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