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

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

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 During the bloom coccolithophorides can out compete other photosynthetic phytoplankton, because they reflect incoming light. In this conditions they represent 90% of phytoplankton species.

The Coccolithophore Emiliana huxleyi 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

Effects on oceanic chemistry Pre-industrial atmospheric [CO2]: 280 ppm Today atmospheric [CO2]: 380 ppm CO2 obeys Henry’s 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 (>500 Gt C02)  pH decrease of 0,1 units since pre-industrial times 8,18  8,07: Buffer efficiency of oceans (CaCO3) to neutralise change in acidity due to CO2 uptake/dissolution decreases (….CO2 uptake decreases) Estimation based on current measurement of ocean pH, analysis of [CO2] in ice cores, understanding of the rate of CO2 absorption and retention in the surface oceans, knowledge of the CaCO3 buffer. -0,1 seems not so high…but because of logaritmic skala corresponds to about 30% increase in [H+] Studies predict a possible decrease in pH of 0,5 units by the year 2100 in the surface oceans…it corresponds to a 3fold increase in the [H+]

Effects on oceanic chemistry Oceanic absorption of atmospheric CO2: relevant processes

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%

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) Calcifying marine organisms are dependent on the presence of bicarbonate and carbonate forms to build CaCO3 structures below the saturation horizon: seawater undersaturated and CaCO3 will tend to dissolve Above the saturation horizon: seawater is supersaturated and CaCO3 will tend to be preserved CaCO3 solubility depends also on [CO32-] and therefore indirectly on pH

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

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

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) Example: reduced pH would lower the concentrations of ammonia (NH3) in seawater in preference to ammonium (NH4+)

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: Δ[CO2]= +80 ppm in 6000 years Current change occur 100 folder stronger DURING OUR LIFETIMS: it will take tens of thousands years for ocean chemistry to return to a condition similar to that at pre-industrial times (even assuming that emissions will be fall to “zero” in the year 2500!) .. [CO2] in the atmosphere decreases slowly and pH will continue to rise in deepwater) Recent ice age? When? Changes in ocean pH are outside the range of natural variability  They could have a substantial affect on biological processes in the surface oceans

Effects on biology Photosynthesis (POC) Laboratory Field POC: particulate organic carbon. Fertilizing effect is small, because of carbon concentrating mechanisms. Photosynthesis is saturated with inorganic carbon. Left: E. hux. (circles) and Gephyrocapsa oceanice (squares), boths coccolithophorids. Right: subartic north pacific phytoplankton assemblages (mostly coccolithophores). Lines: 280 ppm (preindustrial), 365 ppm (present) and 750 (triple preindustrial, future)

Effects on biology Calcification Laboratory Field requirement of more energy to actively raise pH after encapsulation of seawater in order to increase the carbonate concentration. If pH is lower a shift in carbonate concentration will require more energy and this may reduce growth of the organisms. Field: calcification is reduced by order of 10!

Effects on biology Calcite/POC Laboratory Field Reduced Calcite production overcompensates POC increase (for lab). This should be seen in the lab. experiments more than in the field: Calcite << POC with increasing CO2 (true for Gephyrocapsa oceanica (squares), because calcification is very small)

Effects on biology Malformation E. Huxleyi G. oceanica 300 ppm In lab. and field observed. There is a direct effect of seawater acidity on calcification, with resluting corrosion or incomplete calcification (with weakening of calcite structure). 780-850 ppm

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. Ca2+ + 2HCO32-  CaCO3 + H2CO3. Flux from atmosphere to ocean

Effects on biology Also others organisms are affected:

Effects on biology Coralls will have also others problems, such as pollution, overfishing, temperature change, …

Changing acidity Question: In which areas are marine organisms mainly affected by ocean acidification? Measurements of top 50 meters. Because of upwelling of deeper water, such as in the east equatorial pacific. Here changes can be seen in the future. Cold waters are naturally less supersaturated than warm ones for the various forms of calcium carbonate: high latitude and deep water ecosystems may be the first to suffer from ocean acidification  migration to lower, warmer latitudes (adaption?)

Changing acidity Aragonite saturation of surface waters (light blue: oversaturated, purple: undersaturated) Aragonite saturation of surface waters. Oversaturated: light blue, Undersaturated: purple. In the north pacific the saturation depth ist lowest, due to deep water circulation.

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 Minerals like magnesium Hydroxide could be used  Problem: minerals are relatively rare and could not be obtained at the required scale (+ mining and transportation operation require a great deal of energy…This energy would come from fossil fuel sources and would increases rates of greenhouse gas emission! Effects: - Amounts of carbonate minerals needed make this approach infeasible at the scale required to mitigate global changes in ocean chemistry Concept: capture CO2 from power generation and store them (such as in liquid form) for thousands of years in places that are isolated from the atmosphere (deep oceans or in underground geological structures) Effects: ? Iron-method: ? Reducing the scale of future changes to the chemistry and acidity of the oceans is only possible by preventing the accumulation of CO2 in the atmosphere. Alternative solutions, such as adding chemicals to counter the effects of acidification, are likely to be only partly effective and only at a very local scale.