Ocean acidification

1

• Introduction coccolithophores
• Effects on oceanic chemistry
• Effects on biology
• Discussion and conclusions

2
Coccolithophores

• Etymology carrying round stones
• Characteristics
• Free drifting photosynthetic Phytoplankton
  (phylum Haptophyta)
• One of the most abundant marine calcifying
  phytoplankton
• Building of calcium carbonate scales (coccoliths)
• Ca2 CO32- ? CaCO3
• Ca2 2HCO3- ? CaCO3 H2O CO2

3
Coccolithophores

• Favorable conditions cause algae blooms, with a
  overproduction of coccoliths
• During a bloom the water turns an opaque
  turquoise (white waters)
• Growth is not inhibited by high UV light, such as
  other phytoplankton species
• Diameter of 5-10 µm

4
Coccolithophores

• Occurrence
• Mostly in upper layers of sub polar regions
• Nutrient poor and mild temperature waters

Satellite image of a bloom in the English Channel
off coast of Cornwall (24 July 1999)
The Coccolithophore Emiliana huxleyi
5
Effects on oceanic chemistry

• Pre-industrial atmospheric CO2 280 ppm
• Today atmospheric CO2 380 ppm
• ?CO2 obeys Henrys law
• CO2(atmosphere) ? ? CO2(surface oceans)
• Dissolution of CO2 into seawater releases
  hydrogen ions and therefore causes ocean
  acidification
• ? In the past 200 years the oceans absorbed 50
  of CO2 emitted by human activities (gt500 Gt C02)
• ? pH decrease of 0,1 units since pre-industrial
  times

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Effects on oceanic chemistry

• Oceanic absorption of atmospheric CO2 relevant
  processes

7
Effects on oceanic chemistry

• pH range of seawater 8,2 0,3 (today)
• Relative proportions of the 3 main inorganic
  forms of CO2 dissolved in seawater
• - CO2 (aq) (including H2CO3) 1
• - HCO3- 91
• - CO32- 8

8
Effects on calcium carbonate and saturation
horizons

• Solubility of CaCO3 ? temperature, pressure
  (depth) increasing solubility by decreasing
  temperature and increasing depth
• ? Result of these variables development of
  natural boundary in seawater called saturation
  horizon
• Dissolution of CO2 decreases CO32-, because
  carbonate ions react with protons to become
  bicarbonate (HCO3-)
• Equilibrium shifts to the right
• (Dissolution)

9
Effects on calcium carbonate and saturation
horizons

• Increasing CO2 levels (and resultant lower pH) of
  seawater decreases the saturation state of CaCO3
  and raises the saturation horizon closer to the
  surface
• Two main forms of calcium carbonate aragonite
  and calcite

Aragonite Calcite
Structure orthorhombic trigonal
Solubility high low
Calcifying species Corals, pterods, macroalgae Foraminifera, macroalgae, coccolithophores, crustacea
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• Saturation horizon of calcite and aragonite
• Aragonite SH nearer the surface of the oceans
  because higher solubility than calcite
• Calcifying organisms producing aragonite form of
  CaCO3 are more vulnerable to changes in ocean
  acidity

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Ocean acidification vs. chemistry of nutrients
and toxins

• Metals exist in two forms in seawater complex
  and free dissolved
• ?pH
• - generally increases the proportion of free
  dissolved forms (most toxic forms)
• - release of bound metals from the sediment to
  the water column
• - effects on nutrient speciation (phosphate,
  ammonia, iron, silicate)

12
Ocean acidification past and future

• Ocean acidification is essential an irreversible
  process during our lifetimes
• Fastest natural change in atmospheric CO2 at the
  end of the recent ice age
• ?CO2 80 ppm in 6000 years
• Current change occur 100 folder stronger

Changes in ocean pH are outside the range of
natural variability ? They could have a
substantial affect on biological processes in the
surface oceans
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Effects on biology

• Photosynthesis (POC)

Laboratory
Field
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Effects on biology

• Calcification

Laboratory
Field
15
Effects on biology

• Calcite/POC

Laboratory
Field
16
Effects on biology

• Malformation

G. oceanica
E. Huxleyi
300 ppm
780-850 ppm
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Effects on biology

• Negative feedback for atmospheric CO2
• Reduced calcification leads to reduced CO2
  production from calcification. This results in an
  increased CO2 storage in the upper part of the
  ocean.

18
Effects on biology

• Also others organisms are affected

19
Effects on biology
20
Changing acidity
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Changing acidity
Aragonite saturation of surface waters (light
blue oversaturated, purple undersaturated)
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Approaches to mitigate ocean acidification

• Addition of alkalinity to the oceans
• Direct injection of CO2 into the deep oceans
  (CCS-programm carbon capture and storage)
• Fertilization of the upper oceans with iron
• Preventing accumulation of CO2 in the atmosphere