HIGH-MOUNTAIN LAKES AS A HOT SPOT FOR PRODUCTION OF DISSOLVED ORGANIC 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 indicators Organisms on edge of environmental tolerance Same processes as downstream forested and grassland ecosystems Less capacity! Less “buffering” Snow: moderates soil temperature, stores water and chemical, released at once

ABER SPAGHETTI DIAGRAM

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

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

NIWOT RIDGE NADP

Changes 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 DEPOSITION CAUSING CHANGES IN ECOSYSTEMS? WHAT KIND OF CHANGES SHOULD WE BE LOOKING FOR? WHERE WILL WE SEE THOSE CHANGES FIRST?

APPROACH Carbon and nitrogen cycling in Alpine/Subalpine ecosystem Carbon and nitrogen cycling in Alpine/Subalpine ecosystem –Characterize DOM using fractionation and fluorescence techniques –Understand temporal and longitudinal changes in the reactivity and source of DOM –Ecological importance of DOM

DOM BACKGROUND –DOC in aquatic ecosystems Heterogeneous class of substancesHeterogeneous class of substances Energy for heterotrophic growthEnergy for heterotrophic growth Metal complexation and light absorbanceMetal complexation and light absorbance

Albion Green Lake 4 Saddle Stream Como Creek

METHODS Fractionation Fractionation –Chromatographic separation based on hydrophobicity –Separation of hydrophobic acids (fulvic acids) from hydrophilic acids and low molecular weight compounds –Provides information on reactivity and elemental content

COLUMN FRACTIONATION Chromatographic separation Chromatographic separation Isolate hydrophobic acids (fulvic acids) from hydrophilic acids and low molecular weight compounds Isolate hydrophobic acids (fulvic acids) from hydrophilic acids and low molecular weight compounds

DOC Concentrations DOC (mg/L) Discharge (m 3 /day)

DOC Concentrations DOC (mg/L)

Percent Fulvic Acid Discharge (m 3 /day) % Fulvic Acid

Percent Fulvic Acid % Fulvic Acid

DOM Fractions: N export Red = non-humic green = fulvic

DOM Fractions: N content (June)

DOM fractions: 15N Isotopes C:N Ratio  15 N

DOM Fractions: Aromatic carbon C:N Ratio Aromatic C (%)

FRACTIONATION SUMMARY Fulvic acids highest on rising limb Fulvic acids decrease on recession limb Greater non-humic in alpine lakes DOM has more N in alpine lakes Higher the C:N in DOM, the more recalcitrant it appears to be

METHODS: Fluorescence Fluorescence Fluorescence –All humic substances fluoresce –At least 2 main fluorophores –Provides information on precursor organic material of fulvic acids Excitation emission matrices (EEMS) different for microbial vs terrestrial DOCExcitation emission matrices (EEMS) different for microbial vs terrestrial DOC

Fluorescence Endmembers

Fluorescence Index Ratio of 450 /500 nm emission at 370 nm excitation Ratio of 450 /500 nm emission at 370 nm excitation Simple interpretive tool Simple interpretive tool End-member mixing provides qualitative assessment of sources End-member mixing provides qualitative assessment of sources Fluorescence Index

MaySample May Sample JulySample July Sample SeptemberSample September Sample Fluorescence Index

Source and Quality September July May Fluorescence Index % Fulvic Acid

Fluorescence Index

AQUATIC vs TERRESTRIAL PRODUCTION

AQUATIC vs TERRESTRIAL YIELDS: 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 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 LARGER AREA Sediment cores in GL4 Sediment cores from other lakes DOC and mercury in alpine lakes

GL4 sediment core dated by 210 Pb activity: 9 cm~1940

Recent sediments: more OM and diatom pigments, more microbial 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

Changes in Green Lake 4 since about 1940 Changes in Green Lake 4 since about 1940 Associated with nitrogen enrichment and climatic changes More growth of benthic diatoms, change in lake ecosystem structure More accumulation of sediment organic matter Shifts in the dominant diatom species in the water column

NUTRIENT ENRICHMENT PRISTINE LAKE

NUTRIENT ENRICHMENT ENHANCES Hg IN LAKES

Mercury and DOC in Streamwaters Mast (unpublished), 2001

Even with controls on N deposition, climate change may enhance 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

SUMMARY Fractionation Fractionation –Quality of dissolved organic carbon Fluorescence Fluorescence –Source of source of dissolved organic material Insight into ecological controls on DOM Insight into ecological controls on DOM Climate change will enhance lake DOM Climate change will enhance lake DOM