Eutrophication, Hypoxia, and Ocean Acidification Puget Sound Oceanography 2011
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Transcript of Eutrophication, Hypoxia, and Ocean Acidification Puget Sound Oceanography 2011
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Eutrophication, Hypoxia, and Ocean Acidification
Puget Sound Oceanography2011
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Eutrophication:
The enrichment of a body of water with dissolved nutrients to the point that phytoplankton are released from nutrient-limited growth.
Cultural / anthropogenic eutrophication-- River inputs influenced by urbanization + agriculture-- Run-off / Septic systems-- Sewage Treatment Plants
Natural eutrophication-- River inputs-- Run-off
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Findings of NOAA’s 2004 National Estuarine Eutrophication Assessment:
Extent of eutrophication (measured as number and severity of symptoms)
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Findings of NOAA’s National Estuarine Eutrophication Assessment:
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Kemp et al., 2005
System of feedbacks in eutrophication:
Nu
trie
nt
Fe
ed
ba
ck
Wa
ter
cla
rity
fee
db
ack
Large-scale / long-term stresses
Short-term / regional-scale stresses
Large phytoplankton standing stockShading of benthos (loss of sea grasses)increased turbidityimpacts on benthic communitylower filtering
….biological feedbacks
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(a) The structural diversity afforded by the plants and the availability of oxygen in the sediment promote a diverse community of animals.
(b) The loss of structural diversity and oxygen from the sea-bed causes the animal community to be replaced by one of bacterial decomposers.
(Open University)
.
Alternate Stable StatesChanges in sea floor communities in shallow coastal waters following eutrophication.
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Hypoxia and anoxia in natural and in eutrophied systems
Hypoxia: Low dissolved oxygen. Various thresholds, often
defined as <2 mg DO l-1
Anoxia: An absence, or near-absence (below detection
limits), of dissolved oxygen
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The fundamental metabolic processes driving hypoxia
BacteriaZooplankton
Benthic macrofauna
Sin
king
Thermocline
Upper mixed layer:Generation of organic matter(Release of O2, use of CO2)
Lower layer:Breakdown of organic matter(use of O2, release of CO2)
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Conditions for bottom hypoxia:
• Sufficient nutrients• Excess phytoplankton production (exceeding grazing)• Stratification • Sinking material• Low flushing/long residence time
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Chesapeake Bay -- from Zhang et al., 2006
Ox
yg
en
(m
l L
-1)
1996 1997 1999 2000
April
July
October
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Extent of hypoxia in Chesapeake Bay is increasing:
1950 2000
DO<0.2 mg/l
DO<1.0 mg/l
DO<2.0 mg/l
109
m3
109
m3
109
m3
ObservedModeled (Observed flow)Modeled (Avg Flow)Modeled (Low Flow)Modeled (High flow)
Hagy et al., 2004
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Rate of oxygen drawdown:
Typical = 75 days from winter level to anoxia.
Hagy et al., 2004
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Main Stem Hood Canal oxygen patterns:
Ocean end Hoodsport
Density
Oxygen
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Hood Canal oxygen profiles:
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Hood Canal ORCA buoy oxygen profiles:
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CO2 + CaCO3 + H2O 2HCO3- + Ca2+
CO2 + H2O ⇌ H2CO3 (carbonic acid) equilibrium
H+ + HCO3− (bicarbonate ion) ⇌ H+ + CO3
2− (carbonate ion)
Ocean Acidification – lowered pH of the ocean due to increased CO2 concentrations.
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Feely et al., 2010
• ‘Anthropogenic’ acidification• Increased atmospheric CO2 concentrations
• ‘Natural’ acidification• Respiration increased CO2
Atmosphere
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Feely et al., 2002
Calcium carbonate (as aragonite) saturation depths: from 1991-1996 cruises.
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