Eutrophication, Hypoxia, and Ocean Acidification

Puget Sound Oceanography2011

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

Findings of NOAA’s 2004 National Estuarine Eutrophication Assessment:

Extent of eutrophication (measured as number and severity of symptoms)

Findings of NOAA’s National Estuarine Eutrophication Assessment:

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

(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.

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

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)

Conditions for bottom hypoxia:

• Sufficient nutrients• Excess phytoplankton production (exceeding grazing)• Stratification • Sinking material• Low flushing/long residence time

Chesapeake Bay -- from Zhang et al., 2006

Ox

yg

en

(m

l L

-1)

1996 1997 1999 2000

April

July

October

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

Rate of oxygen drawdown:

Typical = 75 days from winter level to anoxia.

Hagy et al., 2004

Main Stem Hood Canal oxygen patterns:

Ocean end Hoodsport

Density

Oxygen

Hood Canal oxygen profiles:

Hood Canal ORCA buoy oxygen profiles:

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.

Feely et al., 2010

• ‘Anthropogenic’ acidification• Increased atmospheric CO2 concentrations

• ‘Natural’ acidification• Respiration increased CO2

Atmosphere

Feely et al., 2002

Calcium carbonate (as aragonite) saturation depths: from 1991-1996 cruises.