Miller and Spoolman (2018:79) define biodiversity as, ‘the variety of life on Earth’ and in this article, I will explore the main threats to aquatic biodiversity such as, overfishing and climate change, as well as the importance of sustaining biodiversity within the marine environment.
The Biological Pump and Ocean Acidification
Figure 1. Graph showing monthly average CO2 measurements since 1960 in parts per million (ppm) Source: Lindsey (2022).
According to the United Nations (2020), the ocean absorbs ~23% of annual anthropogenic CO2 emissions. This is because the wide variety of aquatic flora in the euphotic zone, such as phytoplankton, take in CO2 for photosynthesis and when these organisms die, they sink to the bottom of the ocean, trapping carbon within the organic matter.
Marine calcifying organisms, such as, crabs and lobsters also turn carbon into calcium carbonate to build their shells and bones. When these organisms die, the carbon in their shells and skeletal debris is stored within marine sediments on the seafloor for millions of years (Sala et al, 2021). This process known as, the biological pump helps to offset CO2 emissions and mitigate climate change. Therefore, it is important to sustain marine biodiversity.
However, anthropogenic activities are degrading marine biodiversity through ocean acidification. The burning of fossil fuels as well agriculture and deforestation has increased atmospheric CO2 (Fig.1). As a result, the ocean is absorbing more CO2 which reacts with water to form carbonic acid. This increases the amount of hydrogen ions found within the ocean which can dissolve the shells and bones of calcifying organisms (Miller and Spoolman, 2018).
An increase in hydrogen ions within the ocean can also lower the amount of carbonate ions present as the carbonate ions react with hydrogen ions to form bicarbonate ions. Consequently, there are less carbonate ions available for calcification (Miller and Spoolman, 2018). The average acidity of our oceans has risen by 15% since the 1990s and as ocean acidification continues in the future, calcifying organisms will struggle to survive and reproduce, whilst green algae and jellyfish (which thrive in acidic waters) will become dominant (ibid). This will disrupt food webs and threaten marine biodiversity.
Ocean acidification also affects non-calcifying organisms such as, clownfish by disrupting the GABAA receptor in the nervous system (Georgia Institute of Technology, 2014). As a result, clownfish struggle to smell their predators and locate their habitats, lowering their chances of survival and thus, threatening biodiversity (National Oceanic and Atmospheric Administration (NOAA), 2020).
Ocean Warming and Coral Bleaching
Figure 2. Graph showing heat energy in the top half mile of the ocean compared to the 1955-2006 average. Heat content in the global ocean has been above average (red bars) since the mid 1990s. Source: Lindsey and Dahlman (2020).
Figure 3. Coral Bleaching. Source: Miller and Spoolman (2018).
Global mean atmospheric temperature has increased by ~1°C during the last century due to an amplification of the greenhouse effect caused by human activity (Marshak, 2012). According to the United Nations (2020), the ocean has absorbed ~90% of the excess heat in the climate system, causing ocean heat content to increase over time (Fig.2). This has led to thermal expansion of oceans, which paired with glacial melting has caused sea levels to rise by 8-9 inches since 1880. (Lindsey, 2022).
As a result, the algae which resides within the coral reef structures receives less sunlight for photosynthesis. This paired with warmer ocean temperatures, has caused corals to expel their colourful algae (Fig.3), a process known as, coral bleaching which makes them more susceptible to disease and mortality. 50% of the world’s coral reefs have been destroyed or degraded since the 1950s and it is estimated that another 25-33% could be destroyed within the next 20-40 years (Miller and Spoolman, 2018). Although the coral reefs occupy only 0.1% of the world’s ocean, they provide shelter and important breeding, feeding and nursery ground to 25% of the world’s marine fish species (ibid). Therefore, if the coral reefs are destroyed, the fish that depend on them will become vulnerable to extinction, putting stress on the marine species that feed on these fish.
The Socio-economic significance of Coral Reefs
Coral reefs and the biodiversity they support are vital within medicine. Organisms found within coral reef habitats produce chemical compounds that can be used to treat leukaemia, lymphoma, and skin cancer as well cardiovascular diseases within humans (Coral Reef Alliance, 2023).
According to the NOAA (n.d), coral reefs dissipate 97% of the energy from storms, waves, and floods and therefore, act as a natural coastal defence against flooding and erosion. This helps to reduce coastal property damage, prevent loss of life, and protect coastal habitats, helping to preserve biodiversity on land.
The coral reefs are vital within the tourism and fishing industries. It is estimated that 6 million people depend on reef fisheries for their livelihood (Cinner, 2014) and according to the Great Barrier Reef Marine Park Authority (2022), 2 million tourists visit the Great Barrier Reef in Australia every year to go snorkelling and scuba-diving. Therefore, the biodiversity within coral reef ecosystems provides vital economic and ecosystem services and so it is important that Environmental Managers take action to protect it.
Overfishing Keystone Species
Overfishing is when vessels catch fish faster than the fish can breed and naturally replenish their populations (World Wildlife Fund, 2023). Overfishing is currently threatening the biodiversity found within the Galápagos Marine Reserve (GMR), off the coast of Ecuador. Only small-scale, sustainable fishing methods are allowed within the Marine Protected Area, but vessels have been luring fish outside this zone by using Fish Aggregating Devices (FADs) (Collyns, 2017). These are wooden structures with hanging nets that attract fish by appearing to provide protection against predators (Marine Stewardship Council, 2023). In the GMR, FADs are used to catch tuna but attract other pelagic fish as well, such as dolphins, sharks, sea lions and turtles (Collyns, 2017). The tuna is kept for food production whilst the other unwanted fish is discarded back into the sea as bycatch. One-third of the world’s annual fish catch consists of bycatch species (Miller and Spoolman, 2018) which depletes these species and puts stress on the species that feed on them, thus, threatening biodiversity.
According to Mills et al (1993:219), keystone species are ‘crucial in maintaining the organisation and diversity of their ecological communities.’ They feed on and therefore, regulate the population density of various species within an ecosystem, preventing the best competitors from becoming dominant and outnumbering the less competitive species. This demonstrates how trophic interactions within ecology determine biodiversity. However, as we can see with the GMR case study, keystone species, such as, sharks and tuna are being overfished. Consequently, 37% of the world’s sharks are threatened with extinction and some species of tuna such as, the southern blue fin tuna are endangered (International Union for Conservation of Nature, 2021), threatening the overall health and balance of marine ecosystems.
Invasive Species: The Lionfish in North America
An invasive species is ‘one that arrives in a habitat it had not previously occupied’ (Simberloff (2010:131). When this happens, the organism escapes the predators, competitors, parasites, and viruses that had previously regulated its population within its native habitat (Miller and Spoolman, 2018). This allows their population to grow and outnumber the native species. Consequently, the native species struggle to compete with the invasive species for resources. The invasive species can also transmit diseases that the native species are not immune to, making the native species vulnerable to extinction. When the invasive species first start to prey on the native species, the native species are naïve to their new predators and because they have not co-evolved with the invasive species, they have not developed any defence mechanisms or counterstrategies against them. Consequently, they struggle to survive under the threat of predation and as a result, the invasive species can become dominant and threaten species evenness within the ecosystem. .
Figure 4. The lionfish invading the North Atlantic Ocean. Source: Miller and Spoolman (2018).
An example of an invasive species within the marine environment is the lionfish which is native to the Western Pacific Ocean but has invaded the North Atlantic Ocean, off the east coast of North America. Hurricane Andrew in 1992 damaged outdoor aquariums in Miami, Florida causing these lionfish to escape into the North Atlantic Ocean (Miller and Spoolman, 2018). Lionfish are quick to reach sexual maturity, produce many offspring and are protected by venomous spines (ibid) (Fig.4). Therefore, in the North Atlantic Ocean, they have few, if any predators and their populations grow rapidly. The lionfish outcompete reef fish for food and feed on their young. As a result, they have become the dominant species in areas, such as the Bahamas, threatening biodiversity. The lionfish also prey on the parrotfish who consume and therefore, regulate the algae population around coral reefs (ibid). This causes the algae to overgrow and kill the corals, thus, threatening biodiversity once more.
Oxygen Production and Mutualistic Relationships
The biodiversity within our oceans provides vital ecosystem services. For example, marine autotrophs such as phytoplankton produce oxygen through photosynthesis, that us humans need to survive. According to NOAA (n.d), half of the oxygen on Earth comes from the ocean and so it is important that Environmental Managers use their ecological knowledge to protect marine biodiversity.
Figure 5. The clownfish and sea anemone’s mutualistic relationship. Source: National Geographic (2023).
Organisms depend on their mutualistic relationships with one another, in which they ‘interact to their mutual benefit’ (Begon et al, 2006:381). An example of this within the marine environment is the interaction between clownfish and sea anemones. Clownfish live within sea anemones (Fig.5) as it provides them with protection against predators. It also provides them with food supply as they feed on the sea anemones’ waste matter (Miller and Spoolman, 2018). In return, the clownfish provides the sea anemones with protection against some predators and parasites (ibid). Therefore, it is important to sustain marine biodiversity because no aquatic organism is completely self-sufficient.
Conclusion
This article has highlighted the various threats to marine biodiversity such as, ocean warming and acidification as well as overfishing and invasive species. We have also explored the importance of sustaining marine biodiversity by looking at the environmental and economic services that the oceans provide- proving that the marine world is worth protecting.
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