HIGH-MOUNTAIN LAKESAS A HOT SPOT FOR

PRODUCTION OF DISSOLVEDORGANIC MATTER IN A

CHANGING CLIMATE

Mark Williams, Diane McKnight,Eran Hood and Dave Manthorn

PROBLEM:N DEPOSITION INCREASES

MOUNTAINS AT RISK

Alpine areas: early warning Alpine areas: early warning indicatorsindicatorsOrganisms on edge of environmental

toleranceSame processes as downstream

forested and grassland ecosystemsLess capacity! Less “buffering”Snow: moderates soil temperature,

stores water and chemical, released at once

ABER SPAGHETTI DIAGRAM

Green Lake 4: View southwest towards Green Lake 4: View southwest towards Arikaree Peak (4008 m) from the shore Arikaree Peak (4008 m) from the shore near the outlet in late September.near the outlet in late September.

Decrease in ice cover thickness Decrease in ice cover thickness in late March: more snow on in late March: more snow on ice?ice?

NIWOT RIDGE NADP

Changes in Rocky Mountain lakesChanges in Rocky Mountain lakes

Increased atmospheric deposition of nitrate from agricultural use of fertilizers and urban development- upslope winds bring pollution to alpine environments

Observed changes in climate include decrease in ice-cover thickness in late March, may be related to delay in winter snowfall

IS INCREASING N DEPOSITIONCAUSING CHANGES IN ECOSYSTEMS?

WHAT KIND OF CHANGES SHOULDWE BE LOOKING FOR?

WHERE WILL WE SEE THOSECHANGES FIRST?

APPROACHAPPROACH

Carbon and nitrogen cycling in Carbon and nitrogen cycling in Alpine/Subalpine ecosystemAlpine/Subalpine ecosystem– Characterize DOM using fractionation and Characterize DOM using fractionation and

fluorescence techniquesfluorescence techniques– Understand temporal and longitudinal Understand temporal and longitudinal

changes in the reactivity and source of DOMchanges in the reactivity and source of DOM– Ecological importance of DOMEcological importance of DOM

DOM BACKGROUNDDOM BACKGROUND

– DOC in aquatic ecosystemsDOC in aquatic ecosystems

• Heterogeneous class of substancesHeterogeneous class of substances

• Energy for heterotrophic growthEnergy for heterotrophic growth

• Metal complexation and light Metal complexation and light absorbanceabsorbance

AlbionAlbion

Green Lake 4Green Lake 4

Saddle StreamSaddle Stream

Como CreekComo Creek

METHODSMETHODS

FractionationFractionation– Chromatographic separation based on Chromatographic separation based on

hydrophobicityhydrophobicity– Separation of hydrophobic acids (fulvic acids) Separation of hydrophobic acids (fulvic acids)

from hydrophilic acids and low molecular weight from hydrophilic acids and low molecular weight compoundscompounds

– Provides information on reactivity and elemental Provides information on reactivity and elemental contentcontent

COLUMN FRACTIONATIONCOLUMN FRACTIONATION

Chromatographic Chromatographic separationseparation

Isolate hydrophobic Isolate hydrophobic acids (fulvic acids) acids (fulvic acids) from hydrophilic from hydrophilic acids and low acids and low molecular weight molecular weight compoundscompounds

DOC ConcentrationsDOC Concentrations

0

2

4

6

8

2-May 21-Jun 10-Aug 29-Sep

0

20000

40000

60000

GREEN LAKE 4 (GL4)

DO

C (

mg/

L)

Dis

char

ge (

m3 /

day

)

DOC ConcentrationsDOC Concentrations

0

2

4

6

8

10

2-May 21-Jun 10-Aug 29-Sep

GL4

Albion

SLPI

CCMRS

DO

C (

mg/

L)

Percent Fulvic AcidPercent Fulvic Acid

20

30

40

50

60

70

80

2-May 21-Jun 10-Aug 29-Sep

0

20000

40000

60000

GREEN LAKE 4

Dis

char

ge (

m3 /

day

)

% F

ulv

ic A

cid

Percent Fulvic AcidPercent Fulvic Acid

20

30

40

50

60

70

80

2-May 21-Jun 10-Aug 29-Sep

GL4

Albion

SLPI

CCMRS

% F

ulv

ic A

cid

DOM Fractions: N exportDOM Fractions: N exportGL4 - June

46%54%

SLP - June

44%56%

GL4 - September

73%

27%

SLP - September

58%42%

Red = non-humic

green = fulvic

DOM Fractions: N content (June)DOM Fractions: N content (June)

Fulvic Acid Hydrophilic Acid

N Content (%) C:N Ratio N Content (%) C:N Ratio

GL4 2.2 27.8 2.8 18.3SLP 1.2 52.8 1.7 34.3

DOM fractions: 15N IsotopesDOM fractions: 15N Isotopes

C:N Ratio

15N

0

1

2

3

0 20 40 60

DOM Fractions: Aromatic carbonDOM Fractions: Aromatic carbon

0

10

20

30

0 20 40 60

C:N Ratio

Aro

mat

ic C

(%

)

FRACTIONATION SUMMARYFRACTIONATION SUMMARY

Fulvic acids highest on rising limbFulvic acids decrease on recession limbGreater non-humic in alpine lakesDOM has more N in alpine lakesHigher the C:N in DOM, the more

recalcitrant it appears to be

METHODS: FluorescenceMETHODS: Fluorescence

FluorescenceFluorescence– All humic substances fluoresceAll humic substances fluoresce– At least 2 main fluorophoresAt least 2 main fluorophores– Provides information on precursor organic Provides information on precursor organic

material of fulvic acidsmaterial of fulvic acids• Excitation emission matrices (EEMS) Excitation emission matrices (EEMS)

different for microbial vs terrestrial DOCdifferent for microbial vs terrestrial DOC

Fluorescence EndmembersFluorescence Endmembers

Emission Wavelength (nm)

Suwannee River, GA

250

300

350

400

400 450 500 550

Excitation W

avele

ngth

(nm

)

Fluorescence EndmembersFluorescence Endmembers

Emission Wavelength (nm)

Suwannee River, GA

250

300

350

400

400 450 500 550

Exc

itatio

n W

ave

length

(nm

)

Emission Wavelength (nm)

Lake Fryxell, Antarctica

250

300

350

400

400 450 500 550

Exc

itatio

n W

ave

length

(nm

)

Fluorescence IndexFluorescence Index

Ratio of 450 /500 nm Ratio of 450 /500 nm emission at 370 nm emission at 370 nm excitationexcitation

Simple interpretive toolSimple interpretive tool End-member mixing End-member mixing

provides qualitative provides qualitative assessment of sourcesassessment of sources

1

1.2

1.4

1.6

1.8

2

Suwannee River Lake FryxellF

l uo r

esc e

nce

Inde

x

Fluorescence IndexFluorescence Index

1.2

1.3

1.4

1.5

1.6

1.7

1.8

6-May 25-Jun 14-Aug 3-Oct

Lake Fryxell

Suwannee River

GREEN LAKE 4MayMay SampleSample

JulyJuly SampleSample

SeptemberSeptember SampleSample

Flu

ores

cen

ce I

nd

ex

Source and QualitySource and Quality

1.2

1.3

1.4

1.5

1.6

1.7

1.8

6-May 25-Jun 14-Aug 3-Oct

20

30

40

50

60

70

80

GREEN LAKE 4

SeptemberSeptemberJulyJuly

MayMay

Flu

o res

cen

c e I

nd

ex

% F

ulv

ic A

c id

Fluorescence IndexFluorescence Index

1.2

1.3

1.4

1.5

1.6

1.7

1.8

6-May 25-Jun 14-Aug 3-Oct

GREEN LAKE 4

SILVER LAKE INLET

COMO CREEK

Flu

ores

cen

ce I

nd

ex

AQUATIC vsTERRESTRIALPRODUCTION

AQUATIC vs TERRESTRIAL AQUATIC vs TERRESTRIAL YIELDS: AREAL BASISYIELDS: AREAL BASIS

Alpine lakes produced 14x more DOC than surrounding terrestrial ecoysystem

Treeline lakes produced 5x more DOC than surrounding terrestrial ecosystem

Lake area is 4-7% but produces 26-40% of catchment DOC yield

Lake areas are hotspots of DOC production

FLUORESCENCE SUMMARYFLUORESCENCE SUMMARY

Terrestrial production main source of DOM during snowmelt

Terrestrial sources decrease on recession limb

On recession limb, alpine lakes have greater proportion of aquatic sources

Aquatic lakes produce more non-humic (labile) DOM than terrestrial ecosystems

LONGER TIMEPERIOD AND LONGER TIMEPERIOD AND LARGER AREALARGER AREASediment cores in GL4Sediment cores from other lakesDOC and mercury in alpine lakes

GL4 sediment core dated by GL4 sediment core dated by 210210Pb activity: 9 cm~1940Pb activity: 9 cm~1940

Recent sediments: more OM Recent sediments: more OM and diatom pigments, more and diatom pigments, more microbial source for OMmicrobial source for OM

PCA shows that first axis explains 78.6% of variation

Sample scores for the first axis have increased steadily since 1940, corresponding to introduction of N fertilizer

ChangesChanges in Green Lake 4 since in Green Lake 4 since about 1940about 1940Associated with nitrogen enrichment

and climatic changesMore growth of benthic diatoms, change

in lake ecosystem structureMore accumulation of sediment organic

matterShifts in the dominant diatom species in

the water column

NUTRIENT ENRICHMENT NUTRIENT ENRICHMENT PRISTINE LAKEPRISTINE LAKE

NUTRIENT ENRICHMENT NUTRIENT ENRICHMENT ENHANCES Hg IN LAKESENHANCES Hg IN LAKES

Mercury and DOC in StreamwatersMercury and DOC in StreamwatersMast (unpublished), 2001

Even with controls on N Even with controls on N deposition, climate change may deposition, climate change may enhance trendenhance trend

Net effect: Longer ice-free period with greater N availability, more algal growth, continuing changes in algal species, increase in DOC in lakes and effect on water quality

Oblique aerial view west of lower Green Lakes Valley in late June

SUMMARYSUMMARY

Fractionation Fractionation – Quality of dissolved organic carbonQuality of dissolved organic carbon

FluorescenceFluorescence– Source of source of dissolved organic Source of source of dissolved organic

materialmaterial Insight into ecological controls on DOMInsight into ecological controls on DOMClimate change will enhance lake DOMClimate change will enhance lake DOM