zone that in most cores was just a few decimetres thick (the exception was a core obtained in an area of strong turbulent currents), and an underlying anoxic zone with DOP values of around 60%. Pyritic iron increased at the expense of the HCl-extracted fraction in approximately the top metre, but no further pyrite formation occurred at greater depths. Pyrite formation in these lower levels was not limited by lack of either HCl-extracted iron or metabolizable organic matter, but apparently by lack of sulphate ion, the diffusion of which to these depths was probably prevented by the fine texture of the sediments. Our findings highlight the interest of studying transitional sedimentary environments in order to explore in detail the geochemical parameters controlling pyrite formation.

A millennial record of environmental change in peat deposits from the Misten bog (East Belgium)

In this study, palaeoenvironmental changes recorded in the top metre of a peat profile (Misten bog, East Belgium) were investigated using a multiproxy approach. Proxies include bulk density, Ti and Si content, pollen, macrofossils, δ13C on specific Sphagnum stems, and δ13C–δ18O on Sphagnum leaves. A high-resolution chronology was generated using 210Pb measurements and 22 14C AMS dates on carefully selected Sphagnum macrofossils. δ13C only records large change in mire surface wetness. This is partly due to the fact that the core was taken from the edge of a hummock, which may make it difficult to track small isotopic changes. The δ13C signal seems to be dependent upon the Sphagnum species composition. For example, a change between Sphagnum section Cuspidata towards Sphagnum imbricatum causes a significant drop in the δ13C values. On the whole, the C and O isotopes record two shallow pool phases during the 8th–9th and the 13th centuries. Pollen and atmospheric soil dust (ASD) fluxes records increased human occupation in the area. There may be some climatic signals in the ASD flux, but they are difficult to decipher from the increasing human impact (land clearance, agriculture) during the last millennium. The variations in the proxies are not always synchronous, suggesting different triggering factors (temperature, wetness, windiness) for each proxy. This study also emphasizes that, compared to studies dealing with pollution using geochemical proxies, palaeoclimatic inferences from peat bogs need as many proxies as possible, together with highly accurate and precise age-models, in order to better understand climate variability and their consequences during the Holocene.

Holocene Carbon Stocks and Carbon Accumulation Rates Altered in Soils Undergoing Permafrost Thaw

Permafrost soils are a significant global store of carbon (C) with the potential to become a large C source to the atmosphere. Climate change is causing permafrost to thaw, which can affect primary production and decomposition, therefore affecting ecosystem C balance. To understand future responses of permafrost soils to climate change, we inventoried current soil C stocks, investigated ∆14C, C:N, δ13C, and δ15N depth profiles, modeled soil C accumulation rates, and calculated decadal net ecosystem production (NEP) in subarctic tundra soils undergoing minimal, moderate, and extensive permafrost thaw near Eight Mile Lake (EML) in Healy, Alaska. We modeled decadal and millennial soil C inputs, decomposition constants, and C accumulation rates by plotting cumulative C inventories against C ages based on radiocarbon dating of surface and deep soils, respectively. Soil C stocks at EML were substantial, over 50 kg C m−2 in the top meter, and did not differ much among sites. Carbon to nitrogen ratio, δ13C, and δ15N depth profiles indicated most of the decomposition occurred within the organic soil horizon and practically ceased in deeper, frozen horizons. The average C accumulation rate for EML surface soils was 25.8 g C m−2 y−1 and the rate for the deep soil accumulation was 2.3 g C m−2 y−1, indicating these systems have been C sinks throughout the Holocene. Decadal net ecosystem production averaged 14.4 g C m−2 y−1. However, the shape of decadal C accumulation curves, combined with recent annual NEP measurements, indicates soil C accumulation has halted and the ecosystem may be becoming a C source. Thus, the net impact of climate warming on tundra ecosystem C balance includes not only becoming a C source but also the loss of C uptake capacity these systems have provided over the past ten thousand years.

Controlled activation of species typical behaviour to low salinity, seawater movement and seawater depth in Aurelia labiata (Scyphozoa) Jellyfish in Roscoe Bay, Canada

Controlled observations in Roscoe Bay examined whether Aurelia labiata medusae would respond to three environmentally significant stimuli: low salinity, seawater movement and seawater depth. Significantly more upward-swimming medusae turned and swam down or sideways when they encountered an experimentally created low-salinity plume than did upward-swimming medusae that encountered a seawater plume or a seawater plume containing milk powder. Downward-swimming medusae that encountered a plume containing freshwater continued swimming down. Significantly more upward-swimming medusae exposed to an experimentally created stream of seawater that gently pushed them horizontally turned and swam down than did upward-swimming medusae not exposed to the seawater stream. Downward-swimming medusae continued to swim down when pushed horizontally. A stronger seawater stream that tumbled as well as pushed medusae horizontally was less effective. In a shallow area near the time of a low tide, most medusae were in the top metre of a 2 m water column whilst in an adjacent area where the water was 4–6 m deep, almost all medusae were below 1.5 m. These observations add to a growing body of evidence that the ecological distribution of A. labiata is influenced by their ability to respond adaptively to significant environmental stimuli.

Field Observations During the Eighth Microwave Water and Energy Balance Experiment (MicroWEX-8): from June 16 through August 24, 2009

In order to really accurately predict weather and near-term climate, it is necessary to take into consideration soil moisture in the top meter of soil: it governs moisture and energy fluxes at the land-atmosphere interface, and it plays a significant role in partitioning of the precipitation into runoff and infiltration. This 68-page report presents the observations of an experiment using remotely sensed microwave observations with a view towards incorporating the findings into the models used to estimate energy and moisture fluxes and improve root zone soil moisture estimates. Written by Tara Bongiovanni, Heather Enos, Alejandro Monsivais-Huertero, Blaire Colvin, Karthik Nagarajan, Jasmeet Judge, Pang-Wei Liu, Juan Fernandez-Diaz, Roger De Roo, Yuriy Goykhman, Xueyang Duan, Daniel Preston, Ramesh Shrestha, Clint Slatton, Mahta Moghaddam, and Anthony England, and published by the UF Department of Agricultural and Biological Engineering, August 2011.
 http://edis.ifas.ufl.edu/ae476

Soil Carbon Pools in California’s Annual Grassland Ecosystems

Rangeland ecosystems cover approximately one-third of the land area in the United States and half of the land area of California. This large land area, coupled with the propensity of grasses to allocate a considerable proportion of their photosynthate belowground, leads to high soil carbon (C) sequestration potential. Annual grasslands typical of the Mediterranean climates of the western United States differ in their life history strategies from the well-studied perennial grasslands of other regions and thus may also differ in their soil C pools and fluxes. In this study we use the literature to explore patterns in soil C storage in annual grass-dominated rangelands in California. We show that soil C is highly predictable with depth. Cumulative soil C content increased to 2–3-m depth in rangelands with a woody component and to at least 1-m depth in open rangelands. Soil C within a given depth varied widely, with C content in the top 1-m depth spanning almost 200 Mg C ? ha 21 across sites. Soil C pools were not correlated with temperature or precipitation at a regional scale. The presence of woody plants increased C by an average of 40 Mg ? ha 21 in the top meter of soil. Grazed annual grasslands had similar soil C content as ungrazed grassland at all depths examined, although few details on grazing management were available. Soil C pools were weakly positively correlated with clay content and peaked at intermediated levels of aboveground net primary production. Our results suggest that annual grasslands have similar soil C storage capacity as temperate perennial grasslands and offer an important resource for mitigation of greenhouse gas emissions and climate change. Resumen

Sediment Characterization and Dynamics in Lake Pontchartrain, Louisiana

FLOCKS, J.; KINDINGER, J.; MAROT, M., and CHARLES, C., 2009. Ssediment characterization and dynamics in Lake Pontchartrain, Louisiana. Journal of Coastal Research, SI(54), 113–126. West Palm Beach (Florida), ISSN 0749-0208. Lake Pontchartrain in southeastern Louisiana is the largest of several shallow estuaries that together cover over 15,000 km 2 . Wetlands, forests, and large urban areas surround the lake. Primary transport mechanisms of sediments to Lake Pontchartrain include urban runoff, major diversions of the Mississippi River, discharge from streams along the north and west shores, and tidal circulation. Sediments deposited in Lake Pontchartrain are subjected to resuspension and mixing by natural and human activities. Bioturbation and water turbulence throughout the lake are the major mixing agents, and mechanical shell dredging has reworked much of the lake bottom over the last century. Sediment characterization through direct sampling and geophysical surveys indicates that these processes continually rework the top meter of sediment. The lake receives discharge from roadways and industrial and agricultural sources. Contaminants from these sources accumulate in the lake sediments and are an important contributor to the degradation of the estuary. Decline in populations of various benthic organisms, such as shrimp and clams, has been documented in the lake. To characterize the health of this important estuary, the U.S. Geological Survey (USGS) conducted a comprehensive evaluation of the geology, geomorphology, coastal processes, and environmental condition of the Pontchartrain Basin from 1994 to 1997. This report presents an assessment of sediment distribution and quality using a multidisciplinary approach to characterize the influence of various physical and chemical parameters: nearsurface stratigraphy, major trace metal concentrations (Cu, Pb, Zn, and Ni), and short-lived radionuclides ( 210 Pb, 7 Be, and 137 Cs). The results are compared with water-circulation patterns to determine high-resolution sedimentation patterns in the lake. The data show a significant increase in trace metals in the top 1 m of lake sediments. Above this horizon, pollen analysis indicates a correlation with land clearing in the area, a proxy for increasing human development of the surrounding landscape and an increase in surface run-off. The data also show that the top meter of sediment undergoes frequent resuspension during high-energy circulation events and via circulation gyres in the lake. This regular turnover does not allow stratification of recently deposited sediments, restricting the sequestration of contaminated material that enters the lake.