SUPPLEMENTARY INFORMATION DOI: 10.1038/NGEO2507
Supplementary Information
Sources of and processes controlling CO2 emissions Sources of and processes controlling CO emissions change with the size of streams and rivers change with the size of streams and rivers 2
E.R. Hotchkiss1,*,†, R.O. Hall, Jr.2, R.A. Sponseller1, D. Butman3, J. Klaminder1, H. Laudon4, M. Rosvall5, and J. Karlsson1 1
Department of Ecology and Environmental Science, Umeå University, Umeå, Sweden 901 87 Department of Zoology & Physiology, University of Wyoming, Laramie, WY, USA 82071 3 School of Environmental and Forest Sciences & Civil and Environmental Engineering, University of Washington, Seattle, WA, USA 98195 4 Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden 901 83 5 Integrated Science Lab, Department of Physics, Umeå University, Umeå, Sweden 901 87 2
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Corresponding author:
[email protected]
†
Current address: Département des sciences biologiques, Université du Québec à Montréal, Montréal, Québec, Canada H3C 3P8
Table of Contents S1. Additional sources of CO2 ……………………………………………………………….…2 S2. Comparing freshwater ecosystem metabolism with CO2 fluxes. …...………………….…...3 Table S2-1. Range of -NEP and CO2 emissions (fCO2) binned by stream and river discharge (Q). ...............................................................................................…....6 Figure S2-1. Frequency distributions of CO2 and metabolism estimates by stream discharge. ...………………………………………………………………..….…7 Figure S2-2. Regional estimates of metabolism and CO2 emission fluxes. ……....…….7 Figure S2-3. Density distributions of calculated pCO2, CO2, modeled k, and estimated CO2 efflux. …………………………………………………………………...….8 Figure S2-4. Estimating parameter uncertainty with Markov chain Monte Carlo (mcmc) sampling. …………………………………………………………………….......8 References for Supplementary S1 & S2 …………………………………………….…………..9 S3. Supplementary Resource: Citations for stream and river metabolism estimates ……….…11 S4. Supplementary Resource: USGS parameter codes and site IDs for CO2 estimates ……….14
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S1. Additional sources of CO2 Geochemical weathering and photochemical processes also contribute to carbon dioxide (CO2) emissions from inland watersS1-3, but we did not separate these from “external” CO2 sources in this first investigation linking metabolism with CO2 emissions along a size gradient of streams and rivers. There are currently no large-scale predictions about the relative role of abiotic processes in CO2 emissions from inland waters, which will vary depending on geology, pH, light, water residence time, and availability of photoreactive organic matter. Weathering can act as a source or sink of CO2 to inland waters S4-5. Increases or decreases in pH may precipitate or dissolve carbonates, which can remove or release inorganic carbon (C) from/to the water column, but will not bias metabolism estimates calculated using diel changes in dissolved oxygen (O2). Light reactions with organic solutes can also influence CO2 and/or O2 dynamics, but there are few, if any, measurements of these processes at the whole-ecosystem level. Most metabolism estimates are made using nighttime calculations of ecosystem respiration (ER) from changes in O2, so potential error in O2-based metabolism estimates due to photochemical and biological light reactions will most likely be for gross primary production (GPP) and net ecosystem production (NEP). O2-based measurements of NEP may be biased by photooxidation, photorespiration, and photostimulated respiration, which consume O2S2,6. Based on recent researchS2, we imagine there is a larger role for photooxidation than photostimulated respiration in water column O2 dynamics. If photooxidation rates significantly influence whole-ecosystem (benthic and water column) O2 cycling, corrected estimates of CO2 production from NEP would be less negative. Complete light-driven oxidation of organic matter (photomineralization) may contribute to CO2 emissionsS2-3 but will not bias estimates of NEP via O2, and are thus included with other “external” CO2 sources in our simulations (Methods). Depending on the spatial scale of C flux and metabolism measurements, upstream inputs of autochthonous (internally fixed) organic C (OC) and dissolved CO2 from the respiration of autochthonous C could be grouped with respiration of terrestrially derived OC and included in downstream NEP and CO2 measurements as “external” C. Quantitative examples of this are few, but one recent study found downstream export of autochthonous dissolved and particulate OC was a principal fate of newly fixed C in a temperate mountain streamS7. The fate of autochthonous C after export from a defined study reach is unknown, but some portion of the exported C likely stimulates downstream respiration that is not included in upstream metabolism estimatesS7-8. The upstream-downstream linkages within streams and rivers certainly deserve future attention in C budgeting research, as the timing and location of metabolism and CO2 estimates may only provide a snapshot of stream and river contributions to larger-scale net C fluxes.
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S2. Comparing freshwater ecosystem metabolism with CO2 fluxes Approaches used to infer the role of metabolism in freshwater CO2 budgets Aquatic net ecosystem production (NEP) may be used to budget internal carbon dioxide (CO2) production in streams and rivers via mineralization of terrestrial organic carbon (OC). NEP reflects the balance between gross primary production (GPP) and ecosystem respiration (ER), where GPP is C fixation by autotrophs and ER is the mineralization of all OC. Annual NEP estimates better reflect mineralization of terrestrial subsidies, but we included all published stream and river NEP estimates for this large-scale comparison of internal CO2 production with other CO2 fluxes in running waters. When simulating lateral input fluxes along a theoretical continuum (Fig. 2), we did not include data from 10-100 m3 s-1 rivers, as these metabolism summary statistics were likely biased by the large proportion (>80%) of single season (in most cases, only a few days) estimates during productive summer months (Table S2-1). Comparisons of freshwater ecosystem metabolism with CO2 fluxes in site-specific and catchment C budgets are rare. Earlier studies related water column (pelagic) rates of metabolism with freshwater CO2 fluxes and found a potentially high role for pelagic mineralization of terrestrial OC in governing CO2 supersaturation and emissions from heterotrophic lakesS9-10. One notable stream study found that pelagic respiration, measured using biological oxygen demand assays, contributed to
