Regulation of DOC by climate and atmospheric deposition on various temporal scales. Use of long-term data records from headwater catchments

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Bertram Perry
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1 Regulation of DOC by climate and atmospheric deposition on various temporal scales Use of long-term data records from headwater catchments Heleen de Wit Norwegian Institute for Water Research Oslo Structure Known examples of chemical controls of DOC in soils Exploring the relation between atmospheric chemistry, soil chemistry and DOC in surface water Are climatic and chemical effects on DOC separable? Are quantity and quality affected? Simple concept of chemical streamwater DOC controls 1

2 Effects of chemistry on DOC in soils Field experiment in Norway spruce forest Test toxicity of Al on trees by increasing the soil solution concentration of Al 5 treatments from low to high Al addition On addition of AlCl 3 to soil solution, DOC levels in soil solution drop O horizon leachate DOC (mg/l) AL DOC C A0 A1 A2 A4 Treatment Al (µmol/l) De Wit ea 2001, WASP ; De Wit ea 2010, FEM Effects of chemistry on DOC in soils Batch experiment with soil from field experiment O horizon soil, add solution, shake Measure ph, DOC, Al Adding acidity decreases ph, lowers DOC, mobilizes inorganic Al High soil Al content results in lower DOC Mechanistic models like WHAM can describe soil-solution phase interactions of DOC and metals ph C A ph Simulations C Simulations A DOC Al-qr Base - Acid (10-3 mol -1 L) De Wit ea 2001, SSAJ Al-qr (10-6 mol/l), DOC (10-3 mol/l) 2

Hydrologists thought it was Hydrology (hydrological pathways) Microbiologists

3 Effects of chemistry on DOC in soils Podzolization What causes the region-wide increase in DOC? Hydrologists thought it was Hydrology (hydrological pathways) Microbiologists thought it was Microbiology (enzymes) Limnologists thought it was in-lake processes (photo-oxidation) Acid rain scientists thought it was... anything but acid rain 3

4 The browning of the waters Hypotheses More DOC production in soils in a warmer climate destabilization of organic matter with important consequences for terrestrial C sink. Upward trend could continue Chemical response to recovery from acid deposition return to pre-industrial concentrations of DOC, expecting that DOC trends will level off Exploring the acid-rain hypothesis Not very hot compared with climate change, but a valuable and complementary perspective What mechanistic explanations are available? Can DOC quality change given changes in atmospheric chemistry? 4

5 DOC trends in >500 lakes from monitoring networks (reporting to International Cooperative Programme for Assessment and Monitoring of Rivers and Lakes - ICP Waters) Monteith ea 2007, Nature 5

ΔDOC explained by ΔSO4 and ΔCl Variable Estimate p Cumulative R 2 Intercept 0.83 0.0004. Change in SO 2-4 (µeq/l/yr) -0.57 <0.0001 0.

6 ΔDOC explained by ΔSO4 and ΔCl Variable Estimate p Cumulative R 2 Intercept Change in SO 2-4 (µeq/l/yr) < Change in Cl - (µeq/l/yr) < (a) CANADA FINLAND NORWAY SWEDEN UK USA (b) CHANGE IN DOC (% yr -1 ) DOC CHANGE IN DOC (% yr -1 ) DOC CHANGE SO4 IN SO4 2- (µeq l -1 yr -1 ) -10 Cl Umeå April CHANGE IN Cl - (µeq 2010 l -1 yr -1 ) Empirical relations suggest: Declining atmospheric SO4 and Cl Negative changes give positive TOC trends Catchment acid-sensitivity the more base cations, the lower the acidsensitivity, and the lower the change in TOC 6

Separation of climate and chemical controls on DOC?

climatic drivers and deposition Norway, headwater streams De Wit ea

small lakes Haaland ea 2010, ES&T Norwegian Catchments included in

7 Separation of climate and chemical controls on DOC? Higher frequency of DOC measurements Empirical approach including climatic drivers and deposition Norway, headwater streams De Wit ea 2007, ES&T Sweden, 35 larger rivers Erlandsson ea 2008, GCB Norway, small lakes Haaland ea 2010, ES&T Norwegian Catchments included in National Monitoring Program since early 1970s Langtjern Storgama Langtjern Birkenes Storgama Birkenes 7

8 Methods Relate to climate (T, precip, snow, discharge) and deposition (Cl, SO4, NO3) at various time intervals (from days to years) (Forward) stepwise regression Weekly TOC concentrations in 3 small Norwegian catchments ( km2) 8

Model variables weekly TOC annual mean slope T snow 1 prec disch dep Cl dep SO 4 dep NO 3 r 2 r 2 µg C yr -1 Birkenes observed 64 regression x 0.43 0.06 9 x x x 0.50 0.05 5 x x x x 0.52 0.

9 Model variables weekly TOC annual mean slope T snow 1 prec disch dep Cl dep SO 4 dep NO 3 r 2 r 2 µg C yr -1 Birkenes observed 64 regression x x x x x x x x x x x x x Storgama observed 94 regression x x x x x x x x x x x x x x Langtjern observed 130 regression x x x x x x x x x x x x x Climate (1-60 days) explains most variation in weekly TOC Deposition (6 months - 3 yrs) explains most of the interannual variation and long-term trend in TOC TOC in 35 Swedish rivers Catchments km 2 Synchronous patterns, strong oscillations, upward trends Multiple linear regressions shows that most variation is explained by discharge and S deposition Discharge and S deposition equally important Erlandsson ea 2008, GCB 9

10 Are trends in color stronger than in TOC? Lake data SE Norway compared with streamwater south Norway Trends in color are related to changes in atmospheric chemistry Precipitation adds to the annual variation TOC konsentrasjon (mg C/L) % Birkenes Storgama Langtjern Haaland ea, 2010 ES&T NIVA data +1-2% How important is catchment size? Is discharge a more important control of DOC in larger catchments? Is this because of presence of lakes? Discharge and color i Glomma, NorthWatch Norway s DOC largest controlsriver Bømo ea, 2007 Vann 10

11 Upward trends in the color to DOC ratio in the UK Not linked to discharge Soil chemistry? Location and magnitude of trend in color: DOC ratio Worrall and Burt 2010, GB Controls on DOC Recovery from acid rain correlated with increase in DOC a stronger increase in color also related to atmospheric chemistry? Discharge contributes strongly to interannual variations in DOC and color Is color more sensitive to discharge than DOC? Is discharge a more important factor in larger catchments? 11

12 Mechanisms linking chemistry and DOC ph or ionic strength? Has TOC charge density changed? And can we interpret higher charge density as a higher solubility of OM? What is the role of complexation with Al? Austnes ea, in preparation Returning to the monitoring data... Austnes ea in prep 27 sites with lake/river records from acid monitoring in Norway, UK, Finland, Sweden Full streamwater chemistry Calculation of charge deficit and charge density using speciation model Wham-Water (Tipping 1994) Al speciation and alkalinity Trend analysis using Mann-Kendall test and Sen-slopes 12

13 ph or ionic strength? TOC trends are correlated best with trends in E(SO4+Cl) Ionic strength and ph effects cannot be separated in monitoring studies TOC trend TOC trend TOC trend 0.50 R 2 = Ionic strength trend 0.5 R 2 = R 2 = ECl+ESO4 H+ trend trend Austnes ea in preparation Effects of increased ph on DOC Humic compounds are anions OF VARIABLE CHARGE! Reactive groups (carboxylic groups) can bind protons and metals R-C =O -OH R-C =O -O - + H + An increase in ph = decrease in H+ leads to deprotonation of humic compounds AND INCREASED CHARGE Humic acids consist partly of apolar, hydrophobic compounds that prefer to stick to the solid phase Increased charge will help them overcome their fear of water and promote desorption into to solution phase 13

14 Estimation of charge deficit/density The negative charge deficit is usually calculated by estimating a charge balance of major cations and anions DOC charge density = charge deficit / DOC Low ph, high I DOC Low charge density R-C =O -OH Higher ph, lower I R-C DOC - High charge density =O -O - + H + Reduced acidity increases humic charge A higher charge density increases tendency to desorb from solid phase 14

15 Charge deficit and charge density Positive DOC trends under reduced acidity are associated with increased organic charge deficit ( more negative charge ) higher charge density ( DOC with higher negative charge ) Both sig TOC sig -0.1 Def sig Charge deficit trend None sig Both sig TOC sig -0.1 CD sig TOC Charge density trend None sig TOC trend TOC trend 15

16 Charge density Positive DOC trends under reduced acidity are associated with higher charge density ( DOC with higher negative charge ) Correct for most sites (especially with sign TOC trends), but NOT in the UK! TOC trend 0.0 Both sig TOC sig -0.1 CD sig TOC Charge density trend None sig TOC trend 0.0 Norw ay Finland -0.1 UK TOC charge density trend Sw eden Why so different in UK? Maybe complexation with Al has increased, so less sites are available for deprotonation? Trend test of ratio non-labile Al to TOC If anything, complexation with Al has declined ratio org Al to TOC Both sig TOC sig ratio sig TOC trend None sig Afon Gwy Allt amarcaidh Coneyglen Burn Beaghs Burn River Etherow Org Al to TOC trend 16

17 Summary Color has increased more rapidly than TOC, suggesting a change in DOC quality DOC increases are associated with a larger (more negative) organic charge deficit DOC charge density has decreased (more negative charge pr g DOC) increased (less negative charge pr g DOC) The latter can be explained by a larger contribution of DOC with a lower charge density Is the newly mobilized DOC highly aromatic, with few reactive groups? newly mobilized DOC was formerly sorbed to the solid phase, but now its fear of water (hydrophobicity) has been overcome by increasing its negative charge? Conclusions Reduced acid deposition has caused an increase in DOC Recovery of acid deposition may have caused a change in quality of DOM (more aromatic, more color, lower charge density) Similar to changes in DOM quality caused by precipitation events? Changes in DOC and DOC quality might not only be linked to the usual suspect (hydrological pathways) but also to atmospheric chemistry If climate change results in more frequent storm events, sea salt deposition will affect freshwater DOC Episodic drought-induced acidification in peatlands may lead to oxidation of S and decline in DOC (see work by Jo M. Clark) 17

18 Al vs sum of (SO4+Cl) TOC vs sum of (SO4+Cl) Al TOC SO4 + Cl SO4 + Cl Upland Stream Peat Al vs sum of (SO4+Cl) TOC vs sum of (SO4+Cl) Al TOC SO4 + Cl SO4 + Cl Upland Stream Peat 18

19 Conclusions Reduced acid deposition increases DOC in soil- and peat water decreases Al Increased soil DOC results in increased stream water DOC Under lower acid deposition, less DOC is removed from stream (and soil?) water by coagulation Relevance of changes in DOC Export of DOC is a significant part of the (boreal/temperate) C cycle Soil C uptake maybe severely overestimated when ignoring DOC export to rivers and lakes Fish in brown-water lakes have high concentrations of methyl-hg When DOC increases, MeHg in fish increases? 19

20 Thank you! 20

Lecture 13 More Surface Reactions on Mineral Surfaces. & Intro to Soil Formation and Chemistry

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