The Decline of Coral Reefs: How Humans are Contributing. Caitlin Harris. Longwood Biology and Environmental Sciences Department.

Transcription

1 The Decline of Coral Reefs: How Humans are Contributing Caitlin Harris Longwood Biology and Environmental Sciences Department Sophomore Seminar BIOL 288 April 10 th, 2018 Abstract

2 1. Introduction By 2040 to 2050, most of the worlds warm-water coral reefs will be gone. Coral reefs have been rapidly declining over the past 30 to 50 years with roughly half of all tropical reefs already damaged or completely degraded (Hoegh-Guldberg et al. 2017). These underwater rainforests are commonly known because of their vast biodiversity. This biodiversity refers to the number of different species within a specific area (Wilson et al. 2009). In this case, the exact number of species that inhabit corals are unknown but could likely be in the millions (Knowlton, 2008). We do know however that roughly 1/3 of all fish can be found in these reefs (Crabbe, 2008). Other than fish, reefs are also home to many marine species such as crustaceans, turtles, and sponges. They also provide protection for land by disrupting the rough waves during a storm which helps slow the erosion of coastal shores. However, within recent years, many anthropogenic-driven stressors have drastically taken a negative toll on the structure and composition of coral reefs and will likely kill off corals faster than they can recover (Crabbe, 2008). Rinkevich (2015), senior scientist at the National Institute of Oceanography, speculates that if no action is taken to drastically improve the survivorship of coral reefs, the world will lose up to 70% of the current existing reef coverage by the year This literature review will determine the main anthropogenic-driven stressors contributing to the decline of coral reefs and will review some conservation methods that could help recover damaged reefs. 2. Coral Reefs

3 The most well-known reefs are Australia s Great Barrier Reef and those found in the Caribbean which are situated in the tropical belt (Fig. 1). The clear, warm water is required for the photosynthesis of a specific algae, known as zooxanthellae (Fig. 4A&B), that grows on the corals (Knowlton, 2008). These algae are the color that we see when looking at a bright coral reef (Crabbe, 2008). This mechanism is known as a symbiotic relationship because the algae provides the corals with sugar (glucose) and oxygen, and the corals then give the algae nutrients and protection in return (Hoegh- Guldberg et al. 2017). Under the algae is the coral itself that is composed of a calcium-based exoskeleton (Fig. 4D) (Knowlton, 2008). With a large variety of corals, each kind reproduces in a different way. Asexual budding is a common process when a mature coral develops polyps which then leads to the growth of that colony. The cues for sexual reproduction are unknown but is specifically timed so that the eggs from the female corals are released at the same time as the male corals sperm. This method is usually called broadcast spawning. Once the egg is fertilized it is carried by the current until it settles on the ocean bottom or on top of another coral and starts to mature. Another method of sexual reproduction is brooding, where the eggs are fertilized from within a coral and are allowed to develop before being released (Knowlton, 2008).

4 3. The Effect of Coral Reef Fisheries Most coral reef fisheries worldwide are overexploiting reefs close to the point of complete ecological collapse. Newton et al. (2007) analyzed data from 49 island countries and determined that over half (55%) were conducting unsustainable fishing practices (Fig. 2). This means that the fisheries are catching fish at rates 64% higher than the reef and fish populations can replenish. Removal of certain fish species can cause a negative cascading effect on the structure of the coral reef itself (Newton et al. 2007). For example, parrotfish are common herbivores found in coral reef areas. These fish help maintain algae coverage and they promote coral recovery (Graham et al. 2007). The possible removal of this species at high rates could cause the reef to become less productive due to overgrown of algae and lower recovery rates. Even though coral reef fisheries only make up roughly 2 to 5% of all global fisheries combined, they have put roughly 1/3 of reefs at risk for overexploitation. Based upon Newton et

5 al. (2007) results, in order to compensate for the worlds ever growing population and increasing want for fish (Fig. 3), the Earth would need to produce roughly 4 more reefs the size of the Great Barrier Reef in order to reach sustainable fishing practices worldwide. 4. The Effect of Global Climate Change Global climate change is notably the most significant cause related to the decline of coral reefs (Coker et la. 2009). Climate change refers to the change of average global climate patterns over a long period of time. This is not a new concept to coral reefs and their evolution. However, the problem with the current climate change is that it is rapidly accelerating due to the increased human consumption of fossil fuels such as coal, oil, and natural gases (Hoegh-Guldberg et al. 2017). When these fossil fuels are burned they release carbon dioxide (CO2) into the air. CO2 is known as a greenhouse gas and in great abundance, will cause the entrapment of warm air inside the atmosphere therefore increasing the average temperature. This increase in temperature can lead to such things as, more intense and frequent storms, coral bleaching, as well as ocean acidification (Rinkevich, 2015). 4.1 Extreme Weather Storms, such as hurricanes, can cause major damage to coral reefs due to rough waves. Coral reefs protect the near-by coasts by taking the full force of these incoming waves. Strong waves have the ability to break corals apart and cause the sediment on the ocean floor to be stirred up. Once the water has calmed down, the sediment will settle back on top of the corals causing some to suffocate (Knowlton, 2008). With evidence showing that these kinds of storms

6 are becoming stronger and more frequent, coral structural complexities will more than likely decrease with each hit of these storms (Wilson et al. 2009; Rinkevich, 2015). Another problem that is underrepresented is the melting of the ice caps. Due to the buildup of greenhouse gases and therefore the increased air temperature, polar regions are shrinking. This is leading to rise in sea levels by roughly 3.2 mm year -1. This may not sound very drastic but warm water corals live at a certain shallow depth to maximize the zooxanthellae s ability to photosynthesize. Once the coral is too deep, the algae will not be able to produce as much nutrients (due to less sunlight) causing the algae to leave or die (Hoegh-Guldberg et al. 2017). Without the nutrients from the algae, these corals will not be able to grow upwards fast enough and will eventually drown (Knowlton, 2008). This was seen in Hawaii roughly 14,700 years ago. A once shallow reef drowned due to the rapid increase of the sea level leaving the reef now 150 meters below sea level (Webster et al. 2004). 4.2 Ocean Acidification Coral reefs are sensitive systems and function within a specific range of ph. Any kind of deviation (above or below) causes reefs to fall apart (Coker et al. 2009). Ocean acidification refers to the lowering of the oceans natural ph levels which are roughly around 7-8. With the constant and increasing burning of fossil fuels, we have created a large abundance of CO2 into the atmosphere. The ocean will try to absorb some of this excess CO2 in order to maintain an equilibrium of CO2 in the air and water (Hoegh-Guldberg et al. 2017) which then drops the ph of the water (Knowlton, 2008). This is dangerous to corals because they are composed of a calcium-carbonate base. This means that the corals are being dissolved as the ph continues to drop lower and lower. Since 1990, calcification rates have decreased by at least 15 to 30% and

7 have been predicted to further decrease by 78% by the year 2100 (Rinkevich, 2015). The overall drop in ph levels does not look that significant because it is only by a few units (Hoegh- Guldberg et al. 2017), but this small change can have a significant impact on the corals ability to successfully grow (Knowlton, 2008). 4.3 Coral Bleaching As recent as 2016, there was a mass bleaching event that has been one of the worst on record resulting from one of the warmest years (Hoegh-Guldberg et al. 2017). Natural events like this have been recorded for , , and As these events become more frequent and intense, some people like Crabbe (2008) begin to predict what the future will look like. He determined that the most susceptible areas are the Caribbean, Southeast Asia, and the Great Barrier Reef (Crabbe, 2008). Coral bleaching is caused by the increase of sea surface temperatures (SSTs). This process happens when the SST rises above the maximum temperature at which the algae can withstand. Once it is too warm, these algae either die off or they leave their corals to find other better suitable environments. The bare white corals (Fig. 4D) are then left susceptible to starvation, disease, and eventually death (Hoegh-Guldberg et al. 2017; Knowlton, 2008). As a result of bleaching, coral

8 growth in stunted (Crabbe, 2008) and causes short and long-term effects on the organisms around them (Graham et al. 2007). Short-term, bleaching will likely cause a decrease in survival of organisms that gain food and shelter from the corals, but long-term, bleaching will collapse the reefs overall species richness either by higher predation rates (Fig. 5) or causing species to find new homes (Graham et al. 2007). Graham et al. (2007) study conducted at the same time each year between 1995 to They studied the effects of mass bleaching on the coral reef fish. They surveyed 7 areas of shallow fringing reefs (5 that had fishing disturbances and bleaching and 2 that were marine protected areas (MPAs)). They determined that immediately after a bleaching event, they saw a decline in juvenile fish which would eventually lead to a decline in adult fish. This confirmed their theory of a lag-effect. This lag-effect means that over time, with increased bleaching events, there could be a considerable effect on fish composition due to the continued decline of juvenile fish. On the other hand, some areas are being identified as resistant and resilient against climate change and these researchers suggested that these areas become MPAs. MPAs were believed to increase the resilience of the ecosystem but were later found to have no significant effect compared to normally fished areas (Graham et al. 2007). During the same time period as the previous study, Wilson et al. (2009) surveyed 10 different locations on the Great Barrier Reef looking at the coral coverage, structure, and species richness. There were 7 sites having already been disturbed and 3 control sites that had no visible damage. The researchers found that at all 10 sites, 46-96% of all the coral reefs declined due to extreme storms or bleaching. They also determined that species richness did not necessarily correlate to the structural complexity of the reef itself (Wilson et al. 2009).

9 The Lizard Island Research Station (LIRS) in the Great Barrier Reef conducted an aquarium-based experiment where they observed the predation rates on coral fish under different coral conditions (healthy, bleached, dead, and algal-covered). They found that the predation rates of coral fish increased when they were near bleached or dead corals (Fig. 5). Also, when the predators were given the option to choose which corals to feed on, they favored the bleached or dead corals. They speculated that this may be due to the contrasting colors of the fish against the white corals (Coker et al. 2009). Corals are naturally very bright in color and the fish that inhabit them will camouflage themselves with matching bright colors. When corals become bleached, fish are left standing out making it easier for predators to find them. 5. Conservation Methods Other than the number one solution of cutting down the global usage of fossil fuels, therefore reducing the rate of CO2 being released into the Earth s atmosphere, there have been many conservation methods being tested that may help in managing the current damage to coral reefs. Some of these include transplantation, marine cloud brightening (MCB), and largeamplitude internal waves (LAIW).

10 In 1998, a study conducted in Puerto Rico used concrete artificial reefs (made by the Reef Ball Development Group. Ltd.) to study the survivorship of transplanted corals at 3 different locations. After one year, the researchers not only found that overall 90% of their corals lived, but that they also saw the successfully recruitment of additional species as well. This experiment could help in the recovery of damaged corals and increase their established habitat areas (Ortiz- Prosper et al. 2001). Latham et al. (2013) conducted a study with the help of computers in which researchers studied the effect of a solar radiation management (SRM) system referred to as marine cloud brightening (MCB). This ultimately means that with the release of micro-seawater droplets into certain clouds, they would be able to increase the size of the clouds and therefore reducing the warming effect on the ocean. This system theoretically works and would only take roughly a year for the cooling to start and is reversible, but the problem is that it would cost $40 million to set up designated vessels only around major reefs and the cooling of the ocean s surface doesn t help with ocean acidification and could potentially disrupt photosynthesis of the algae (Latham et al. 2013). With these hurdles to still overcome, this system should still be a possible solution. Wall et al. (2014) set up base on the islands on the Thai continental shelf. They wanted to understand what kind of effect did these large-amplitude internal waves (LAIW) have on shallow corals. LAIW consists of strong undercurrents that can bring cold water to coral reefs. They selected 12 sites (7 that were exposed to LAIW and 5 that were sheltered by the islands) and monitored temperatures and coral mortality. After analyzing their data, they found that LAIW reduced the bleaching and mortality rates of the shallow watered reefs (Wall et al. 2014). These are naturally occurring internal waves but maybe, with some engineering, there could be the of

11 development of a mechanism that could help recreate this sensation in other reefs around the world. 6. Conclusions and Future Directions The main major anthropogenic-driven stressors that are contributing to the decline of coral reefs are overfishing and climate change. However, the worst stressor on coral reefs is due to bleaching with ocean acidification right behind it. These two stressors have major impacts on the structural complexity of the coral reefs speeding up their detrition rates. Without drastic positive changes, coral reefs will likely lose a majority of their current habitat area within a few decades and are very much unlikely to ever return back to their historic conditions (Rinkevich, 2015). There will likely be a mass extinction of many marine species that only inhabit coral reefs and will eventually cause a negative cascading effect that could disrupt the entire food web of the ocean. There are still many hurdles to come overt on the conservation side like more support and funding that may come from conducting more studies to help convey the destruction of corals to the general public. As a community, can come together to help save the rainforests of the sea (coral reefs). 7. References Coker D. J, Pratchett M. S, and Munday P. L Coral bleaching and habitat degradation increase susceptibility to predation for coral-dwelling fishes. Behavioral Ecology [Internet]. [cited 27 Feb 2018];20(6): Available from

12 Crabbe M. J. C Climate change, global warming and coral reefs: Modelling the effects of temperature. Computational Biology and Chemistry [Internet]. [cited 19 Feb 2018];32(5): Available from Graham N. A. J, Wilson S. K, Jennings S, Poulunin N. V. C, Robinson J. Bijoux J. P, and Daw T. M Lag Effects in the Impacts of Mass Coral Bleaching on Coral Reef Fish, Fisheries, and Ecosystems. Conservation Biology [Internet]. [cited 27 Feb 2018];21(5): Available from Hoegh-Guldberg O, Poloczanka E. S, Skirving W, and Dove S Coral Reef Ecosystems under Climate Change and Ocean Acidification. Frontiers in Marine Science [Internet]. [cited 25 Feb 20180;4(158):1-20. Available from Knowlton N Coral Reefs. Current Biology [Internet]. [cited 19 Feb 2018];18(1): Available from Latham J, Kleypas J, Hauser R, Parkes B, and Gadian A Can marine cloud brightening reduce coral bleaching? Atmospheric Science Letters [Internet]. [cited 28 Feb 2018];14(4): Available from Ortiz-Prosper A. L, Bowden-Kerby A, Ruiz H, Tirado O, Cabán A, Sanchez G, and Crespo J. C Planting Small Massive Corals on Small Artificial Concrete Reefs or Dead Coral Heads. Bulletin of Marine Science [Internet]. [cited 28 Feb 2018];69(2): Available from ?crawler=true Rinkevich Climate Change and Active Reef Restoration Ways of Constructing the Reefs of Tomorrow. Journal of Marine Science and Engineering [Internet]. [cited 28 Feb 2018];3(1): Available from Wall M, Putchim L, Schmidt G. M, Jantzen C, Khokiattiwong S, and Richter C Largeamplitude internal waves benefit corals during thermal stress. Coral Reefs [Internet]. [cited 28 Feb 2018];282(1799):1-9. Available from

13 Webster J. M, Clague D. A, Riker-Coleman K, Gallup C, Braga J. C, Potts D, Moore J. G, Winterer E. L, and Paull C. K Drowning of the -150m reef off Hawaii: A casualty of global meltwater pulse 1A? The Geological Society of America [Internet]. [cited 10 April 2018];32(3): Available from Wilson S. K, Dolman A. M, Cheal A. J, Emslie M. J, Pratchett M. S, and Sweatman H. P. A Maintenance of fish diversity on disturbed coral reefs. Coral Reefs [Internet]. [cited 19 Feb 2018];28(1):3-14. Available from