Ocean AcidificationReid Bergsund and Catherine Philbin

9.7 Billion Metric Tonnes per Year of Anthropogenic Carbon Emissions in 2012

Anthropogenic emissions are the result of human combustion of fossil fuels

Anthropogenic carbon emissions grew 2.1% since 2011

Global Greenhouse Gas Emissions by Source

Balance of carbonate species shifts right from CO2 to HCO3

- and CO3-

Ocean absorbs about 1/3 of excess atmospheric CO2

The addition of CO2 to seawater causes the formation of carbonic acid

CO32- decreases with

increasing levels of CO2

Carbonic acid then undergoes two dissociations and releases H+ ions into the ocean, causing ocean pH to decrease

CO2 (g) ⇔ CO2 (aq)

CO2 dissolves in ocean water

CO2 + H2O ⇔ H2CO3

CO2 and H2O form carbonic acid

H2CO3 ⇔ HCO3- + H+

The first dissociation

HCO3- ⇔ CO3

2- + H+

The second dissociation

pH = -log10[H+]

The pH is determined by the concentration of H+ ions

The Carbonate Buffer System Helps to Maintain Equilibrium

CO2, HCO3- and CO3

2- make a carbonate buffer system

The addition of CO2 will cause the equilibrium equation to move to the right.

An increase in CO2

decreases CO32- in

order to maintain chemical equilibrium

CO2 + CO32- + H2O ⇔ 2HCO3

-Equilibrium reaction for CO2

in seawater

An Increase in Atmospheric CO2 Causes a Decrease in the Supersaturation levels of CaCO3

Surface waters are supersaturated with CaCO3

The decrease in CO32-

caused by an increase in CO2 results in less abundant dissolved CaCO3

Corals produce CaCO3 more slowly as ocean pH and CaCO3 supersaturation levels both decrease

Ca2+ + CO3

2- ⇔ CaCO3Calcification reaction

Coral with Calcite shells

Reef after ocean acidification?

Healthy Coral Reef

Time for Discussion…

Iglesias-Rodriguez et al (2008) Argue that Rising Atmospheric CO2 Partial Pressures Won’t Reduce Marine Organism Calcification

Grew cultures of Emiliania huxleyi coccolithophores (photosynthetic plankton)

Bubbled different partial pressures of CO2 through cultures ranging from 280 ppmv (pre-industrial levels) to 750 ppmv (end of 21st century worst case scenario)

E. huxleyi coccolithophore

E. huxleyi is the Most Abundant Species of Coccolithophore Found in

temperate, subtropical, and tropical oceans; forms base of large proportion of marine food webs

E. huxleyi’s abundance and importance is the reason it’s often used for studies of this nature

Relative abundance of different species of Coccolithophore

Coccolith Volume Changed During Experiments at Different CO2 Levels

Coccolith volume and CaCO3 levels generally increase with rising CO2 partial pressures

Discovered that coccolith calcification increased under higher-CO2 conditions

Experimental Results Show Coccolith Productivity Rising with Increased pCO2

Particulate Inorganic Carbon (PIC), Particulate Organic Carbon (POC): suspended, particulate inorganic and organic carbon species produced by phytoplankton at the sea surface

PIC (graph A) levels and POC (graph B) levels double at 750 ppmv CO2

PIC production rates (graph C) and POC production rates (graph D) double at 750 ppmv CO2

CO2 ppmv

CO2 ppmv

Different colored dots represent different independent experiments

Experimental Results Show Coccolith Growth Rates Decreasing with Increased pCO2

If the range of experiments is viewed as providing error bars, the experiments are consistent with each other

Different colored dots represent different independent experiments

Field Observations Confirm Upward Trend in Coccolith Mass Witnessed in Experiments

Change in coccolith mass has accelerated over recent decades

Increase in average coccolith mass correlates with rising atmospheric CO2

Atmospheric CO2 data taken from Siple ice core and Mauna Loa

Observatory

Riebesell et al (2008) Argue that Experimental Protocol is Flawed Particulate Inorganic Carbon production

rates and Particulate Organic Carbon production rates don’t increase when normalized to Particulate Organic Carbon biomass for E. huxleyi cultures under different CO2 partial pressures

Different colored dots represent different independent experiments

PIC Production Rates

POC Production Rates

Riebesell et al (2008) describe specific procedural oversights in Iglesias-Rodriguez et al (2008) experiments

E. huxleyi precultures were grown at 5x-10x natural cell density, affecting cell carbon levels

E. huxleyi precultures may have experienced nitrogen limitation during transfer to experimental flasks

Nitrogen limitation in E. huxleyi increases cell size and carbon levels

Experimental incubations only lasted 1.5 to 3 days; only 1 to 2 cell generations

With only 2 generations, differences in carbon quota, cell growth rates could be attributed to preconditioning, not CO2

differences

E. Huxleyi cultures, magnified

Procedural errors could have driven cell growth rate

and carbon levels artificially high

Iglesias-Rodriguez et al (2008) argue that experiments accurately depicted ocean conditions Alkalinity is the capacity of

an aqueous solution to neutralize an acid

Due to the long residence time of alkalinity in the ocean, alkalinity was held constant in the experiment, contrary to what Riebesell et al (2008) suggest

CO2 bubbling mimics the ocean alkalinity and the changes in the biocarbonate ion of high CO2 seawater

ALOHA Data Shows that the Surface Ocean pH has Decreased as Atm CO2 and pCO2 has Increased

Red: atmospheric CO2 at Mauna Loa in ppmv Blue: surface ocean pH

Tan: pCO2 inμatm in the subtropical North Pacific Ocean

Oceanic uptake of CO2 is a cause for increases in DIC and decreases in CaCO3 saturation

Since preindustrial age, atmospheric CO2 and the pCO2 in the ocean have increased, whereas pH has decreased by 0.1 units

Aragonite is a More Soluble Form of CaCO3 than Calcite

The crystal structure of Aragonite

The crystal structure of Calcite

Aragonite and Calcite Saturation Levels Mirror Changes in Total CaCO3

Coral reefs are defined by the ability to produce a net surplus of CaCO3

In the last 17 years the saturation of both calcite and aragonite have both decreased

Blue: saturation of aragonite

Gray: saturation of calcite

Calcite and Aragonite Saturation Levels Have Gotten Less Deep in the Ocean since Preindustrial Times

Saturation levels of aragonite and calcite are based on temp, pressure, and pH

Above the saturation levels, calcite and aragonite tend to form crystal structure

Saturation levels at present (solid) have changed since preindustrial times (dashed)

Latitude

Saturation Levels of Aragonite and Calcite Vary Across Different Oceans

Anthropogenic CO2 penetration is highest in the N. Atlantic because of deep water formation

Saturation levels of Calcite and Aragonite change most where CO2

has penetrated

Organisms Lose Calcite Shells in More Acidic Seawater Degree of sensitivity to

ocean acidification varies between species

Chronic exposure is difficult to demonstrate in laboratories

Calcification rates decrease as ocean pH and aragonite and calcite saturation states decrease

Some corals completely lose their skeletons when grown in highly acidic water Coral grown for 12

months in: (a) pH = 8.2 and (b) pH = 7.4

Varied Responses to Acidification Across Different Species

Calcification, photosynthesis, nitrogen fixation and reproduction were studied across different species

Ocean acidification has widely varied affects on different biological processes

Columns a-d represent linear positive, linear negative, level, and parabolic responses to increasing seawater pCO2

Global Warming Causes a Change in Nutrient Availability

Species, such as phytoplankton, are dependant on nutrient rich waters

Global warming increases thermal stratification of the upper ocean, and decreases nutrient upwelling Nutrient rich water is cold and

deep, and is eventually pushed to the surface. This is the

process of upwelling.

Ocean Acidification Impacts Food Webs and Ecosystems

Calcified structures provide protection from predators

With acidification, calcifiers will need to adapt, or be adversely affected

Organic matter grown at high CO2 levels have higher carbon/nitrogen ratios, affecting food quality for microbial consumers

Acidification isn’t the only effect of a changing climate, so it is difficult to predict outcomes of ocean acidification on food webs

Phytoplankton are at the base of the oceanic

food web

There are many uncertainites in the impact of increasing CO2 levels on the global CaCO3 budget

Decreasing CaCO3 saturation states cause increasing dissolution rates

If all carbonate production stops, atmospheric CO2 would decline 10-20 ppmv

Uptake rate of anthropogenic CO2 could overwhelm the natural carbonate buffer system

Oceanic CaCO3 Budget Affects Carbon Uptake

CaCO3 flux estimates for the ocean

Chemical Speciation in Seawater Changes with Ocean pH Weak acid species will

undergo speciation changes with acidification, ex: boron, phosphorous, silicon, nitrogen

These changes can affect bioavailability

More research is needed to understand effects of ocean acidification on trace metals and organic mater Changes in phosphoric

acid speciation due to changes in pH

Ocean acidification will definitely occur, but how will the oceanic ecosystems

respond?

A descriptive title

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caption, explaining exactly what’s shown in the figure including both axes and all lines, etc

A brief take home message. Everything on slide in 24 points or larger