Trends in Chesapeake Hypoxia/Anoxia

Modeling Subcommittee Quarterly ReviewFeb. 2, 2010

Rebecca R. Murphy and William P. BallJohns Hopkins University,

Department of Geography and Environmental Engineering

Outline Chesapeake Bay Environmental

Observatory (CBEO) Hypoxic volume trends Stratification trends Statistical models Possibility that large-scale climatic forces

are affecting hypoxia/stratification Brief comparison to model output

CBEO and Hypoxia AnalysisData Sets

Chesapeake Bay Program dataUSGS River Monitoring

Historic CBI dataModel outputs

etc…

New Tools/MethodsCBEO website: http://cbeo.communitymodeling.org

investigate

Generated from data in Hagy et al. (2004) in Estuaries

DO Analysis Approach: Analyze hypoxic volume with as much temporal and spatial resolution as possible

Interpolate DO and calculate hypoxic volume for each data collection cruise

Examine time series Look at where/when changes are occurring in

DO patterns

Interpolated DO: June-July 2 years

Data Sources: USGS, CBP

rN,vol = 0.31 (p=0.07)

rN,vol = - 0.44 (p=0.006)

rN,vol = 0.50 (p=0.001)

rN,vol = 0.88 (p=4e-17)

Example bottom DO at CB4.3C

Example bottom DO at CB4.3C

Early July DO:More hypoxic volume than anticipated from

nutrients alone

Late July DO:Hypoxic volume

follows trend expected from nutrient loads

Nut

rient

s

June DO: Oxygen depletion is happening earlier in summer in recent

years

Stratification:Plays a role in oxygen depletion

Approach: Calculate pycnocline

strength Examine time series Identify where/when

changes are occurring Look for significant

relationships between stratification and hypoxic volume trends

Density with Depth at CB4.4 on 6/13/2005

0

5

10

15

20

25

300 5 10 15Density (sigma units: +1000 kg/m3)

dept

h (m

)

Stratification Strength Calculation

Pycnocline

Data Source: CBP

Brunt Vaisala Frequency with Depth at CB4.4 on 6/13/2005

0

5

10

15

20

25

300 0.01 0.02

BVF (s-2)

dept

h (m

)

Stratification Strength Calculation

zgN

2

1. Calculate Brunt

Väisälä Frequency

2. Interpolate maximum

Brunt Väisälä Frequency

(N2)

3. Average max N2 in region of

interest

Average = 0.015

Density with Depth at CB4.4 on 6/13/2005

0

5

10

15

20

25

300 5 10 15Density (sigma units: +1000 kg/m3)

dept

h (m

)

Stratification Trends

Analysis: Stratification to Early July Hypoxic Volume (‘85-’09)

HypoxicVolEarlyJuly = 6 + 0.8 (StratificationJune) + 2 (StratificationEarlyJuly) +

R2 = 0.74 (p=3e-07)

JuneStratification:

• Increasing over time

• Causes less vertical mixing of

oxygen

July Stratification: • Not increasing

over time• Very variable

Nut

rient

sEarly July DO:More hypoxic volume than anticipated from

nutrients alone

Late July DO:Hypoxic volume

follows trend expected from nutrient loads

June DO: Oxygen depletion is happening earlier in summer in recent

years

Changing ClimaticForces (NAO,

wind, GSI, sea level

rise?)

ClimaticForces Wind

speeds much more

variable year-to-

year in July

Possible Climatic Factors North Atlantic

OscillationIndex has been

higher for last 25 years – influences mid-Atlantic climate in multiple ways

Wind direction shifts: (related to NAO)Winds from south

tend to weaken stratification

NAO

Graph from Jeremy Testa (UMCES)

Wind shift and possible relation to hypoxic volume identified by Malcolm Scully (Old Dominion), data=NOAA

Possible Climatic Factors: Salinity Changes

Gulf Stream Position (related to NAO)Index has been higher

for last 25 years – means saltier mid-Atlantic waters (Lee and Lwiza 2008)

Sea Level RiseEvidence of increased

Bay salinity (Hilton et al. 2008)

NOAA data

Plymouth Marine Laboratory data

Continuing Work

Investigation of the causes of the increase in early summer stratification and hypoxia

Expanded interpolation efforts into tributaries to examine hypoxia vs. stratification trends

Comparison to Model Output

57k output from Mark Noel March ‘09

Acknowledgements

Chesapeake Bay Environmental Observatory (CBEO) team, including:Dr. Michael Kemp and Jeremy Testa (UMCES)Drs. Dominic Di Toro and Damian Brady (UDel)Dr. Randal Burns and Eric Perlman (JHU CS)

Data sources: Chesapeake Bay Institute, Chesapeake Bay Program, USGS, NOAA

NSF funding of the CBEO project

References Guo, X. and Valle-Levinson, A. (2008). “Wind effects on the lateral structure of density-driven

circulation in Chesapeake Bay.” Continental Shelf Research, 28, 2450-2471. (Univ of Florida) Hagy, J. D., Boynton, W.R., Wood, C.W., Wood, K.V. (2004). "Hypoxia in the Chesapeake Bay,

1950-2001: long-term changes in relation to nutrient loading and river flows." Estuaries, 27(4), 634-658.

Hilton, T.W., Najjar, R.G., Zhong, L., and M. Li. (2008). “Is there a signal of sea-level rise in Chesapeake Bay salinity?” Journal of Geophysical Research, 113, C09002. (Penn State, UMCES)

Lee, Y.J. and K.M.M. Lwiza. 2008. Factors driving bottom salinity variability in the Chesapeake Bay. Continental Shelf Research 28:1352-1362. (Stony Brook)

Scully, M.E. (2009). “The importance of decadal-scale climate variability to wind-driven modulation of hypoxia in Chesapeake Bay.” Nature Precedings, Posted Jun 2, 2009. (Old Dominion)

Data Sources: CBP: http://www.chesapeakebay.net/data_waterquality.aspx CBI and other historic data:

http://archive.chesapeakebay.net/data/historicaldb/historicalmain.htm USGS: http://va.water.usgs.gov/chesbay/RIMP/ and http://waterdata.usgs.gov/nwis/monthly/ NCDC (wind data, Patuxent Naval Air Station:

http://cdo.ncdc.noaa.gov/pls/plclimprod/poemain.accessrouter?datasetabbv=DS3505 NOAA (sea level): http://tidesandcurrents.noaa.gov/sltrends/sltrends.shtml GSI: http://web.pml.ac.uk/gulfstream/default.htm

Jan-May Susquehanna Nitrogen Load

0

20

40

60

1949 1954 1959 1964 1969 1974 1979 1984 1989 1994 1999 2004

N lo

ad (G

g)Spring N loadDecadal average4 to 10 year average

Average July Hypoxic Volume (DO<1 mg/L)

0

2

4

6

8

10

1949 1954 1959 1964 1969 1974 1979 1984 1989 1994 1999 2004

Vol

ume

(km

3 )

Data Source: USGS

Theory: Stratification and Mixing

BVF Richardson Number, Ri

(Ri: balance between buoyant and shear forces)2

zuz

g

Ri

ai

zz bR

EE

]1[0,

BVF

vertical velocitygradient

Ez,0 = Eddy diffusivity with no stratificationa,b = coefficients

Ri Vertical Diffusivity, Ez

(Munk and Anderson 1948)

Flow