Bleaching Coral Reefs and Ocean Acidification. By Rachel Gallaher

Transcription

1 Bleaching Coral Reefs and Ocean Acidification By Rachel Gallaher

2 Warming Oceans, Increasing Acidification About 25% of all CO2 released from anthropogenic sources enters the ocean where it then reacts with the water to produce carbonic acid. Through evaluation of data taken from the Vostok Ice Core, scientists were able to compare the ocean temperatures and carbonate-ion concentrations of present times to those during the past 420,000 years. Image taken from Coral Reefs Under Rapid Climate Change and Ocean Acidification Science magazine (December 14, 2007)

3 Notes for Slide One As anthropogenic sources release larger amounts of carbon dioxide each year, about 25% of the total is absorbed into the oceans. Upon entering the water, it reacts with the water and produces carbonic acid, which in turn dissociates to form bicarbonate ions and protons. These bicarbonate ions and protons reacts with carbonate ions and as a result produce more bicarbonate ions. The overall result of these processes is that there is a reduced carbonate availability for biological systems such as coral reefs. Carbonate is important to these systems and its reduction in turn reduces the rate of calcification of such organisms as corals. It is these levels of carbonate-ion concentration (which prove crucial to the life and survival of coral reefs) that become severely affected by the world s output of CO2. The graph on this slide plots various points of carbonate-ion concentration (lower x-axis) from the past 420,000 years, showing the atmospheric carbon levels when they occurred (on the upper x-axis), as well as the deviation in degrees Celsius from today s sea temperatures (y-axis) at the time of their occurrence. The units which the carbonate concentration are measured in are micromoles; one micromole is equal to 10^-6 of a mole. The thresholds presented on the graph for each axis indicate the point at which major changes would be seen in coral communities. For example, the thermal threshold is set on the graph at +2 degrees C, which means that at a rise of this amount, coral communities would begin to see major shifts in such aspects as growth, population, fecundity, etc. The three letters shown on the graph represent three different scenarios based on increasing levels of carbon emissions presented in the IPPC s Fourth Assessment Report in These will be discussed in more detail in the following slides. Image on title slide taken from: Image on Slide One taken from Coral Reefs Under Rapid Climate Change and Ocean Acidification, article appearing in Science magazine (December 14, 2007).

4 Responses to Increased CO2 Not only are increased concentrations of CO2 detrimental to the continuation and survival of coral reefs, but the rate at which the CO2 change occurs proves crucial, as most modern types of coral are unable to adapt quickly enough to sudden changes that occur in their environments. Some of the potential responses of reefbuilding coral to such an event are: Concentrations 1. Decreased linear extension rate 2. Reduced skeletal density 3. Maintained skeletal growth through greater energy investment in calcification

5 Notes for Slide Two As this slide notes, sudden changes to the environments in which coral reefs are found can prove extremely detrimental to the life and survival of the reef-making coral. If these sudden changes (such as rapidly increased CO2 concentrations over a short period of time) cause reduced calcification there are various ways in which corals react. For clarification purposes I should mention that the calcification of coral is the product of the linear extension rate and skeletal density; in short it is the process in which the mineral calcium builds up in an organism s tissue and eventually hardens. 1. Decreased linear extension rate: As the name might suggest, the linear extension rate of the coral is the outward growth of the individual branches of a coral. Reduced calcification would therefore, obviously have adverse effects on this. The pictures at the bottom of the slide illustrate examples of linear extension. 2. Reduced Skeletal Density: Sometimes corals will maintain their linear extension only through detriment to their skeletal density; skeletal density in coral is similar to bone density in human beings. Just as an older person with low bone density is more likely to break their leg than a younger person, a lower skeletal density in coral means a weaker or more fragile organism that is more susceptible to damage or injury. Also, the decreased density makes the coral more susceptible to the grazing of certain animals such as parrotfish who prefer the carbonates from these lower-density corals. 3. Maintained Skeletal Growth Through Greater Energy Investment in Calcification: If a coral does not decrease either its skeletal density or extension rate, and maintains its calcification rate in order to do so, but there are less carbonates available, it will be forced to use more energy for this growth. The effect of this is that resources are taken from other vital processes such as reproduction. Scientists cannot currently say with full confidence that the observed decrease in coral growth and calcification are a direct result of ocean acidification, but these decreases seem to coincide with decreasing levels of ph and carbonate-ion concentrations. Images on Slide Two taken from galleries at

6 Projected Future Scenarios According to the IPPC, global temperatures are projected to rise anywhere from 1.8 degrees C to 4 degrees C as a result of raised levels of CO2 emission. The three projected future carbon emission scenarios that would all have adverse effects on coral reefs are: 1. Scenario A: Stabilized CO2 levels at 380 ppm 2. Scenario B: Raised CO2 levels to ppm 3. Scenario C: Raised CO2 levels exceeding 500 ppm

7 Notes For Slide Three The three pictures on this slide show visual representations of the projected physical state of coral reefs under each of the three IPPC scenarios listed below. The letters in the upper left hand corners of each picture not only match the scenarios, but also the letters displayed on the graph on Slide One showing carbonate concentration in the water in relation to CO2 concentration in the atmosphere. I will now briefly go through each scenario and explain the possible effects of each on coral reefs. SCENARIO A: This is the Business As Usual scenario in which we continue to release CO2 at the same levels we are today and do not increase them (more than 1 ppm per year) and so the atmospheric concentration becomes stabilized at around 380 ppm. In this scenario, coral reefs may experience a slight change in their inhabitants as well as the growth rate of the coral, but they will still be dominated by coral in their current areas of habitation. Local, non-global warming related factors will also play a role in their survival (i.e. fishing pressure, pathogens etc.) SCENARIO B: Under this scenario, in which atmospheric concentrations of CO2 rise to ppm, reef erosion will begin to exceed reef calcification (reef-building). Coral density will decline, as will coral diversity as certain corals become weak and die out. This will lead to an overall loss of biodiversity in the reefs. Coral will also become more susceptible to bleaching. As coral dies out, it allows for settlement of marcroalgae which interferes with coral reproduction and growth because these two compete for the same resources. SCENARIO C: In this drastic scenario, CO2 levels would increase to over 500 ppm and raise sea temperatures more than 2 degrees C above their current levels. This scenario would prove disastrous and fatal to coral reefs, reducing them to the crumbling ruins seen in picture C on this slide. The communities of organisms that were able to survive under the first two scenarios would not be able to do this same in this one. Image on Slide Three taken from Coral Reefs Under Rapid Climate Change and Ocean Acidification, article appearing in Science magazine (December 14, 2007).

8 Coral Bleaching Coral Bleaching is a process in which coral experiences a loss of color, hence the bleaching due to the stress-induced expulsion of symbiotic algae, or loss of pigmentation in the algae. This process can occur as a result of many different factors including: INCREASED WATER TEMPERATURES CHANGE IN WATER CHEMISTRY

9 Notes For Slide Four As can be seen from the pictures in this slide, coral bleaching is a very accurate name for the results of the process listed. Both pictures feature coral that has undergone bleaching and is therefore left as mere skeletons. When the coral organisms are faced with certain stressors, not only does their calcification rate have the potential to decline, but also they become more susceptible to bleaching. During bleaching, the coral expel the Zooxanthellae, which is a photosynthesizing, unicellular algae that live in their tissues. Changes in ocean temperature as outlined in the graph in Slide One, as well as decreasing ph levels, both conditions which are projected to be caused by increased atmospheric levels of CO2, are stressors that cause bleaching. Bleaching not only affects individual coral organisms and the reefs in which they resides, but also the other organisms that have habitats in among them and depend on them for survival. Once bleaching begins it continues, sometimes for long periods after the stressor has been removed or altered. It can take weeks, or even months for coral populations to regain their original density. Coral can then either be colonized by their original species of Zooxanthellae, or a new kind altogether. However, after bleaching coral often lose some of their reproductive ability and growth rate, and can die altogether with a temperature rise of over 2 degrees C. The IPPC Report estimates that the temperature thresholds at which the bleaching occurs will be crossed more frequently if CO2 concentration increases in the future. Images in Slide Four taken from

10 Consequences, Impacts and Intervention Possibilities CONSEQUENCES Decrease in Coastal Protection Decreases in Tourism in Certain Areas Decreased Reef Rugosity INTERVENTION METHODS Reduce Local Stresses Facilitate Fish Grazing Setting Catch Limits

11 Notes For Slide Five Clearly then, given the potential amount of damage that I have outlined due to both decreased coral calcification and potential coral bleaching, it is important to realize that the impacts of these events do not only rest under the waves. There are heavy possible human impacts as well. For example, many coral reefs growing on coastal lines act as barrier from the impacts of waves and storms. With a decrease of existing coral reefs, people, infrastructure and coastal ecosystems will become more vulnerable to these impacts. Coral Bleaching and decreasing reefs due to climate impact will also cause a decrease in tourism in many areas as the reefs lose many of their interesting organisms as well as their ability to attract tourists. In some countries this would be catastrophic as up to half of their Gross Domestic Product depends on tourism. Reef Rugosity is essentially the amount of wrinkles or curves in a coral reef. This is an element that proves vital for many different fishing industries including industrial as well as those that provide tropical fish for aquariums. Fish density is likely to be higher with a higher reef rugosity, but decreased rugosity gives fish fewer hiding places, making them less likely to settle in certain reefs. Of course the reduction of anthropogenic CO2 output is vital for not only coral reefs, but for the future climate of the earth in general. Some smaller ways in which humans can help counteract the destruction of coral reefs include the reduction of coastal pollution, the management of fish grazing as some species that are left to graze on coral have in the past decimated entire reef populations, and also a setting of catch limits on fish that would help provide natural management of these coral-eating species. SOURCES FOR PRESENTATION: O. Hoegh-Guldberg, et al. Coral Reefs Under Rapid Climate Change and Ocean Acidification. Science. Vol. 318 (2007) , IPPC, Climate Change 2007: Impacts, Adaptation and Vulnerability. M.L. Perry, et al., Cambridge University Press: New York, 2007.