Literature review

First, this literature review examines basic coral biology.

Secondly, coral bleaching and other coral stressors are discussed. After which, literature on the coral health status in the Caribbean and Aruba specifically are evaluated.

Then, citizen science projects and CoralWatch specifically are discussed.

2.1 Coral reefs: the basics

Corals are colonial organisms: they are made up of hundreds to hundreds of thousands of organisms called polyps, as seen in figure 1. These polyps range in size from one to three millimetres in diameter (NOAA, 2018a). Corals feed themselves at night by capturing their food with stinging cells, called nematocysts, that come out of these polyps called nematocysts (NOAA, 2018b). Reef building polyps secrete calcium carbonate, which forms a protective cup called a calyx around the polyps (NOAA, 2018a). This reef building is very important, as it creates the structures that are known as corals.

Figure 1 – Coral polyp anatomy (NOAA, 2018a)

The majority of reef building corals have a symbiotic relationship with photosynthetic algae called zooxanthellae that live in their tissue. Together they facilitate a tight recycling of nutrients in nutrient-poor tropical waters.

The coral gives the zooxanthellae compounds needed for photosynthesis, and the zooxanthellae provide the coral with photosynthesis products, oxygen and waste removal.

As much as 90% of the organic material photosynthetically produced by the zooxanthellae is transferred to the host coral tissue. This is the reason corals thrive in nutrient poor tropical waters (NOAA, 2018c).

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108 In addition to providing an environment for the zooxanthellae, corals also perform ecosystem services that benefit humans. Corals provide three-dimensional structure and substrate that houses and feeds fish and other marine animals, which are important to the livelihoods of fishers (Moberg & Folke, 1999). Coral reef ecosystems also perform cultural services, like tourism and a sense of belonging for the local community (Waite et al., 2014).

They contain great amounts of biodiversity, rivalling with terrestrial rainforests on diversity scales. This biodiversity is important for resilience of ecosystems, for example a diverse reef is better equipped to deal with major catastrophic events like hurricanes. Moreover, the reef has a role as a nursery and general habitat to countless fish species, many of them edible. Additionally, reefs protect and create land, like the limestone rocks on Aruba. They can also dissipate wave energy from storms and tsunamis, which reduces impact on the land (Carilli, 2013). A decrease in reef resilience, for example by coral bleaching, endangers these ecosystem services.

2.2 Coral bleaching and other stressors

Corals and their ecosystem services are threatened by a wide range of stressors, the biggest global one being warming oceans. Due to global climate change, sea temperatures have risen by approximately 1C over the past 100 years, and are expected to increase another 1-2 °C this century. This seemingly small increase, for example due to extremely warm seasons (El Niño), causes great stress for the reef building corals, that are already living close to their thermal maxima. An exceedance of their thermal maxima results in coral bleaching; the zooxanthellae become increasingly vulnerable to damage by light and are released by the coral host. If the temperatures do not go back within desired temperature range, corals do not reacquire symbionts and die in great numbers. Coral bleaching events have been observed since 1980 and they are expected to increase over the next few decades as carbon dioxide levels continue to increase, to an annual occurrence in most tropical oceans

by the end of the next 30-50 years. For the Caribbean, yearly bleaching is already expected by 2020. Corals may adapt to these rising temperatures. However, the current rate of warming is exceptionally fast compared to evolutionary time scales. An adaption like this would take at least hundreds of years (Hoegh-Guldberg, 2017).

Coral bleaching is most detrimental when coral health is suboptimal. Globally, coral health is affected by ocean acidification. Reefbuilding corals need carbonate ions for their skeleton. As atmospheric carbon dioxide levels increase, more carbon dioxide is taken up by the ocean. The CO2 reacts with water to produce carbonic acid. Carbonic acid dissociates to form bicarbonate ions and protons, which in turn react with carbonate ions to produce more bicarbonate ions, reducing the availability of carbonate to biological systems. This decrease in carbonate-ion availability reduces the rate of calcification of marine organisms such as reef-building corals, and ultimately favours erosion (Hoegh-Guldberg et al., 2007).

Corals are also affected by local, anthropogenic stressors like nutrient pollution, increased sedimentation and disturbances by (over)fishing, tourism and industries (Gil

& Osenberg, 2010). The main sources of water pollution on Aruba are the Parkietenbos dumpsite, the water and electricity plant (WEB) and the industries and tourist activities along the coastline (Zetten, Meulen, & Brink, 2001).

Even seemingly trivial things, like sunscreen usage, have been shown to negatively impact coral health (McCoshum, Schlarb, & Baum, 2016). Additionally, the reefs are still impacted by the oil refinery (operational from 1925 to 1985 and 1990 to 2012) on the south-eastern coast (C. M. Eakin, Feingold, & Glynn, 1994). The reefs near the refinery are likely still more severely damaged compared to surrounding areas (C. M. Eakin et al., 1994). There are efforts to reopen the refinery again.

2.3 Coral health in the Caribbean

Over the past 40 years, most complex reefs have disappeared in the Caribbean on shallow (<6 m), mid-water (6-20 m) and deep (>20 m) depths. The loss of architectural complexity coincides with the key events in the Caribbean’s ecological history: “the loss of structurally complex Acropora corals, the mass mortality of the grazing urchin Diadema antillarum and the 1998 El Nino Southern Oscillation-induced worldwide coral bleaching event.” (Alvarez-Filip, Dulvy, Gill, Cote, & Watkinson, 2009). Across the Caribbean basin, hard coral cover, like Acropora, has reduced by 80%, from about 50% to 10%.

These corals form the basis for the rest of the reef, and their decline has a big impact on the reef’s health (Gardner et al., 2003). Additionally, the mass mortality of the grazing urchin in 1983 has led to more macroalgal blooms (Mumby, Hastings, & Edwards, 2007). A combination of loss of structural complexity and increase in algal cover has negative effects on reef health.

Unfortunately, Aruban corals have not been well studied.

Curacao and Bonaire’s corals, the two (Dutch) islands closest to Aruba, have been studied in relation to coral bleaching.

Bonaire has always had the highest coral cover, architectural complexity, and net carbonate accretion, though the reefs have suffered over the past 40 years. Bonaire also has the highest architectural complexity. The 2010 bleaching event hit Bonaire hard, resulting in a 10% coral mortality.

However, the corals recovered quickly to pre-bleaching levels in 2017 due to their proactive management. A high abundance of herbivores prevents a shift towards an algal-dominated reef (DCNA, 2017a). In contrast, the bleaching event had a smaller impact on Curacao. Twelve percent of the bottom covered by reef building coral “bleached” and of those 10% subsequently died (DCNA, 2017b). The reefs on Bonaire and Curacao are likely in a much better state than the Aruban reefs, so the magnitude of the impact of the bleaching event on the Aruban reefs is hard to estimate.

If the Aruban reefs are in a worse state than the Curacao

and Bonaire reefs, it is likely that they were hit harder by the bleaching event.

Recently the XL Catlin Global Reef Monitoring project has mapped some of Aruba’s reefs with coral reef imagery (González-Rivero et al., 2016; XL Catlin SeaviewSurvey, 2015). They found struggling coral colonies and associated reef communities, with only 5-8% coral cover (XL Catlin Seaview Survey, 2013a, 2013b). However, there was still a lot of marine life, including soft corals, sponges and smaller reef fish. The abundance of juvenile fish in particular stressed the ecological importance of these reefs (XL Catlin SeaviewSurvey, 2015). This paints a hopeful picture.

2.4 Citizen science research

This lack of data could be remedied by introducing a citizen science reef health project. Citizen scientists are non-scientists who volunteer their time to help collect or analyse data in a scientific project led by a researcher (Gura, 2013). Citizen science projects are useful for two reasons. Firstly, citizen science results in data collection that would not be performed otherwise, for example due to a lack of time and resources. Citizens can easily perform longitudinal research in their own surroundings (McKinley et al., 2017). This benefit is confirmed by the previously mentioned lack of available data on coral reef health on Aruba. Additionally, they can monitor rare events (Starkey et al., 2017), like sudden bleaching events. Secondly, citizen science projects engage the public in scientific endeavours in a way traditional research projects cannot (McKinley et al., 2017). For example, the Lost Ladybug project asks kids to log ladybug observations, while its bigger purpose is not to monitor ladybugs but rather to “help children become confident and competent participants in science, identifying personally with science, so that we develop a generation of adults who are engaged in scientific discussions, policy, and thinking.” (Lost Ladybug Project, 2018). likewise, having Aruban residents engage with the reef will not just provide more data, but also engage them with their surrounding

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110 ecosystem. However, the key question is whether data collected by citizens can be reliable enough to analyse trends and answer relevant questions. The reliability of citizen data collection should be tested before programmes are set up (McKinley et al., 2017). Another obstacle to the scientific use of citizen science data is the perceptions of trained scientists. Some have concerns about data quality, like unsuitable study designs, insufficient training of the citizen scientists, and a lack of standardization and verification methods (Burgess et al., 2017). Addressing these issues is vital for citizen science efforts to be properly incorporated in scientific discourse. An example of successful validation is a study on the population variability of butterflies in the UK.

Since 1976, the UK Butterfly Monitoring Scheme (UKBMS) has been collecting data on butterfly abundancy. Analysis of these data concluded that variability in species can be deduced from these opportunistic citizen science records (Mason et al., 2018). This shows the potential monitoring value of citizen science records.

2.5 CoralWatch

The appropriate citizen science project for combatting the lack of coral health data on Aruba would be CoralWatch.

CoralWatch launched in 2002 after two massive bleaching events following each other shortly in 1998 and 2002.

The project’s goal is to stimulate education about coral bleaching together with increasing global monitoring. They use the monitoring network to educate the general public about coral reef ecology, the effects of climate change and environmental stewardship (Marshall, Kleine, & Dean, 2012). They developed their Coral Health Chart in 2006 (Siebeck, Marshall, Klüter, & Hoegh-Guldberg, 2006).

Figure 2 - CoralWatch Health chart (CoralWatch, n.d.-b)

This CoralWatch Health Chart has made coral monitoring as easy as comparing paint chips. The standardised colour reference card requires the observer to match the colour of the coral with one of the hues on the chart (Siebeck, Logan,

& Marshall, 2008). Spectrophotometric and photographic colour quantification was used to produce the colour series, which indicates the symbiont density and thus bleaching in the monitored coral. It doesn’t require extensive training and can be used for rapid assessment of corals (Siebeck et al., 2008). The method has been used while reef-walking, snorkelling and scuba diving (Siebeck et al., 2008). Changes in coral colour can be detected by citizen scientists if the change is at least 2 units (Siebeck et al., 2008). In April 2018, CoralWatch had 4902 members who had performed 9969 surveys distributed over 1705 reefs, coming to a total of 207141 datapoints (CoralWatch, 2018b). Between January and April 2018, about 5000 corals have been surveyed

(CoralWatch, 2018b). Annual Reefblitzes, inspired by Bioblitzes, have also used the CoralWatch Health Chart on the Great Barrier Reef. In October 2016, over 1,100 people collected over 18,000 data points for a range of marine related citizen science programs. In this effort, 1,396 coral colonies had data contributions recorded for CoralWatch, which was a 44% increase on the monthly CoralWatch average (Great Barrier Reef Foundation, 2016).

On the chart, bleaching of coral is reflected in a change of 2 units or more. A reef can be classified as “healthy” (5-6), “partially bleached” (3-4), or “bleached” (1-2) (Siebeck et al., 2006). When testing the health chart, the standard deviation was ± 0.5 when only a single species was used in a laboratory environment, and higher (± 0.59) when there were field conditions with a cloudy sky and rising tide (Siebeck et al., 2006). The initial try-out on an intertidal reef flat of the Great Barrier Reef with non-specialist observers found an inter-observer error of ± 1 colour score (Siebeck et al., 2006). Interobserver variability is the amount of variability, or error, between two or more observers examining the same thing. In this study, that is the variation in scores given by multiple observers to the same coral organism. Low interobserver variability indicates reliable monitoring.