Present Day Sea Level Rise

The IPCC’s Fourth Assessment Report (2007) predicts that global mean sea level will have risen between 0.18 – 0.59m by 2090, relative to 1980 levels (although there has been much debate over the accuracy of these predictions). Sea level rise is in response to a warming climate, which is currently causing the melting of polar ice and glaciers and the thermal expansion of the oceans. Coral reef ecosystems are dependent upon being able to maintain a certain level within the water, in order to receive sufficient solar energy. Global sea level rise is likely to affect coral reefs by reducing the amount of available sunlight and causing 'drowned' reefs. This happens when the sea level rises at a faster rate than coral growth, meaning there is insufficient levels of sunlight for the maintenance of zooxanthellae within the coral (Hoegh-Guldberg, 1999). Coral reefs are able to adapt to gradually rising sea levels by growing vertically. However, it is predicted that present day sea level rise coupled with ocean acidification (which reduces the calcification rate), will limit the ability of coral reefs to adapt.

An early paper that attempted to assess the threat of sea level rise to coral reef ecosystems published in 1988 by Buddemeier and Smith. This paper estimated that sea levels will rise at an average rate of 15mm per year, which is five times the modal rate of vertical coral reef growth. The authors expect the vertical growth rates to increase in response to rapidly rising sea levels, however they still conclude that this accelerated rate will be insufficient. The paper attempts to predict the response of coral reefs to sea level rise by assessing the maximum growth capacity of coral organisms. This is approached through the analysis of coral reef response during the period of rapid post-glacial sea level rise at the start of the Holocene and the last interglacial period. Their study suggests that rapidly growing corals will be more abundant, as this will enable the reefs to adapt. However, the success of coral reef adaptation is dependent on the level of sea level rise; if it exceeds the maximum growth rate of corals then many reefs will be lost. It is interesting to note that this article does not mention ocean acidification, which is now recognised as a limiting factor for coral reef growth. 




Done (1999) suggests that coral reefs will easily be able to adapt to sea level rise as they are capable of vertical growth rates of up to 20cm per year (compared with current sea level rise which is given as less than 1cm per year). However, he admits that the response will vary between and within regions, reflecting differences in species and the events and processes that operate at that scale.




Figure 1: Photos taken 6 hours apart on the Pacific island of Tuvalu, indicating the potential impact of sea level rise on low lying islands (Source: Gary Braasch).




Aside from the impact on coral reefs, sea level rise will also detrimentally affect coastal populations, especially those in low lying tropical regions where coral reefs are common. One of the most well publicised cases is the island of Tuvalu, which has been featured in the press and academic papers (Lewis, 1989; Church et al, 2006). Tuvalu is a low lying coral attol, where the highest point is less than 5m above sea level and is therefore likely to be one of the first casualties of a rising sea level. Vafeidis et al (2008) developed a global coastal database to assess sea level rise, within the DINAS-COAST project. It aims to support impact and vulnerability analysis to sea-level rise at a range of scales, by collecting high resolution data and will therefore facilitate future modelling exercises. It is this form of monitoring and modelling projects that will enable the better understanding of sea level rise, and the potential impact this will have on coral reef ecosystems and coastal regions.

In the news this week . . .

An article published in the Scotsman this week highlights the importance of Scotland's cold water coral resource. As outlined in the previous post, these deep water corals are highly vulnerable to disturbances and environmental change. A research team from Heriot-Watt University, Edinburgh is investigating the impact of ocean acidification on these ecosystems over the next few years through laboratory experiments and expeditions to the reefs.

The economic impacts of coral reef degradation was outlined in a Guardian article this week. It refers to the double blow currently being dealt to coral reefs from anthropogenic climate change and direct human impacts, such as damaging fishing activities, tourism and pollution.
 


The environmentalists favourite oil company Shell has been heavily crticised this week for planning an oil and gas drilling site thirty miles from the Ningaloo reef. Apparently the burning of fossil fuels isn't destroying coral reefs quick enough, so instead they have decided to take on a more direct approach. 

Ocean Acidification

Rising carbon dioxide emissions from human activities poses another threat to coral reefs in the form of ocean acidification. More than 30% of anthropogenic carbon dioxide emitted to the atmosphere is taken up by the ocean, a process which lowers the pH Sabine et al (2004). Increased ocean acidity affects coral reefs by reducing the calcification rate of reef builders and making coral susceptible to dissolution. The ability of coral reefs to produce calcium carbonate defines these ecosystems, and ensuring that a net surplus is produced enables reefs to build up. This documentary by the Natural Resource Defence Council provides a good deal of information regarding ocean acidification, and the potential impact to coral reef ecosystems:







Inorganic carbon, which is dissolved in the oceans, is used by coral organisms to deposit calcium carbonate. Increasing uptake of atmospheric carbon dioxide by the oceans requires carbonate ions that would otherwise be used by marine organisms to build shells or skeletons, as in the case of coral reef ecosystems. Decreasing carbonate ion concentrations is therefore likely to cause weak and brittle coral skeletons and slow growth rates. The chemistry of this process is relatively well understood, but the impact it will have on the global coral resource is heavily debated. Gattuso et al (1999) reviewed the carbon and carbonate cycle in coral reefs and the potential effects of human induced environmental change. 

  

Crustose coralline algae are an important calcifying organism in most marine habitats. In coral reefs they produce carbonate sediments and form coral reef structures out of carbonate fragments. Kuffner et al (2007) set up an eight week mesocosm experiment to quantify changes to calcifying components as a result of decreasing calcium carbonate saturation, which occurs with increasing carbon dioxide uptake by the oceans. They showed that the recruitment rate and growth of crustose coralline algae was significantly inhibited in mesocosms that had higher carbon dioxide levels. This study shows that ocean acidification has the potential to cause severe changes to benthic community structure in coral reefs, which will threaten coral reef ecosystems. 

 

This problem is not just restricted to tropical coral reefs, cold-water and deep water corals are severely threatened by ocean acidification. Their slow growth rate and limited ability to recover from disturbances make them more vulnerable to the effects of human activities. Turley et al (2007)  review the potential impact of ocean acidification on cold water corals, highlighting the lack of knowledge regarding these inaccessible ecosystems. They refer to a study by Guinotte et al (2006)  which predicts that 70% of known cold water coral ecosystems will be in water that has very low carbonate ion saturation levels, meaning it is unlikely that the corals will be capable of calcifying.
 


The ability of coral reefs to form calcium carbonate rapidly ensures that they are able to migrate upwards in order to adapt to changing sea levels. The threat posed to coral reef ecosystems by ocean acidification will limit their ability to adapt and could result in significant global losses of coral reef ecosystems. However, the impact of ocean acidification is not restricted to the reef building organisms. Many coral reef species produce calcium carbonate shells or skeletons that are central to their survival. Although there has been little research carried out on these species, Klepyas and Yates (2009) summarise knowledge regarding the effects of ocean acidification on these taxa in a special feature in the journal Oceanography. Deep sea sediment cores have provided a lot of information regarding past ocean acidification events, and have enabled an understanding of how these important coral species may be affected. Kump et al (2009) compares previous ocean acidification events to the human induced changes that are occurring today. Once again it appears the environmental changes that are taking place in our oceans are occurring at an unprecedented rate, a fact which questions the ability of coral reef ecosystems to adapt and survive.  

In the news this week . . .

Following on from the last post about the threats posed to Coral Reefs by rising sea surface temperatures, there is some good news from researchers at Southern Cross University's National Marine Science Centre. A team has been investigating coral bleaching at Lord Howe Island, which is located 500km off the coast of New South Wales. Warmer than average sea surface temperatures in 2010 caused mild to severe bleaching events in large areas of the coral reef, but according to Doctor Steve Dalton most sites are now showing signs of recovery. The full story can be viewed here.
 
A research team led by Dr Johnathan Kool of the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University, has published a paper which reinforces the importance coral reef ecosystems have in maintaining marine biodiversity. The paper, 'Connectivity and the development of population genetic structure in Indo-West Pacific coral reef communities', presents results that suggest the Coral Triangle, which is the richest marine area in the world (located between Indonesia, Malaysia and the Philippines), is highly dependent on coral and fish larvae swept in from the South China Sea and Solomon Islands. Strong connections between these regions are responsible for the high diversity and resilience in the Asia-Pacific region. Dr Kool can be heard talking about his research in this region on ABC radio. The original news article can be viewed here.

Coral Bleaching

Coral bleaching refers to the whitening of coral following the loss or reduction in the symbiotic single celled organism zooxanthellae, which provide nutrients to the polyp through photosynthesis. This was first described by Peter Glynn (1984) following the 1982-3 El Nino event that caused mass coral bleaching in the Panama and Galapagos regions of the Pacific. Before the 1980s, most coral mortality events were localised and largely linked to factors such as storms and exposure to air during low tides. However, since the ‘80s there has been a marked increase in coral mortality events that are spread over large areas and due to coral bleaching, as a response to increasing sea surface temperatures (Glynn, 1992). Although the monitoring of coral reefs has been more extensive in recent years, anomalously high sea surface temperatures (linked to anthropogenic climate change) have been more frequent and caused widespread coral mortality.

Figure 1. Bleached coral at Reunion Island in the western Indian Ocean. (Source: John Pascal/ Agence pour la Recherche et la Valorisation Marines).

The global rise in sea surface temperatures that has occurred and which is predicted to continue as a response to increasing atmospheric concentrations of greenhouse gases, is likely to result in widespread coral mortalities, due to the sensitivity of coral to periods of thermal stress. One of the best sources of information regarding coral bleaching is the book ‘Coral Bleaching: Patterns, Processes, Causes and Consequence’ (van Oppen and Lough, 2008). The book looks at all aspects of coral bleaching, as the title indicates, at a number of scales and includes attempts to predict future scenarios. In this book a comparison is made between coral bleaching events in the Caribbean and Indo-Pacific, with those in the Caribbean being much more significant. The Caribbean coral reefs are much more vulnerable ecosystems, since they contain less biodiversity than those in the Indo-Pacific and have been more negatively affected during past climate changes (Smith and Buddemeier, 1992).

Severe bleaching events occurred in the Caribbean in 1995, 1998 and 2005, but they all had different impacts to the ecology of the reefs affected. ‘Caribbean Coral Reefs after Bleaching and Hurricanes’ (Wilkinson and Souter, 2008) reviews that status of reefs in this region following the 2005 event, predicts future changes and assesses the susceptibility of reefs. The 1995 event resulted in minimal mortality, even though widespread regions of coral reef suffered from bleaching. 1998 was the most detrimental event, with a reduction in coral reef cover of 50% in Belize. While the 2005 event affected the most widespread area, minimal mortality was reported compared to the two previous significant events. This has been attributed to the number of storms during this period, which mixed the waters and prevented ‘doldrum conditions’ that were linked with previous events. Looking to the future, this book suggests that the climatic conditions that lead to coral bleaching events are projected to occur more regularly over the next 20-30 years. Whether or not bleaching events will occur depends on the adaptive ability of corals and their symbionts. Current scenarios indicate that more than 75% Caribbean reefs will have to increase their thermal tolerance by 1-1.5°C over the next half century to avoid bleaching events occurring more than once every five years. Many biologists believe this time frame is too short to prevent widespread loss of corals.

While many of these papers present a fairly depressing picture of the future for coral reefs, there is some evidence that they have the ability to adapt to changing sea surface temperatures. Baker et al (2004) present findings from a study investigating the thermal tolerance of coral reefs across the tropics. Their results suggest that corals containing thermally tolerant symbiotic algal species are more abundant on reefs after severe bleaching events. Adaptive shifts to these species will improve the resistance of affected coral reefs to future bleaching events, and therefore has the potential to prevent mass extinctions. However, the speed of change may well prevent these adaptations from occurring.