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Redox conditions affect DOC quality in stratified freshwaters Tallent Dadi, Mourad Harir, Norbert Hertkorn, Matthias Koschorreck, Philippe Schmitt-Kopplin, and Peter Herzsprung Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04194 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017

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Environmental Science & Technology

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Redox conditions affect DOC quality in stratified freshwaters

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Tallent Dadi1, Mourad Harir2, Norbert Hertkorn2, Matthias Koschorreck1, Philippe Schmitt-

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Kopplin2, Peter Herzsprung*1

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1

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Brückstr. 3a, 39114, Magdeburg, Germany

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2

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(GmbH), Research Unit Analytical BioGeoChemistry, Ingolstaedter Landstr. 1, 85764

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Neuherberg, Germany

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*

UFZ - Helmholtz Centre for Environmental Research, Department of Lake Research,

Helmholtz Zentrum München, Deutsches Forschungszentrum für Gesundheit und Umwelt

Corresponding author:

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Tel.: +49-391-8109 330

11

Fax: +49-391-8109 150

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[email protected]

1 ACS Paragon Plus Environment


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Abstract

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The quality of dissolved organic carbon (DOC) affects both carbon cycling in surface

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waters as well as drinking water production. Not much is known about the influence of

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environmental conditions on DOC quality. We studied the effect of redox conditions on the

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chemical composition of DOC in a drinking water reservoir by Fourier transform ion

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cyclotron resonance mass spectrometry (FTICR-MS) in combination with sediment core

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incubation experiments under manipulated redox conditions. We observed clear differences in

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DOC quality between oxic epilimnion, anoxic hypolimnion and sediment pore-water.

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Sediment pore-water showed relative high intensities of polyphenol-like components with

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H/C ratios < 1 and O/C ratios > 0.6. Consistent with this, anoxic incubation of sediment core

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resulted in an accumulation of these components in the overlying water. The observed pattern

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of DOC quality change can be explained by redox dependent adsorption/desorption of DOC

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on iron minerals. Under oxic conditions the polyphenol-like components bind on freshly

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formed iron hydroxides, a process which both affects DOC stability in surface waters as well

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as treatability in drinking water production.


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Introduction Dissolved organic carbon (DOC) in lakes originates both from allochthonous

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1

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(terrestrial) and autochthonous (produced by aquatic organisms) sources.

Mobilization of

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DOC from the catchment during high discharge events (snowmelt, rain events etc.) is leading

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to the observed increase in DOC concentration in surface waters of the Northern Hemisphere.

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2 -5

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heterotrophic bacteria.

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composition (“quality”) and this affects how they react and are processed in lake systems.

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Characterization of DOC components is important both for water treatment applications and

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for understanding carbon cycling processes in the water bodies. The choice of DOC removal

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method in drinking water treatment depends on the DOC quality.8, 9

Autochthonous DOC is mobilized from decaying algae, exudated by living algae and 6, 7

The allochthonous and autochthonous DOC has different chemical

39

With DOC quality we mean measurable chemical properties of DOC as specified

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below. Up to date we are far from being able to identify each single compound (isomer) of

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DOC. FTICR-MS is the most advanced instrumentation with the highest resolution for the

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analysis of DOC composition.

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solid phase extractable dissolved organic matter (SPE-DOM)

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electrospray ionisation) and thus measurable by FTICR-MS. The DOC quality is

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characterised by thousands of elemental compositions (containing carbon, hydrogen, and

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oxygen, so called CHO components, and others containing nitrogen and / or sulfur) as

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calculated from the exact mass from molecule ions. Here we mainly focus on DOC quality as

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derived from elemental compositions, as allocated in van Krevelen diagrams, and sorting

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(ranking) them according to their mass peak intensities. In addition, we calculated some bulk

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parameters like the unsaturation of molecules expressed by their aromaticity index or the

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average oxidation state of carbon as two examples, which also express DOC quality in

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another specific way.

10

In our study the DOC quality is exclusively referred to the 11

, which is ionisable (by



53

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The DOC quality can be controlled in lakes by in-lake processes like primary 12

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production, flocculation, decomposition, photodegradation.

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and magnitude of these processes within lake systems is expected. This implies that inflow

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zones in lakes with are expected to have different DOC signatures compared to the outflow

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zones. Many lakes exhibit heterogeneous redox conditions as result of stratification and

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subsequent depletion of oxygen in the hypolimnion from summer until autumn mixing.13, 14

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The stratification is influenced by lake morphometry (shape and depth). Redox conditions at

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the sediment water interface are important in the mobilization and immobilization of

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dissolved organic carbon (DOC).15,

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sensitive Fe species and DOC components.17, 18 These DOC mobilization and immobilization

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processes are influenced by the DOC quality.19, 20

16

Heterogeneity in occurrence

This has been linked to the association of redox

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Differences in DOC quality between inflow and open water zones of lakes can be

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expected because anoxia being more prone in the open water zone.21. Inflow zones are

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predominantly characterised by a high input of allochthonous DOC and therefore expected to

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have more aromatic lignin derived DOC. On the other hand open water zones have more

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autochthonous less aromatic algae derived DOC.22 Vertical differences in inflow zones are

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expected to be minimal due to complete mixing of the water. However a vertical gradient in

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both DOC quantity and quality is expected in the open water zone due to their stratified nature

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between spring and autumn.

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We investigated the effects of redox conditions on DOC quantity and quality in a

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drinking water reservoir pre-dam. In particular we address: 1) spatial (vertical and horizontal)

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DOC quality gradients, and 2) the effects of redox conditions on the DOC quality using a

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sediment incubation method. We aimed to answer the major question: Is the DOC quality in

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the water affected by the sediment and/or redox conditions?

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Materials and methods

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Study site

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Rappbode pre-dam is located in the Harz Mountains (51o 42' 15" N, 10o 47' 30" E), 23 – 25

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Germany and is part of the Rappbode reservoir system.

A pre-dam is a small reservoir

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upstream of the main reservoir, which serves as a sedimentation basin to reduce loads of

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suspended particles and dissolved nutrients, especially phosphorus, into the main reservoirs.26,

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27

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occur; hence they are interesting for catchment reservoir interaction studies. Rappbode pre-

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dam is mesotrophic and stratifies in summer, developing an anoxic hypolimnion. This

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resulted in selection of two sampling points; a shallow point (4 m) without stratification and a

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deep point (16 m) with stratification (Fig. 1, Fig. S1). The concentration of DOC ranges

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between 0.3 - 0.6, and 0.4 - 2.7 mmol l-1 in water and pore-water respectively. Refer to Table

90

S1 for morphometry and hydrological parameters.

Pre-dams are important biological reactors in which various biogeochemical processes

91 92

Sediment and water sampling

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Six sediment cores (60 cm long and 9 cm in diameter PVC tubes) were sampled in

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September 2014 from each site using a gravity corer (UWITEC, Mondsee, Austria); four

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cores for sediment incubation and two sediment cores for sediment pore-water extraction. The

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water height in the cores was on average 22 cm (1.3 L water volume). Plexiglas sediment core

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covers were used to close the four sediment incubation cores immediately after they were

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taken. 28 The core covers were air and water tight, through use of three O-rings. They also had

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various openings for oxygen sensors (2 mm diameter, closed by a silicon stopper outside use

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times), pH electrode (20 mm, closed by a screw cap with an O-ring), sampling and gas/air

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bubbling (2 openings, 4 mm diameter, closed by a two-way stop cock). The sampling and 5 ACS Paragon Plus Environment


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gas/air bubbling opening had an extended, height adjustable silicon tube. The second opening

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was for gas/air release during bubbling and also replacement of sampled water volume. The

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covers were fitted with an inbuilt motor system with a magnetic stirrer. The covers were

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designed to be powered by battery in the field and by alternating current in the laboratory.

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Immediately after closing the sediment cores with the covers, bottom water was used to

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completely fill the cores and all bubbles were removed. Bubbles can interfere with oxygen

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measurements.

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sediment by a standard 2 L water sampler (Hydrobios, Kiel, Germany). Bottom water was

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also collected for use in control incubations and replacement of sampling water during core

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incubation. The stirrers were switched on to ensure the water column was mixed all the time.

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The cores were transported in insulated boxes to the laboratory to maintain the in situ

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temperature.

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Bottom water here refers to water sampled approximately 50 cm above the

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The upper 3 cm sediment layer of the two sediment pore-water cores were sectioned

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on site and combined into one sample. Part of the mixed sample was used for pore-water

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extraction and the other part was used for sediment quality analysis i.e. water content and loss

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on ignition. Pore-water was extracted by centrifugation at 3750 rpm for 15 minutes at the

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incubation temperature.

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For determination of vertical and horizontal DOC quality gradients, surface and

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bottom water samples were taken from both shallow and deep sites using a standard 2 L water

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sampler (Hydrobios, Kiel, Germany). An additional sample was taken at the deep site at 4 m,

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which was the depth of the oxycline. At each site, in-situ vertical profiles of oxygen,

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temperature, and turbidity measurements were conducted, using multi-parameter probes (Sea

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& Sun Technologies, Germany).


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Sediment incubation

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Four sediment cores per site were incubated at in-situ conditions (oxic, 14oC for

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shallow and anoxic 6oC, for deep), in the dark for 13 days after which redox conditions were

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switched and the sediment cores were further incubated at switched redox conditions and in-

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situ temperature (anoxic, 14oC for shallow and oxic, 6oC for deep) for 14 days. Redox

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conditions were judged by the presence or absence of molecular oxygen. Since nitrate was

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always below the detection limit under anoxic conditions (data not shown) we can assume

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reducing conditions in the absence of oxygen. Sediment cores were incubated in climate

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chambers at in situ temperature.

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with aluminium foil. The sediment cores were allowed to stand for one night during which

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oxygen measurements were carried out using optodes (Pyroscience, Aachen, Germany). Oxic

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cores were controlled for oxygen at the start of the experiment and frequently during the

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course of the experiment which also included bubbling the cores with air to keep them oxic.

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Anoxic cores were controlled for oxygen using an optode before the start of the experiment

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and at the time of each water sampling. In case of oxygen contamination, the cores were

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bubbled with a mixture of nitrogen and carbon dioxide (99.96 % N2 / 0.04 % CO2) until they

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were anoxic. Mixed nitrogen and carbon dioxide gas was used to buffer the pH. The

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concurrent withdrawal and replacement of the sample was used to sample the overlying water

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in anoxic cores to prevent oxygen contamination. A 30 mL syringe was filled with 30 mL of

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anoxic hypolimnion water and attached to the gas/air release/sample replacement opening on

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the sediment cover and an empty syringe was fixed to the gas/air bubbling/sampling opening

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with an extended silicon tube. The sample was withdrawn and concurrently the syringe with

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replacement water was emptied into the core. During the sediment incubation 30 ml of the

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overlying water were sampled every 3 days and analysed for DOC and Fe using standard

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methods16, 28 Samples for DOC were filtered using glass fiber (Whatman GF/F) filters, and

16, 28

To prevent light penetration, the cores were wrapped



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samples for Fe were filtered using a 0.2 µm syringe filter. The sampled volume was replaced

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by bottom water samples. Controls without sediment were prepared by incubating bottom

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water samples in glass bottles at in situ temperature and redox conditions. The controls were

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sampled at the same time as the overlying water and likewise analysed.

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Per site three DOC quality samples (15 mL from each of the four cores were

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combinedto 60 mL for SPE) were taken from the incubated cores; 1) at the start of the in-situ

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redox condition incubation, 2) before redox manipulation/switch, and 3) at the end of the

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manipulated redox condition incubation. Altogether there were 13 samples analysed for DOC

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quality (5 lake water, 2 pore-water, and 6 incubation samples). The redox conditions switch

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was carried out by bubbling oxic cores with N2 / CO2 until anoxic and allowing them to stand

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for a day before the start of the sampling under anoxic conditions. The anoxic cores were

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made oxic by bubbling the overlaying water with air until oxic and allowing them to stand for

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a day before the start of the sampling under oxic conditions, during which oxygen

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consumption measurements were also performed. Fluxes were calculated from the linear

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change of concentrations in the water overlying the incubated sediment cores. Concentration

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increase, which means a flux from the sediment into the water, was defined as a positive flux.

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DOC quality determinationAnalysis of samples

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Aliquots of 60 mL water from lake water samples or pore-water or overlying water

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from incubation samples (all 13 samples are listed in Fig. 2) were filtered through pre-

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combusted Whatman GF/F glass fiber filters and acidified with HCl to pH 2.0. Subsamples of

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50 mL of the acidified filtrate were processed by solid phase extraction (SPE).

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described briefly in the supplementary information (SI). The details of FTICR-MS

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measurements (electrospray ionization in negative mode) are described briefly in SI3 and in

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more detail in Hertkorn et al 30. Molecular formulas were calculated considering the following

29

The SPE is


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elements 1H0-200,

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(DBE: double bound equivalents) were considered for further data evaluation. The reasons for

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these selection criteria were elucidated in Herzsprung et al.31,

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solutions were recorded for one mass peak by the mass calculator, the rules from Herzsprung

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et al.31,

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Herzsprung et al.31 explained formula assignment rules derived from data sets containing

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almost only CHO and in

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was elucidated for components containing up to 7 N and 3 S (SI, page S3, Fig. S2 therein). 33

32, 33

12

C0-100,

16

O0-80,

14

N0-6,

32

S0-3. Only formulas with -10 ≤ DBE - O ≤ +10

32

If two or more formula

were used to find out the most reliable one from all recorded solutions.

32

for those data sets containing many CHOS. Formula assignment

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Calculations and statistics

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For each sample (separately) the percentage intensities were calculated for each

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component (% intensity = [intensity of component / sum of intensities] × 100 %). Intensity

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weighted bulk parameters were calculated as described in SI5.

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Exclusively all 1147 CHO components which were present in all 13 samples were

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extracted from the complete data set to reveal principle differences in DOC composition.

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Components not present in all samples will be not reconsidered due to statistical reasons

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(ranking of components with intensity = 0). The exclusion of the other components (CHON,

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CHOS, CHONS) is shortly explained in the results section and in SI4. The new data set is

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named Data_13. The ranks of mass peak intensities are shown in the database.xlsx (SI) in

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spreadsheet “Data_13”, column L. They were calculated for each sample as described in SI6

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and Herzsprung et al.34

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Hierarchical cluster analysis (Ward method, HCA) was performed (using

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STATISTICA Version 12, StatSoft GmbH) from the Data_13 set using the ranks of mass

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peak intensities. Herzsprung et al

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calculation of HCA (in section 2.3.4. therein). The Ward method with squared Euclidian

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explained the use of mass peak intensity ranks for



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distances was used because it is based on a classical sum-of-squares criterion, yielding groups

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that minimize within-group dispersion at each binary fusion. 36 With a z-score scaling as data

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pre-treatment the expected value (average) of each series became 0 and the standard deviation

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1. All calculated ranks were used in the next step for an inter-sample rankings analysis in

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spreadsheet “Table_S13” of the database.xlsx. The principle is shown in Tables S13 – S15

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and explained in SI6. The inter-sample rankings analysis is a tool for the visualization of

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DOC quality differences in van Krevelen diagrams. It makes abundance differences of

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biogeochemical components classes (for example lipids, lignins, tannins) visible. This tool

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was already used to differentiate 4 or 5 river samples 34, 22 peatland samples 35, 6 or 7 pore-

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water samples 37, 5 time dependent samples in flumes 38, 4 pristine river samples 39.

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We performed inter-sample rankings for the entire dataset as well as for particular subsets. In

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particular we used inter-sample rankings by combining three inter-sample ranks for the

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shallow site, four for the deep site (Fig. S4), three of them both for the manipulated redox

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conditions at the shallow site and the deep site (Fig. 3), four of them for comparing the oxic

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and anoxic incubations ends of the shallow and deep site (Fig. S5) and combining 13 inter-

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sample ranks for comparison of all 13 samples (Fig. S3).

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In order to answer our question how DOC quality is affected by redox conditions we focused

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on the behaviour of CHO (components containing only C, H, and O) and of those oxygen-rich

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and relatively unsaturated, polyphenol-like components (Ox-Unsat-Comp) with H/C < 1 and

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O/C > 0.6 in relation to the remaining part of the SPE-DOM (all other components detected

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by FTICR-MS).


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Results

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Spatial DOC gradients and redox conditions effects (sediment core incubation)

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The temperature profiles showed that the water column at the shallow site was

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completely mixed. Consistent with this the DOC concentration at the surface and above the

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sediment was the same (Fig. 1). The deep site was stratified; the epilimnion had a higher DOC

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concentration than the hypolimnion. The pore-water DOC concentration at both the deep and

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shallow sites were higher than the overlying water DOC concentration (Fig. 1), hence

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potential for diffusion of DOC from the pore-water into the overlying water. During the initial

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phase of incubation (in-situ redox conditions) cores under oxic conditions (shallow site) had

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constant DOC, and those under anoxia (deep site) released DOC (Fig. 1). When redox

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conditions were switched, DOC magnitude and flux direction changed. Fe fluxes just like

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DOC exhibited similar pattern (Fig. S6).

233 234 235 236

Fig. 1. DOC characteristics depending on location: The size of the dots indicate DOC concentration, the color indicates the abundance of oxygen-rich and relatively unsaturated, polyphenol-like components (Ox-Unsat-Comp): green = low, orange = intermediate, red = 11 ACS Paragon Plus Environment


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high (the ranking model is explained in SI7 / Table S16). DOC concentrations ranged between 0.28 and 1.9 mmol C l-1. The column graphs show the DOC flux at the two sites, the dots indicate abundance of Ox-Unsat-Comp before, in between and at the end of the respective experiment. The right graph shows vertical profiles of O2 and temperature in the reservoir as measured with a CTD probe. Error bars represent standard deviation of four replicates cores.

242 243 244

Calculation of bulk DOM quality parameters and balancing components presences We expect that of all calculated bulk DOM quality parameters the average carbon 40

245

oxidation state (OSC ≈ 2 × O/C - H/C)

would especially respond to redox switches. The

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average OSC (Table S12) was lowest (-0.038) in the shallow oxic incubation end sample. The

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highest values were found in the deep pore-water sample (+0.073) and deep anoxic incubation

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end sample (+0.044). More details including other bulk parameters are shown in SI5 and

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Table S12. While all components intensities were used for calculation of bulk parameters,

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only the CHO components which were present in all samples were used for statistical analysis

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(SI4). They make up 52.1 % of all formulas (Table S10) and 81.7 % of the total percentage

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intensity in all 13 samples (Table S11). The major part of all detected components can be

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assigned to three different biogeochemical groups (condensed hydrocarbons, lignins, tannins)

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whose van Krevelen coordinates were provided by Mann et al

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Other biogeochemical groups were not important in our dataset (Table S19).

41

(page 3, Fig. 1 a therein).


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Hierarchical cluster analysis (HCA) of DOC quality samples The DOC quality in different water layers and under different redox conditions was

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compared by a HCA (Fig. 2). The HCA showed on the first view four different groups of

259

samples with similar DOC quality (1,2; 3,4; 5,6,7; 8,9,10,11,12,13). Quality of DOC was

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similar in all samples from the epilimnion (shallow 0 m and 4 m; deep 0 m and 4 m). The

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deep 16 m (hypolimnion) sample was somewhat different from the epilimnion samples. Pore-

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water samples from both shallow and deep sites were different from all other pelagial samples

263

and also different from each other. The deep anoxic incubation start sample was similar to the

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deep16 m sample as expected, and this water was used for incubation of the sediment.

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Analogous to that, the shallow oxic incubation start sample was similar to the shallow 4 m

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sample and all other epilimnion samples. The deep anoxic incubation end sample was similar

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to the deep pore-water sample, but different from all other samples. The shallow oxic

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incubation end sample was similar to the shallow pore-water sample and different from all

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other samples. The redox conditions manipulation from anoxic to oxic at the deep site resulted

270

in similarity to that at the anoxic incubation start and the hypolimnion water (16 m). At the

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shallow site, the redox conditions switch from oxic to anoxic resulted in similarity to the oxic

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incubation start sample. However the oxic incubation start sample was different from the oxic

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incubation end sample. The differences in samples for both shallow and deep site, when

274

comparing the start of incubation and the samples after redox condition switch were more

275

pronounced at the shallow site. The HCA does differentiate the samples in a quantitative

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(statistical) manner, showing the squared Euclidean distances. The HCA contains no

277

information about the difference in chemical DOC composition between samples. The inter-

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sample rankings can visualize such DOC quality differences.

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Inter-sample ranking analysis of DOC quality samples

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The deep epilimnion samples 0 m and 4 m were similar to each other (Fig. S4 right),

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confirming the results from the HCA (Fig. 2). The deep hypolimnion sample (16 m) showed

283

relative high intensities (first ranks) in the region 1 < H/C < 1.5 and O/C < 0.6. Relative low

284

intensities (last/fourth ranks) could be observed for H/C < 1 and O/C > 0.6. The deep pore-

285

water sample showed opposite (to deep 16 m, hypolimnetic water) ranking of intensities. First

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ranks were found for Ox-Unsat-Comp and for many components with H/C < 1 and 0.4 < O/C

287

< 0.6. In addition many components with O/C > 0.5 and 1 < H/C < 1.3 showed first and

288

second ranks in the deep pore-water sample. Components with 1 < H/C < 1.5 and O/C < 0.5

289

mainly showed last ranks (fourth ranks). Some higher saturated components with H/C > 1.5

290

showed

first

ranks

in

the

pore-water

sample.

291 292 293 294 295

Fig. 2. Cluster analysis (Ward’s method squared Euclidean distances) of CHO components of vertical depth samples and pore-water from the deep site (red) and from the shallow site (green); of sediment incubation samples from the deep site (orange) and from the shallow site (blue).. 14 ACS Paragon Plus Environment

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Comparing the three samples from the shallow site, the pore-water as the most

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different (deduced from HCA in Fig. 2) showed first ranks for components with O/C > 0.5

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and 1 < H/C < 1.5, for components with O/C < 0.6 and 0.5 < H/C < 1.0 and some components

299

with H/C > 1.4. Both many (but not all of them) components with 1 < H/C < 1.5 and O/C
0.7 from the deep pore-water

308

sample (first and second ranks) (Fig. S3).

309

The incubation experiments showed DOC quality change. At the deep site, anoxic

310

incubation of sediment cores changed the intensity ranking dramatically as shown by the

311

anoxic start and end of the incubation. (Fig. 3 right). Oxygen-rich components (O/C > 0.6)

312

now showed first ranks and oxygen-poor (O/C < 0.5) components third (last) ranks. After the

313

redox conditions manipulation from anoxic to oxic the DOC quality changed nearly to the

314

initial status at the start of the anoxic incubation, a result similar to that shown by the HCA

315

(Fig. 2). At the shallow site the sediment cores were initially incubated under oxic conditions.

316

The oxygen-rich components (O/C > 0.6) showed last ranks whereas oxygen-poor (O/C < 0.5)

317

components showed first ranks (Fig. 3 left). The redox conditions switch from oxic to anoxic

318

again changed DOC quality at the shallow site. At the end of the anoxic incubation, the

319

sample showed now first ranks for oxygen-rich components (O/C > 0.6) and third ranks for

320

oxygen-poor components (O/C < 0.5). 15 ACS Paragon Plus Environment


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322 323 324 325

Fig. 3. Van Krevelen diagrams showing the inter-sample rankings analysis of DOC quality of in-situ redox (upper 2 charts) and manipulated redox conditions (bottom chart) sediment incubation samples from Rappbode shallow (left) and deep (right) sites.

326 327

Anoxic incubation led to an increase of Ox-Unsat-Comp intensities whereas oxic

328

incubation led to a decrease of these components intensities. However the DOC quality at the

329

shallow and the deep site was rather different at the anoxic incubation end as shown in Fig. 2.

330

It was also different for both sites at the oxic incubation end. These DOC quality differences

331

are elucidated by another (more specific) inter-sample rankings analysis as shown in Fig. S5

332

and described in the SI (page S21).


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The inter-sample rankings analysis generally showed that the DOC quality was

334

different in epilimnion and hypolimnion water, different in hypolimnion and deep pore-water.

335

Oxic and / or anoxic sediment incubation considerably changed DOC quality of overlying

336

water. Redox switch led to the original start DOC quality (Fig. 2, sample 6 versus sample 5;

337

sample 9 versus sample 8).

338

339

Discussion

340

The study was carried out for two purposes. The first was to identify influences on

341

DOC quality which can be observed by variation of water sources (different water depths,

342

epilimnion, hypolimnion, sediment, redox conditions). The second was to explain the

343

identified influences by processes (release, adsorption, production, consumption, transport of

344

components).

345

346

Epilimnetic spatial (inflow vs outflow) effects?

347

The differences in DOC signature in the shallow and deep site epilimnion (horizontal

348

gradient) were negligible (Fig. 2, Fig. S3) indicating that there was relatively small change in

349

DOC quality as water moves from the inflow (shallow site) to the outflow (deep site). The

350

Rappbode pre-dam has a mean water residence time of 52 days.

351

DOC degradation in Rappbode epilimnetic water and found a degradation of 13-20 % over a

352

period of 91-131 days, which indicates that DOC reaching the outflow in the reservoir is still

353

dominated by allochthonous DOC from the catchment. DOC production by pelagic algae was

354

probably not relevant in our case since algae abundance was low in the pre-dam and

355

incubation assays had shown only low DOC exudation. 3 This confirms results of Friese et al

24

Morling et al 42 measured



Page 18 of 28

356

24

357

Rappbode pre-dam. Obviously in the presence of only water the DOC quality change is slow

358

43, 44

359

is a redox gradient.

who also found DOC of high aromaticity and thus, most probable allochthonous origin in

. In contrast, there was a dramatic quality change in the presence of sediments and if there

360

361

Redox conditions effects on DOC quality?

362

Surprisingly the average OSC was more positive in anoxic samples and smaller or

363

negative in oxic samples (Table S12, Fig. S2). Redox switch to anoxic did not result in overall

364

reduction of DOC and switch to oxic did not result in oxidation of DOC. Anoxia showed

365

evidently relative higher intensities of more oxidised DOC components. This fact let us have a

366

look on the behaviour of Ox-Unsat-Comp. For clarification of observations concerning DOC

367

quality change we deduce three different qualitative abundance levels of Ox-Unsat-Comp

368

from Fig. S3 with following van Krevelen diagrams coordinates: H/C < 1; O/C > 0.6 (in SI7

369

we elucidate why these coordinates are appropriate for this limited consideration of

370

components). As shown in Fig. 1 epilimnion water showed intermediate Ox-Unsat-Comp

371

abundance, hypolimnion water and shallow pore-water low abundance and deep pore-water

372

high abundance. The assignment of samples to these three abundance levels in Fig. 1 was

373

calculated in Table S16 a. From this clarified viewpoint deep pore-water must be a source and

374

shallow pore-water or hypolimnion water a sink of Ox-Unsat-Comp. A sink of components

375

can be caused by consumption and/or adsorption, a source can be caused by release

376

(desorption) or production of components. This principle was discussed in Herzsprung et al 37

377

for DOC quality analysed by inter-sample rankings in iron-rich sediments of an acidic mining

378

lake. Suggestion of new biogeochemical synthesis of Ox-Unsat-Comp seems unrealistic

379

inside the investigated environment. As well the decomposition (complete mineralisation, not


Page 19 of 28


380

chemical modification) of Ox-Unsat-Comp seems unrealistic due to their high recalcitrance

381

against biodegradation in the dark.45

382

The most probable explanation for sources is the release of Ox-Unsat-Comp by

383

decomposition of terrestrial material (leaves) or desorption from minerals. For sinks the

384

adsorption to minerals would be a realistic explanation. Adsorption processes were already

385

described to be responsible for alteration of DOC quality. A loss of carboxyl groups and

386

aromatic groups was observed in soil solutions passing through forested soils.46 The

387

adsorption / desorption hypothesis is supported by Fig. 1. The DOC flux was positive related

388

to the flux of dissolved iron. The shallow oxic incubation end sample showed less abundance

389

of Ox-Unsat-Comp (average rank value 12 as calculated in Table S16 a); 14.3 % total

390

percentage intensity as calculated in Table S17) compared to the shallow oxic incubation start

391

sample (average rank value 7.6; 15.6 % total percentage intensity). The loss of intensity and

392

higher average rank value was probably caused by the adsorption of Ox-Unsat-Comp.

393

Obviously, oxic incubation led to further establishment of equilibrium between DOC and

394

mineral phases.

395

We confirmed earlier results that redox conditions influence the DOC flux.15, 16 Ox-

396

Unsat-Comp can be adsorbed to ferric iron minerals under oxic conditions. After redox switch

397

to anoxic conditions they can be released (desorbed) if solid ferric iron is reduced to dissolved

398

ferrous iron. Recent literature supports this iron redox cycling (desorption / adsorption)

399

hypothesis of Ox-Unsat-Comp.9, 19, 20, 37, 47, 48 The hypothesis is further confirmed by our redox

400

switch experiments. We observed release of Ox-Unsat-Comp after anoxic incubation of

401

hypolimnion water from the deep sediment. Redox switch to oxic led to loss of Ox-Unsat-

402

Comp which can be explained by adsorption to generated / precipitated ferric iron minerals.

403

The oxic incubation of epilimnion water resulted in further loss of Ox-Unsat-Comp, what can

404

be explained by adsorption to ferric minerals in the shallow sediment. Redox switch to anoxic 19 ACS Paragon Plus Environment


Page 20 of 28

405

then caused release of the before adsorbed Ox-Unsat-Comp and the original DOC signature

406

(oxic incubation start, Fig. 3 left) was nearly restored. Yang et al

407

with oxic and anoxic incubations in parallel but not by switching redox conditions. They

408

observed a release of two different humic like components (accompanied by an increase of

409

the humification index HIX) under hypoxic conditions. Such components and HIX were

410

shown to correlate with abundance of Ox-Unsat-Comp by rank correlation.35, 50, 51

49

performed experiments

411 412

Sediment effects on DOC quality

413

Pore-water is known to have high DOC concentration.37 In Rappbode the pore-water

414

concentration was 4 times higher than the hypolimnion concentration. It would therefore be

415

expected that diffusion of DOC from the pore-water into the hypolimnion influences the DOC

416

quality in the hypolimnion if neither transformation nor adsorption of specific components

417

took place at the sediment surface. But this was not the case in Rappbode. As shown in

418

Figs. 1, S3, S4, the anoxic hypolimnion DOC quality was different from that in pore-water.

419

Therefore DOC quality in the hypolimnion was probably influenced by the DOC pool prior to

420

mixing and DOC mobilized from settling particles from the epilimnion. Epilimnetic DOC

421

concentration was higher than hypolimnetic. This can be explained by import of DOC from

422

the catchment to the epilimnion. This is plausible because the main source of DOC in the pre-

423

dam was reported to originate from the catchment.4 A second explanation might be a loss of

424

Ox-Unsat-Comp to the particulate organic carbon pool (POC) via adsorption to ferric iron

425

mineral particles in the hypolimnion.

426

It could be expected that the hypolimnion has a higher DOC concentration as result of

427

the accumulation of DOC from the DOC rich pore-water. Even though there was DOC

428

accumulation in the hypolimnion, the accumulation was not homogenous. A closer look at bi-

429

weekly routine monitoring data (Fig. S7) revealed that there was an increasing concentration 20 ACS Paragon Plus Environment

Page 21 of 28


430

of DOC in the hypolimnion probably as a result of diffusion from the pore-water. However,

431

the hypolimnetic DOC concentration was heterogeneous from the start of anoxia, with the

432

layer just above the sediment having concentration twice than the water 3 m above the

433

sediment. We conclude that the direct influence of the sediment on DOC quality in the

434

hypolimnion was small.

435 436

Benefits and limitations

437

The described results are based on the assumption of similar recovery rates for SPE-

438

DOM and similar ionisation efficiency for all components in all 13 samples. The recovery rate

439

of DOC is controlled by many factors which are not yet completely understood.29, 52, 53 The

440

recovery rates in our experiment (44.6 % to 79.1 %; Table S2) were in the range known from

441

the literature as discussed in Kamjunke et al.

442

ionisations efficiencies were also discussed in Kamjunke et al.

443

of Ox-Unsat-Comp in order to simplify the comparison of DOC quality in different samples

444

(Fig. 1) in view of our suggested model by adsorption / desorption via iron redox cycling. We

445

ignored some specific DOC quality differences, for example between the shallow pore-water

446

and the shallow oxic incubation end samples (Fig. S3). These differences base mainly on

447

components with van Krevelen diagram coordinates different from Ox-Unsat-Comp. The

448

adsorption / desorption of Ox-Unsat-Comp evidently might disguise redox processes of other

449

DOC components. As shown in Fig. S2 components with minor abundance (CHON, CHOS,

450

CHONS) show different tendencies of OSC values in all samples compared to CHO.

38

The limitations of potential interactive 38

We focused on the ranking

451

As a benefit, we can deduce from our observations, that anoxic sediments are a

452

temporary source for ferrous iron and Ox-Unsat-Comp, both which can be precipitated at the

453

oxic/anoxic boundary. This is a relevant process at the autumn lake overturn during which the

454

anoxic hypolimnetic water mixes with the oxic epilimnetic water. This promotes burial and 21 ACS Paragon Plus Environment


47

Page 22 of 28

and persistence of aliphatic components.54 For drinking water

455

preservation of carbon

456

treatment this implies that coagulation alone might not be enough to remove the DOC since

457

the more aliphatic components cannot be removed by this method. 9, 55

458

The envisaged climate change effects on temperature increase might induce longer and

459

stronger summer stratification, therefore effects of anoxia on both DOC quantity and quality

460

might gain prominence in the future.

461 462

Acknowledgements This work was financially supported by the TALKO project (BMBF 02WT1290A).

463 464

We thank Corinna Völkner for assisting with field sampling, Ina Siebert for DOC

465

determination and Ines Locker for SPE and determination of recovery rates. Furthermore we

466

thank four anonymous reviewers whose comments considerably improved the manuscript.

467 468

Supporting Information

469

Additional details on study site, SPE, FTICR-MS measurements, inter-sample rankings,

470

solute fluxes, lake stratification. A database containing assigned elemental formulas and

471

calculated inter-sample rankings values.

472 473

Notes

474

The authors declare no competing financial interest

475


Page 23 of 28

476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526


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Intensity low intermediate high

DOC Fe2+

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DOC Fe2+ DOM quality alteration

anoxic incubation

ACS Paragon Plus Environment

DOC

Fe(OH)3 DOC

oxic incubation

DOC

Redox conditions affect DOC quality in stratified ... - ACS Publications

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