Rachel H. R. Stanley 1 , Zoe O. Sandwith 1 , Bill J. Williams 2
description
Transcript of Rachel H. R. Stanley 1 , Zoe O. Sandwith 1 , Bill J. Williams 2
Rates of Summertime Biological Productivity in the Beaufort Gyre:
A Comparison between the Record-Low Ice Conditions of August 2012 and Typical
Conditions of August 2011
Rachel H. R. Stanley1, Zoe O. Sandwith1, Bill J. Williams2
1Woods Hole Oceanographic Institution, USA2 Institute of Ocean Sciences, Canada
Motivation1) Few rates of primary production have been measured in the
Arctic Ocean
2) How do rates of production depend on sea ice coverage?
Image from National
Snow and Ice Data Center
1978 1993 2011 year
8
10
12
Ice E
xte
nt
(10
6 k
m2)
Motivation1) Few rates of primary production have been measured in the
Arctic Ocean
2) How do rates of production depend on sea ice coverage?
– 2012 had record-low sea ice extent. 2011 was more typical.
Image from NASA website
Yellow = typical minimum sea ice area
Red = study area
2012 Sea Ice Extent
Motivation1) Few rates of primary production have been measured in the
Arctic Ocean
2) How do rates of production depend on sea ice coverage?
– 2012 had record-low sea ice extent. 2011 was more typical.
Image from NASA website
2012 Sea Ice Extent
We used gas tracers to determine rates of NCP and GPP in mixed layer
in Beaufort Gyre in Summer 2011 and 2012
Why measure NCP and GPP?• Advantages to concurrent measurements of NCP and GPP
– If NCP is low, is it because of low photosynthesis or tightly matched photosynthesis and respiration?
– Determine NCP/GPP ratio, analagous to “f-ratio”, a measure of efficiency of biological pump
Gas Tracers for NCP and GPP
To calculate NCP:
•d(O2/Ar)dt = NCP – gas exchange
– Correct for thermal effects by using O2/Argon
– Assume steady state and calculate gas exchange
Gas Tracers for NCP and GPP
To calculate GPP:
•Triple Oxygen Isotopes (16O, 17O, 18O)
•Photosynthetic O2: Mass dependentAir: Mass independent
•17, a measure of the three isotopes, does not change with respiration
NCP in Beaufort Gyre: 2011 vs. 2012• Rates of mixed layer NCP similar between the two years
GPP in Beaufort Gyre: 2011 vs. 2012• Rates of mixed layer GPP doubled in the record low sea-ice
year of 2012
2011 to 2012 Comparison• Histogram of mixed layer rates of NCP and GPP
– Similar distribution for NCP in 2011 and 2012
– Broader range, higher rates for GPP in 2012
2011 to 2012 Comparison• Histogram of mixed layer rates of NCP and GPP
– Similar distribution for NCP in 2011 and 2012
– Broader range, higher rates for GPP in 2012
What is Happening?• Higher photosynthetic rates in
response to less ice (more light?) but heterotrophs keep up with increase in production
• Result: No change in net carbon uptake
What is Happening?• Higher photosynthetic rates in
response to less ice (more light?) but heterotrophs keep up with increase in production
• Result: No change in net carbon uptake
What is Happening?• Higher photosynthetic rates in
response to less ice (more light?) but heterotrophs keep up with increase in production
• Result: No change in net carbon uptake
• Supporting Data: No rates of respiration were measured but slightly higher bacterial abundances observed in 2012 at the stations that were no longer ice covered (Bill Li, DFO, Canada)
Data from Bill Li, DFO
Rates at Depth?
• NCP and GPP rates only measured in the mixed layer
• In the Beaufort Gyre, deep chlorophyll maximum observed and production at depth likely is important
• Can roughly scale up mixed layer numbers to whole euphotic zone using depth relations determined from ARCSS-PP Database
Figure from Arrigo and Dijken 2011
Primary Production (mg C m-3 d-1)0 60 120
0
50
100D
epth
(m
)
Effect of ice coverage within 2011
• Within the 2011 cruise, regions where higher total ice concentration had
– Larger GPP
– Similar NCP
– Smaller N:G Ratio
Effect of ice coverage within 2011
• Within the 2011 cruise, relationships of GPP and NCP/GPP with ice cover are significant
• NCP vs. ice cover not significant
• Lots of scatter but implication is that actively melting ice increases GPP but no change in NCP
Conclusions
• Summer with record low ice (2012) has higher GPP but similar NCP to more typical summer (2011)
• Implication is that as ice continues to decrease, GPP may rise in Beaufort Gyre but NCP may not change no net change in carbon uptake by biological production
• Does this occur elsewhere in Arctic? At other times during the year? Time-series would be fantastic!
Acknowledgements• Thanks to
– Captain and crew of CCGS Louis S. St-Laurent
– Sarah Zimmermann and rest of JOIS/BGOS Team
– National Science Foundation, Fisheries and Ocean Canada, WHOI Climate Institute
Photo from JOIS Cruise Report
Equations• Equations used to calculate NCP and GPP
Measuring TOI• Measurements are hard to make - cannot be made at sea!
• Processing line to remove nitrogen, then measure oxygen isotopes on isotope ratio mass spectrometer
• Precision in my lab = 4 per meg
Method: Triple Oxygen Isotopes
• Standard is air so by definition 17 of air = 0
• Respiration does not change 17• Photosynthesis has 17 = 250 per meg (as determined from
terrarium experiments by Luz et al (1999), Luz and Barkan (2005) )
Modified after Luz and Barkan,2005
Mass Dependent Fractionation
ln(
17O
/1000+
1)
ln(18O/1000+1)0
Respiration Slope = 0.52
Stra
tosp
here
Slo
pe =
1.7
MASSINDEPENDENT
17 = ln(17O/1000+1)-0.52*ln(18O/1000+1) x106
Method: Triple Oxygen Isotopes
• Standard is air so by definition 17 of air = 0
• Respiration does not change 17• Photosynthesis has 17 = 250 per meg (as determined from
terrarium experiments by Luz et al (1999), Luz and Barkan (2005) )
Modified after Luz and Barkan,2005
ln(
17O
/1000+
1)
ln(18O/1000+1)0
Respiration Slope = 0.52
17 = ln(17O/1000+1)-0.52*ln(18O/1000+1) x106
17
17Air=0
17Photo =250
Vocabulary Primer• Gross Primary Production (GPP): Total photosynthetic flux. Base of food
chain.
• Net Primary Production: Photosynthesis minus autotrophic respiration
• Net Community Production (NCP): Photosynthesis minus autotrophic and heterotrophic respiration
• New Production: Fueled by input of new nutrients into euphotic zone
• Export Production: Flux of organic matter that leaves euphotic zone
Over long temporal and spatial scales, NCP, New, and Export Production should be approximately equal.
Why NCP and GPP?
• asdasd