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1.1 Overview of ocean acidification
1.1.1 Mechanism of ocean acidification
1.1.2 Aragonite saturation state, what is it?
2. IMPACTS OF OCEAN ACIDIFICATION
2.1 Overview of the calcification
2.2 Impact of OA on selected marine organisms
2.2.4 Coralline algae and seagrass
2.2.5 Mussels and oysters
2.2.6 Fish larvae
2.2.8 Biogeochemical impacts of OA
3.1 Ecological impacts of OA
3.2 Impacts of OA on social systems
4. CONCLUSIONS AND OUTLOOK
Ocean acidification (OA) refers to the ongoing decrease in the pH of the earth's oceans that is caused by the uptake of Carbon dioxide (CO2) gas from the atmosphere. It is postulated that the pH in ocean surface waters could decline further from the pre-industrial values by the end of the century (2100) if atmospheric CO2 concentration reaches 1000 mg/l. These chemical changes in the ocean are likely to have subsequent biological consequences in marine ecosystems. Despite the wideness of the topic of ocean acidification, this brief review attempts to bring into focus, in simplified way, some of the key aspects relating to ocean acidification. OA is introduced and viewed in terms of its impacts on selected marine organisms as well as on social systems.
Keywords: Ocean acidification, calficification, biogeochemical impact, ecological impact, social systems
The surface ocean and the atmosphere are in equilibrium in terms of the CO2 concentration through constant gas exchange. The ocean however, contains about 50 times more CO2 than the atmosphere because the physical and biological carbon pumps provide a mechanism to remove CO2 from the exchange zone at the surface through transfer into deeper water (Auel et al., 2009). This sequestration of CO2 by the oceans is currently estimated to be approximately 2 Petagrams (Pg) of Carbon per year while the anthropogenic uptake has been estimated to be 118 ± 19 Pg of Carbon from 1800 to 1994 (Koch et al., 2013). Despite the ongoing discussion on the potential rates of increase of atmospheric CO2 concentration due to variable estimates of future burning of fossil fuels, and strong positive and negative feedbacks, it is projected that the present atmospheric CO2 concentration of ̴ 394 mg/l is likely to approach or exceed 1000 mg/l by the year 2100 and reduce pH in ocean surface waters by 0.3-0.4 pH units under the ‘business as usual’ CO2 emissions scenario (Doney et al., 2009, Fabry et al., 2008, Koch et al., 2013).
Increasing atmospheric CO2 concentration influences the oceanic dissolved inorganic carbon (DIC) system that exists in three major forms: bicarbonate ion [HCO3–], carbonate ion [CO3–], and aqueous carbon dioxide [CO2] which also includes carbonic acid [H2CO3]. This is one of the most important chemical equilibria in the ocean that is largely responsible for the control of the pH of seawater (Fabry et al., 2008). A simplified detail on chemical mechanisms that lead to a decrease in seawater pH as a result of the increasing atmospheric [CO2] is given below:
The constant exchange of CO2 between the atmosphere and seawater results in the dissolution of CO2 in seawater into its aqueous form resulting in the formation of carbonic acid [H2CO3]. [Abbildung in dieser Leseprobe nicht enthalten]
Most of the carbonic acid dissociates into a hydrogen ion [H+] and bicarbonate ion [HCO3–]. [Abbildung in dieser Leseprobe nicht enthalten]
The bicarbonate ion so formed further dissociates into the carbonate ions [CO3–]. [Abbildung in dieser Leseprobe nicht enthalten]
This being a reversible reaction, H+ can then react with CO3– to form HCO3[Abbildung in dieser Leseprobe nicht enthalten]
These reactions are fully reversible with the resultant net effect of adding CO2 to seawater being the increase in the concentrations of H2CO3, HCO3– and H+ and a decrease in the concentration of CO3– thereby effectively lowering the pH of seawater (Fabry et al., 2008 and references therein, Koch et al., 2013).
Aragonite refers to the mineral form of Calcium carbonate while "Aragonite saturation state”, (Ω), describes its level of saturation in seawater. Supersaturated waters with respect to calcium carbonate concentration have Ω values greater than 1 (Ω>1) while under-saturated waters have Ω values less than 1 (Ω<1). Sustained OA through increasing addition of CO2 to the atmosphere and the ocean may thus influence the rates of formation and dissolution of aragonite and calcite, bio-minerals that are critical to diverse marine taxa. Aragonite supersaturation favours calcifying organisms while aragonite undersaturation is unfavourable to calcifying organisms. Declining seawater pH increases aragonite undersaturation and its subsequent effects on calcifying organisms (Checkley et al., 2009, IGBP et al., 2013).
Calcification is the process of combining calcium [Ca+] and carbonate [CO3−] ions to form the mineral CaCO3. This secretion of CaCO3 skeletal structures is widespread across animal phyla (Anderson and Gledhill, 2013, Fabry et al., 2008, Kleypas et al., 2006). Most of the calcium carbonate in the ocean is made by calcifying organisms (such as corals) benthic shelly animals, plankton species such as coccolithophores and foraminifera and pteropods where it takes place under direct metabolic control (Ridgwell and Zeebe, 2005).
Organisms that deposit shells, tests, and skeletons made of CaCO3 gain a range of benefits, including structural support against hydrodynamic forces, protection from predators, increased surface area and competitiveness for space, and mechanisms to maintain elevation of the organism above the sediment-water interface into higher light and better flow conditions as well as keep up with sea level rise (Anderson and Gledhill, 2013, Kleypas et al., 2006). Skeletal growth also plays a key role in the reproductive success of some coral species like Goniastrea aspera, whose reproductive maturity is achieved by size rather than age and in Acropora palmata where it promotes potential for sexual reproduction although increased skeletal fragmentation in A. palmata can also promote asexual propagation (Kleypas et al., 2006).
While changes in ocean chemistry as a result of OA are likely to have cascading biological consequences among broad taxonomic groups in marine ecosystems (Fabry et al., 2008, Kroeker et al., 2013, Koch et al., 2013, Martin et al., 2008), this work, limited by its scope, has given special focus to selected ecologically and economically important marine organisms which have recently been shown to be most vulnerable to increasing OA. These include some species of phytoplankton, pteropods, foraminifera, coralline algae and seagrass, mussels and oysters, fish larvae and corals.
Phytoplankton are photosynthetic planktonic protists and plants that consist of single-celled organisms or of chains of cells. They are the basis of primary production of the aquatic environment and hence a foundation of the aquatic food web. They serve as a source of food for zooplankton, small fish and invertebrates, which are then fed upon by bigger aquatic organisms. Examples include diatoms, dinoflagellates, cyanobacteria, coccolithophores, cryptophytes and silicoflagellates (Castro and Huber, 2010, Levinton, 2001).
Some phytoplankton like coccolithophores are unicellular and are covered with a series of calcium carbonate plates and are thus vulnerable to OA (IGBP et al., 2013, Levinton, 2001). While some species of coccolithophores appear to be tolerant to OA, others show decreased calcification and growth rates in acidified waters (Kleypas et al., 2006). For example, some studies have reported a contrary trend of increased calcification in Emiliania huxleyi at elevated CO2 partial pressure (pCO2) (Hoffman et al., 2010, Iglesias-Rodriguez et al., 2008). The study of Riebsell et al. (2000) demonstrated a decrease in calcification with increasing CO2 levels in two coccolithophores species (E. huxleyi and Gephyrocapsa oceanica) indicating intraspecific diversity in the responses of E. huxleyi strains to increased CO2 levels as pointed out by Hoffman et al. (2010).
Pteropods are holoplanktonic gastropod molluscs found globally in aquatic environments. They have higher biomasses in polar areas as well as on the continental shelves and areas of high productivity (Bednarsek et al., 2014, Levinton, 2001). As ubiquitous holoplanktonic calcifiers, they are particularly important for their role in carbon flux and energy transfer in pelagic ecosystems, building shells of aragonite and contributing about 20 - 42% towards global carbonate production. Their highly soluble aragonitic shells render them sensitive to ocean acidification (Bednarsek et al., 2014, Fabry et al., 2008, Tsurumia et al., 2005).
Thecosome pteropods, commonly known as ‘sea butterflies’, are prey for zooplankton, commercially important fish (e.g. salmon, cod and herring), whales, seals, and birds. They also predate on phytoplankton and other pteropods, and as such, factors that affect pteropod abundance will influence many trophic levels (Bednarsek et al., 2014, Hoffman et al., 2010).
Bednarsek et al. (2014), studying the impacts of OA on the pteropod species, Limacina helicina, in the California Current Ecosystem note that, relative to preindustrial CO2 concentrations, the extent of undersaturated waters in the top 100 m of the water column along the CCE has increased over six fold and has strong vertical gradients in aragonite saturation in the first 100 m that was accelerated by anthropogenic OA. The researchers were able to demonstrate a strong positive relationship between the proportion of pteropod L. helicina affected by severe dissolution and the percentage of aragonite undersaturated water in the upper 100 m of the water column.
Foraminifera are single-celled protists secreting calcite shells and are found in all marine environments, from the intertidal to the deepest ocean trenches, and from the tropics to the poles. They may be planktic or benthic in mode of life and are an important part of the marine food chain preyed upon by many different organisms including worms, crustacea, gastropods, echinoderms and fish (Hoffman et al., 2010).
Changes in ocean chemistry associated with OA will result in reduced calcification of foraminifera. Lombard et al. (2010) investigated the effect of CO3− concentration on calcification rates in the planktonic foraminifera Orbulina universa and Globigerinoides sacculifer. Results showed that in both species, low CO3− concentration resulted in a reduction of calcification rate as well as the final shell weight suggesting that OA would impact on calcite production by foraminifera and may decrease the calcite flux contribution from these organisms.
Coralline algae pioneer the colonization of seagrass, making the blade more hospitable for other species such as diatoms, sponges, foraminifera, worms or turf algae and also contribute significantly to sediment accumulation in seagrass (Martin et al., 2008). Martin et al. (2008) found a significant reduction in epiphytic coralline algal cover on P. oceanica meadows with increasing acidification of seawater due to natural CO2 vents. The authors note that these findings suggest that ocean acidification may have dramatic effects on the diversity of seagrass habitats and lead to a shift in the biogeochemical cycling of both carbon and carbonate in coastal ecosystems dominated by seagrass beds. A reduction in recruitment of coralline algae (Agegian) with elevated pCO2 when exposed to undersaturated aragonite concentration has also been demonstrated (Kleypas et al., 2006 and references therein).
Interestingly, Hoffman et al. (2010) reported a variation in response of seagrass to OA. Citing the study of Hendriks et al. (2010), the authors note that seagrass growth increases as pCO2 levels increase with hints suggesting that other primary producers in coastal systems like seaweeds and phytoplankton could respond similarly.
These findings point to the need for more research to fully understand the mechanisms by which coralline algae and seagrass respond to OA.
Molluscs are the second most valuable group of marine food species after fishes. Important food molluscs include squids, cuttlefishes, octopuses, clams (Macoma), oysters (Ostrea), mussels (Mytilus) scallops (Pecten) and abalones (Haliotis). Most molluscs have soft body enclosed in a calcium carbonate shell (Castro and Huber, 2010). Ocean acidification has been shown to markedly degrade the mechanical integrity of larval shells in mussels. Gaylord et al. (2011) cultured larvae of Mytilus californianus in seawater containing CO2 concentrations expected by the year 2100 (i.e. 540 or 970 mg/l) and found that it precipitated weaker, thinner and smaller shells than individuals raised under present-day seawater conditions (380 mg/l), and in addition exhibited lower tissue mass. The authors note that these trends suggest potential consequences including increased vulnerability of new settlers to crushing and drilling attacks by predators; poorer larval condition, leading to increased energetic stress during metamorphosis; and greater risks from desiccation at low tide due to shifts in shell area to body mass ratios. Since early life stages drive general patterns of distribution and abundance of marine species, the ecological success of this vital species may be tied to how OA proceeds in coming decades.
Oyster hatcheries have already started noticing that oyster larvae fail to thrive in acidified coastal seawater (Grossman, 2011). Media reports indicate that shellfish hatcheries are changing strategies in order to stay in business in light of increased acidification of seawater. This includes changing the pH balance of the water being pumped from coastal water into tanks in order to keep the oyster larvae alive.
Early life stages determine recruitment and general patterns of distribution of marine species. Understanding the impact of OA on early life stages of fish larvae is thus vital to predict future patterns in light of increasing OA. Checkley et al. (2009) found that the otoliths of fish larvae of white sea bass (Atractoscion nobilis) grown in seawater with high CO2, and hence lower pH and aragonite saturation, were significantly larger than those of fish grown under simulations of present-day seawater conditions. The authors pointed out that information lacks on whether these results would apply to other taxa with aragonite sensory organs, such as squid and mysids (statoliths) or other fish species. They further noted that information also lacks on whether larger otoliths have a deleterious effect on fish larvae but point out that asymmetry between otoliths can be harmful.
Clearly, there are substantially knowledge gaps to be filled in order to fully understand the impacts of OA on early life stages of fish.
Corals calcify to form the massive three-dimensional structures that define coral reefs that create habitat supporting an extraordinarily high biodiversity. Coral reefs support many functions of a coral reef community that includes but not limited to: (1) the ability to keep up with sea level rise, (2) the creation of spatial complexity that supports diversity, (3) the depth gradient that also supports diversity, and (4) the structural influence on the local hydrodynamic regime (Hoffman et al., 2010, Kleypas et al., 2006).
Reef building requires reef calcification to exceed reef dissolution. This is achieved through production of more calcium carbonate by corals than is removed. Apart from light, temperature, food and nutrients and hydrographic regime, seawater CO2 chemistry (CO2, H+, HCO−, CO3−, and CaCO3 saturation state) is one of the most important variables that directly affect coral calcification and growth rates (Anderson and Gledhill, 2013). With decreasing seawater pH, unfavourable surface water conditions for tropical coral reef growth are created that will decrease reef-building due to a decline in calcification rates. Further, as the overlying water becomes less supersaturated, it is hypothesized that the rates of dissolution in the sediments and reef framework will increase (IGBP et al., 2013, Kleypas et al., 2001, Kleypas et al., 2006, Marubini et al., 2001).
However, coral responses to OA are more diverse than currently thought and the findings by Jury et al. (2010) raise questions on the reliability of using [CO3–] or aragonite saturation state (Ω) as the sole predictor of the effects of OA on coral calcification. The study found strong responses of the coral Madracis auretenra to variations in HCO3– concentration with very little, if any, responses to changes in CO3– concentration, Ω or pH indicating a lack of full understanding of the physiological processes that drive coral calcification hence highlighting the need for more research in this study area.
Precipitation of CaCO3 in the marine environment is primarily driven by corals, benthic shelly animals, plankton species such as coccolithophores and foraminifera, and pteropods where it takes place under direct metabolic control. Of the major planktonic CaCO3 producers, coccolithophores, foraminifera, and euthecosomatous pteropods account for nearly all the export flux of CaCO3 from the upper ocean to the deep sea (Fabry et al., 2008, Ridgwell and Zeebe, 2005). Coccolithophores and foraminifera are important in calcite precipitation in the open ocean and a reduction of calcification rates of these organisms as a result of increasing OA would impact on calcite production and may subsequently lead to a decrease in calcite flux contribution from these organisms (Kleypas et al., 2006, Lombard et al., 2010, Ridgwell and Zeebe, 2005). Likewise, pteropods are important for their role in carbon flux and energy transfer in pelagic ecosystems, building shells of aragonite and contributing about 20 - 42% towards global carbonate production (Bednarsek et al., 2014, Tsurumia et al., 2005). Corals also play a significant role in the world’s calcium balance by precipitating approximately half of the calcium delivered to the sea each year (Moberg and Folke, 1999). Increasing OA means a decrease in calcification rates of corals and an increase in the rates of dissolution in the sediments and reef framework (Kleypas et al., 2001, Kleypas et al., 2006, Marubini et al., 2001). Further, by reducing the colonization of seagrass by diatoms, sponges, foraminifera, worms or turf algae, OA can lead to a shift in the biogeochemical cycling of both carbon and carbonate in coastal ecosystems dominated by seagrass beds (Martin et al. 2008).
Significant buffering of ocean chemistry and of atmospheric CO2 results from this biologically driven carbonate deposition. By adversely impacting calcifying organisms, OA will impact the oceans as net carbonate sinks, the effects of which require deeper understanding of the marine carbonate cycle (Ridgwell and Zeebe, 2005).
This section summarizes the impacts of OA advanced in section 2 and also introduces the link between OA and its effects on social systems.
The ecological consequences of OA are profound. Early life stages drive general patterns of distribution and abundance of marine species. By having direct impact on fish larvae as demonstrated by Checkley et al. (2009), OA will influence recruitment and subsequent abundance and distribution of fish, thereby potentially leading to a decline in catches. Further, phytoplankton are the foundation of the aquatic food web, hence even slight changes have potential cascading effects including impacts on fishery. As demonstrated by Riebsell et al. (2000), OA has the potential to alter primary production by adversely affecting coccolithophore growth. This will affect fishery which will in turn impact food security negatively. However, this will depend on non-coccolithophorid primary production responses to OA (Flynn, et al. 2015).
Molluscs are the most successful marine animals in terms of abundance of species and are also the second most valuable group of marine food species after fishes (Castro and Huber, 2010). Direct impact of OA on molluscs (for example degradation of the mechanical integrity of larval shells in mussels as demonstrated by Gaylord et al. (2011) and media reports of failure of oyster larvae to thrive in acidified seawater in oyster hatcheries) means that there will be altered patterns of distribution and abundance of these species and shellfish in general. This will not only affect the ecology of these species in the wild but will also have significant adverse effects on the aquaculture of these species as is being currently noticed in oysters.
There are potential cascading ecological effects due to reduced calcification rates in pteropods and foraminifera as a result of increasing OA. Fabry et al. (2008) note that negative impacts on pteropods would result in greater predation on juvenile fish, for example salmon since most of the carnivorous zooplankton and fish (e.g. cod, pollock, haddock, mackerel) that feed on euthecosomatous pteropods would have to switch to other prey types. Still, those species that prey exclusively on shelled pteropods (for example gymnosomes) would likely shift their geographic distribution in concert with their euthecosome prey. Similar effects will be expected as a result of reduced calcification rates on foraminifera that form a vital part of the marine food chain preyed upon by many different organisms including worms, crustacea, gastropods, echinoderms and fish (Hoffman et al., 2010) .
The most complex yet ecologically significant effects of OA will be those related to impacts of OA on corals and seagrass. Increasing OA means a decrease in reef-building (due to a decline in calcification rates) as well as an increase in the rates of dissolution in the sediments and reef framework (Kleypas et al., 2001, Kleypas et al., 2006, Marubini et al., 2001). Some coral reef organisms migrate back and forth between adjacent ecosystems for example fish that migrate to mangroves and sea-grass beds and use them as nursery grounds. The net effect of this migration is the transfer of energy from the system where feeding or development occurs to the system that shelters the adults. Furthermore, pelagic juvenile stages of many reef organisms that drift into adjacent ecosystems serve as food sources for commercially important fishes. Thus, apart from the loss of the high biodiversity supported by coral reefs, OA will lead to a loss of ecosystem services provided by coral reefs like provisioning services (subsistence and commercial fisheries), cultural services (tourism and recreation), regulating services (protection of beaches and coastlines from storm surges and waves) and supporting services (cycling of nutrients and nursery habitats) (Castro and Huber, 2010, Moberg and Folke, 1999, UNEP-WCMC, 2006).
Impact of OA on seagrass is associated with a reduction in epiphytic coralline algal cover on seagrass, that pioneer the colonization of seagrass, hence influencing diversity of seagrass habitats (Kleypas et al., 2006, Martin et al., 2008). Not only do seagrasses provide critical habitat for diverse groups of marine organisms, but also play an important role in fisheries production by providing refuges and nurseries for larvae and juveniles of many fish species. Other functions range from providing food for many macro-grazers and micro-grazers, sediment stabilisation and protection from coastal erosion, filtering water, absorption of organic nutrients, oxygen production and taking up CO2, supporting adjacent habitats (through flows of organisms and physical resources), including salt marshes, mangrove forests and coral reefs (Di Carlo and McKenzie, 2011, Moberg and Folke, 1999).
Probably one of the most worrying possibilities that require urgent attention is the impact of OA on upwelling ecosystems. Upwelling ecosystems are highly productive regions of seas that comprise the most valuable fishing grounds of the world including small and medium pelagic fish as important economic resources (Auel et al., 2009, Castro and Huber, 2010). While upwelling regions like Eastern Boundary Upwelling Systems (Benguela, Humboldt, Canary and California systems), have naturally lower pH levels, increased ocean acidification and aragonite undersaturation in the California Current System as reported by Gruber et al. (2012) is alarming since these regions are most productive ecosystems. These researchers note that specific attention should be given to the development of ocean acidification in Eastern Boundary Current Systems where similar conditions prevail.
While there are significant variations in responses of marine organisms to ocean acidification, there will be general reductions in survival, calcification, growth, development and abundance of marine organisms in response to ocean acidification as pointed out by Kroeker et al. (2013) and various studies that have been cited here.
Clearly, possible worldwide ecological and resultant economic impacts as a result of increasing ocean acidification necessitate continued focus of research in this area to fully understand its scale of impacts and possible mitigating strategies.
Ocean acidification is only one of several physical changes occurring in coastal systems. Water temperature, stratification and hypoxia are all increasing with climate change and all will interact with ocean acidification (Hoffman et al., 2010). These multiple stressors will not only have dire consequences for biological systems but also for social systems globally. According to RCP4.5 (Representative Concentration Pathways 4.5, a modelling tool for projecting ocean biogeochemical parameters), by the year 2100, approximately 2.02 billion coastal people will live in countries with medium to high ocean biogeochemistry change; of those, 1.12 billion live in countries of medium to high ocean dependence; and of these, 870 million live in low-income countries (Mora et al., 2013).
The concentration of people in coastal areas is not surprising since linkages between the natural and social systems are readily apparent with coastal systems providing goods and services that are essential to human well-being. In view of the various studies presented in this short review, potential impacts of OA and associated stressors on coastal resources are likely to include changes in the distribution and abundance of finfish and shellfish resources, reductions in the yield and profitability of wild capture fisheries and shellfish aquaculture, suppression of recovery of species and stocks already depleted through overfishing or habitat loss (Hoffman et al., 2010, Mora et al., 2013, IPCC, 2014). The potential of OA to alter ecosystems (by impacting shellfish, fisheries and the health of coral reefs) will affect the society and the economy on a global scale (ISRS, 2008). In light of this threat, the call for new approaches for coastal resource management to mitigate on the effects of OA and associated stressors on aquatic ecosystem is well placed and in order.
Failure to effectively reduce CO2 emissions will sustain the increase of OA and associated stressors with resultant negative ramifications on ecosystem goods and services. This is likely to increase poverty and economic shocks, which according to IPCC (2014) are key drivers of violent conflicts in the form of civil wars and inter-group violence.
The most effective way to combat OA is to prevent CO2 build up in the atmosphere, either through reduction in fossil fuel emissions, or carbon sequestration technologies (ISRS, 2008). This fact effectively links combating OA to efforts of combating climate change and the politics involved therein with differences apparent among nations.
From the ground breaking Copenhagen Accord in which developed and developing nations committed to reduce their respective CO2 emissions by implementing economy-wide emissions targets for 2020 (UNFCCC, 2009), CO2 emissions were projected to reach a record high of 40 billion tons in 2014. The 'big 11' nations in terms of CO2 emissions based on their share of global CO2 emissions contributed a combined total of 66% of global CO2 emitted in 2013 (Oliver et al., 2013). This is a very high percentage of emission from a few nations. While they lead in CO2 emissions, these nations could also take a leading role in bridging the gap of differences that has characterized the failure to successfully combat climate change.
While there seems to be more of the talk with less of the walk when it comes to combating climate change through reduction of CO2 emissions, it is still necessary to:
- Maintain the political will to combat climate change by reducing CO2 emissions. Nations should have a more humanitarian approach than economic approach in these talks, remembering that it is eventually self-destructive if the increase in CO2 emissions is not curtailed and reduced.
- Direct more effort in scientific research with a view of fully understanding the implications of OA. This will enhance better understanding of OA from which mitigating measures will be derived.
- Adopt a conservation ecosystem management approach in which managers are well informed and empowered by scientific evidence that efficiently trickles down from research findings to managers.
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