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09 Sep. 2002 Diane McKnight et al., INSTAAR, Email: Diane.McKnight@Colorado.Edu“Ecological response to nitrogen deposition and climate change in alpine lakes in the Colorado Front Range".Seminar given at INSTAAR, University of Colorado. Copyright 2002 Diane McKnight. All Rights Reserved.McKnight presentation (1.3 Mb PDF).

ALGAL AND BIOGEOCHEMICAL

RESPONSES TO ENVIRONMENTAL

CHANGE IN AN ALPINE LAKE

Summer B. Waters, Diane M. McKnight,

Alexander P. Wolfe, Rolf Vinebrooke, Sean

Sunderman, and Meaghan O’Brien

Institute of Arctic and Alpine Research, University of Colorado

And results of Koren R. Nydick, CSU

Support from NSF-LTER program

Sources of N

Colorado Front Range

N emission trends

Diatom changes in Sediment Core

1950-

1970

Alternative Hypothesis

Input of P-rich windborne agricultural dust and

volatile organic compounds?

Phosphorus Fertilizer, S. Platte Basin, CO

0

200

400

600

800

1945 1955 1965 1975 1985 1995

kg

x 1

000 p

er

yr

missing

data

Niwot Ridge LTER site

Green Lake 4

Green Lake 4: View southwest towards Arikaree Peak

(4008 m) from the shore near the outlet in late September.

Green Lake 4: Alpine lake surrounded by

talus, ice-covered from October to June

Green Lake 4

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

Increases in N deposition and increases

in annual minimum nitrate

Decrease in ice cover thickness in

late March: more snow on ice?

Potential Climate Change Effects

• Delay in formation of ice-cover (longer

autumn)

• Earlier break-up of the ice-cover in early

summer (longer summer)

• Warmer summer temperatures

• Net effect: Longer ice-free period with

greater nitrate availability

Shallow Mountain Lake

Benthos

Pelagic Zone

....

...

.. .. ..

..

.. . .

.....

...

.. ..

Phytoplankton

Epipelic

Algae Epilithic

AlgaeSediment

Paleolimnological approach to

understanding response of Green Lake 4

• Few previous samples of phytoplankton,

difficult to sample in winter and ice-out

• Monitored phytoplankton and water quality

during the summer at surface, 3 m and 9 m

• Deployed sediment trap during summer

• Collected sediment core from lake center

Green Lake 4: sampling sites

Spring Bloom:

Ice cover breaks up

Phytoplankton grow

Diatoms and chlorophytes

are abundant

Nitrate decreases

Phytoplankton species distribution

• Diatoms about 50% or more of biovolume

• Small coccoid algae (chlorophytes?)abundant

• Dominant diatom species: Aulacoseira distans,Fragilaria construens, F. crotonensis, Meridioncirculare

• High species diversity and variability withdepth and time

GL4 sediment core dated by 210Pb

activity: 9 cm~1940

Recent sediments: more OM and diatom

pigments, more microbial source for OM

Diatoms in lake sediments represent

species in water column and in benthos

• Diatom species richness: watercolumn<sediment trap<sediment core

• most diatom species in water column werealso in sediment trap and core

• most diatom species in sediment trap werealso in sediment core

• most diatom species in sediment core werenot found in water column

Recent sediments: more benthic diatoms(Fragilaria pseudoconstruens, F.brevistrata)

Recent sediments: different planktonic

diatoms (less Aulacoseira distans, A.

periglabra, A. lirata, more A. ambigua)

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

Why would increased N and thinner

ice cause more benthic diatoms?

• Benthic diatoms havegreater supply of Pfrom weathering insediments and canutilize increased N

• Benthic diatomsgrowing underthinning ice-coverare not flushed fromlake during snowmelt

2) Nutrient Enclosures-Snowy Range Lakes (Low N)

Lake: Frame w/out plastic

C: Control (no treatment)

N: NO3

P: PO4

NxP: NO3 + PO4

Phytoplankton

Epilithon (tiles)

Epipelon

2 week duration

2 experiments in 2 lakes

Treatments in Triplicate

Our mesocosms

2001 phyto chla

0

5

10

15

20

25

30

35

mg

m-3

0

5

10

15

20

25

30

35

17-Jul 24-Jul 31-Jul 7-Aug 14-Aug

mg

m-3

aaa

a

a

Snowy Range (low N)

RMNP (hi N)

a

NxAxP

N,

NxA

Lake,

Cont,

P

NxAxP

Lake, Cont, P,

N, NxA,

Phytoplankton Chlorophyll a

2001 tile chla

0

5

10

15

20

25

30

17-Jul 24-Jul 31-Jul 7-Aug 14-Aug

mg

m-2

0

5

10

15

20

25

30mg

m-2

Snowy Range (low N)

RMNP (hi N)

N

NxAxP

Epilithic (tile) Chlorophyll a

Water Clarity

N Treatment

Control (with plastic)

N x P Treatment

P treatment

NO3 uptake

NO3 Uptake

0

50

100

150

200

250

300

350

400

C N NA NAP P

mg

N p

er

Meso

co

sm

denitrificationepilithonsediment 0.5-5 cmepipelonphytoplankton

C N NA NAP P

Low NO3 Lake High NO3 Lake

20% 8% 12% 13% 26% 7% 7% 7% 13% 20%

Changes in Green Lake 4 since

about 1940

• Associated with nitrogen enrichment andclimatic changes

• More growth of benthic diatoms, changein lake ecosystem structure

• More accumulation of sediment organicmatter

• Shifts in the dominant diatom species inthe water column

Because ice cover limits light regime,

climate change may enhance trend

• Delay in ice-cover

formation (more algal

growth in autumn)

• Earlier ice-out (more

algal growth in

summer)

• Warmer summer

temperatures (more

algal growth)

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