Progression of the Coral-Algal Phase Shift in the Caribbean: A Case Study in Bonaire, Dutch Caribbean

Global climate change and local anthropogenic stressors have been identified as primary causes for coral reef degradation and are targets of research to assess anthropogenic impacts (Aronson & Precht, 2006; Crabbe, 2010; Fung et al., 2011;Muthukrishnan & Fong, 2014). The first scientific observation of a coral-algal phase shift (Figure 1), which is defined as a transition from coral to algal dominance, was in a 25-year study in the Caribbean by Antonius and Ballesteros (1998). It is evident that coral-algal phase shifts occur as environmental conditions on reefs cross thresholds for optimal coral growth and make reversal to a coral-dominated state unlikely (Fung et al., 2011). Case studies in the Caribbean including the U.S. Virgin Islands (Rogers & Miller, 2006), Jamaica (Crabbe, 2010; Goreau, 1992; River & Edmund, 2001), Bonaire (Bak et al., 2005; Bries et al., 2004), Curacao (Bak et al., 2005; Bries et al., 2004), and other locations have documented declines in coral reef health by measuring the percentage of benthic substrate occupied by live coral versus algae over time. As an example, strong hurricanes have caused the degradation of reefs in Jamaica, which has led to live coral cover of less than 5% and an algal cover of more than 90% (Goreau, 1992; River & Edmunds, 2001). In some cases, allelopathic algae species have been shown to produce feedback supNovember/Decem pression of coral resilience, preventing recovery of coral colonies after a disturbance while promoting stability of algal beds (Rasher & Hay, 2010). Increased presence of algal species can affect coral communities on the microbial level with these chemical defenses, eventually affecting overall reef health in ways that are difficult to predict (Morrow et al., 2012). Even in well-studied marine ecosystems, it is difficult to determine the baseline condition of coral and algal balance due to a lack of knowledge of historical reef states (Bruno et al., 2014). A rapid increase of algal benthic cover can affect the reef system in multiple ways. Algae recruit on dead substrate and compete with coral colonies ber 2014 Volume 48 Number 6 33 for space (Aronson & Precht, 2006), where an overabundance of algae inhibits coral recruitment (Titlyanov & Titlyanova, 2008). Furthermore, algal contact may also serve as a trigger for onset of coral disease (Nugues et al., 2004). Similar to other populated tropical islands, marine environments on Bonaire experience anthropogenic pressures (Debrot et al., 2013; Govers et al., 2014; Slijkerman et al., 2013). The ecosystems on Bonaire are subject to pressure from coastal populations including litter contamination (Debrot et al., 2013), release of coastal waste products and sewage (Govers et al., 2014; Slijkerman et al., 2013), and physical damage from high recreational diver traffic (Lamb et al., 2014). The release of waste products and coastal runoff contributes to eutrophication of tropical water, which has detrimental affects on the stability of coral reef systems and also decreases resilience in other marine ecosystems (Govers et al., 2014; Slijkerman et al., 2013; Stokes et al., 2010; Thurber et al., 2014). Overfishing of herbivorous reef fish also affects coral communities because these species play an important role in depressing algal abundance on coral reefs (Edwards 34 Marine Technology Society Journa et al., 2014; Loh & Pawlik, 2014; McManus et al., 2000). Additionally, a rise of storm frequency and intensity, increased ocean temperatures, and increasing ocean acidity are factors in increasing reef vulnerability to damage and stress, often resulting in coral mortality (Bries et al., 2004). On Bonaire, reefs serve as an important economic resource and biological habitat. Increased stressors that affect the marine environment have resulted in a degradation of reefs around the island in the last 30 years (Stokes et al., 2010). A study completed by Stokes and colleagues (2010) gave a comprehensive view on the state of coral-algal phase shifts at multiple sites in 2008. This study reported that, at the sites surveyed, live coral cover was half the recorded values in 1982, while algal cover increased between 2and 20-fold. The researchers described an algae-dominated reef as one that has an algae-coral benthic cover ratio of higher than 1:1. Small-scale rate of change assessments of benthic composition are useful for studying phase shifts and have ecological implications (McManus & Polsenberg, 2004). This described study is a small-scale survey on the island of Bonaire in which video tranl sects and laboratory video analyses were used to determine the percent of coral and algal cover as well as disease presence. By studying coral and algal cover percentages, conclusions can be drawn on current reef health, degradation over time, and resilience (Stokes et al., 2010). This principle served as the base for this study. Verification of study hypotheses provides evidence of and reason to evaluate human impacts on near-shore reefs at Bonaire and to increase conservation efforts. It is hypothesized that the algae-coral ratio on Yellow Sub reef will be greater than 1:1 and will be higher than ratios observed in 2008. It is also expected that several coral diseases and coral bleaching will be frequently observed on the study site. Materials and Methods Study Site The study site is located in the southern Caribbean Sea on the island of Bonaire, Dutch Caribbean. Benthic cover and coral disease transect surveys were conducted at a study site, commonly known as Yellow Sub reef (12°09′36.47′′N 68°16′55.16′′W), at Kralendijk, on the leeward side of Bonaire. The historical data according to Stokes and colleagues (2010) were collected at the Cliff study site (68°17′26.62′′W 12°10′26.44′′N) 1,812 m north of Yellow Sub reef (Figure 2). The Yellow Sub study reef is located within close proximity to the capital city of Kralendijk and is subject to island pollution and runoff, boat travel, and diver traffic. No previous data have been collected at this site in concern to a coral-algal phase shift. Field Research At the Yellow Sub reef study site, underwater video transects were used FIGURE 1 Example of a coral-algal phase shift from a coral-dominated reef (left) and an algae-dominated reef (right) in the Dutch Caribbean (photo credit: Dutch Caribbean Nature Alliance, 2013).