Case Study: Effect Of Climate Change On Ningaloo Reef

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Climate change:

Climate change is any long term change in the measurable properties of a climate. Climate change may occur due to natural processes such as changes in temperature due to volcanoes, internal pressures of a climate, the sun’s radiation or human induced climate change which occurs due to changes in composition of the Earth’s atmosphere because of harmful human processes (Australian Academy of Science, 2019).

Solar energy is the driving force of the climate, the amount of energy the sun emits changes due to how far away the sun is from the earth, a portion of solar energy is reflected off the atmosphere and never makes it to earth, however due to changes in the atmosphere caused by humans the amount of solar energy that comes to energy is increased. The surface of the earth also produces heat, known as infrared energy, due to human induced climate change the amount of infrared energy that is released into space is decreased and the amount that is reflected back to earth is increased (Figure 1) (Australian Academy of Science 2019). These changes impact sea levels, melting ice sheets, ocean temperatures (Graph 1), rainfall, plant growth, etc. The many organisms that live in all of the ecosystems are unable to adapt fast enough to survive these changes which threatens the population.

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Coral Reefs:

Corals are colonial organisms that are made up of individual polyps, beneath the soft bodies of stony coral’s polyps secrete a calcium carbonate skeleton that becomes the foundation of coral reef ecosystems( P. Henkel, 2010). However, the major abiotic factors that allow for the biotic factors of the reef to survive such as sea temperature, salinity, water density, light penetration and water acidity are very vulnerable to the effects of climate change. Climate change is the biggest global threat to coral reefs ecosystems (NOAA, 2018) rising sea temperatures, driven by climate change, are increasing the frequency and severity of mass coral bleaching events and reducing the opportunities that coral has to recover (Climate Council Of Australia, 2017), ocean acidification, a result of increased carbon dioxide dissolving into the ocean, decreases growth rates and structural integrity of coral and changes in storm patterns lead to the destruction of reef structure (NOAA, 2018). Both temperature and salinity affect the calcification of the reef and while affect the abundance of coral, this will result in less diverse reef communities.

Hermatypic corals rely on photosynthetic zooxanthellae to grow quickly enough to produce coral reefs, this severely limits their chances of distribution (P. Henkel, 2010)

Ningaloo Reef:

The Ningaloo reef is the largest fringing coral reef in Australia (ningaloo atlas pdf), it spans over 260 kilometres of coastline in Western Australia (Figure 2), this reef is a world heritage site due to its biodiversity and exceptional conservation values of the reef including the vast array of marine habitats that the reef harbours. The region’s economy is based on tourism, fishing, mining, horticulture and livestock, while nature based and wilderness tourism is the main source of income in Exmouth and Coral Bay, and is marketed nationally and internationally as a premier tourism destination (Western Australian Tourism Commission, 2003). The reef has an incredibly diverse population of fish, corals, algaes and hosts migrating species of sharks every year in warmer months.

Species in the Ningaloo Reef:

White Sharks:

The apex predator white sharks (Carcharodon Carcharias) are frequently found in high temperate coastal waters such as the Ningaloo reef. White sharks were observed to have in population abundance that were found to be influenced by abiotic factors such as sea surface temperature, ocean acidification, tidal height were all related to daily sightings of white sharks (Robbins). When ocean temperatures are too warm, sharks heart rates spike increasing their oxygen needs unlike most species of sharks white sharks have a complex circulatory system that conserves heat that is generated by the movement of the muscles of the shark and then redistributed throughout the body, this adaptation allows white sharks to be active in waters that most other species of predatory shark would find too cold but white sharks are also more vulnerable than other species to higher sea temperatures. (Robbins) concluded that female white sharks were present when ocean temperatures ranged from 15.7 to 18.1 degrees celsius whereas males were observed at temperatures ranging from 14.3 to 17.8 degrees celsius. Temperature variability, driven by climate change has been shown to have a direct impact in the male to female ratio and population abundance and distribution of white sharks.

White sharks are distributed in temperate and subtropical waters with very specific sea temperature niches. Temperature variability of the ocean is a significant predictor of daily and monthly variability in predation rates (Skubel). Observed over 18 years of monitoring at Seal Island in False Bay, South Africa (Figure 3) rising ocean temperatures are directly influencing the abundance of both male and female white sharks, in 2017 and 2018 it was recorded that over prolonged periods of consecutive sampling says that there was a complete absence of white sharks. The decrease and then the eventual disappearance of the white shark population prompted the emergence of a new apex predator, the sevengill shark, Notorynchus cepedianus (see Graph 1.1). This species has been recorded attacking a live seal in the absence of white sharks, this data provides evidence for behavioural changes in a marine predator following the decline of white sharks (Hammerschlag, 2019).

These changes in behaviour have an impact on the entire marine ecosystem, predators such as the sevengill shark are scavengers and eat a wider variety of prey that white sharks ignored. In False Bay, sevengill sharks frequently hunted inshore kelp beds, sites that white sharks were unable to hunt in due to the dense kelp that obstructed white shark movement provided shelter to their prey, sevengills aggregating these kelp beds benefited from reduced competition for shared prey. Due to the sevengill being able to move through the inshore kelp beds the fish species that sheltered in the kelp will be affected due to the lack of defenses that these organisms have against the sevengill. The Ningaloo reef’s environment, where the population of white sharks is also decreasing in relation to sea temperature, could just as easily be affected by the effects of climate change and could bring devastating effect onto the organisms of the reef.

Figure 3: (A), Location of Seal Island off the Western Cape of South Africa. (B), False Bay. (C), Area where white sharks historically patrolled the waters of Seal Island in colder months.

Graph 1.1: Relative abundance of white sharks and sevengill sharks over 18 years of monitoring at Seal Island in False Bay, South Africa. Following the drop in white shark abundance the sevengill sharks emerged in great numbers.

Common Brown Kelp

Algae, more commonly known as seaweed, is a vital part of most marine ecosystems, it is one of the most common producers found in the Ningaloo reef. Seaweed are multicellular organisms that bind to rock surfaces due to their lack of a true root system, this plant provides the basis of the majority of food chains and is essential to most organisms that live in the reef. Seaweed in the Ningaloo reef covers hectares of the reef and usually grows in the spring and the summer before breaking off and floating away with the current. This plant has a slick surface to prevent organisms in the reef from attaching to it, the stem of the plant is also very flexible and strong so the organism can withstand currents. However, increased storm rates, fluctuating seasonal temperatures and ocean acidification, driven by climate change, the seaweed known as Common Brown Kelp, Ecklonia radiata commonly found in the Ningaloo reef, has been breaking off rock platforms and dying. Climate change is increasingly affecting marine ecosystems including habitat forming sessile organisms (Polocsanska, 2013), seaweed provides a complex habitat for many adolescent fish to mature in, without the abundance of the common brown kelp these adolescent fish will be unable to fully mature and reproduce and thus continue the species. The lack of kelp will seriously affect coral reef productivity and biodiversity (J. Fulton).

Coral reefs show great resilience to the effects of climate and the destruction of reefs is often slow. Ningaloo reef in particular was able to escape major coral bleaching that affected other reefs in Australia such as the Great Barrier reef. However this trend can not be expected to continue, if rising sea temperatures and ocean acidification continue to rise it can be concluded that the reef would not be able to recover and will eventually sustain damages that the ecosystem will not be able to recover from.


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