Relationship Between Winter Downwelling
Conditions and Summer Hypoxia Severity
Along the Oregon Coast
Helen Walters1, Yvette Spitz2, Harold Batchelder3
1.
2.
3.
BioResource Research, College of Agriculture Oregon State University
Oregon State College of Earth, Ocean, and Atmospheric Sciences
North Pacific Marine Science Organization
What is Hypoxia and Why Does it Matter?
• Dissolved oxygen levels in the ocean reach a point unfit for
marine organisms to survive (1.4 ml/l.)
• Two
different types: anthropogenic and natural
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May 28, 2014
Photo: NOAA, annauniv.org, wri.org
Anthropogenic Hypoxia
• Causes: high nutrient
(nitrate, phosphorus)
load from
• Discharge of treated
sewage
• Agricultural runoff
Large phytoplankton bloom
that will decompose on
the bottom
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June 9, 2014
Photo: NOAA
Natural Hypoxia
• Causes:
• Depletion of oxygen due to stratification
• Decomposition of biomass by bacteria on the ocean floor after
large phytoplankton bloom formed during upwelling
Negative Effects on Higher Trophic Levels
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June 9, 2014
Photo: NOAA
Natural Hypoxia Cycle: Seasonal Variation in wind
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June 9, 2014
Photo: NSF
Places of Natural and Anthropogenic Occurrence
• Natural: Eastern Boundary Currents
California
Current System
Benguela Current
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June 9, 2014
Photo: iopscience.org
Humboldt Current
North Pacific Basin
Oregon
Shelf
Oregon Centric
May 28, 2014
Hypoxia off Oregon?
Dissolved oxygen (ml l-1)
Depth (m)
1950 to 1999
1.4 ml l-1
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May 28, 2014
Chan et al. (2008, Science)
Newport
Hypoxia off Oregon?
Dissolved oxygen (ml l-1)
1950 to 1999
Depth (m)
2000 to 2005
8
1.4 ml l-1
May 28, 2014
Chan et al. (2008, Science)
Hypoxia off Oregon?
Dissolved oxygen (ml l-1)
1950 to 1999
2000 to 2005
Depth (m)
2006
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May 28, 2014
1.4 ml l-1
Chan et al. (2008, Science)
Case Study July 2002: Grantham et al., 2004
• Impact on benthic organisms
• Impact on nekton
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June 9, 2014
California Current
System
Case Study: Grantham et al., 2004
August 2000
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June 9, 2014
July 2002
Seasonal (June-Oct); most severe in Aug/Sept
Seasonal Variability of NH5 O2
at 50 m (bottom is 62 m)
-1
Oxygen concentration (mL L )
7
6
5
2005
2006
2007
2008
2009
4
3
2
1
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan
Month of the Year
May 28, 2014
Figures courtesy of Bill Peterson
Why Downwelling Season?
• Previous research in the summer time examining severity and
duration of hypoxia but not much research into what sets up
this summer event.
• The seasonal cycle of summer hypoxia is well explained but the
cause of inter-annual variation of the severity in summer
hypoxia is more of a mystery.
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June 9, 2014
Hypothesis:
• The severity and extent of summer hypoxia on the Oregon
shelf depends not only on the strength of the upwelling during
the summer season but on the strength of the downwelling
during the preceding winter/spring as well.
Alternative Hypothesis:
• Alternatively, there may be no relationship between the winter
atmospheric forcing (downwelling favorable wind stress) and
the ensuing hypoxia event, but a late winter (February) bloom
may set the stage for the hypoxic conditions instead.
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June 9, 2014
Study Site: Newport Hydrographic Line
• Samples were taken
5, 10, 15, 20, 25, 35,
45, 65, and 85
nautical miles off
shore.
• Contour Lines show
isobaths.
165 nautical miles
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June 9, 2014
Method -Two fold approach
• Analysis of Atmospheric Conditions as a proxy of intensity of
downwelling and upwelling as well as bloom formation.
•
•
•
•
Upwelling Index
Wind stress
Oxygen
Chlorophyll
• Model Simulations with different atmospheric forcing.
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June 9, 2014
Upwelling Index
• From Pacific Fisheries
Environmental Laboratory
(NOAA)
• Calculated using geostrophic
winds.
• Used to calculate the date of
the spring transition for each
year and length of winter
season.
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June 9, 2014
Length of
End Day of
Winter
Winter
Year
Season
Season
97-98
229
26-Mar
98-99
174
29-Mar
99-00
237
13-Jun
00-01
200
1-May
2001-2002
191
17-Apr
2002-2003
166
20-Apr
2003-2004
207
21-Apr
Wind Stress
• The wind stress in this dataset was derived from observed winds
at Newport, Oregon.
• Our winter analysis begins in January because wind stress levels
are fairly uniform throughout years in this period.
Cumulative Wind Stress 1998-2003
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June 9, 2014
Pierce et al., 2007
Oxygen
• NH Line spans 1960-2012
• Not a complete time span.
• The Next Ten Years of Oceanography (TENOC)
encompassed data spanning from 1961-1971
• Global Ocean Ecosystems Dynamics (GLOBEC) Long
Term Observation Program (LTOP) collection period
from 1997-2004
• Interpolated over 26 year gap in data
• General decreasing trend
• 4-7 GLOBEC cruises a year
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June 9, 2014
Pierce et al., 2006
Converting Chlorophyll to Oxygen
Chlorophyll-A
(mg m-3)
Phytoplankton
Biomass
(mmol m-3)
Detritus
(mmol m-3)
Consumed
Dissolved
Oxygen
(ml l-1)
Equation 1: O2 (ml l-1) = Total Chl (mg m-3)* 0.0247
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June 9, 2014
Chlorophyll:
• Obtained in 2 protocols
• In situ data from GLOBEC/LTOP
• Satellite data from SeaWiFS
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June 9, 2014
Photos: NOAA
SeaWiFS
• NASA Goddard Space Flight Center that
produces 9 km resolution imagery
compiled into a 8-day composite form.
• Alternate SeaWiFS from Dr. Ricardo
Letelier (CEOAS, OSU) shows isolated
Oregon coast SeaWiFS data gridded to 4
km resolution and a 7-day composite.
• 4 km resolution was used because higher
resolution means less interpolation
between datasets.
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June 9, 2014
Photo: Ricardo Letelier, CEAOS OSU
Relating SeaWiFS and GLOBEC
• An aggregation of the specific longitudes of the NH line
provided a good comparison with the GLOBEC data.
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There is variability in GLOBEC data based on
when the cruise was taken.
2002
Downwelling
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Upwelling
Converting chlorophyll to consumed oxygen
1998
1999
2000
2001
2002
2003
27.507
33.581
18.759
0.680
0.894
0.464
January 1 – End of March
Chlorophyll-a
Pigment (mg m-3)
14.882
Consumed
Dissolved Oxygen
(ml/l)
0.369
Chlorophyll-a
Pigment (mg m-3)
Consumed
Dissolved Oxygen
(ml/l)
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June 9, 2014
11.841
18.696
0.293
0.462
January 1 – June 1
26.127
43.994
35.366
44.055
69.736
37.283
0.646
1.088
0.874
1.089
1.764
0.922
Results of Analysis of Atmospheric Conditions:
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June 9, 2014
2002 is an anomalous year
1999
2000
2001
2002
2003
Cumulative Wind Stress 1998-2003
2001-2002
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June 9, 2014
2002 has high chlorophyll content in April and a fairly
short downwelling season preceding.
Cumulative algorithm to show total amount of
chlorophyll (mg m-3) and wind (N m-2)
28
Using
SeaWiFS and wind stress from Barth et al., 2006.
June 9, 2014
Model
• Therefore, three different years:
2000, 2001, 2002 were simulated in
a model to compare different initial
atmospheric forcing.
• Coupled biological and physical
model
• NAPZDO
•
•
•
•
•
•
Nitrate
Ammonium
Phytoplankton
Zooplankton
Detritus
Oxygen
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June 9, 2014
Adapted from Spitz et al., 2005
Atmosphere
O2 gain
O2 loss
Oxygen
Phytoplankton
Nitrate
Zooplankton
Ammonium
Detritus
• Circulation
model: ROMS
forced by uniform wind stress and
heat flux
• Topography: realistic continental
shelf topography with 10m minimum
and 1200 maximum
• Vertical resolution: 45 levels with
higher resolution at the surface and
bottom (S – coordinate system)
• Horizontal resolution: rectangular
grid with 1 km at the coast to about 6
km offshore, and 1 km alongshore
• Three open boundaries: periodic
channel (N-S)
(drawback = no remote forcing, short
simulations)
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May 28, 2014
Chlorophyll Levels
2000
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June 9, 2014
2001
2002
2000 April 11
Model vs. GLOBEC
2000
Overall, it appears that the model is
High
oxygen levels
a little lower in oxygen levels
then
on the
the in situ GLOBEC data, but
in shelf.
general they are relatively accurate.
Green: 2.5 ml/l
Red: 3.5 ml/l
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June 9, 2014
Model 2001
April 11:
Low oxygen levels
on the shelf
Model 2002
Green: 2.5 ml/l
Red: 3.5 ml/l
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Bottom Oxygen Levels: NH 10 & 25
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Strong Downwelling
Mean wind stress Januaryend of March (N/m2)
Mean: 0.0299 Standard Dev.: 0.1204
Strong Upwelling
Mean: -0.025 Standard Dev.: 0.1504
Mean: -0.0128 Standard Dev.: 0.1436
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June 9, 2014
Model Simulation with no biological components.
• A simulation was conducted without any biological
components to as further narrow down what impacts the
initial conditions.
• If there is not much of a difference between the 2002 run
without biology versus the 2002 run with all of the biological
components, then the initial conditions are driven by the
physics (wind stress) as oppose to the biological bloom.
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June 9, 2014
Bottom Oxygen Levels: Biology vs. No Biology: NH 10 & 25
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June 9, 2014
Summary from modeling
• 2000 winter had higher bottom oxygen levels and increased
downwelling favorable wind in the winter/spring season resulting in
less surface chlorophyll
• 2001 and 2002 had lower bottom oxygen levels and higher
chlorophyll levels. Wind stress during winter/spring season were
more varied with intermixed periods of upwelling and downwelling
favorable winds.
• When a model simulation was conducted without biology, there was
minimal difference, verifying further that the winter wind stress
plays a major role in oxygen regeneration/removal on the shelf and
summer hypoxia.
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June 9, 2014
General Conclusions
Analysis of Atmospheric Conditions:
• Large inter-annual variations in atmospheric conditions
could affect inter-annual variations in the severity of
summer hypoxia off the coast of Oregon at Newport.
We believe this weak statement is due the scarcity of
data
• SeaWiFS – cloud cover problems –
the white is all cloud cover and
therefore lack of data.
• 4-7 cruises annually from GLOBEC
In the future
• There are gliders that could be used
for analysis of more recent years
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June 9, 2014
Year
1998
1999
2000
2001
2002
2003
Average %
of Missing
Data
57
28
35
51
48
40
Conclusions – Continued
Analysis from modeling effort:
Atmospheric forcing conditions in the winter/spring season have an
effect on the set up of initial conditions for the summer and
consequently hypoxic event.
Future simulations for multiple years with remote forcing (not period
channel) are necessary to refine our understanding of the effect of the
winter season on the summer low oxygen content of the Oregon Shelf.
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June 9, 2014
Acknowledgements
A Big Thanks to:
Dr. Yvette Spitz
Dr. Harold Batchelder
Dr. Kate Field
Brandy Cervantes
Wanda Crannell
Friends and Family who have supported me
through this process
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June 9, 2014