selected presentation from the instaar monday noon seminar...
TRANSCRIPT
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Selected Presentation from the INSTAAR Monday Noon Seminar Series.
Institute of Arctic and Alpine Research, University of Colorado at Boulder.http://instaar.colorado.edu
http://instaar.colorado.edu/other/seminar_mon_presentations
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09 Sep. 2002 Diane McKnight et al., INSTAAR, Email: [email protected]“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).
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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
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Sources of N
Colorado Front Range
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N emission trends
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Diatom changes in Sediment Core
1950-
1970
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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
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Niwot Ridge LTER site
Green Lake 4
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Green Lake 4: View southwest towards Arikaree Peak
(4008 m) from the shore near the outlet in late September.
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Green Lake 4: Alpine lake surrounded by
talus, ice-covered from October to June
Green Lake 4
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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
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Increases in N deposition and increases
in annual minimum nitrate
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Decrease in ice cover thickness in
late March: more snow on ice?
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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
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Shallow Mountain Lake
Benthos
Pelagic Zone
....
...
.. .. ..
..
.. . .
.....
...
.. ..
Phytoplankton
Epipelic
Algae Epilithic
AlgaeSediment
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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
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Green Lake 4: sampling sites
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Spring Bloom:
Ice cover breaks up
Phytoplankton grow
Diatoms and chlorophytes
are abundant
Nitrate decreases
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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
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GL4 sediment core dated by 210Pb
activity: 9 cm~1940
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Recent sediments: more OM and diatom
pigments, more microbial source for OM
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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
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Recent sediments: more benthic diatoms(Fragilaria pseudoconstruens, F.brevistrata)
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Recent sediments: different planktonic
diatoms (less Aulacoseira distans, A.
periglabra, A. lirata, more A. ambigua)
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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
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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
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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
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Our mesocosms
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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
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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
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Water Clarity
N Treatment
Control (with plastic)
N x P Treatment
P treatment
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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%
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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
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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)
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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