Life under the water

How does coastal eutrophication influence the severity of coral bleaching events in the Great Barrier Reef?

Raeha Boyer

Introduction

The United Nations shares 17 goals of sustainable development ranging from poverty to marine ecosystems. Life under the water is goal number 14. This entails conservation and sustainable use of the oceans, seas and marine resources for sustainable development. The emergencies that surround the ocean contain coastal eutrophication, coastal acidification, ocean warming, plastic pollution and over-fishing. (United Nations, Goal 14 | Department of Economic and Social Affairs)

The great barrier reef stretches across 2,300km along the coastal line of Australia and is a huge tourist attraction for the continent. This ecosystem holds the most ecological significance being home to a multitude of marine animals such as 1,500 species of fish, 400 species of coral, mollusk, sponges and other invertebrates, over 2,900 individual reef structures, 900 islands. The Great Barrier Reef is a refuge space for mammals such as the green sea turtle, dolphins, whale sharks and white tip reef sharks (Reef Biosearch 2012) that keep predator and prey relations balanced which is crucial for the marine ecosystem and the terrestrial ecosystem.

The most important part of the coastal area is the mass amount of coral entities that live on the sea floor. These corals have grown in this area for millions of years and in symbiosis with dinoflagellate whom give the corals their vibrant and beautiful colors and is crucial for health of the corals. These algae provide the corals with their essential nutrients with photosynthesis whilst the corals provide a sheltered environment for the dinoflagellates to thrive and grow. Looking more intricately to their relationship, it's shown that they contribute to calcification from that photosynthesis that contributes to the corals calcium carbonate skeleton. If this relationship starts to strain from natural and unnatural stressors such as change in temperature, pollutions and radiation. This could be detrimental to the corals health as they start to expel they dinoflagellates and become more susceptible to diseases and famine. In 2003, it was unbeknownst how important these relationships are to the severity to coral bleaching believing they were just anomalies. (Andrew Baker, 2003)

Looking into how changes in the microbial communities living on these bleached corals within the Great Barrier Reef play a part in the resilience of corals during and after the bleaching, how can these shifts contribute to potential recovery or continued degradation of the reefs ecosystem.

Coral bleaching 101 - coral bleaching explained | Great Barrier Reef Marine Park Authority

Here we have Dr. Dave explain coral bleaching, an expert from the Great Barrier Reef marine park authority.

METHODS

The Australian Great Barrier Reef is a major tourist attraction bringing in millions (approx. 1.6M) of tourists to the northeastern coast (Nat Geo, 2023). The biodiversity is rich and fulfilled with invertebrates and a multitube of vertebrate animals. Though the main attraction, the coral reefs are at risk.

Global climate change in Australia varies around a range of space and time over thousands of years. The orbital position of the earth and the spatial relations between each island and reef contribute to the rate of climate change though it varies naturally (Lough, 2007).

Globally the temperatures have been rising from human activities since the 18th centuries in the industrial revolution which have had a major increase for our already natural greenhouse gasses though without this effect the earth would be 30 degrees (Celsius) cooler though with enhanced greenhouse effect more heat gets trapped within the climate system (Lough, 2007). Greater rises in temperature happen around the fall months than in the summer months, typically the range is 0.4-0.6 degrees whilst the maximum range is 2-3 degrees Celsius based on large scale averages and the range of sea surface temperatures (SST) observed on coral reefs can be much greater than observed (Lough, 2007).

Corals survive in a narrow range of temperature and environmental conditions meaning they are super sensitive to the slightest change in environment. The sensitivity of ecosystem properties such as calcification and productivity is inevitably derived from the sensitivity of species groups such as corals and plankton. Damages to the photosystem (dinoflagellates) and the host body (corals) cause a "bleaching" to occur (Lough 2007b).

Temperature rise contributes to specific direct and indirect effects including the outbreaks of crown-of-thorns starfish. Large drought breaking floods get associated with the outbreaks of this species. Once this outbreak has been initiated, they can spread to reefs either north or south to where the outbreak started.

SPATIAL ANALYSIS

There are a multitude of different reefs along the overall great barrier reef as shown in the map provided. More specifically, 2,900 different reefs ranging across the coast.

Each reef sits within a heated area of SST that constantly changes its perimeters. It's shown over time the environmental differences in Fig. 2 from 1989-2021, the constant rise and fall of the temperatures throughout the months.

With the knowledge given prior it is to understand the stress all these environmental changes that the corals undertake. This will cause the bleaches to occur more often with little time to recuperate and regrow their bodies of calcium carbonate for the dinoflagellates to come back to a safe protected area and give the corals their vibrant color's and nutrients to survive, it is very possible for the corals to come back to life if the temperature change doesn't stick around for a prolonged amount of time.

The northern reefs do not have as much chance to cool down like the southern reefs do, also shown in fig. 2. The first recorded island to begin bleaching was the was in the north. Major outbreaks happened in 2016 where the northern and the central both went white, central recovered quickly whilst the north still struggled with their comeback.

ATTRIBUTE ANALYSIS

Figure 3 shows the distributions of the invasive species, the Crown-of-thorns starfish (COTS). Being the worlds second largest Echinodermata, COTS prey on nearly all corals and can eat their way through 10 square metres of it a year creating a major disruption in the ecosystem. During an outbreak, which is when 15 or more COTS are found in a one hectare area, the starfish can strip a reef of 90% of its living coral tissue (Crown of Thorns Starfish 2023).

They have become an issue within the GBR from how quick they can consume the corals, not giving them enough time to regenerate and grow to their full state again. Reef managers have found a way to control the starfish without harming the ecosystem around them by injecting the invertebrate with either Ox liver or vinegar (Crown of Thorns Starfish 2023).

The removal project of the COTS budget is $41.5million as well as other financial budgets, feasibility study ($1.5 million), research and development ($8.3 million), and Independent scientific reviews ($250,000)

With this being the fourth major outbreak since 1960, management have found that now is the right time. COTS control is ensuring the most effective and efficient decisions are being made when controlling the current COTS outbreak, and finding innovative ways to address this key threat to the Reef into the future.

DENSITY ANALYSIS

In figure 4 it's shown where in the reef bioregions the COTS occupy the most.

In the southern part of the reef they cluster within the Swains inter reef and Central inter reef. Moving up there is more clustering in multiple different bioregions, Outer Central inter reef, Western Pelagic platform, Intermediate broad slope and outer shelf inter reef - central and the furthest north, steep slope, far northern offshelf, offshelf and Princess Charlotte bay outer shelf. Aside from the mass clustering in specific areas, the other COTS spots lay at a random cluster.

The COTS stay close to the shoreline where the sunlight hits the most for the optimal coral growth positions to feed off of and do not travel far into the Eastern plateau and the far northern offshelf.

CONCLUSION

In conclusion it was found that temperature and environmental changes have a mass effect on the bleaching of coral reefs specifically within the Great Barrier Reef of Australia. Though seeing climate changes rapid growth over the span of approximately 100 years and the resilience that the Anthozoa's are equipped with to these changes the effects have become extremely dangerous for their wellbeing.

These extreme environmental changes are not the only issue within these waters, a simple food web and predator-prey relations play within the causes of the death of the reef. Crown of thorn starfish, corals main predator within the reef, has gotten uncontrollable in the 4 major outbreaks starting in the 60's.

To continue rehabilitation of this beautiful ocean monument, the control of these poisonous starfish must be maintained over years. It is impossible to say that sea surface temperatures must stop changing especially when it is a natural occurrence however, finding ways to cut back on human emissions to the greenhouse gas emissions will greatly increase the chance of a slow on the detrimental rise of the earths temperatures. The life under the water is in dire need of assistance and it all depends on a collection of efforts from communities around the local area as well as the world around it.

References

Baker, A. C. (2003). Flexibility and specificity in coral-algal symbiosis ... - annual reviews. Flexibility and Specificity in Coral-Algal Symbiosis. https://www.annualreviews.org/doi/abs/10.1146/annurev.ecolsys.34.011802.132417

Reef Biosearch. (2012). Sharks of the great barrier reef. Reef Biosearch Sharks of the Great Barrier Reef Comments. https://www.greatbarrierreefs.com.au/sharks-great-barrier-reef/index.html

Sheppard, C., Davy, S. K., Pilling, G. M., & J., G. N. A. (2018). The biology of coral reefs. Oxford University Press.

Lough, J. (2007). Part I: Introduction chapter 2 - gbrmpa. Climate change in the barrier reef. https://elibrary.gbrmpa.gov.au/jspui/bitstream/11017/535/1/Chapter-2-Climate-and-climate-change-on-the-Great-Barrier-Reef.pdf

Lough, J. (2007b). Part III: Habitats chapter 17 - gbrmpa. Chapter 17 -  Vulnerability of coral reefs of the Great Barrier Reef to climate change. https://elibrary.gbrmpa.gov.au/jspui/bitstream/11017/549/1/Chapter-17-Vulnerability-of-coral-reefs-of-the-Great-Barrier-Reef-to-climate-change.pdf

Brown, T. (n.d.). Great Barrier Reef. Education. https://education.nationalgeographic.org/resource/great-barrier-reef/

Slezak, M., & Timms, P. (2020, April 6). Scientists say the Great Barrier Reef is getting “tougher” as seas get warmer. ABC News. https://www.abc.net.au/news/2020-04-07/great-barrier-reef-most-widespread-coral-bleaching-on-record/12107054

Crown of Thorns Starfish. Great Barrier Reef Foundation. (2023). https://www.barrierreef.org/the-reef/threats/Crown-of-thorns-starfish