How do Various Iron Sources Fuel
Phytoplankton Growth in the Southern
Ocean?
What Factors Control Phytoplankton
Abundance in Coastal Polynyas?
Kevin R. Arrigo
Gert van Dijken
Stanford University
1
Antarctic polynyas
Areas of open water
surrounded by sea ice
Can be identified with
SSM/I satellite data
Based on days of ice
cover
Red = Max
Blue = Min
Polynyas are blue
areas near coast
2
Antarctic polynyas
40
80
120
160
200
Hot spots of primary
production (g C m-2 yr-1)
3
Antarctic polynyas
Can support high
densities of marine
mammals and birds
Adelie Penguins vs. Polynya Productivity
Mean colony size (1000 pairs)
Hot spots of primary
production (g C m-2 yr-1)
Ann R2 = 0.63
Annual production (mg C m-2 yr-1)
Arrigo and Van Dijken (2003)
4
Antarctic polynyas
Can support high
densities of marine
mammals and birds
Seal Pups vs. Polynya Productivity
Weddell seal pups weaned
Hot spots of primary
production (g C m-2 yr-1)
R2 = 0.75
Annual production (mg C m-2 yr-1)
Paterson et al. (submitted)
5
Antarctic polynyas
Hot spots of primary
production (g C m-2 yr-1)
CO2 flux (g C m-2 yr-1)
Can support high
densities of marine
mammals and birds
Ross Sea
Large sinks for
atmospheric CO2
Arrigo et al. (2008)
6
Question:
What factors control phytoplankton abundance in Antarctic
coastal polynyas?
Factors investigated for each polynya:
Area of open water
Number of days of open water
Mean daily PAR during growing season
Sea surface temperature
Continental shelf width
Basal melt rate of nearby glaciers (Rignot
et al. 2013)
Photo:
John Weller
Phaeocystis antarctica
7
Approach
SeaWiFS
For 1998-2013:
Used satellite data to identify
polynyas, track changes in
sea ice, measure SST and
chlorophyll a concentration,
and calculate primaryAVHRR
production
SSM/I
8
Atlantic
Ocean
17 18
1516
14
13
12
11
9
19 20
21
22
23
24
25
26
27
28
10
8
7
6
29
ANTARCTICA
30
5
4
3
Pacific
Ocean
2
1
45
46
44
43
42
41
40
31
32
33
34
35
36
37
38
39
Indian
Ocean
9
Annual mean phytoplankton abundance in 46 coastal polynyas
Chlorophyll a (mg m-3)
2.5
Pacific
Atlantic
Indian
Pacific
2.0
1.5
1.0
0.5
0.0
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45
Polynya number
10
Annual mean phytoplankton abundance in 46 coastal polynyas
• Related to annual mean sea surface temperature
• Warmer temperatures promote higher Chl a
• Strongest relationship in Atlantic sector
Pacific
Atlantic
Indian
Pacific
0
-0.2
2.0
-0.4
1.5
-0.6
-0.8
1.0
-1.0
0.5
-1.2
Temperature (°C)
Chlorophyll a (mg m-3)
2.5
-1.4
0.0
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45
Polynya number
11
2.0
1.5
Pacific
Atlantic
2.5
R2 = 0.41
p < 0.01
2.0
1.5
1.0
Indian
Pacific
0.5
0
-0.2
0.0
-1.5
-1.0
-0.5
0.0
Temperature (°C)
-0.4
-0.6
-0.8
1.0
-1.0
0.5
-1.2
Temperature (°C)
Chlorophyll a (mg m-3)
2.5
Chlorophyll a (mg m-3)
Annual mean phytoplankton abundance in 46 coastal polynyas
-1.4
0.0
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45
Polynya number
12
What other factors control phytoplankton abundance in coastal
polynyas?
Chlorophyll a (mg m-3)
2.5
Pacific
Atlantic
Indian
Pacific
2.0
1.5
1.0
0.5
0.0
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45
Polynya number
13
What other factors control phytoplankton abundance in coastal
polynyas?
• Related to width of local continental shelf
• Relationship about equally strong in all sectors
Pacific
Atlantic
Indian
Pacific
700
600
2.0
500
1.5
400
300
1.0
200
0.5
100
Shelf width (km)
Chlorophyll a (mg m-3)
2.5
0
0.0
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45
Polynya number
14
2.0
1.5
Pacific
Atlantic
R2 = 0.40
p < 0.01
2.5
2.0
1.5
Indian
Pacific
1.0
700
600
0.5
500
0.0
0
200
400
600
Shelf width (km)
1.0
400
300
200
0.5
100
Shelf width (km)
Chlorophyll a (mg m-3)
2.5
Chlorophyll a (mg m-3)
What other factors control phytoplankton abundance in coastal
polynyas?
0
0.0
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45
Polynya number
15
Why the relationship between phytoplankton abundance and
continental shelf width?
•  Highest seawater iron concentrations are on the shelf
•  Wider shelves have larger iron inventory and permit greater
entrainment of iron into surface waters (Bruland et al. 2005, Biller et al. 2013)
•  More iron = higher phytoplankton biomass
Blue = Low iron concentration
Orange = High iron concentration
visibleearth.nasa.gov 16
Why the relationship between phytoplankton abundance and
continental shelf width?
A
Marsay et al. (2014)
nM
0
2
200
1.5
400
B
A
1.0
600
0.5
800
0
164°E
Best example is in
Ross Sea
Large continental
shelf
Depth (m)
B
168°E
172°E
176°E
180°E
Longitude
0
Gerringa et al. (submitted)
200
400
600
800
Blue
iron concentration
Low= Low
Fe near
RIS
Orange = High iron concentration
Ross Ice Shelf
Latitude
visibleearth.nasa.gov 17
What other factors control phytoplankton abundance in coastal
polynyas?
Chlorophyll a (mg m-3)
2.5
Pacific
Atlantic
Indian
Pacific
2.0
1.5
1.0
0.5
0.0
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45
Polynya number
18
What other factors control phytoplankton abundance in coastal
polynyas?
• 39 of 46 coastal polynyas are adjacent to melting ice shelves
Chlorophyll a (mg m-3)
2.5
Pacific
Atlantic
Indian
Pacific
2.0
1.5
1.0
0.5
0.0
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45
Polynya number
19
Chlorophyll a (mg m-3)
2.5
Pacific
Atlantic
Indian
Pacific
180
160
140
2.0
120
100
1.5
80
60
1.0
40
20
0.5
0
Basal melt rate (Gt yr-1)
What other factors control phytoplankton abundance in coastal
polynyas?
• 39 of 46 coastal polynyas are adjacent to melting ice shelves
• Phytoplankton abundance related to basal melt rate
• Relationship strongest in Pacific sector
0.0
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45
Polynya number
20
2.0
1.5
Pacific
Atlantic
2.5
R2 = 0.59
p < 0.01
2.0
1.5
Indian
Indian
Pacific
180
1.0
160
0.5
140
0.0
120
0
50
100
150
200
Basal melt rate (Gt yr-1)
100
80
60
1.0
40
20
0.5
0
Basal melt rate (Gt yr-1)
Chlorophyll a (mg m-3)
2.5
Chlorophyll a (mg m-3)
What other factors control phytoplankton abundance in coastal
polynyas?
0.0
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45
Polynya number
21
Why the relationship between phytoplankton abundance and
basal melt rate of nearby ice shelves?
•  Ice sheets grind up bedrock as they flow to sea
•  Ice sheets accumulate Fe-containing dust
•  Particulate and dissolved iron is released into seawater
•  Facilitated by upwelling of warm CDW
•  Iron taken up by growing phytoplankton
Phytoplankton bloom (CDW) 22
Why the relationship between phytoplankton abundance and
basal melt rate of nearby ice shelves?
Best example is in Amundsen Sea
• Most rapidly melting glaciers
• Large Fe flux at face of Pine Island Glacier
• Fe consumed by phytoplankton in surface waters
DynaLiFe Cruise
Depth (m)
0
Dissolved iron (nM)
Chlorophyll > 10 mg m-3
0.8
Chlorophyll > 5 mg m-3
100
0.6
0.4
200
0.2
300
250
200
150
100
50
0
0
Distance from Pine Island Glacier (km)
Modified from Gerringa et al. (2012)
23
How much variance in chlorophyll a can be
explained by environmental variables?
Multiple regression model included:
Shelf width
SST
Basal melt rate
Open water area
*Number of days of open water
*Mean daily PAR
*Removed because they had no effect on regression model
24
Some of the predictor variables were significantly
but weakly correlated
Predictor
Predictor
R2
p-value
Shelf width
SST
0.19
p<0.01
SST
Basal melt
0.16
p<0.01
Shelf width
Basal melt
0.17
p<0.01
Shelf width
Open water
0.20
p<0.01
25
Multiple linear regression: R2 = 0.77
Parameter
p-value
VIF
% of R2 explained
Basal melt rate
(Gt yr-1)
<0.001
1.34
43.6%
Sea surface temp
(°C)
<0.01
1.42
24.5%
Shelf width
(km)
<0.01
1.67
19.1%
Mean open water
(km2)
<0.05
1.26
12.8%
VIF = Variance Inflation Factor (should be below 2.5)
26
CONCLUSIONS
Even coastal Antarctic polynyas are iron-limited
Light does not control phytoplankton productivity
Higher SST increases phytoplankton abundance directly
through increased growth rates but possibly also indirectly
through increased glacial melting
Wide shelves are more productive than narrow ones
Greater resuspension of sediment iron
Very productive polynyas require input of glacial meltwater
Presumably because of additional iron supplies
Future increases in glacial melt are likely to increase
biological productivity of coastal ecosystems
27
THANK YOU!
Ocean Biology and
Biogeochemistry
Cryosphere Science
Program
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