ALBERT

Description: Abstract Lakes are conduits of greenhouse gases to the atmosphere; however, most efflux estimates for individual lakes are based on extrapolations from a limited number of locations. Within‐lake variability in carbon dioxide (CO2) and methane (CH4) arises from differences in water sources, mixing, atmospheric exchange, and biogeochemical transformations, all of which vary across multiple temporal and spatial scales. We asked, how variable are CO2 and CH4 across the surface of a single lake, how do spatial patterns change seasonally, and how well does the typical sampling location represent the entire lake surface? During the 2016 ice‐free period, we mapped surface water concentrations of CO2 and CH4 approximately weekly in Lake Mendota (USA) and modeled diffusive gas exchange. During stratification, CO2 was generally lower than atmospheric saturation (mean 19.81 μM) and relatively homogenous (mean coefficient of variation 0.12), whereas CH4 was routinely extremely supersaturated (mean 0.29 μM) with greater spatial heterogeneity (mean coefficient of variation 0.65). During fall mixis, concentrations of both gases increased and became more spatially variable, but their spatial arrangements differed. In this system, samples collected from the lake center reasonably well represented the spatially weighted mean CO2 concentration but overestimated annual CO2 efflux by 21%. For CH4, the lake center underestimated annual diffusive efflux by only 8.6% but poorly represented lakewide concentrations and fluxes on any given day. Upscaling from a single site to the whole lake requires consideration of spatial variation to assess lakewide carbon dynamics due to heterogeneity in within‐lake processing, transport to the lake surface, and exchange with the atmosphere.

Description: A warming and shifting climate in the Arctic has led to significant declines in sea ice over the last several decades. Although these changes in sea ice cover are well documented, large uncertainties remain in how associated increases in solar radiation transmitted to the underlying ocean water column will impact heating, biological and biogeochemical processes in the Arctic Ocean. In this study, six under-ice marine, two ice-free marine, and two ice-free terrestrially-influenced water samples were irradiated using a solar simulator for 72 hours (representing ~10 days of ambient sunlight) to investigate dissolved organic matter (DOM) dynamics from the Chukchi and Beaufort seas. Solar irradiation caused chromophoric DOM (CDOM) light absorption at 254 nm to decrease by 48 to 63%. An overall loss in total DOM fluorescence intensity was also observed at the end of all experiments, and each of 6 components identified by parallel factor analyses (PARAFAC) was shown to be photoreactive in at least one experiment. DOM fluorescence (FDOM) also indicated that the majority of DOM in under-ice and ice-free marine waters was likely algal-derived. Measurable changes in dissolved organic carbon (DOC) were only observed for sites influenced by riverine runoff. Losses of CDOM absorbance at shorter wavelengths suggest that the beneficial UV protection currently received by marine organisms may decline with the increased light transmittance associated with sea ice melt ponding and overall reductions of sea ice. Our FDOM analyses demonstrate that DOM irrespective of source was susceptible to photobleaching. Additionally, our findings suggest that photodegradation of CDOM in under-ice waters is not currently a significant source of carbon dioxide (CO 2 ) (i.e., we did not observe systematic DOC loss). However, increases in primary production and terrestrial freshwater export expected under future climate change scenarios may cause an increase in CDOM quantity and shift in quality throughout Arctic Ocean surface waters. As arctic temperatures continue to warm and summer sea ice further declines, examination of the resulting enhanced photodegradation processes and their impacts on the interplay between primary production, carbon cycling, and surface ocean heating processes will be paramount.

Description: Free Air CO 2 Enrichment (FACE) experiments provide a remarkable wealth of data which can be used to evaluate and improve terrestrial ecosystem models (TEMs). In the FACE Model-Data Synthesis project (FACE-MDS), 11 TEMs were applied to two decade-long FACE experiments in temperate forests of the south eastern US—the evergreen Duke Forest and the deciduous Oak Ridge forest. In this baseline paper, we demonstrate our approach to model-data synthesis by evaluating the models' ability to reproduce observed Net Primary Productivity (NPP), transpiration and Leaf Area Index (LAI) in ambient CO 2 treatments. Model outputs were compared against observations using a range of goodness-of-fit statistics. Many models simulated annual NPP and transpiration within observed uncertainty. We demonstrate however, that high goodness-of-fit values do not necessarily indicate a successful model, because simulation accuracy may be achieved through compensating biases in component variables. For example, transpiration accuracy was sometimes achieved with compensating biases in leaf area index and transpiration per unit leaf area. Our approach to model-data synthesis therefore goes beyond goodness-of-fit to investigate the success of alternative representations of component processes. Here, we demonstrate this approach by comparing competing model hypotheses determining peak LAI. Of three alternative hypotheses—(1) optimisation to maximise carbon export, (2) increasing specific leaf area (SLA) with canopy depth and (3) the pipe model—the pipe model produced peak LAI closest to the observations. This example illustrates how datasets from intensive field experiments such as FACE can be used to reduce model uncertainty despite compensating biases, by evaluating individual model assumptions.

Description: We characterized peat decomposition at the Marcell Experimental Forest (MEF), Minnesota, USA, to a depth of 2 meters to ascertain the underlying chemical changes using Fourier Transform Infrared (FT IR) and 13 C Nuclear Magnetic Resonance ( 13 C NMR) spectroscopy) and related these changes to decomposition proxies C:N ratio, δ 13 C and δ 15 N, bulk density and water content. FT IR determined peat humification increased rapidly between 30 and 75 cm, indicating a highly reactive intermediate-depth zone consistent with changes in C: N ratio, δ 13 C and δ 15 N, bulk density and water content. Peat decomposition at the MEF, especially in the intermediate depth zone, is mainly characterized by preferential utilization of O-alkyl-C, carboxyl-C, and other oxygenated functionalities with a concomitant increase in the abundance of alkyl- and nitrogen-containing compounds. Below 75-cm, less change was observed but aromatic functionalities and lignin accumulated with depth. Significant correlations with humification indices, identified by FT IR spectroscopy, were found for C: N ratios. Incubation studies at 22 °C revealed the highest methane production rates, greatest CH 4 :CO 2 production ratios and significant O-alkyl-C utilization within this 30 and 75 cm zone. Oxygen-containing functionalities, especially O-alkyl-C, appear to serve as excellent proxies for soil decomposition rate, and should be a sensitive indicator of the response of the solid phase peat to increased temperatures caused by climate change and the field study manipulations that are planned to occur at this site. Radiocarbon signatures of microbial respiration products in deeper porewaters at the MEF resembled the signatures of more modern dissolved organic carbon (DOC) rather than solid phase peat, indicating that recently photosynthesized organic matter fueled the bulk of subsurface microbial respiration. These results indicate that carbon cycling at depth at the MEF is not isolated from surface processes.

Description: Accurate simulation of soil thermal dynamics is essential for realistic prediction of soil biogeochemical responses to climate change. To facilitate ecological forecasting at the Spruce and Peatland Responses Under Climatic and Environmental change (SPRUCE) site, we incorporated a soil temperature module into a Terrestrial ECOsystem (TECO) model by accounting for surface energy budget, snow dynamics, and heat transfer among soil layers and during freeze-thaw events. We conditioned TECO with detailed soil temperature and snow depth observations through data assimilation before the model was used for forecasting. The constrained model reproduced variations in observed temperature from different soil layers, the magnitude of snow depth, the timing of snowfall and snowmelt, and the range of frozen depth. The conditioned TECO forecasted probabilistic distributions of soil temperature dynamics in 6 soil layers, snow and frozen depths under temperature treatments of +0.0, +2.25, +4.5, +6.75, and +9.0 o C. Air warming caused stronger elevation in soil temperature during summer than winter due to winter snow and ice. And soil temperature increased more in shallow soil layers in summer in response to air warming. Whole ecosystem warming (peat + air warmings) generally reduced snow and frozen depths. The accuracy of forecasted snow and frozen depths relied on the precision of weather forcing. Uncertainty is smaller for forecasting soil temperature but large for snow and frozen depths. Timely and effective soil thermal forecast, constrained through data assimilation that combines process-based understanding and detailed observations, provides boundary conditions for better predictions of future biogeochemical cycles.

Description: Vast stores of arctic permafrost carbon that have remained frozen for millennia are thawing, releasing ancient dissolved organic carbon (DOC) to arctic inland waters. Once in arctic waters, DOC can be converted to CO 2 and emitted to the atmosphere, accelerating climate change. Sunlight-driven photoreactions oxidize DOC, converting a portion to CO 2 and leaving behind a photomodified pool of dissolved organic matter (DOM). Samples from the Kolyma River, its tributaries, and streams draining thawing yedoma permafrost were collected. Irradiation experiments and radiocarbon dating were employed to assess the photolability of ancient permafrost-DOC in natural and laboratory generated samples containing a mix of modern and ancient DOC. Photolabile DOC was always modern, with no measurable photochemical loss of ancient permafrost-DOC. However, optical and ultrahigh resolution mass spectrometric measurements revealed that both modern river DOM and ancient permafrost-DOM were photomodified during the irradiations, converting aromatic compounds to less conjugated compounds. These findings suggest that although sunlight-driven photoreactions do not directly mineralize permafrost-DOC, photomodification of permafrost-DOM chemistry may influence its fate and ecological functions in aquatic systems.

Description: [1]  The regulation of bacterial metabolic activity by viruses and dissolved organic carbon (DOC) was examined using natural microbial communities in three treatments (active viruses, inactive viruses, and virus-free) at two contrasting coastal sites (pristine vs. eutrophic) with substantial differences in environmental conditions during the wet and dry seasons. Our results showed that net growth rates and production of bacterioplankton were reduced primarily by viruses via repressing metabolically active bacteria with high nucleic acid (HNA) content which had a high capacity for incorporating carbon, while bacterial respiration was primarily regulated by DOC lability. The quality of organic matter played a more important role in regulating bacterial growth efficiency (BGE) than the supply of organic matter in eutrophic coastal waters. The lack of HMW-DOC and high carbon demand in the virus-free treatment resulted in a significant increase in cell-specific bacterial respiration, which was responsible for the lowest bacterial growth efficiency among the three treatments. The presence of viruses did not necessarily lower bacterial growth efficiency since viral-induced mortality alleviated bacterial carbon demand and enhanced carbon cycling. Virus-induced mortality was greater in relatively pristine waters than eutrophic waters, likely since the high supply of substrates alleviated the pressure of viral infection, through extracellular proteases produced by bacteria, which might result in the hydrolytic destruction or modification of viral capsids. An important implication of our results was that the input of riverine DOC and nutrients improved bacterial metabolic activity by alleviating virus-induced mortality of bacteria in estuarine and coastal waters.

Description: [1]  Organic and inorganic carbon dynamics were studied over a 6-year period with a range in discharge conditions in the Santa Fe River Sink-Rise eogenetic karst system, north Florida, USA. Spatiotemporal variations in dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), major ions concentrations and other geochemical parameters including stable carbon isotopes of DIC (δ 13 C DIC ), dissolved oxygen, pH and alkalinity were measured in surface water and deep and shallow well-water samples. Three endmember water sources were identified; one DOC-rich/DIC-poor/δ 13 C DIC -depleted, one DOC-poor/DIC-rich/δ 13 C DIC -enriched, and one distinguished by enrichment in major ions. Given their spatiotemporal distributions, they were presumed to represent soil water, upper aquifer groundwater, and deep aquifer water sources, respectively. Using assumed ratios of Na + , Cl - , and SO 4 2- for each endmember, a mixing model calculated the contribution of each water source to each sample. Then, chemical effects of biogeochemical reactions were calculated as the difference between those predicted by the mixing model and measured species concentrations. In general, carbonate mineral dissolution occurred throughout the Sink-Rise system, surface waters were net autotrophic and the subsurface was in metabolic balance, i.e. no net DOC or DIC production or consumption. However, there was evidence for abundant chemolithoautotrophy, perhaps by hydrogen oxidizing microbes, at some deep aquifer sites. Mineralization of this autochthonous natural dissolved organic matter (NDOM) led to localized enhanced carbonate dissolution as did surface water-derived NDOM supplied to shallow well sites during the highest flow periods. This study demonstrates the heterogeneity of aquifer biogeochemical processes and linkages between hydrology, abiotic processes, microbial metabolism and carbon dynamics in karst systems. These results have important implications for groundwater quality, karst morphologic evolution, and hydrogeologic projects such as aquifer storage and recovery in karst systems.

Description: Sphagnum mosses are the keystone species of peatland ecosystems. With rapid rates of climate change occurring in high latitudes, vast reservoirs of carbon accumulated over millennia in peatland ecosystems are potentially vulnerable to rising temperature and changing precipitation. We investigate the seasonal drivers of Sphagnum gross primary production (GPP)—the entry point of carbon into wetland ecosystems. Continuous flux measurements and flux partitioning show a seasonal cycle of Sphagnum GPP that peaked in the late summer, well after the peak in photosynthetically active radiation. Wavelet analysis showed that water table height was the key driver of weekly variation in Sphagnum GPP in the early summer and that temperature was the primary driver of GPP in the late summer and autumn. Flux partitioning and a process-based model of Sphagnum photosynthesis demonstrated the likelihood of seasonally dynamic maximum rates of photosynthesis and a logistic relationship between the water table and photosynthesising tissue area when the water table was at the Sphagnum surface. The model also suggested that variability in internal resistance to CO 2 transport, a function of Sphagnum water content, had minimal effect on GPP. To accurately model Sphagnum GPP we recommend: 1) understanding seasonal photosynthetic trait variation and its triggers in Sphagnum ; 2) characterizing the interaction of Sphagnum photosynthesising tissue area with water table height; 3) modelling Sphagnum as a ‘soil’ layer for consistent simulation of water dynamics; and 4) measurement of Sphagnum ‘canopy’ properties: extinction coefficient (k), clumping (Ω), and maximum SAI.

Description: Data on past peatland growth patterns, vegetation development, and carbon (C) dynamics during the various Holocene climate phases may help us to understand possible future climate-peatland feedback mechanisms. In this study, we analyzed and radiocarbon dated several peat cores from Kalevansuo, a drained bog in southern Finland. We investigated peatland succession and C dynamics throughout the Holocene. These data were used to reconstruct the long-term atmospheric radiative forcing, i.e. climate impact of the peatland since initiation. Kalevansuo peat records revealed a general development from fen to bog, typical for the southern boreal zone, but the timing of ombrotrophication varied in different parts of the peatland. Peat accumulation patterns and lateral expansion through paludification were influenced by fires and climate conditions. Long-term C accumulation rates were overall lower than the average values found from literature. We suggest the low accumulation rates are due to repeated burning of the peat surface. Drainage for forestry resulted in a nearly complete replacement of typical bog mosses by forest species within 40 years after drainage. The radiative forcing reconstruction suggested positive values (warming) for the first c. 7000 years following initiation. The change from positive to negative forcing was triggered by an expansion of bog vegetation cover and later by drainage. The strong relationship between peatland area and peat type with radiative forcing suggests a possible feedback for future changing climate, as high latitude peatlands may experience prominent regime shifts, such as fen to bog transitions.