High Carbon Dioxide Evasion from an Alpine PeatlandLake: The Central Role of Terrestrial DissolvedOrganic Carbon Input

Dan Zhu & Huai Chen & Qiu’an Zhu & Yan Wu &

Ning Wu

Received: 2 September 2011 /Accepted: 8 December 2011 /Published online: 27 December 2011# Springer Science+Business Media B.V. 2011

Abstract We measured carbon dioxide (CO2) fluxesacross air–water interface with floating chambers inLake Medo (a small, shallow lake in peatland) on theeastern Tibetan Plateau in the warm season of 2009.During the study period, mean CO2 fluxes was488.63±1,036.17 mg CO2m

−2 h−1. The flux rate washigh compared to those of lakes in other regions, andrepresented a “hotspot” of CO2 evasion. Temporal var-iation of CO2 fluxwas significant, with the peak value in

the beginning and lowest point in the end of warmseason. High concentration of dissolved organic carbon(DOC) in lake water (WDOC) was found to highlycorrelated to CO2 flux (R00.47, P<0.01, n054). Be-sides, fluorescence index of WDOC showed its terres-trial origin character. In accordance with lakes innorthern and boreal regions, terrestrial DOC concentra-tion in water column was the most important regulatorof CO2 flux from this lake. We suggest that large area ofpeatlands in catchments support high concentration ofDOC in this lake, and consequently high CO2 evasion.

Keywords CO2 flux . Temporal variation . Shallowlake . Terrestrial DOC . The Zoige peatlands

1 Introduction

Carbon cycle is a fundamental biogeochemical pro-cess linking lakes and their catchments (Karlsson et al.2010). Previous studies have suggested that a majorityof the world’s lakes are net sources of CO2 to theatmosphere (Kling et al. 1991; Cole et al. 1994), andthe flux of CO2 from lakes largely represent the de-composition of terrestrial DOC in water column or/and sediment (Lennon 2004; Sobek et al. 2006).Changes of terrestrial DOC input to lakes are extensivereported (Clark et al. 2007; Tranvik and Jansson 2002)and it could have strong effect on CO2 evasion fromlakes (Hope et al. 1996; Sobek et al. 2003).

Lakes in catchments with high proportion of peatlandare thought to have comparative high CO2 evasion

Water Air Soil Pollut (2012) 223:2563–2569DOI 10.1007/s11270-011-1048-6

D. Zhu :Y. Wu :N. WuChengdu Institute of Biology, Chinese Academy of Sciences,Chengdu 610041, China

D. Zhue-mail: zhudan_m@cib.ac.cn

D. Zhu :Y. Wu :N. WuKey Laboratory of Mountain Ecological Restoration andBio-resources Utilization, Chinese Academy of Sciences,Chengdu 610041, China

H. Chen :Q. ZhuLaboratory for Ecological Forecasting and Global Change,College of Forestry,Northwest Agriculture and Forest University,Yanglin 712100, China

H. Chen :Q. ZhuInstitute of Environment Sciences,University of Quebec at Montreal,Montreal C3H 3P8, Canada

N. Wu (*)International Centre for Integrated Mountain Development,Kathmandu, GPO Box 3226, Nepale-mail: wuning@cib.ac.cn

(Sobek et al. 2003). This could be attributed to the highexport rate of DOC from peatland to lake (Larsen et al.2011). However, most of studies monitoring CO2 fluxfrom peatland lake are limited in the northern and borealregions (Casper et al. 2000; Huttunen et al. 2002; Repoet al. 2007; Marchand et al. 2009; Schrier-Uijl et al.2011). The Zoige peatlands, which located in the easternTibetan Plateau, is approximately 3,500 km2 in areawith complex network of rivers and lakes (Bai et al.2008). The potential effects of terrestrial export of DOCon those lakes are still uncertain. In order to obtain theprimary knowledge about the carbon cycle of thoselakes, CO2 flux from lakes and its controlling factorsare urgently needed to be monitored.

In this paper, a small, shallow lake in Zoige peat-lands was chosen to study: (1) CO2 flux from lake andits temporal pattern and (2) relationship between CO2

and its controlling factors.

2 Materials and Methods

2.1 Lake Description

Lake Medo was chosen for this study, a small, shallowlake (1.6 km2 in area and maximal depth<1 m) inZoige National Wetland Reserve (33°6′ N, 102°2′ E,3,430 ma.s.l.). The region is characterized by coldQinghai-Tibetan climatic conditions with average an-nual precipitation 650 mm and temperature 1.7°C. Thewarmest monthly temperature is 9.1–11.4°C in July.Mean annual wind speed is 2.4 ms−1.

The catchment of Lake Medo is mainly coveredwith peatland (30%) and meadow (70%), estimatedfrom a remote sensing imagery. There are two visibleinlets entering the Lake Medo in the east and onevisible outlet in the west (Fig. 1). Chemical and phys-ical characteristics of water and sediment in LakeMedo are listed in Table 1. The dominant submergedplants in Lake Medo are Ceratophyllum demersumand Potamogeton pectinatus, which grew mixed andpatch-like. They occupied peripheral zone of the lakebottom and the central zone was almost soft sediment.

2.2 Sampling Plots and Gas Flux Measurement

Three parallel transects were set from upper to down-stream in central zone of the lake, each transect withfive plots (distance between plots <5 m). Gas fluxes

were sampled between 23 May to 23 August 2009with intervals between 15 and 20 days.

CO2 across the air–water interface were collectedwith floating chambers. Mean annual wind speed of thisregion is lower than 3 ms−1, therefore floating chambermethod is a feasible method to quantify gas fluxes fromlakes (Soumis et al. 2004; Cole et al. 2010). Chambers(40 cm in height and 30 cm in diameter) were made ofclear polycarbonate pipe with a close end, equipped witha small lateral vent stopped by silicon septum for sam-pling. When the measurement began, the chamber wasplaced on a buoy and the chamber wall extended 5 cminto the water. The buoy was fastened to an anchor fixedin the sediment. Four gas samples from the chamber airheadspace were taken into 5 ml airtight vacuumed vialsat 0, 5, 10, and 15 min after deployment. All sampleswere transported to the laboratory in a cool box.

The CO2 concentration was determined by a gaschromatography (PE Clarus 500, PerkinElmer, Inc.,USA), equipped with a flame ionization detector operat-ing at 350°C and a 2 m Porapak 80–100 Q Column. Thecolumn oven temperature was 35°C and the carrier gaswas N2 with a flow rate of 20 cm3 min–1. The minimumdetectable concentration was 1×10−3 μl L–1 (ppb). Cer-tified CO2 standard in 303 μl L–1 (China National Re-search Center for Certified ReferenceMaterials, Beijing)was used for calibration. We used linear regression anal-ysis on every group of four gas samples from eachchamber, and only three groups with the highest corre-lation (R>0.9) were used in flux calculation.

The flux J (mg CO2m−2 h−1) of CO2 was calculated

as:

J ¼ dc

dt� MV0

� P

P0� T0T

� H

Where dc/dt is the rate of concentration change; Mis the molar mass of CO2; P is the atmosphere pressureof the sampling site; T is the absolute temperature ofthe sampling time; V0, P0, T0 is the molar volume,atmosphere pressure, and absolute temperature, re-spectively, under the standard condition; H is thechamber height over the water surface.

2.3 Water and Sediment Sampling and Analysis

Lake water and sediment were sampled from the sameplots during every sampling campaign. Lake waterand 10 cm sediment were collected in polyethylenebottles. After collection, samples of lake water were

2564 Water Air Soil Pollut (2012) 223:2563–2569

filtered through glass-fiber syringe filter (WhatmanGD/X, 0.45 μm). All samples were kept at 4°C untilfurther processing. Samples of sediment were centri-fuged at 12,880×g for 10 min. The supernatant aliquotwas used to measure DOC and dissolved nitrogen(DN) concentration. DOC and DN concentration wasdetermined by a total organic carbon analyzer (multiN/C 2100, Analytik Jena AG, Jena, Germany).

Lake water samples for fluorescence analysis wereroom-tempered (20°C) determined. Fluorescence wasmeasured on a scanning spectrofluorometer (HitachiF-7000), followed the method described by McKnightet al. (2001). The fluorescence index (FI) was calculatedas the ratio of the emission of 450:500 nm at excitationat 370 nm.

Chlorophyll a (Chl a) stands for net primary pro-duction (NPP) of phytoplankton was determined bythe method from Jespersen and Christoffersen (1987).Total phosphorus (TP) content in water was deter-mined by UV–VIS spectrophotometer (PerkinElmer

Fig. 1 Location and satellitic image of Lake Medo (the yellow line encircles open water area)

Table 1 Chemical and physical characteristics of water andsediment in Lake Medo

Parameter Valuec

Lake watera

Depth 25±13 cm

pH 7.5±0.5

Dissolved nitrogen 0.82±0.51 mg L−1

Total phosphorus 6.27±6.37 μg L−1

Sedimentb

pH 7.2±0.1

Organic carbon 331.2±34.76 mg kg−1

Organic nitrogen 30.05±1.97 mg kg−1

Organic phosphorus 0.65±0.04 mg kg−1

a Lake water data used samples between 23 May and 23 Augustin 2009b Sediment data used samples in 23 August 2009c Data used in this table are means ± standard deviation

Water Air Soil Pollut (2012) 223:2563–2569 2565

Lambda 35), after H2SO4–HClO4 digestion (Saarnioand Silvola 1999).

2.4 Measurements of Physical Factors

A portable digital meter (EcoScan pH6, Eutech Instru-ments Pte Ltd, Singapore) was used to measure tem-peratures. Lake water temperatures at 10 cm, sedimentsurface temperatures, and sediment temperatures at10 cm were manually recorded. Lake water depthswere recorded with a ruler.

2.5 Statistical Analysis

Analyses of variance, in which the sampling campaignwas treated as the independent variable, were used totest the differences of gas fluxes. Linear regressionwas carried out between instantaneous gas fluxes andenvironmental factors. The effect of a certain variablewas considered significant for P<0.05 and highlysignificant for P<0.01. All statistical analyses werecompleted with SPSS 11.0.

Sampling date

CO

2 f

lux,

mg

CO

2 m

-2 h

-1

-1000

-500

0

500

1000

1500

2000

5.23 6.03 6.28 7.18 8.08 8.23

Fig. 2 Temporal variation of CO2 fluxes from Lake Medo

Sampling date

WD

OC

co

ncen

trat

ion,

mg

C L

-1

0

20

40

60

SDO

Cco

ncen

trat

ion,

mg

C L

-1

12

18

24

30

36

Chl

aco

ncen

trat

ion,

ug L

-1

0

4

8

12

Sedi

men

t te

mpe

ratu

re, ¡

æ

10

15

20

25

a

b

c

d

5.23 6.08 6.28 7.18 8.08 8.23

Fig. 3 Temporal variationsof WDOC (a), SDOC (b),Chl a (c), and sedimenttemperature (10 cm; d)

Sampling date

FI

1.45

1.50

1.55

1.60

1.65

1.70

WDOCSDOC

5.23 6.08 6.28 7.18 8.08 8.23

Fig. 4 Temporal variations of FI of both WDOC and SDOC

2566 Water Air Soil Pollut (2012) 223:2563–2569

3 Results

3.1 Temporal Variation of CO2 Flux andEnvironmental Factors

Temporal variation of CO2 flux was significant (P<0.01).The CO2 flux rates reached their peaks in the spring–summer transition (1,131.14±677.55 mg CO2m

−2 h−1),and decreasing gradually to the lowest points in thesummer–autumn transition (−126.92±371.63 mg CO2

m−2 h−1; Fig. 2).WDOC concentrations ranged from 8.84 to

58.11 mg CL−1, with its peak values in spring–sum-mer transition and declining through the samplingperiod, SDOC concentrations ranged from 13.16 to32.96 mg CL−1, Chl a concentrations ranged from0.08 to 16.23 μg L−1, sediment temperatures (10 cm)ranged from 11°C to 22°C, with their peak values all

in mid-summer (Fig. 3a–d). FI of WDOC and SDOCranged from 1.46 to 1.64 and 1.46 to 1.69, respectively.However, the values ofWDOCwere lower than those ofSDOC in all sampling dates (Fig. 4).

3.2 Relationships Between CO2 Flux and Variables

Regression analysis showed that CO2 fluxes werehighly correlated to WDOC (R00.47, P<0.01, n054;Fig. 5). However, no significant correlation was foundbetween CO2 fluxes and Chl a concentrations. Be-sides, neither SDOC nor sediment temperature wassignificantly correlated to CO2 fluxes in this study.

4 Discussion

Small peatland lakes are thought to have high CO2

flux among lakes (Hope et al. 1996). In the presentstudy, CO2 flux rate from Lake Medo was 488.63±1,036.17 mg CO2m

−2 h−1 that was much higher thanthose of other small peatland lakes, and even lakeswith smaller area than Lake Medo (summarized inTable 2). We primarily considered Lake Medo a “hot-spot” of CO2 flux among lakes have been monitored.In addition, more data on other small peatland lakes inother alpine regions of the world are needed in thefuture.

The concentrations of WDOC in Lake Medo werehigher than most other studies reported (Fig. 3a)(Algesten et al. 2004; Sobek et al. 2003). The exportrates of DOC from peatlands were high compared tomost other terrestrial ecosystems due to their high

WDOC concentration, mg C L-10 10 20 30 40 50 60 70

CO

2 f

lux,

mg

CO

2 m

-2 h

-1

-2000

-1000

0

1000

2000

3000

4000

Y=42.86X-381.984 R=0.47, P<0.01

Fig. 5 Relationship between CO2 fluxes and WDOCconcentrations

Table 2 Comparison of CO2 fluxes from Lake Medo and other peatland lakes

Lake Climate zone Area (km2) CO2 flux (mg CO2 m−2 h−1) Reference

Priest Pot Temperate 0.01 73.33a Casper et al. 2000

Jänkäläisenlampi Boreal 0.01 22.00 Huttunen et al. 2002

Mistumis Boreal 4.00 78.07b Marchand et al. 2009

Schutsloterwiede Temperate 1.41 80.00c Schrier-Uijl et al. 2011

MT Subarctic 0.005 66.67d Repo et al. 2007

Medo Alpine 1.60 488.63 This study

a Calculated from millimoles of CO2 per square meter per dayb Calculated from milligrams of C per square meter per dayc Estimated from referenced Calculated from grams of CO2 per square meter per day

Water Air Soil Pollut (2012) 223:2563–2569 2567

concentrations of DOC (Aitkenhead and McDowell2000; Mattsson et al. 2005). In this study, the coverageof peatlands was high in the catchment of Lake Medo.Besides, McKnight et al. (2001) suggested that a lowFI of DOC (1.40<FI<1.65) indicate its terrestrial origin.In this study, low FI of WDOC showed that the domi-nance of terrestrial origin DOC in Lake Medo (Fig. 4).Therefore, we suggested that DOC in Lake Medo waslargely loaded from its adjacent terrestrial ecosystems,especially the peatlands which are large in area.

Regression analysis showed that CO2 fluxes werehighly correlated to WDOC (R00.47, P<0.01, n054;Fig. 5). Many studies have documented that heterotro-phic respiration in the lakes is dependent on DOCconcentration (Hanson et al. 2003; Lennon 2004;Jonsson et al. 2003). Additionally, large datasets haveshowed that the concentration of DOC could explainsignificant variation in lake pCO2 (Sobek et al. 2005).In this study, high WDOC content load from adjacentpeatlands has supported large CO2 evasion from LakeMedo. Temporally, CO2 fluxes from Lake Medo de-clined with decreasing concentration of terrestrialDOC. These results support the hypothesis that terrestrialcarbon subsidies increase CO2 flux from lake.

Actually, we found temporal pattern of CO2 fluxesfrom Lake Medo was similar to that of Lake Donghuwhich in subtropical zone, though the key factor wasnot DOC but NPP of phytoplankton in Lake Donghu(Xing et al. 2005). In Lake Medo, seasonal Chl aconcentrations indicated relative low productivity ofphytoplankton (Fig. 3c), while high DOC input fromadjacent peatlands could largely subsidized heterotro-phic processes (Del Giorgio et al. 1999). In lakes ofnorthern and boreal regions, phytoplankton communi-ty plays a less important role of CO2 exchange be-tween lake and atmosphere as well, especially lakeswith medium or high DOC concentrations (Jonsson etal. 2001; Hanson et al. 2003).

In our study, neither SDOC nor sediment tempera-ture was correlated to CO2 fluxes. The relative contri-bution of sediment respiration (SR) to ecosystemrespiration (ER) was highly variable from 2% to44% according to other studies reported (den Heyerand Kalff 1998; Kortelainen et al. 2006; Sand-Jensenand Staehr 2009). The ratio of SR/ER was observed tocorrelate positively to TP in water column, whilenegatively to allochthonous DOC input (Algesten etal. 2005; Pace and Prairie 2005; Liboriussen et al.2011). Therefore, SR was not the major contributor

to the whole lake CO2 evasion due to the high terrestrialDOC input and low TP concentrations (Table 1) inLake Medo.

5 Conclusion

In warm season, CO2 flux rate from Lake Medo was488.63±1,036.17 mg CO2m

−2 h−1. This result wasmuch higher than other studies reported. Temporalvariation of CO2 flux was significant, with the peakvalue in the beginning and lowest point in the end ofwarm season. DOC concentrations in Lake Medo werehigh, and FI of WDOC showed its terrestrial origincharacteristic. WDOC was found to correlate positivelyto CO2 flux. We conclude that large area of peatlands incatchments support high concentration of DOC in thislake, and consequently high CO2 evasion.

Acknowledgments This study was financially supported byChinese Academy of Sciences (Y1B2021100), National NaturalScience Foundation of China (30972345, 40971178, 31100348).The Administrative Bureau ofWetlandNational Nature Reserve ofZoige is thankful for the logistic assistance and guide in Zoige. Wemust give personal thanks to Hua Li and Ming Zhang for theirsuggestions and logistic arrangement on our field measurements.The technical assistance of Yufei Li in laboratory is much appre-ciated. Yuyuan Wu, Fangmiao Lin, and Xianshu Liu gave muchhelp on laboratory analyses. Ms. Wan Xiong, an expert for ESP, isthanked for her great and patient help in our writing and reasoning.The anonymous reviewer was thanked for her or his detailedevaluation and constructive suggestion on our manuscript.

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