Differences in riverine and pond water dissolved organic matter

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Differences in riverine and pond water dissolved organic matter composition and sources in Canadian High Arctic watersheds affected by active layer detachments Jun-Jian Wang, Melissa Lafrenière, Scott Lamoureux, Andre J Simpson, Yves Gélinas, and Myrna J Simpson Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05506 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 6, 2018

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Environmental Science & Technology

Wang et al. Graphical Abstract

ACS Paragon Plus Environment


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Differences in riverine and pond water dissolved organic matter composition and

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sources in Canadian High Arctic watersheds affected by active layer detachments

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Jun-Jian Wang†,#, Melissa J. Lafrenière‡, Scott F. Lamoureux‡, André J. Simpson†, Yves

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Gélinas§, and Myrna J. Simpson†,*

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†

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University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario M1C 1A4,

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Canada

Environmental NMR Centre and Department of Physical and Environmental Sciences,

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‡

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Kingston, Ontario K7L 3N6, Canada

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§

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7141 Sherbrooke West, Montréal, Quebec H4B 1R6, Canada

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*Corresponding

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[email protected] (M.J. Simpson)

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#

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Laboratory of Soil and Groundwater Pollution Control, Southern University of Science

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and Technology of China, Shenzhen, Guangdong 518055, China

Department of Geography and Planning, Queen’s University, 68 University Ave.,

GEOTOP and the Department of Chemistry and Biochemistry, Concordia University,

Author:

Tel:

416-287-7234;

Fax:

416-287-7279;

e-mail:

Present address: School of Environmental Science and Engineering and Shenzhen Key

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1 ACS Paragon Plus Environment

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Abstract

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Regional warming has caused permafrost thermokarst and disturbances, such as active

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layer detachments (ALDs), which may alter carbon feedback in Arctic ecosystems.

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However, it is currently unclear how these disturbances alter DOM biogeochemistry in

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rivers and ponds in Arctic ecosystems. Water samples from the main river channel, ALD-

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disturbed/undisturbed tributaries, and disturbed/undisturbed ponds within a catchment in

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the Canadian High Arctic were collected and analyzed using carbon isotopes and

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spectroscopic methods. Both river and pond samples had large variations in dissolved

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organic carbon (DOC) concentrations. Ponds, particularly ALD-disturbed ponds, had

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much older 14C DOC ages than rivers. Results from δ13C and absorption and fluorescence

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analyses indicate higher autochthonous contributions in ponds than rivers and increasing

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autochthonous contributions from upper to lower reaches of the main channel. The

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disturbed samples had less carbohydrates but more carboxyl-rich alicyclic molecules in

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samples also contained less terrestrial-humic-like but more oxidized-quinone-like

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components in the fluorescence spectra. Interestingly, the disturbed pond DOM displayed

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the greatest DOM oxidation with ALDs compared to undisturbed areas. Compared to

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Arctic rivers, small Arctic ponds have DOM predominantly from permafrost and

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microbial sources and may have a disproportionally stronger positive feedback on climate

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warming.

H nuclear magnetic resonance spectra than undisturbed samples. These ALD-impacted

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Keywords: Permafrost thawing; Active layer detachments, Nuclear magnetic resonance

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spectroscopy, Fluorescence spectroscopy, Parallel factor analysis PARAFAC, Cape

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Bounty Arctic Watershed Observatory


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INTRODUCTION

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The Arctic environment is a critical zone for biogeochemical processes because of

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its high carbon storage (~1300 Pg; ~50% of global soil organic carbon1,2) and high

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vulnerability to climate change.2-4 Variations in organic matter storage and cycling in

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Arctic ecosystems are major sources of uncertainty in modeling of carbon, nutrient, and

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pollutant cycling and climate projections on local to global scales.5-7 Dissolved organic

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matter (DOM) represents one of the most mobile but the least characterized carbon pools

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in Arctic environments because of the high complexity of DOM composition and

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difficulty to access the Arctic region, particularly the High Arctic.8-10 A better

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understanding of DOM chemistry and degradation and the controlling factors in Arctic

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ecosystems will help predict global climate change feedback in the future.7-9,11,12

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Previous studies have focused on the river, lake, and estuary DOM character and

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explored the terrigenous carbon transport from Arctic watersheds to lakes, northern seas,

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and the Arctic Ocean.6,13-16 Arctic rivers are estimated to transport 34–38 Tg yr−1 of

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dissolved organic carbon (DOC) to the Arctic Ocean and surrounding basins,15 whereas

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37–84 Tg yr−1 of DOC is delivered to inland waters but degraded to the atmosphere or

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buried in lakes and streams before entering the ocean.17,18 Characteristics of Arctic river

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DOM present strong spatial-temporal patterns due to varying DOM sources (e.g., soil and

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plant materials, permafrost and glaciers, and microbes9,19) and varying degrees of

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degradation.9,14,20 While large amounts of high molecular weight, relatively fresh, and

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lignin-rich aromatic DOM was exported in spring freshets to rivers, relatively lower

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concentrations of DOM with lower molecular weights, less aromaticity, and older ages

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was exported later in the summer season.10,21-23 Importantly, increasingly severe glacier



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melt and permafrost thaw may reduce the DOC concentration (proxy for DOM

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concentration) and specific ultraviolet absorbance at 254 nm (SUVA, proxy for DOM

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aromaticity), and increase the

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studies have shown the lack of a permafrost signal in DOC exported via major Arctic

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rivers to the ocean, likely due to the high lability of the ancient permafrost-derived

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DOM.26-29

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C age of DOC in Arctic rivers.24,25 In contrast, some

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Compared to rivers, Arctic ponds, which are typically a few square meters in area,

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do not directly contribute to DOM flux from land to ocean, but disproportionally

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contribute to atmospheric carbon fluxes and biodiversity within Arctic ecosystems.30-32

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For example, their greenhouse gas emissions as a function of surface area are higher than

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those of large lakes.33 In addition, ponds have been considered as excellent indicators of

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climate change because of their hydrological isolation, small water volumes, and large

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surface area-to-depth ratios.34,35 Significant declines in Arctic pond area and abundance

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are occurring.30,31 Although the ponds account for about half of the total surface water in

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permafrost landscapes,36 DOM biogeochemistry within these small ponds has not been

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studied to the same extent as rivers.32,37 Moreover, most current knowledge of Arctic

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DOM characteristics has been derived from the Sub-Arctic and Low Arctic environments

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and few from the High Arctic that has shorter growing seasons, cooler summers, and

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reduced diversity of flora and fauna.10,14,37,38 Whether and how the DOM characteristics,

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and their responses to climate change, differ between the rivers and ponds remain

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unknown in the High Arctic, which is a region subject to increasing warming and

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precipitation and higher risks of permafrost degradation and disturbance.39


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As a representative type of permafrost disturbance, active layer detachments

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(ALDs) are slope failures that displace soil along the upper permafrost boundary due to

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active layer thickening and ground ice thawing, and are increasing in frequency and

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severity in Canadian High Arctic.40,41 Such disturbance can alter soil organic matter

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characteristics by increasing the release of labile compounds and stimulating microbial

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degradation of previously unavailable soil organic matter.42-44 These processes are

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expected to alter the terrestrial carbon export to downstream waterbodies. For example,

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the DOC export was found higher in an ALD-disturbed catchment than an undisturbed

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catchment in the Canadian High Arctic 2 years post-disturbance.21 Also, DOM in streams,

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rivers and lakes in recently disturbed catchments appeared to contain more low molecular

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weight fluorescent components10 and showed higher biological and photochemical

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lability than those from undisturbed systems.14 Despite this progress, the relatively long-

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term (>5 years) impact of ALDs (or impact of “historic” ALDs in contrast to the “recent”

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ALDs (within 2-3 years)) on DOM characteristics is still unclear. To understand the

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chemical characteristics between pond and river DOM and their responses to ALDs, this

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study characterized the molecular-level DOM chemistry from river and pond waters in a

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previously ALD-disturbed catchment using detailed isotopic and spectroscopic analyses.

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The West River watershed at the Cape Bounty Arctic Watershed Observatory (CBAWO),

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Nunavut, Canada was selected because of intensive ALD activity in 2007-08.40

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MATERIALS AND METHODS

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Site information. Water samples from 14 river and 6 pond sites were collected in early

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August of 2014 and 2015 within the CBAWO, southern Melville Island, Nunavut,



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Canada (74°54’N, 109°35’W; Fig. 1; Fig. S1; Table S1). This region has a High Arctic

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climate with a thaw period between June and August. The mean monthly January and

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July air temperatures at the nearest long-term weather station, Mould Bay (200 km to the

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west) are -33.1 °C and 4.0°C, respectively.45 The mean annual precipitation recorded at

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Mould Bay includes 102 mm of snowfall and 16.3 mm of rainfall.46 The 14 river sites

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included 7 main channel sites (from upper to lower: WR-1 to WR-7), 5 undisturbed

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tributary sites (WRE-1, WRE-2, WRW-1, WRW-2, and Goose), and 2 ALD-disturbed

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tributary sites (Ptarmigan-D and ALD-D; Fig. 1; Fig. S1; Table S1). The 6 small (< 30 m2)

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ephemeral ponds included 4 undisturbed ponds (Pond-2, Pond-4, Pond-5, and Pond-6)

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and 2 disturbed ponds (Pond-1-D and Pond-3-D; Fig. 1; Fig. S1; Table S1). More

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information about the CBAWO and the sites is available in the supplementary

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information.

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Sample analyses. The pH and electrical conductivity of water were measured in the field

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using Orion Star A215 benchtop meter, with the Orion Ross Ultra pH and DuraProbe

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conductivity probes. Surface water samples were collected at a depth of 10-20 cm,

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filtered through precombusted 0.7 µm glass fiber filters, and transported to the lab at 4 oC

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for analyses of DOC, radiocarbon, and optical properties. Samples for radiocarbon were

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collected in pre-cleaned certified 500 mL glass amber bottles. Sample aliquots destined

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for δ13C and 14C analyses were acidified to a pH of 2 in the field, using trace metal grade

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HCl. Samples for nuclear magnetic resonance (NMR) analyses were collected in 1 L

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amber HDPE bottles, frozen immediately, and transported to the University of Toronto.

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The concentrations of DOC and total dissolved nitrogen (TDN) were quantified using a


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Shimadzu (TOC-V) organic carbon analyzer with a nitrogen detection unit (Shimadzu

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TMN-1; Kyoto, Japan).10

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Radiocarbon analyses of the DOC were performed on a 3 MV tandem accelerator

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mass spectrometer (AMS) at the A.E. Lalonde AMS Laboratory (Ottawa, Canada). After

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removing the inorganic carbon using 85% phosphoric acid, DOC was oxidized to CO2

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using a sodium persulfate wet oxidation technique.47,48 The CO2 was converted to

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elemental C using semi-automated graphitization lines in the presence of hydrogen. The

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fraction of modern carbon, F14C, is calculated as the ratio of sample

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standard

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years. Sample and standard

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measured 13C/12C ratio.50 Ages are in years before present (years BP; CE 1950).51,52 The

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stable isotope composition was measured at the Concordia University, using high

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temperature combustion and continuous flow IRMS, and reported as δ13C (in ‰), relative

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to the Pee Dee Belemnite (PDB) reference standard according to methods detailed in

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Lalonde et al.53

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C/12C using Oxalic II as a reference,49 and the Libby 14

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C/12C and the

C half-life of 5568

C/12C ratios are background-corrected using the AMS-

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For solution-state NMR analysis, 1 liter of each water sample was filtered, frozen,

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freeze-dried and dried under vacuum over P2O5. Depending on the abundance of

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dissolved components, 4-40 mg of dried sample was reconstituted in 60 µL NaOD/D2O

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solution (pH=12), centrifuged to remove insoluble materials, and transferred into Bruker

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BioSpin 1.7 mm NMR tubes.54 The analyses were conducted using a Bruker BioSpin

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Avance III 500 MHz NMR spectrometer (Karlsruhe, Germany) equipped with a 1H-15N-

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13

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one-dimensional 1H NMR spectra were collected using presaturation utilizing relaxation

C TXI 1.7 mm microprobe fitted with an actively shielded Z gradient.54 Specifically,



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gradients and echoes to suppress the resonances from residual water.55 A total of 256

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scans were collected using a recycle delay of 2 s, and 32K time domain points. Spectra

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were processed using a zero filling factor of 2 and were apodized by multiplication with

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an exponential decay corresponding to 0.3 Hz line broadening. Diffusion-edited 1H NMR

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spectra were collected using a bipolar pulse longitudinal encode–decode sequence.56 A

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total of 2048 scans were collected using a 2.5 ms, 53.5 gauss cm−1, sine-shaped gradient

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pulse, a diffusion time of 200 ms, 16K time domain points and 10 Hz line broadening.

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The diffusion edited 1H NMR experiment highlights signals that do not move position in

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the NMR tube.57 In this study, the diffusion filter emphasizes molecules that do not move

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more that (~ 1 µm) within the sample over the diffusion time of 200 ms58 and as such will

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emphasize molecules with little translation motion including macromolecular and

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aggregated compounds. The one-dimensional 1H NMR spectra were integrated into four

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regions: 1) materials derived from linear terpenoids (MDLT), 0.6–1.6 ppm; 2) carboxyl-

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rich alicyclic molecules (CRAM), 1.6–3.2 ppm; 3) carbohydrates, 3.2–4.5 ppm; and 4)

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aromatics and phenolics, 6.5–8.4 ppm.14,59 The percentages for the four regions were

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calculated to indicate the relative abundances of protons (99.98% natural abundance of all

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H isotopes) in different forms of molecular groups. Therefore, the percentages represent

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the total 1H signal from specific molecular groups found within DOM. For example,

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although the aromatic and phenolic constituents accounted for 2%-6% of total 1H NMR

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resonance, the aromatic and phenolic compounds account for higher absolute abundance

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in DOM than 2%-6% because the aromatic and phenolic compounds have low H/C ratio.

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Moreover, to provide better spectral dispersion, two-dimensional 1H-13C heteronuclear

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single quantum coherence (HSQC) spectra were obtained in digital quadrature detection


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mode with echo–antiecho gradient selection. A total of 1200 scans were collected with

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2K and 96 time domain increments in the F2 and F1 dimensions respectively. HSQC

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spectra were processed using a function corresponding to 25 Hz line broadening in F2

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and a sine-squared function shifted by 90o in F1 with a zero-filling factor of 2 in both

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dimensions.

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Ultraviolet-visible absorbance and fluorescence emission excitation matrices

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(EEMs) of the filtered samples were collected at room temperature using a Horiba

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Aqualog spectrometer (NJ, USA) as previously described.10 All EEMs were corrected for

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blanks and inner filter effects and calibrated to a quinone sulfate unit.60 Optical indices

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including specific ultraviolet absorbance at 254 nm (SUVA), E2/E3 ratio, fluorescence

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index (FI), biological index (BIX), and humification index (HIX) were calculated.61,62

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Specifically, the SUVA is indicative of DOM aromaticity and was calculated as

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ultraviolet absorbance at 254 nm divided by DOC concentration.63 E2/E3 ratio was

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calculated as absorbance ratio of 254 nm to 365 nm, and higher E2/E3 ratio indicates

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lower molecular size of DOM.64 Note that the accuracy of SUVA and E2/E3 ratios as

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chemical proxies can be impacted by colloidal material65 or dissolved ions such as iron

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(III),66 which were not measured because of the limited amount of samples. In addition,

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the aromaticity reflected by SUVA and the relative abundance of aromatic and phenolic

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constituents in the 1H NMR signal may not be directly comparable because the former

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estimates the overall aromatic and potential interfering light absorbance at 254 nm and

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the latter measures the 1H NMR resonance (instead of carbon resonance) in aromatic and

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phenolic molecular groups. The fluorescence index (FI), the signal ratio of emission 470

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nm to 520 nm at excitation 370 nm, is an index to differentiate microbial or terrestrial



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origins (~1.8 for microbial origin and ~1.2 for terrestrial origin).67 The biological index

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(BIX), an index for the contribution of recently produced autochthonous DOM, was

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calculated as the signal ratio of emission at 380 nm divided by the maximum emission

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between 420 and 435 nm for excitation at 310 nm.68 The humification index (HIX), an

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index of humic substance content, was determined as the area under the emission range

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435–480 nm divided by that under the emission range 300–345 nm, for excitation at 254

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nm.69 The fluorescent DOM components for the 20 EEM data were determined using

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modelling of the parallel factor analysis (PARAFAC)70 and validation of split-half

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analysis.71 The components were compared to published models using the OpenFluor

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database.72 Specifically, signals with excitation wavelengths lower than 260 nm, emission

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wavelengths lower than 300 nm, and those influenced by 1st and 2nd order scattering were

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removed from the matrices to avoid inaccurate noises.10,71 A five-component model

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explaining 99.0% of the fluorescence variability was selected.

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Statistical analyses. The significant differences in chemical parameters between river

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and pond waters (n = 14 and 6) and among main channel, tributary, and pond waters (n =

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7, 7, and 6, respectively) were examined using one-way analyses of variances (ANOVA)

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with Tukey post hoc test. Several parameters including electrical conductivity, DOC, and

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TDN were logarithmically transformed to satisfy the normality assumption of ANOVA.

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When any assumption of parametric ANOVA (e.g., no outlier, normally-disturbed data,

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and homogeneity of variance) is violated, the nonparametric Kruskal-Wallis test (or

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“ANOVA on ranks”) and Dunn’s post hoc multiple comparison with a Bonferroni

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correction was used instead to compare differences between water source groups. To


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examine the main effects of ALDs and their possible interactions with impacts of water

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source (tributary versus pond) on the water chemistry, two-way ANOVA was used on all

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undisturbed and disturbed tributary and pond waters (n=13). Data related to the general

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water quality, optical properties, and DOM chemistry were standardized and analyzed

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using principal component analysis (PCA). Note that the small sample size did not satisfy

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the PCA requirement and will limit the accuracy of analysis, however, we still use it as an

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exploratory tool to provide a rough overview of the reduced dimensions of variables and

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sample clustering. The loading of different parameters on the first two principal

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components that explained 61.5% of the total variance and the relevant component scores

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of each water samples were calculated. SPSS 15.0 (IL, USA) was used for all statistical

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analyses and the level of significance was set at 0.05.

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RESULTS AND DISCUSSION

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General water quality. The pH, EC, DOC and TDN concentrations differed

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significantly between the river and pond waters (Table 1; Table S2). Specifically, both

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the pH and EC values were lower in rivers (pH: 6.2-7.5; EC: 201-361 µS/cm) than ponds

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(pH: 7.5-8.8; EC: 1104-4310 µS/cm). These pH and EC values are mostly comparable to

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those previously reported for High Arctic rivers and ponds.73-75 However, the relatively

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high EC values of the ponds are likely due to water evaporation in summer and the

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surface and subsurface transfer of inorganic solutes.40,76 The DOC concentration of river

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waters ranged from 1.1 to 5.9 mg/L and that of the pond waters varied highly from 1.9 to

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51.3 mg/L. Similarly, the TDN concentration ranged from 0.06 to 0.29 mg/L for river

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samples and varied markedly from 0.21 to 8.92 mg/L for pond waters. The DOC and



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TDN concentrations of river waters were similar to the base-flow concentrations of

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previous years reported10 and also comparable to other Arctic rivers.6 The relatively high

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DOC and TDN concentrations in most ponds support high microbial activity.32 However,

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their large variations indicate that the ponds may have various carbon and nutrient inputs

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and sinks, and different degrees of DOM concentration (e.g., evaporation) and dilution

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(e.g., by melting ice). In addition, the DOC/TDN ratio was significantly lower in ponds

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(0.2-18.2) than in river waters (15.6-27.7). As the DOC/TDN ratio commonly decreases

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with increasing degradation of overall DOM and increasing autochthonous contributions

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in natural water,77 the lower values in ponds suggest higher contributions from

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autochthonous sources.

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Within the river samples, there was no significant difference in pH, EC, DOC or

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TDN concentrations between the main channel and tributary samples or between the

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undisturbed and ALD-disturbed tributary waters (P>0.05). However, from WR-1 to WR-

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7, significant increases in DOC concentrations and DOC/TDN ratios were observed

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(Table S2), likely due to increasing autotrophic primary production (more detail in the

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allochthonous versus autochthonous sources section). Within the pond samples, the

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disturbed ponds showed much lower DOC concentrations (1.87 and 2.44 mg/L) than

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undisturbed ponds (4.78-51.3 mg/L). One possibility causing low DOC concentrations in

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disturbed ponds could be the higher degree of DOM degradation stimulated by the labile

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permafrost inputs.14 Another possibility could be that the ALDs resulted in the

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translocation of surface soils to areas downslope, leaving previously buried mineral soils

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at the surface42 and thus reducing the pool of soil organic matter and inputs of DOM

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from surrounding catchment area.


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1028 to 4975 years BP, much older than those of the river waters (≤ 440 years BP;

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P

Differences in riverine and pond water dissolved organic matter

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