ALBERT

Description: In the southwest United States, the current prolonged warm drought is similar to the predicted future climate change scenarios for the region. This study aimed to determine patterns in vegetation response to the early 21st century drought across multiple biomes. We hypothesized that different biomes (forests, shrublands, and grasslands) would have different relative sensitivities to both climate drivers (precipitation and temperature) and legacy effects (previous-year's productivity). We tested this hypothesis at eight Ameriflux sites in various Southwest biomes using NASA Moderate-resolution Imaging Spectroradiometer Enhanced Vegetation Index (EVI) from 2001 to 2013. All sites experienced prolonged dry conditions during the study period. The impact of combined precipitation and temperature on Southwest ecosystems at both annual and sub-annual timescales was tested using Standardized Precipitation Evapotranspiration Index (SPEI). All biomes studied had critical sub-annual climate periods during which precipitation and temperature influenced production. In forests, annual peak greenness (EVI max ) was best predicted by 9-month SPEI calculated in July (i.e., January–July). In shrublands and grasslands, EVI max was best predicted by SPEI in July through September, with little effect of the previous year's EVI max . Daily gross ecosystem production (GEP) derived from flux tower data yielded further insights into the complex interplay between precipitation and temperature. In forests, GEP was driven by cool-season precipitation and constrained by warm-season maximum temperature. GEP in both shrublands and grasslands was driven by summer precipitation and constrained by high daily summer maximum temperatures. In grasslands, there was a negative relationship between temperature and GEP in July, but no relationship in August and September. Consideration of sub-annual climate conditions and the inclusion of the effect of temperature on the water balance allowed us to generalize the functional responses of vegetation to predicted future climate conditions. We conclude that across biomes, drought conditions during critical sub-annual climate periods could have a strong negative impact on vegetation production in the southwestern United States.

Description: We present nearly 9 yrs (June 2005–December 2013) of measurements of upper-ocean (0 m to 125 m) dinitrogen (N 2 ) fixation rates, coupled with particulate nitrogen (PN) export at 150 m, from Station ALOHA (22° 45′N, 158°W) in the North Pacific Subtropical Gyre. Between June 2005 and June 2012, N 2 fixation rates were measured based on adding the 15 N 2 tracer as a gas bubble. Beginning in August 2012, 15 N 2 was first dissolved into filtered seawater and the 15 N 2 -enriched water was subsequently added to N 2 fixation incubations. Direct comparisons between methodologies revealed a robust relationship, with the addition of 15 N 2 -enriched seawater resulting in twofold greater depth-integrated rates than those derived from adding a 15 N 2 gas bubble. Based on this relationship, we corrected the initial period of measurements, and the resulting rates of N 2 fixation averaged 230 ± 136 μmol N m −2 d −1 for the full time series ( n  = 71). Analysis of the 15 N isotopic composition of sinking PN, together with an isotope mass balance model, revealed that N 2 fixation supported 26–47% of PN export during calendar years 2006–2013. The N export derived from these fractional contributions and measured N 2 fixation rates ranged between 502 and 919 μmol N m −2  d −1 , which are equivalent to rates of net community production (NCP) of 1.5 to 2.7 mol C m −2  yr −1 , consistent with previous independent estimates of NCP at this site.

Description: Ice shelves surround ∼ 75% of Antarctica's coastline and are highly sensitive to climate change; several have recently collapsed and others are predicted to in the near future. Marine waters beneath ice shelves harbor active ecosystems, while adjacent seas can be important areas of bottom water formation. Despite their oceanographic significance, logistical constraints have resulted in few opportunities to directly sample sub-ice shelf cavities. Here, we present the first data on microbial diversity and biogeochemistry beneath the McMurdo Ice Shelf (MIS) near Ross Island, Antarctica. Physicochemical profiles obtained via a 56 m deep borehole through the MIS revealed three vertically layered water masses (Antarctic Surface Water [AASW], Ice Shelf Water [ISW], and modified High Salinity Shelf Water [mHSSW]). Metabolically active, moderately diverse (Shannon diversity from 2.06 to 5.74) microbial communities were detected in the AASW and mHSSW. Heterotrophic bacterial production and dissolved organic matter concentrations were higher (12–37% and 24%, respectively) in mHSSW relative to AASW. Chemoautotrophic production was 5.3 nmol C L −1 d −1 and 6.0 nmol C L −1 d −1 in the AASW and mHSSW, respectively. Phytoplankton cells were more abundant and larger in the mHSSW sample relative to the AASW, which indicates sinking of phytoplankton produced in surface waters and, together with southerly flowing currents (0.09–0.16 m s −1 ), horizontal advection of phytoplankton from McMurdo Sound. Advected phytoplankton carbon together with in situ chemoautotrophic production provide important sources of organic matter and other reduced compounds to support ecosystem processes in the dark waters in the ice shelf cavity.

Description: Global modeling efforts indicate semiarid regions dominate the increasing trend and interannual variation of net CO 2 exchange with the atmosphere, mainly driven by water availability. Many semiarid regions are expected to undergo climatic drying, but the impacts on net CO 2 exchange are poorly understood due to limited semiarid flux observations. Here we evaluated 121 site-years of annual eddy covariance measurements of net and gross CO 2 exchange (photosynthesis and respiration), precipitation, and evapotranspiration (ET) in 21 semiarid North American ecosystems with an observed range of 100 – 1000 mm in annual precipitation and records of 4-9 years each. In addition to evaluating spatial relationships among CO 2 and water fluxes across sites, we separately quantified site-level temporal relationships, representing sensitivity to interannual variation. Across the climatic and ecological gradient, photosynthesis showed a saturating spatial relationship to precipitation, whereas the photosynthesis-ET relationship was linear, suggesting ET was a better proxy for water available to drive CO 2 exchanges after hydrologic losses. Both photosynthesis and respiration showed similar site-level sensitivity to interannual changes in ET among the 21 ecosystems. Furthermore, these temporal relationships were not different from the spatial relationships of long-term mean CO 2 exchanges with climatic ET. Consequently, a hypothetical 100-mm change in ET, whether short- or long-term, was predicted to alter net ecosystem production (NEP) by 64 gCm −2 y −1 . Most of the unexplained NEP variability was related to persistent, site-specific function, suggesting prioritization of research on slow-changing controls. Common temporal and spatial sensitivity to water availability increases our confidence that site-level responses to interannual weather can be extrapolated for prediction of CO 2 exchanges over decadal and longer timescales relevant for societal response to climate change. This article is protected by copyright. All rights reserved.

Description: The daytime evolution of the atmospheric boundary layer under high pressure, anticyclonic synoptic systems over a homogeneous terrain can be successfully described by a penetrative free convection model, evolving from an initially stably stratified environment. In this article dimensional analysis has been employed to derive a new set of scaling parameters, which are functions of external, time-dependent, fluid properties and boundary conditions. This novel theoretical framework has been adopted to compare laboratory scale experiments, performed using a thermally controlled water tank and a Large Eddy Simulation (LES) numerical model. Both these have been developed for the characterization of the instabilities associated with the development of the Convective Boundary Layer (CBL). The characteristic length and time scales of the thermal plumes, their spatial distribution and interaction with the overlying stable layer are analyzed. The proposed scaling parameters appear to be representative of the bulk and turbulent properties of the CBL, as confirmed by both numerical and laboratory results.

Description: Carbon uptake by forests is a major sink in the global carbon cycle, helping buffer the rising concentration of CO 2 in the atmosphere, yet the potential for future carbon uptake by forests is uncertain. Climate warming and drought can reduce forest carbon uptake by reducing photosynthesis, increasing respiration, and by increasing the frequency and intensity of wildfires, leading to large releases of stored carbon. Five years of eddy covariance measurements in a ponderosa pine ( Pinus ponderosa ) dominated ecosystem in northern Arizona showed that an intense wildfire that converted forest to sparse grassland shifted site carbon balance from sink to source for at least 15 years after burning. In contrast, recovery of carbon sink strength after thinning, a management practice used to reduce the likelihood of intensewildfires, was rapid. Comparisons between an undisturbed-control site and an experimentally thinned site showed that thinning reduced carbon sink strength only for the first two post-treatment years. In the third and fourth post-treatment years, annual carbon sink strength of the thinned site was higher than theundisturbedsite because thinning reduced aridity and drought limitation to carbon uptake. As a result, annual maximum gross primary production occurred when temperature was 3°C higher at the thinned site compared to the undisturbed site. The severe fireconsistently reduced annual evapotranspiration (range of 12 to 30%), whereas effects of thinning were smallerand transient, and could not be detected in the fourth year after thinning. Our results show large and persistent effects of intense fireand minor and short-lived effects of thinning on southwestern ponderosa pine ecosystem carbon and water exchanges.

Description: Abstract Aerobic methane production in aquatic ecosystems impacts the global atmospheric budget of methane, but the extent, mechanism, and taxa responsible for producing this greenhouse gas are not fully understood. Lake Bonney (LB), a perennially ice‐covered Antarctic lake, has cold hypersaline waters underlying an oxygenated freshwater layer. We present temporal methane concentration profiles in LB indicating methane production in the oxygenated (〉 200% air saturation) water. Experiments amended with methylphosphonate (MPn) yielded methane generation, suggesting in situ methanogenesis via the carbon‐phosphorus (C‐P) lyase pathway. Enrichment cultures from the lake were used to isolate five bacterial strains capable of generating methane when supplied with MPn as the sole P source. Based on 16S rRNA gene sequencing, the isolates belong to the Proteobacteria (closely related to Marinomonas, Hoeflea, and Marinobacter genera) and Bacteroidetes (Algoriphagus genus). 16S rRNA metagenomic sequencing confirms the presence of these taxa in LB. None of the isolated species were reported to be capable to produce methane. In addition, orthologs of the phosphoenolpyruvate mutase gene (PepM) and methylphosphonate synthase (MPnS), enzymes involved in phosphonate and MPn biosynthesis, were widely spread in the LB shotgun metagenomic libraries; genes related to C‐P lyase pathways (phn gene clusters) were also abundant. 16S rRNA and mcrA genes of anaerobic methanogens were absent in both 16S rRNA and metagenomics libraries. These data reveal that in situ aerobic biological methane production is likely a significant source of methane in LB.

Description: Cloud condensation and ice nuclei in the troposphere are required precursors to cloud and precipitation formation, both of which influence the radiative balance of Earth. The initial stage of hailstone formation (i.e., the embryo) and the subsequent layered growth allow hail to be used as a model for the study of nucleation processes in precipitation. By virtue of the preserved particle and isotopic record captured by hailstones, they represent the unique form of precipitation that allows direct characterization of the particles present during atmospheric ice nucleation. Despite the ecological and economic consequences of hail storms, the dynamics of hailstone nucleation, and thus their formation, are not well understood. Our experiments show that hailstone embryos from three Rocky Mountain storms contained biological ice nuclei capable of freezing water at warm, sub-zero (°C) temperatures, indicating that biological particles can act as nucleation sites for hailstone formation. These results are corroborated by analysis of δD and δ 18 O from melted hailstone embryos, which show that the hailstones formed at similarly warm temperatures in situ. Low densities of ice nucleation active abiotic particles were also present in hailstone embryos, but their low concentration indicates they were not likely to have catalyzed ice formation at the warm temperatures determined from water stable isotope analysis. Our study provides new data on ice nucleation occurring at the bottom of clouds, an atmospheric region whose processes are critical to global climate models but which has challenged instrument-based measurements.

Description: Sustained time-series have provided compelling evidence for progressive acidification of the surface oceans through exchange with the growing atmospheric reservoir of carbon dioxide. However, few long-term programs exist, and extrapolation of results from one site to larger oceanic expanses is hampered by the lack of spatial coverage inherent to Eulerian sampling. Since 1988, the Hawaii Ocean Time-series (HOT) program has sampled CO 2 system variables nearly monthly at Station ALOHA, a deep ocean site windward and 115 km north of the island of Oahu. Surface measurements have also been made at Station Kahe, a leeward site 12 km from the island and on the opposite side of the Hawaiian Ridge. Despite having different physical settings, the sites exhibit identical rates of surface p CO 2 increase and hydrogen ion accumulation, suggesting that atmospheric forcing dominates over local dynamics in determining the CO 2 trend in the surface waters of the North Pacific subtropical gyre.