Statistical upscaling of ecosystem CO2 fluxes across the terrestrial tundra and boreal domain: Regional patterns and uncertainties

The regional variability in tundra and boreal carbon dioxide (CO2) fluxes can be high, complicating efforts to quantify sink-source patterns across the entire region. Statistical models are increasingly used to predict (i.e., upscale) CO2 fluxes across large spatial domains, but the reliability of different modeling techniques, each with different specifications and assumptions, has not been assessed in detail. Here, we compile eddy covariance and chamber measurements of annual and growing season CO2 fluxes of gross primary productivity (GPP), ecosystem respiration (ER), and net ecosystem exchange (NEE) during 1990-2015 from 148 terrestrial high-latitude (i.e., tundra and boreal) sites to analyze the spatial patterns and drivers of CO2 fluxes and test the accuracy and uncertainty of different statistical models. CO2 fluxes were upscaled at relatively high spatial resolution (1 km(2)) across the high-latitude region using five commonly used statistical models and their ensemble, that is, the median of all five models, using climatic, vegetation, and soil predictors. We found the performance of machine learning and ensemble predictions to outperform traditional regression methods. We also found the predictive performance of NEE-focused models to be low, relative to models predicting GPP and ER. Our data compilation and ensemble predictions showed that CO2 sink strength was larger in the boreal biome (observed and predicted average annual NEE -46 and -29 g C m(-2) yr(-1), respectively) compared to tundra (average annual NEE +10 and -2 g C m(-2) yr(-1)). This pattern was associated with large spatial variability, reflecting local heterogeneity in soil organic carbon stocks, climate, and vegetation productivity. The terrestrial ecosystem CO2 budget, estimated using the annual NEE ensemble prediction, suggests the high-latitude region was on average an annual CO2 sink during 1990-2015, although uncertainty remains high

Drivers of ecosystem metabolism in restored and natural prairie wetlands

Elucidating drivers of aquatic ecosystem metabolism is key to forecasting how inland waters will respond to anthropogenic changes. We quantified gross primary production (GPP), respiration (ER), and net ecosystem production (NEP) in a natural and two restored prairie wetlands (one “older” and one “recently” restored) and identified drivers of temporal variation. GPP and ER were highest in the older restored wetland, followed by the natural and recently restored sites. The natural wetland was the only net autotrophic site. Metabolic differences could not be definitively tied to restoration history, but were consistent with previous studies of restored wetlands. Wetlands showed similar metabolic responses to abiotic variables (photosynthetically active radiation, wind speed, temperature), but differed in the direct and interactive influences of biotic factors (submersed aquatic vegetation, phytoplankton). Drivers and patterns of metabolism suggested the importance of light over nutrient limitation and the dominance of autochthonous production. Such similarity in ecosystem metabolism between prairie wetlands and shallow lakes highlights the need for a unifying metabolic theory for small and productive aquatic ecosystems.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Determination of Monomethylmercury and Dimethylmercury in the Arctic Marine Boundary Layer

Our understanding of the biogeochemical cycling of monomethylmercury (MMHg) in the Arctic is incomplete because atmospheric sources and sinks of MMHg are still unclear. We sampled air in the Canadian Arctic marine boundary layer to quantify, for the first time, atmospheric concentrations of methylated Hg species (both MMHg and dimethylmercury (DMHg)), and, estimate the importance of atmospheric deposition as a source of MMHg to Arctic land- and sea-scapes. Overall atmospheric MMHg and DMHg concentrations (mean ± SD) were 2.9 ± 3.6 and 3.8 ± 3.1 (<i>n</i> = 37) pg m⁻³, respectively. Concentrations of methylated Hg species in the marine boundary layer varied significantly among our sites, with a predominance of MMHg over Hudson Bay (HB), and DMHg over Canadian Arctic Archipelago (CAA) waters. We concluded that DMHg is of marine origin and that primary production rate and sea-ice cover are major drivers of its concentration in the Canadian Arctic marine boundary layer. Summer wet deposition rates of atmospheric MMHg, likely to be the product of DMHg degradation in the atmosphere, were estimated at 188 ± 117.5 ng m^–2 and 37 ± 21.7 ng m^–2 for HB and CAA, respectively, sustaining MMHg concentrations available for biomagnification in the pelagic food web

Long-term changes in nutrient dynamics and plankton communities following the creation of a new reservoir

Reservoir creation often leads to a â trophic upsurgeâ of nutrients that may affect communities living near impoundments. We determined the duration of the nutrient upsurge and associated changes in plankton in a study of a new reservoir (L979) in northwestern Ontario that included 2 years pre-impoundment, 16 with impoundment, and 2 post-impoundment. Secondarily, we determined allochthonous versus autochthonous carbon (C) use by zooplankton with reservoir development. For the first 6-14 years of impoundment, average concentrations of total phosphorus (TP), total nitrogen (TN) and dissolved organic carbon increased 3.6, 2.4, and 1.7x above pre-impoundment levels, decreasing thereafter. L979 shifted from a net sink to a source for TP and TN for the first 6 years of impoundment. Average annual biomasses of bacteria, phytoplankton and zooplankton increased 4.6, 17, and 32.6x above pre-impoundment levels, with associated changes in community composition. Bacteria and phytoplankton returned to pre-impoundment levels within 4 and 10 years, respectively, but zooplankton densities remained elevated even after 18 years. Changes in áş 13C suggested that post-impoundment zooplankton biomass was not supported predominantly by allochthonous C.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author