As carbon dioxide (CO2) emissions continue to rise, ocean acidification will continue to change our oceans ecosystems. There has been a recent tumult of research identifying the impacts of ocean acidification on marine organisms with more than 62% of the papers published since 2004 (UNESCO, 2008). A clear focus has been on coral ecosystem SO I'M NOT GOING TO. This paper lays out the fundamental oceanic processes that are affected by ocean acidification and then critically evaluates research methodologies and conclusions compiled for two, a coccolithosphore, Emiliania huxleyi and the bivalve, Mytilus edulis, to determine if any clear correlations exist among research projects and what further research is needed. The conclusion is twofold: ocean acidification is species specific with adaptation taking center stage and past research is not indicative to true environmental conditions. (problem is that research isn't done over 100's of years but w/in 2-3 years â€¦ CO2 is changed dramatically). TOTALLY NEEDS WORK
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INTRODUCTION TO OCEAN ACIDIFICATION 4
CO2 Reservoirs 4
OCEANIC PROCESSES AND CHEMISTRY 6
Oceanic Carbon Chemistry 9
RESEARCH: HISTORICAL AND PREDICTIONS 14
Ocean Research and Predictions 14
Historical Carbon 18
ORGANISMS IMPACTED 19
pH Tolerant Organisms 22
pH Intolerant Organisms 23
Research Recommendations 25
Indian Express (2009) Delegates at Copenhagen meet to emit 40,500 tonnes of CO2. [Online] Available at: http://www.indianexpress.com/news/delegates-at-copenhagen-meet-to-emit-40-500/548977/ (Accessed on 2 January 2010). 32
References for Figures and Graphs 36
INTRODUCTION TO OCEAN ACIDIFICATION
In 2003, Ken Calderia coined the term Ocean Acidification in hopes of drawing attention to a critical issue to our future. He defined it as the decrease in pH levels, and corresponding rise in acidity due to increased uptake of atmospheric carbon dioxide (CO2) into the oceans (Caldeira and Wicket, 2003). Through modeling, Calderia discovered that in the coming centuries the world would see more ocean acidification than in the past 300 million years. Six years later, 15,000 (Indian Express, 2010) scientists and legislators gathered in Copenhagen, Denmark to determine what can be done to slow the inevitable process of rising CO2 levels. Most of the attendees will not have a clear understanding of what ocean acidification is but will walk away with the knowledge that CO2 emissions lead to much more than an increase in global temperatures. In fact, extensive research has been done to determine how marine organisms will adapt to this environmental shift. Each organism's ability to adapt will influence tropic levels and the corresponding ecosystems.
The Industrial Age led to an undeniable rise in mass production and some would argue, a higher quality of life. However, this increase engendered a significant dependence on the combustion of fossil fuels which has lead to a dramatic increase in atmospheric CO2. This prodigious CO2 release has been exacerbated by deforestation and urban development, which is eliminating an important CO2 sink (Miller et al, 2009). Carbon is found not only in the atmosphere but also in the oceans and vegetation with carbon levels at 605 GT in the atmosphere, 433 GT in the oceans and 143 GT in plants (Dickson, 2009). The average concentration of atmospheric CO2 (as of 12/12/09) is 385 parts per million (ppm), which is 38% higher than the pre-industrial level of 280 ppm only 200 years ago (IPCC, 2007). In comparison, CO2 levels prior to the industrial revolution appear to have been fairly constant for the past XXXX years (Calderia and Wicket, 2003). Most alarming, is that 50% of this increase has taken place in the last 30 years (CITE).
Studies by the Royal Society (2005) indicate that the Earth's oceans act as a carbon sink, having absorbed 1/3 of all anthropogenic CO2. This is approximately 2 billion metric tons of carbon per year. Chillingly, many do not realize that atmospheric CO2 concentrations would be 48% higher if the oceans had not been diffusing CO2 proportionately to atmospheric CO2 (Sabine et al, 2004). This accounts for at least half of anthropogenic CO2 emissions that has occurred since 1750 (Sabine et al, 2004). Another way to look at the atmospheric versus oceanic CO2 volumes is to compare the total CO2 concentrations. The ocean holds roughly 50 times greater concentration per unit volume than the atmosphere (World Resources Institute, 2009). Even if the atmospheric CO2 began to decrease, the ocean's CO2 saturation rate will continue to rise as the residence time of carbon in seawater is thousands of years (Nybakken and Bertness, 2005).
Always on Time
Marked to Standard
[Need to tie in / transition to this diagram. The following carbon flow diagram shows how carbon is cycled through the deep ocean and atmosphere. ?? Tie in to lag time??, ]
Global Carbon Flows Fig. X
(Diagram courtesy of NASAscience.com, 2009)
OCEANIC PROCESSES AND CHEMISTRY
To understand the oceanic effects of rising atmospheric CO2 levels it is imperative to review some fundamental oceanography concepts. Carbon is a natural and necessary part of seawater chemistry and without carbon in seawater, marine life would cease to exist. Just as there are several biochemical cycles that are responsible for the movement of chemicals through the biosphere, there are two major pumps in the ocean. The biological pump which involves productivity and particle sinking and the solubility pump which is responsible for ocean circulation and mixing.
Oceanic Solubility Pump Fig. X
The solubility pump is responsible for moving carbon (DIC - Dissolved Inorganic Carbon) through the water column.
(Photo courtesy of Wikkipedia, 2009)
Atmospheric gases get absorbed, dissolved and released from the ocean via the solubility pump. Two ocean properties drive this pump: the thermohaline circulation and CO2 solubility in water. The ocean's thermohaline circulation pattern connects surface waters with deeper waters via vertical and horizontal ocean currents (Nybakken and Bertness, 2005). The pattern is driven by water temperature, salinity and depth (Piney 2009). Colder, more saline water at higher latitudes is denser and therefore sinks (Press et al, 2004). For example, a major downwelling exists between Greenland and Iceland (CITE). This denser water eventually moves towards the equatorial latitudes with upwellings circulating the water towards the surface. When this water breaks the surface, CO2 is out gassed to the atmosphere. This example is part of the ocean conveyor belt. The second property of the solubility pump consists of the solubility of CO2 in seawater which is inversely related to the water temperature. As the water temperature decreases, the solubility of gases increases. Therefore, colder water in higher latitudes carries more CO2 than water at the equator (Pinet, 2009). As the colder water upwells, CO2 is released because of warmer waters lower CO2 capacity.
Oceanic Thermohaline Circulation
(Diagram courtesy of NASA, 2009)
There are two different sources of dissolved CO2 in oceanic waters. The first is from atmospheric CO2 that transfers across the water surfaces. The second is organic matter obtained from dissolved organic carbon (DOC) decomposition. As food is broken down, CO2 is released and plants take up CO2 when photosynthesizing. In a natural system, without excessive anthropogenic CO2, the CO2 values for the ocean and atmosphere are in equilibrium as the ocean is able to buffer small fluctuations. But as more and more atmospheric CO2 enters the oceans, the equilibrium is disturbed. Similar to a production line, as more raw materials are added to the line, the rate of production needs to increase in order to maintain equilibrium. The ocean however can not "speed up" as it functions at a fixed rate. When CO2 levels change over a period of more than 105 years, carbonate minerals buffer the CO2 inputs and negligible pH changes occur (Calderia et al, 1999). However, when a CO2 change occurs over a shorter time interval, less than 105 years, ocean pH is affected as the ocean's natural buffering system can not cope with the higher input of CO2 over the shorter time period (Calderia et al, 1999). As more CO2 dissolves into the ocean, the higher oceanic CO2 level persists without dispersing directly back into the atmosphere.
Oceanic Carbon Chemistry
When CO2 dissolves in seawater, carbonic acid is produced via the following reaction:
(carbon dioxide) + (water) <-> carbonic acid
The carbonic acid is not stable in seawater and immediately disassociates. This causes a release of hydrogen (H+) ions and forms a bicarbonate ion:
(carbonic acid) <-> (hydrogen ions) + (biocarbonate ions)
An increase in hydrogen ion concentration increases the acidity of the seawater. When the hydrogen ions are released, they are able to combine with any carbonate ions in the water to form bicarbonate. Therefore the CO32- is reduced by the number of hydrogen ions (Baird and Cann, 2008).
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(hydrogen ions) + (carbonate ions) <-> (bicarbonate)
Calcifying organisms form calcium carbonate CaCO3 from CO32- (Miller et al, 2009). Therefore the calcification rate declines proportionately with the decline of CO32- availability. The decrease of CO32- (carbonate ions) is inversely proportional to the increasing anthropogenic CO2 concentration(Orr et al, 2005). This process is referred to as lowering the water's saturation state with respect to calcium carbonate per (Sabine et al, 2004). Simply stated, more atmospheric CO2 leads to more oceanic CO2 which increases the hydrogen ion [H+] concentration and lowers the pH. Hence the term ocean acidification, the lowering of the ocean's pH.
Understanding Biomineralization and Calcification
Biomineralization is defined by Simkiss and Wilber as the process that produces the skeletons, shells, and teeth of most animals (1989). There are two different mineralization processes utilized by marine invertebrates: extracellular and intracellular mineralization (Weiner and Dove, 2003). Mollusks, scleratinian corals, bryozoans and some foraminifera use extracellular mineralization which involves secretion or active pumping of ions with active transport of vesicles across cell membranes (Weiner and Dove, 2003). Coccolithophores, echinoderms and other formaminifera utilize intracellular mineralization where specialized vesicles nucleate biominerals within the cell (Weiner and Dove, 2003). Calcification is a division in the field of biomineralization.
Marine calcifiers create shells or skeletons out of calcium carbonate. They employ carbonate ions as fundamental building blocks. This formation of structures utilizing calcium carbonate (CaCO3) is called calcification (Baird and Cann, 2008). This definition may lead the reader to assume that CaCO3 is a unique compound however calcium carbonate is predominate in marine environments as two common polymorphs, aragonite and calcite are the most commonly biomineralized forms of CaCO3 (Miller et al, 2009). The difference is not the chemical makeup but rather their crystalline structure. For example pteropods have shells made of aragonite (Fabry, 2008) whereas, echinoderms and coccolithophores have shells made from calcite (Royal Society, 2005). Regardless of the polymorph, both aragonite and calcite dissolve as partial pressures of CO2 increases (Miller et al, 2009). Calcification rates are sensitive not only to carbonate saturation state but also to temperature, nutrients and light as well (Tuley et al, 2005). Once the solid CaCO3 structure has formed, it then is constantly vulnerable to dissolutioHYPERLINK "http://en.wikipedia.org/wiki/Dissolution_(chemistry)"n as the surrounding seawater must be saturated with carbonate ions (CO32-) to maintain equilibrium. This is analogous to osmosis when the cell has an equivalent concentration to the surrounding liquid, it is in equilibrium and no exchange takes place. If the surrounding liquid has a lower concentration, then the cell donates ions to the water to reach equilibrium. This is true of the calcium carbonate, when the surrounding seawater becomes CO32- poor the shell donates CO32- to the surrounding seawater, leading to shell dissolution. As aragonite becomes less saturated in the surrounding seawater, the organism's metabolic costs for biomineralization increase (Weiner and Dove, 2003) leading to a trade off for energy usage. Note that an organism can use different polymorphs at different life cycle stages. For example, larvae will biomineralize aragonite, the metastable form of calcium carbonate and the more soluble form of CaCO3, as it requires less energy to utilize (Orr et al, 2005). In contrast, calcite is the predominant material used in adult shells (Miller et al, 2009) and is the more stable form of CaCO3 (Orr et al, 2005). The higher the saturation state of aragonite, the greater the relative availability of both Ca2+ and CO2-3 ions, which lowers the energy required for biochemical pumping needed to generate local supersaturation. Note that local supersaturation is required to produce shell material (Miller et al, 2009).
Any mineral in seawater has a saturation state (known as Î©). This is the measure of the energy of a system that allows the mineral to form or to dissolve.
Î© equals the multiple of the concentrations of calcium and bicarbonate ions when the mineral is at equilibrium (Ksp) divided by Ksp when the mineral is neither being created nor dissolved (Atkinson and Cuet, 2008).
The saturation state for CO2 occurs in seawater when the maximum amount of dissolved CO2 is held (Pinet, 2009). The amount is dependent on water temperature, pressure and salinity. If the water is colder, then more carbonate ions are being used to create biocarbonate ions and therefore not available to calcifers to create calcium carbonate. At equilibrium, the carbonate ion is supersatured and the calcite and aragonite are stable. But as ocean pH falls, the carbonate ion concentration also falls at the same rate, and when seawater becomes undersaturated, structures made of calcium carbonate dissolve. Today, most surface waters are supersaturated and therefore, calcium carbonate is insoluble. Descending through the water column, water chemistry changes due to temperature and depth which facilitates calcium carbonate solubility. The natural saturation horizon delineates where the solubility of CaCO3 increases dramatically becoming undersaturated (UNESCO, 2009) . Of the two polymorph structures, aragonite is 50% more soluble than calcite in seawater (Mucci and Morse, 1984) inferring that the aragonite saturation horizon is always above the calcite saturation horizon (Royal Society, 2005). This implies that organisms producing aragonite structures are more susceptible to increased acidity levels than those which produce calcite as the undersaturated levels for argonite are closer to the ocean's surface (Orr et al, 2005).
Above the saturation horizon (Î© >1) calcite and argonite do not dissolve. This is why most calcifying organisms are found in this habitat (Royal Society, 2005). This seawater zone is referred to as being supersaturated and is where CaCO3 can form (Corliss and Honjo, 1981).
Below the saturation horizon (Î© <1) the rate of CaCO3 dissolution, increases dramatically. As the CaCO3 saturation decreases, shells start to dissolve.
Carbonate Compensation Depth (CCD)
The carbonate compensation depth (CCD) is located below the saturation horizon. At this depth, the rate of CaCO3 supplied is equal to the rate of dissolution which means all CaCO3 has dissolved (Pinet, 2009). Since the dissolution rate exceeds the production rate no calcareous plankton or sediment particles are found in the water column below the CCD.
(Calcium carbonate) + (Carbon dioxide) + water <-> (calcium) + (bicarbonate ions)
This concept is critical to understanding where calcifying organisms are found in the ocean. If the sea floor is above the CCD, then when calcareous organism die, the organism will fall to the bottom without dissolving. Otherwise, if the sea floor is below the CCD, all calcareous material will dissolve before reaching the sea floor. Therefore, shallow waters are usually full of shells as the waters are supersaturated in calcite and shells will not dissolve in that environment. The crux is that the CCD is not a fixed point but is rather determined by temperature, pressure and chemical composition of the water. The shallowest CCD has been recorded in Guatemala at 3400 meters, and the deepest at 6000 meters, off the coast of North America. Generally, the CCD is shallow in high latitudes where downwelling conveys young, surface water with relatively low concentrations of carbon dioxide into the deep ocean, depressing the CCD. The various CCD depths depend on the length of time since the bottom water has been exposed to the surface; the concept is expressed as the age of the water. These 'water ages' are based on temporal variations regarding where the water is located on the global conveyor belt. The oldest water masses have the highest concentrations of CO2 which also correlate to the shallowest CCD. The following diagram shows how the calcite dissolution level affects the formation of sediments as carbon is cycled through the water column.
Carbon Transformation Through Water Column Fig X
(Courtesy of Uriarte, 2009)
Ocean pH levels
Oceanic waters are naturally alkaline. The pH level defines how many free hydrogen (H+) ions are available in a sample of water or conversely, how many hydroxyl (OH-) ions are available (Baird and Cann, 2008). All living organisms have a pH tolerance range which is the organism's acidity tolerance. If the pH levels drop or rise beyond the organism's tolerance, then it will cease to inhabit that location. The free hydrogen ions control the acidity of the water (Pinet, 2009). For example, deep, cold waters are naturally too acidic for marine calcifiers to survive (World Resource Institute, 2009). A pH scale (pH = -log10[H+]) represents the acidity of a liquid ranging from 1 to 14 with 1 being extremely acidic, 7 is neutral and 14 is extremely alkaline or basic. The H+ ions react with other compounds in the water and therefore, the more free H+ ions there are, the more acidic the water is (baird and Cann, 2008). As hydrogen ions increase, pH decreases. Since acidity is measured on a logarithmic scale, for every unit decrease on the pH scale, the hydrogen ion concentration increases 10-fold. For example if the pH drops from 8 to 7, the hydrogen ion concentration has increased 10 fold. If the pH dropped 0.4 units that would represent more than a doubling, an almost 150% increase, in acidity. The oceans regularly have slight fluctuations in pH as water temperature, water pressure and salinity all impact the amount of dissolved CO2 that the water can hold and thus hydrogen ions. These small changes in pH are mitigated by buffering capabilities of the natural oceanic system. The pH of pristine seawater ranges between 7.5 and 8.4 depending on the source (World Resource Institute, 2009) (Nybakken and Bertness, 2005) (Pinet, 2009). Since 1760, a 0.1 decrease in pH has been observed which equates to a 30% change in hydrogen ion concentration in the upper 200 meters of the water column (Miller et al, 2009) (Orr et al, 2005).
RESEARCH: HISTORICAL AND PREDICTIONS
Ocean Research and Predictions
A global ocean survey was conducted in 1994, analyzing the water chemistry from 17,000 locations at 37 depths (Dickson, 2009). These readings help provide a picture of the oceanic status throughout the world. One of the major findings highlighted that the ocean is being contaminated with CO2 from the surface waters. Where the highest CO2 concentration was found correlated with a reduced CCD (Dickson, 2009). The North Atlantic Ocean results differed from global findings as dense cooling waters sank to the seafloor bringing CO2 along with it (Dickson, 2009). The importance of this distinction is that when this water hits an upwelling, those higher CO2 are brought closer to the surface. Research has shown that the saturation horizons for calcium carbonate have shifted towards the surface by 50-200 meters since prior to the industrial revolution (Doney, 2006). Also noted from the global data was that continental shelves play a critical role as spawning and nursery habitats (Feely et al, 2008). The global research indicated that temperate coastal locations had a faster rate of acidification than previously predicted (Science Daily, 2009).
In 2004, world renowned marine biologists predicted in Nature that by 2050, 15% to 37% of species and taxa will become extinct (Thomas et al, 2004). More recently, in 2007, the Intergovernmental Panel on Climate Change (IPCC) predicted ocean acidity could increase 150% by 2100 based on current usage predictions for 2007 (IPCC). Many general predictions have been made regarding the oceans fate, in particular the calcifying organisms. At the Ocean Acidification Second Symposium in 2008, it was announced that even if CO2 levels were maintained at 450 ppm the polar oceans may still become corrosive and coral habitats would be lost. Additionally, it was suggested that further research be focused on coastal zones influenced by river inputs or upwellings as these habitats would be indicative of future ecosystems (UNESCO, 2008). The reasoning is that coastal zones have low biodiversity and high counts of invasive species, which could be a similar result with acidification as few organisms can tolerate higher acidities. Also anthropogenic ocean acidification greatly influences coastal regions due to its close proximity of emission sources (Doney et al, 2007). Lastly, based on the 'age of water' concept regarding the oceanic global conveyor belt, it could take thousands of years for water chemistry to return to pre-industrial levels. The following diagram illustrates the correspondence between increasing CO2 levels and decreasing pH Levels.
Global Ocean Acidification (How CO2 Impacts pH Levels) Fig. X(Courtesy of IPCC, 2007)
Calderia and Wickett (2003), Sabine et al (2004) and The Royal Society (2005) all predict a decrease in pH of 0.1 to 0.5 by the end of the century utilizing the IPCC IS92a scenario which correlates with a 100 to 150% increase of hydrogen ion concentration. As the atmospheric CO2 rises, so too will the saturation horizon and the CCD. The saturation horizon has been predicted to rise 50-200 meters relative to its position in pre-industrial times (Doney et al, 2007) with downwelling zones first being affected (Orr et al, 2005) which was proven by the global ocean survey. Currently the National Oceanic and Atmospheric Administration (NOAA) is investigating large bodies of water that are already undersaturated in aragonite in upwellings off the Pacific continental shelf (Feely et al, 2008).
In 1992 the IPCC released projected emission scenarios levels ranging from best case to worst case. These levels were used throughout the scientific community as standard predictions for research. Alarmingly, current CO2 emission levels are now greater than what was projected as the worst case scenario 17 years ago (UNESCO, 2008).
2 x Pre-Industrial
3 x Pre-Industrial
4 x Pre-Industrial
5 x Pre-Industrial
Avg pH of
At current emission rates, CO2 concentration will double in XXXX (projections from IPCC)
The reason why pH is important is that CO2 regulates the acidity of seawater (Pinet, 2009).
Given the input Co2 levels, the projected Oceanic pH levels are shown.
Past, present, and future pH Levels
(Figures based on total alkalinity of 2324 mol/kg and water temperature of 18 â° C)
(IPCC, 2009) (Calderia and Wicket, 2003)
pH Predictions Fig. X
(Courtesy of World Resources, 2009)
Past levels (Pearson and Palmer, 2000)
Future predictions based on IPCC mean scenarios.
Critics to the severity of ocean acidification fall back on the argument that large atmospheric CO2 releases have taken place, due to significant volcanic activity without significant implications. This refers to the Paleocene-Eocene Thermal Maximum (PETM) that occurred 55 million years ago at the end of the Paleocene epoch (Gibbs, 2006). Analyzing geologic sediments for fossil forminifera remains, an oceanic pH history can be recreated demonstrating a much shallower CCD (Orr et al, 2005). The higher CO2 levels were mitigated by corresponding elevated ocean temperatures and the natural oceanic buffering system (Orr et al, 2005). Researchers have estimated over 50 species of foraminifera disappeared with previously rare organisms becoming prominent (CITE). However, the current day CO2 atmospheric rise and PETM comparison is equivalent to comparing a pteropod to a Humpback whale. Yes, both today's CO2 rise and PETM were dramatic increases in CO2 but that is where the similarities end. The PETM duration is estimated to be 1,000 to 10,000 years in length. During this time, the natural buffering process of discarded shells can alleviated the increased CO2 impact. Today's rate of CO2 release is comparable to a 3 times to 30 times quicker release, respectively (Gibbs et al, 2006). The current release levels of CO2 are too high to be absorbed by the ocean's natural buffer capacity .
All organisms, whether referring to bacteria, phytoplankton or multi-celled organism, the goal is the same, to survive and reproduce. Organisms have water chemistry tolerances and differ in their ability to adapt to change. For calcifying organisms changes in water chemistry, including pH levels can alter metabolic efficiencies and demand energy trade-offs between physiological needs and calcification requirements (Miller et al, 2009). There are two major concerns with acidity levels, the length of exposure to higher acidity levels and the tolerance to under saturation of aragonite or calcite (Miller et al, 2009). In most of the calcifying organisms researched in more acidic waters, ranging from single-celled foraminifera, to crustacea, there was a notable decrease in the organism's ability organism to produce calcium carbonate (Feely et al, 2004) (Gazeau et al, 2007) (Bibby et al, 2008) (Findlay et al, 2009) (Berge et al, 2009).
The effects of ocean acidification can vary dependent on existing environmental conditions, (including water chemistry), as well as, organism-specific dependencies such as stage of life cycle and habitat preferences.
Environmental Dependencies Table X
Influences solubility rate
Influences solubility rate
Influences solubility rate
Impacts solar radiation
Whereas, organism specific dependencies range from life cycle stage to habitat preferences that can influence the effects of raised CO2.
Organism Dependencies Table X
Species specific impacts
Larvae, juvenile, adult
Adults have ability to redirect energy reserves (McDougall, 2009)
Juveniles uses aragonite versus adults use more calcite ( , )
Dependent on use of aragonite versus calcite
During testing for susceptibility to higher CO2 (lower pH) , effects covers an immense range of morphological and physiological deformities from reduced growth to respiratory distress and mortality (Royal Society, 2005).
Organism Impacts for Rising CO2 Levels Table X
Ability to construct calcareous structure/shell
Diminished or eliminated growth progress
(Kurihara, 2008) (Riebesell et al, 2000)
Acidosis, build up of carbonic acid in body fluids
Disrupt growth, respiration and reproduction
(World Resource Institute, 2009)
Growth energy diverted to increase calcification and/or energy reserves depleted
(Miller et al, 2009)
Hypercapnia - acidification of body tissues and fluids
Reproductive and physiological effects
(Miller et al, 2009) (Turley et al, 2005)
Decreased rate, ineffective energy use
Food resource limitations
(Royal Society, 2005)
Reallocation of resources from feeding and reproduction to maintain consistent internal pH levels
Reduction in recruitment and growth
All things considered 34 = wiki
Less effective sperm
Decreased fertilization success, mortality
(Scripts Insitute, 2009)
Echolocation distance increases
Communication and predatory consequences
All things considered
34 = wiki
Vulnerable larval development stage
Increased pre-settlement time increases mortality
Impacts habitat, settlement preferences ,
Increased larval stage in water column increases predation leads to mortality
Ocean Acidification Possible Impacts Fig. X
(Courtesy of Kurihara, 2008)
pH Tolerant Organisms
It should be noted that some calcifying species were tolerant to increased CO2 levels. For example, nitrogen-fixing cyanobacteria may benefit from elevated CO2 levels as reported by the Ocean Acidification Symposium in 2008. Increased carbon fixation rates in some calcifying and noncalcifying photosynthetic organisms due to rising CO2 was also noted by Doney (2009). There is also the potential for marine organisms to adapt to increasing CO2 over a period of time. These adaptations could occur over generations and not be observable during a month or two research project.
pH Intolerant Organisms
There is an enormous breadth of species that ocean acidification affects ranging from benthic invertebrates with skeletal elements including crustaceans (barnacles, lobsters, cnidarians, sponges, bryozoans, annelids, brachiopods and tunicates to echinoderms such as the starfish, mollusks (oysters, clams, scallops) (Fabry et al, 2008) . Some of these organisms may be able to adapt relatively quickly to changes in water chemistry, however echinoderms and bivalves have long generation times which means that any adaptations will occur over a period of time (Kurihara, 2008). The remainder of this paper will focus on two different organisms where extensive acidification research has been conducted. The coccolithosphore, Emiliania Huxley and the bivalve, Mytilus edulis. The purpose of the review is to determine if the research has any commonalities, if the methodologies correlate and were appropriate and determine if the analysis are predictive of future research results.
Coccolithosphores - Emiliania huxleyi (Table X)
Coccolithophores are a single-celled member of the haptophyte phylum able to live in nutrient poor surface water environments. It creates a food source for zooplankton and small fish (NASA, 2009). These organisms are considered major planktonic CaCO3 producers distinguished by the calcite (Fabry et al, 2008) plates they surround themselves with called coccoliths (NASA, 2009). Emiliania huxleyi is the most abundant coccolithosphore found throughout the oceans except in polar oceans (Riebesell et al, 2000) and represents one of over 5000 different species of phytoplankton (National Oceanography Center, Southampton, 2009). Being a phytoplankton, the E.huxleyi is an essential foundation of the marine food web. In addition, E.huxleyi's blooms life stage is a primary force in reflecting radiation and regulating oceanic and planetary heat (Riebesell et al, 2000). When the bloom crashes, the coccoliths sink, adding to the oceanic CO2. Coccolithosphores account for the most of the export flux from surface waters to ocean depths of CaCO3 (Fabry, 2008). Note that Emiliania huxleyi is extremely susceptible to lower pH levels, since it has a thin protective calcite covering as shown in the below research analysis. Recent studies have shown that increased CO2 levels led to slower growth rates and deformed coccoliths formation as well as reduced reproductive rates (Riebesell et al, 2000) (Riebesel, 1A, 2A). Six studies focusing on E.huxleyi have been evaluated regarding methodologies and results. Five studies used live organisms and one was model driven. Studies ran from 6 hours to19 days (excluding model testing). Blooms occurred on average by day 12 and then crashed. The biggest difference among methodologies was testing done in situ versus cultured. In E. huxleyi studies done in situ, blooms were encouraged with the addition of nitrogen and phosphorus and population levels of blooms showed a marked decreased with increased CO2 levels (2000 levels versus 2100 predicted levels)(Dehile, 2005)(Engel, 2005). However, the peak timing and subsequent decline reflected no marked changes. Water chemistry was analyzed through all tests since physiological testing on E.huxleyi cannot be done due to it's size and a lack of current knowledge of physiological coccolithophore systems. Chemical water analysis reflected a significant decrease in available bicarbonate available for calcification. This led to the theory that the bloom size of future generations would be limited. A major drawback with a general analysis is the range of timing that E.huxleyi was subjected to higher dissolved CO2 levels (6 hours to 19 days). This species is an integral part of the ecosystem serving as a large nutrient source for higher organisms that tend to be found in nutrient poor environments (NASA, 2009). Thus, smaller population blooms could have extensive impacts on predator species.
Bivalves - Mytilus edulis (Table X)
Mytilus edulis (blue mussel, edible mussel) from the class Bivalvia is found throughout the world from the Arctic to the Mediterranean (UK Marine Special Areas of Conservation, 2009). Unlike Emiliania huxleyi , M. edulis is sessile, using byssal threads to attach to various substrates from exposed shores to sheltered rocky habitats (UK Marine Special Areas of Conservation, 2009). The M. edulis shell is constructed of aragonite and calcite and is used for commercial shell fisheries, habitats for other organisms and as an indicator species to evaluate water conditions. Mussels contain up to 83% aragonite allowing less sensitivity to short term periods of high acidity (Gazeau, 2007). Reviewing the methodologies for five studies involving Mytilus edulis done within the past five years, there are clear consistencies and a few disparities. All studies used collected organisms from temperate climates (warm and cold), utilized collected waters and maintained temperature ranges comparable to habitat organisms were obtained from. Shell dissolution was observed in numerous studies across several phyla, however these specific studies, focused on one species, look at several different attributes ranging from calcification to physiology. The study by Gazeau et al (2007) found M. edulis calcification rates decreased by 25% when subjected to 740 ppmv (IS92a emissions scenario predicted for year 2100), whereas Bibby (2008) recorded immune system deficiencies. Two studies that were more conclusive than directional were Berge (2005) and Findaly (2009). Both studies took different but more holistic approaches than the others. Organisms obtained were both juvenile and adults, which had different results based on the life cycle stage that was studied by Gazeau et al (2007), Bibby et al (2008) and Beesley et al (2008). However, Berge et al (2005) was the only study that incorporated juveniles and adults. This is vital as it enables researchers to note disparate results based on the M. edulis life cycle when subjected to lower pH levels. Larvae appeared more susceptible to growth impairments as adults had stunted growth, displaying their capacity to divert metabolic energy to maintaining shell size. This was also supported by the McDougall research (2009). The Berge et al (2005) study also utilized a constant flow of natural surface seawater obtained at the experiment site. This created a natural range of temperatures and salinities, mimicking environmental conditions. The Findlay et al (2009) study employed an unique approach by utilizing live and deceased specimens. This allowed the researchers to observe energy costs trade-offs directly as the live M. edulis maintained existing shell size while subjected to increased acidity levels. Dead specimen shells dissolved at a rate of 1.5% per day. Further research has shown that effects are not limited to shell construction but also delays in larval settlement (McDougall, 2009). This leads to higher predation as larvae remain in the water column for longer periods of time (Wendt, 1996). Mortalities were also observed across the board with Berge et al (2005) observing significant loss in smaller size organisms attributing the adult higher survival to the ability to transfer energy reserves.
Future studies should include consistent incubation periods with previous studies of similar species whenever applicable. The five studies reviewed were not consistent and, in fact, completely incongruent ranging from 2 hours (Gazeau et al, 2007) to 60 days (Beesely et al, 2008). This leads to a wide range of theories to explain discrepancies (not an apples to apples comparison). The essence of the ocean acidification problem is a rising level of dissolved CO2 levels in the water column. As mentioned previously with the PETM discussion, critical conditions will emerge in the next 100 years, indicating a crisis in the near future.
A large majority of the ocean acidification studies are done "real time" ranging from a few weeks to months, however modeling over a longer time period is required to account for mulit-year impacts and feedback. Modeling may offer a clearer, definitive analysis as a time lag can be constructed. The reason why researchers would want to look at long range trends is to consider the possibility of adaptative mitigation strategies. Modeling will provide foundation data for longer time sequences.
In addition, more recent studies have begun to look at species in comparison. Whether within a specific species life cycle (larvae, juvenile and adult) or two different species within the same genius (mussels to mussel) or across Classes (mussels to oysters). These studies even the playing field for results and clear conclusions may be available as direct comparisons are available.
Use species with rapid, reproductive rates to investigate ocean acidification impacts with rapid, reproductive species. This will allow successive generations to be observed under raised CO2 levels to determine if adaptations are developed.
Compare multiple species under same conditions to evaluate survive, thrive and mortality rates in order to draw conclusions on what species may be more susceptible than others. Comparison is the key here.
Test specific trophic level species that fisheries are reliant on directly or indirectly, to ascertain possible future trophic cascades or collapses due to ocean acidification.
More in field experiments. This research incorporates daily, natural water chemistry fluctuations which are indicative of species habitats.
Focused research in vulnerable habitats (particularly polar, coastal and upwelling). These locations will be impacted first. Coastal habitats are impacted by additional anthropogenic pollutions, as well as, being a nursery habitat for many fisheries larvae and juveniles.
It is clear that the ocean is not a closed system and can change rapidly when biochemical cycles are not in equilibrium. Thus, the broader implications for the entire oceanic ecosystem are unknown. We can only hope that food web dynamics can adapt to the disruption caused by rising CO2 levels. Changes to phytoplankton populations will impact predator organisms inducing a possible trophic structure shift or even collapse. We have seen over geological time, that organisms adapt, evolve and go extinct. Which species will fall into which category is an educated guess at best, as only time holds the answer. As the socioeconomic impacts of ocean acidification start to affect fisheries, the 28 million people employed in fishing and aquaculture, accounting for US $53,000 million (XXXXX, 2001) , will generate additional incentives for poltical action. One thing is dreadfully clear, doing nothing will prevent nothing.
"Prediction is very difficult, especially if it is about the future." (Niels Bohr, 2009)