Ocean acidification

Ocean acidification

Ocean acidification is the continuous decrease of the ocean pH caused by the uptake of carbon dioxide from the atmosphere. Since the industrial revolution there had been a constant CO2 emission to the atmosphere created by land change and burning if fossil. This has indeed increased the level of CO2 in the ocean. Absorption of CO2 in the ocean water caused chemical reactions that increase the number of hydrogen ions and reduces the amount of carbonate ions in the water making the water more acidic. Carbonate ions are an essential building block for coral skeletons and seashells. Therefore, reducing their concentration in the water makes it hard to build and maintain the calcifying organisms such as clams, deep-sea corals, oysters, sea urchins, calcareous plankton and shallow-water coral. Also, the increased acidity will affect the lives of other organisms in the sea, such as some fish ability to detect predators. Altering the chemistry of the ocean put the entire food web at risk. The world as a whole is facing the problem of ocean acidification, including coral reef. The blue economy is at risk of collapse if the problem persists.

The Exposure of Greater Barrier Reef

The Greater Barrier Reef is one of the Seven Wonders of the World. Sea acidificat6ionh has led to the decline of coral calcification below manageable levels. It is impossible to measure the seawater at all individual reef at the Great Barrier Reef. The earth system models used to assess the water pH are too coursing to regulate the variability in water pH in areas with vulnerable reef. Consequently, the current sea chemistry of the reef is unknown.

Complex interactions between the delivery of fresh water, carbon dynamics from open sea and carbon from coastal watersheds leads to variability in aragonite saturation state in the ocean ecosystem. The drivers of aragonite saturation in the Great Barrier Reef have not been quantified due to spatial and temporary variable process that affects the carbon chemistry of the sea. An observation study on a small number of Great Barrier Reef suggests that the process of dissolution and respiration on the reef modifies the aragonite saturation state of the sea. Also, it affects, to a greater extent, the impact of environmental changes in carbon flowing into the reef. To establish the possible result of the processes affecting aragonite saturation, their relative contribution needs to be resolved. Establishment of the likely impact will help incorporate ocean acidification into the conversation and managerial strategy.

The following study was conducted on over 3000 reefs and the novelty of the approach used to estimate the aragonite saturation alongside its drivers. Samples of dissolved inorganic carbon, total alkalinity, salinity and temperatures from 22 sites located inshore of the Great Barrier Reef were used in the experiment. Also, these samples were coupled with hydrodynamics, catchment, sediment and biogeochemical model.

The study showed that the circulation of water, the regional balance of production/respiration and calcification/dissolution moderated the aragonite saturation of every individual reef. These processes make the mean of the aragonite saturation of the GBR to be more than that of the open sea. More ever, it shows a large mean spatial variability within GBR which is equal to the expected tread in a quarter-century. Understanding the regulations of the aragonite saturation suggests how future environmental changes will affect acidification of the individual reef. For instance, changes in hydrological cycles will have no impact as there are no reefs located where rivers plumes. The study showed that there is relatively high aragonite saturation in the outermost of the reef, which is flushed from the Coral Sea. The research shows that the aragonite saturation decreases relatively to upstream value for inner to mid-shelf reef. There is a further decline in aragonite saturation for the north to South GBR caused by net calcification. There are signs that the central reef water is already showing signs of dissolution in the inter-reef regions, and it is likely to expand to the north and south as time goes on. The net of primary production and net calcification is caused by factors that are hard to predict.

The future aragonite saturations of the GBR are dependent on the primary production and balance of calcification. Aragonite saturation has a number of drivers which cannot be extrapolated from forecast of the open sea. To have an accurate trajectory for the GBR exposure and vulnerability regional carbon cycles and the calcification process must be accurately considered. Ocean warming and anthropogenic pressures will also impact the future of aragonite saturation.

Mass Coral Bleaching

The rapidly warming climate threatens the Great Barrier Reef. Mass coral bleaching events ate witnessed and are triggered by the thermal anomalies. In the summer of 2015-216, the worst mass coral bleaching was witnessed. It is estimated that 29% of coral bleach related deaths took place. However, there are no pre-existing records of long-term temperatures measurements of the mesospheric reef environment.

A study on the potential of the deep mesophotic coral ecosystem to offer refuge against thermal bleaching shows the impact created by the long-term temperature. The study showed that there are GBR areas controlled by local oceanographic conditions. As the South Equatorial Current flows westward through the core sea, it hits the Australian continent shelf influencing the reef along the shelf edge of the Northern Great Barrier Reef. The North-Queensland Current controls the possibility of colder deep water masses and also the position of the thermocline. There is the relief provided to mesophotic reef provided by the occurrence of cold water influxes during summer. However, this condition is constrained when the temperature is elevated to months of winter. This was the case of 2016 that lead to the mass bleaching effect. The shutdown of the North-Queensland Current and warm water waves from the Gulf of Carpentaria were the two main contributors to mass bleaching. The deep reef is the home to a substantial number of the overall reef habitat, can act as a thermal refugee during mass bleaching.

Nonetheless, two important caveats surround the deep reefs ability to act as a a refugee despite the incident of 2016. First, the cold water influxes represent seasonal oceanographic processes and are temporally constrained. It is possible that the bleaching impact had a delayed effect and that the outcome was severe than measured at the time of the study. Despite the incident of 2016, there are still prospect that deep reefs will play the role of thermal refugees to help preserve coral reef. However, its ability to play its part is limited to the continuous global warming effect.

Vulnerability of Coral Reef in the Tropic Pacific to Climate Change

To explore the vulnerabilities of coral reef, it is important to look at the conditions necessary for coral to thrive. Corals in the Pacific Ocean thrive under the alkaline, warm and sunlit waters found between 300N and 300S. Under these conditions the corals are able to accumulate calcium carbonate responsible for skeleton formation. The reef structure is glued by marine calcifiers such as crustone coralline algae. The coral framework has spaces within itself that are filled with green algae and sand. The reef structure builds up overtime as this accumulation fight off the forces of biological, chemical and physical erosion. The eventual reef structure is used as the habitat for marine organisms especially invertebrates and many fish.

According to the marine physical and chemical database, corals require an 180C SST, aragonite saturation and ample light to thrive. These are the most important factors for the formation of coral reefs. However, there are other factors that influence formation of coral reefs that include low salinity events, river run-off and storms. For instance, events such as flooding often lead to reduction in salinity and ultimate killing of coral. On the other hand rivers deliver nutrients necessary for the formation of the coral. Besides, storms and cyclone events destroy coral, shift sand and destroy the entire coral structure. The interactions between these events have the ability to change the collar structure and the animals and organisms associated with it.

There structure and ecological function of the reef is undergoing through major changes. These changes are caused by local and major stressors. The study show that the world total reef cover in 2007 was half that of the 1980’s. Therefore, a loss in reef cover causes a huge loss in coral population estimated at 1-2% annually. The survey shows that even the well managed reefs such as the Great Barrier Reefs is prone to the same changes. Studies suggest that the most affected aspect in the Great Barrier Reef is the growth and calcification which has declined by 15% since the 1990. Global warming and ocean acidification are playing a huge role in the death of the coral reef compared to local factors.

Reef building corals have a mutuality symbiosis with dinoflagellates. Dinoflagellates are tiny plant like organisms that are found in the coral reef. They occupy vacuoles of the cells associated with gastro dermal coral tissues and impact a brown colour to the host. As they synthesise within the cells of the host coral, they produce organic carbon abundantly. The carbon produced powers growth, calcification and reproduction in host coral. This relationship results to abundant of energy produced and corals are able to rapidly grow and calcify. In return, the Symbiodinium receives inorganic nutrients from the coral and calcium carbonate produced is beneficial to the reef community.

However, when subjected to stressful conditions, the symbiosis between symbiodiniuma and coral breaks down. A coral turns white when exposed to conditions such as reduced salinity, solar irradiance and increases in toxic chemicals. Under these harsh conditions, the brown symbiodinium cells leave the coral tissues. The phenomenon is known as coral bleaching. The coral becomes more susceptible to competitors such as starvation, seaweeds and eventually death.

For over 80 years, the local reef region has reported cases of mass bleaching. In the 1980’s, reports of mass bleaching in the entire reef region was reported. All incidences were linked to periods of elevated SST which was characterised with warm and still seas. SST broke the symbiosis of corals and symbiodinium that resulted to their population decline in the cells coral cells.

In the periods when the summer exceeds maxima by 10C to 20C for 3-4 weeks, the SST anomalies will strongly show. In these periods, there is a mass bleaching and increase in deaths as the thermal anomalies intensifies and lengthens.

Since the first mass bleaching in the 1980’s, this phenomenon has continued to occur. All other regions have experienced lower mortality cases compared to Indian Ocean where 46% mortality cases were reported in 1998. In the coming decades, it is projected that there will be more cases of reef mortality caused by rise in SST in the tropical reef region in the coming decade. This is due to the fast rising sea surface temperatures in the tropic Pacific.

Effects of Ocean Acidification

It is estimated that of all the carbon dioxide produced into the atmosphere by human activities, 25% is eventually absorbed by the ocean. As a result the interaction of sea water and carbon dioxide leads to a series of reaction that actually forms bicarbonate ions. This process leads to a sharp decline in the carbonate ions that leads to significant reduction in rate of calcification of reef-building corals. Aragonite saturation states inhibit corals from calcifying rapidly as the coral tend to be overpowered by biological and physical erosion.

A balance between reef calcification and erosion represents coral reefs. The loss of coral reef structure and integrity comes when the balanced is tipped in favour of erosion. Most of calcium carbonate laid down is removed by physical and a biological process signifies a close relationship between erosion and calcification. However, the relationship between erosion and calcification is dependent on a number of factors such as latitude, water quality and location.

Today, the corals in the Great Barrier Reef are calcifying at a rate 15% lower than that in the 1990’s. Additional to decrease in calcification rates, biological erosion is likely to be elevated by decrease in carbonate ions concentration. This is occurs through increased dissolut6ion and reduced density of coral skeletons. Moreover, this phenomenon favours the activities of fish and sea urchins as well as worms and sponges.

Interactions and Synergies

There is a compounding effect when ocean acidification is combined with other stressing factors. For instance, when corals are exposed to increase acidic conditions their thermal threshold at which they bleach decreases. Moreover, there is a dramatic loss in productivity of coral reef when subjected to high SST combined with elevated acidity than when it act on its own. Coral skeletons and community weaken by ocean acidification and bleaching with severe damage experienced in case of storms which accelerates degradation.

Interactions between acidification and ocean warming with local factors such as overexploitation and poor water quality will lead to accelerated impact. For instance, crustose coralline is a major settlement cue for large number of vertebrates including corals. Crustose coralline is highly vulnerable to acidity and would negatively impact on corals and other organisms’ settlement. Likewise, if corals were exposed to pathogens and sediments they become susceptible to thermal bleaching hence increased mortality. Local factors amplify the effects of ocean acidification and global warming thus affecting the ability of corals to withstand harsh events.

Projected vulnerability of coral reefs to changes

Today, the rates of change in ocean acidification and atmospheric carbon dioxide are rapid compared to the previous decades. Since the onset of the industrial era, the pH of sea water has decreased by 0.1which represents a 26% increase in hydrogen ions. This is projected to decrease as the concentration of atmospheric carbon dioxide increases. Further, there is a projected 2.4 decrease in aragonite saturation in tropical waters by 2100.

The projected changes will greatly affect the coral reefs due to high sensitivity of crustose coralline and corals algae to declining carbonates ions. Moreover, there will be an increase in bio-erosion due decrease in carbonate ions. Current experiments and field observations shows that when the atmospheric carbon dioxides exceeds 450ppm, the corals and other marine calcifies will struggle to maintain a positive reef carbonate balance as it falls into deficit. Also, concentrations above 350 ppm are above manageable levels.

There is a very low adaptive capacity for marine calcifying organisms. Coral and other calcifying organisms will not be able to adapt to low carbonate ions concentration in the sea water. Therefore, they cannot be able to keep up with the rapid increase in ocean acidification.

Conclusion

In conclusion, it is projected that in the next decade atmospheric CO2 increase to levels between 430 and 450 ppm with SST is expected to increase to a range of 0.50C to 1.00C. Also the sea levels are expected to raise by 6-10 cm. this upwards trajectory of factors undermining coral development will lead to a steady fall of the coral cover. Also, there is expected to be a rise in competition between reef-building coral and non-calcareous microalgae which then reduces growth and calcification. In observation of these consequences, there is need for action. Despite pressure from interest groups, lack of goodwill from the governments across the world has lead to weak international agreement. There is ne4ed to take action on greenhouse gas emission to reduce threat of declining water quality and reduction of overexploitation. The government ought to manage coral reef in a manner that will eventually lead to their adaptive capability to cope with the future chnge4s.