Reef

Coral reef ecosystems

Tropical coral reefs are among the most productive and biologically diverse ecosystems found on earth (Odum and Odum 1955; Connell 1978; Moberg and Rönnbäck 2003). Although these reefs only cover 0.1 – 0.5% of the ocean floor they provide a home to almost one third of the marine fish species and other marine biota (Mcallister 1991; Spalding and Grenfell 1997; Spalding et al. 2001). Like rainforests, their terrestrial equivalent, the three-dimensional habitat complexity underpins the biological success of coral reef systems (Connell 1978; Grigg et al. 1984; Reaka-Kudla 1997). This structural framework is primarily provided through the precipitation of vast quantities of calcium carbonate by scleractinian corals (Goreau 1959b; Goreau and Goreau 1959; Smith and Kinsey 1976). Basic growth of coral skeleton forms the fundament of the reef and facilitates complex ecosystem functioning and niche partitioning to harbour an exceptional heterogeneity of associated biota (Connell 1978; Graham and Nash 2012; Kennedy et al. 2013; Newman et al. 2015). Ancillary to the inexpressible biological value, millions of people worldwide rely in some way on the services provided by coral reefs, most notably for nourishment, but also for services associated with tourism and coastal protection (Costanza et al. 1997; Moberg and Folke 1999; Moberg and Rönnbäck 2003). By increasing frictional dissipation of wave energy, the complex physical structure created by corals protects coastal shorelines from erosion. This has allowed humans to settle and develop coastal areas throughout the tropics. Yet, coral reefs are at present ubiquitously under pressure due to a variety of stressors associated with increased anthropogenic activity on a global and local scale.

The marine environment is continuously exposed to change, but currently this change is more and more the result of human actions (Harvell et al. 1999; Derraik 2002; Orr et al. 2005; HoeghGuldberg and Bruno 2010). The stress exerted by the natural and anthropogenic induced changing global environment works in synergy with stressors that act on a finer spatial scale. Factors such as the overharvesting of fish, pollution, eutrophication, coastal development and the introduction of invasive species can locally trigger shifts in community composition and trophic hierarchy (Hughes 1994; Hughes et al. 2003; Pandolfi et al. 2003; Hughes et al. 2007; Hughes et al. 2017). By destabilising ecosystem functioning and interactions between key species, these stressors reduce reef resilience and therewith the capacity of coral reefs to cope with globally induced sea surface temperature anomalies or ocean acidification (Pandolfi et al. 2003; Bellwood et al. 2004; Hughes et al. 2017). Reefs in the wider Caribbean region seem particularly vulnerable to anthropogenic impact (Jackson et al. 2014). By large this can be ascribed to increased local pressures associated with the unprecedented human population expansion in the region. Since the 1950s, the total population in the Caribbean has more than doubled (United Nations, Department of Economic and Social Affairs, Population, Division, 2015). Natural biological and hydrological conditions are also less favourable compared to, for instance, the Indo-Pacific region (Roff and Mumby 2012). Biological diversity in the IndoPacific exceeds 10-fold the diversity found in the Caribbean (Spalding et al. 2001; Hoeksema et al. 2017), implying limited functional redundancy in the latter (Bellwood et al. 2003; Bellwood et al. 2004; Jackson et al. 2014). In addition, the quality of Caribbean surface water is significantly impacted by discharge from major South-American rivers like the Amazon and Orinoco as well as the North-American Mississippi river. The residence time of the polluted and eutrophic water from these rivers, combined with run-off and sewage water from the numerous islands is relatively long in the Caribbean Sea due to its distinct basin-like morphological and hydrological features (Roff and Mumby 2012). As a consequence of the rapid anthropogenic alteration of the marine environment we now see an ecological degradation of Caribbean coral reef habitats that has not occurred for over 200.000 years (Pandolfi and Jackson 2006).

Compiled abundances of juvenile corals revealed no change over time in the Pacific, but a decline in the Caribbean. Using these analyses as a rationale, we explored recruitment and post-settlement success in determining coral cover using studies in the Caribbean (St John, Bonaire) and Pacific (Moorea, Okinawa). Juvenile corals, coral recruits, and coral cover have been censused in these locations for years, and the ratio of juvenile (J) to recruiting (R) corals was used to measure post-settlement success. In St John and Bonaire, coral cover was stable but different between studies, with the ratio of the density of juveniles to density of recruits (J : R) ~0.10; in Moorea, declines in coral cover were followed by recovery that was related to the density of juvenile corals 3 years before, with J : R ~0.40; and in Okinawa, a decline in coral cover in 1998 was followed by a slow recovery with J/R ~0.01. Coral cover was associated positively with juvenile corals in St John, and in Okinawa, the density of juvenile corals was associated positively with recruits the year before. J : R varied among studies, and standardised densities of juvenile corals declined in the Caribbean, but increased in the Pacific. These results suggest that differences in the post-settlement success may drive variation in coral community structure.

The use of color is seen throughout the animal kingdom. In coral reef ecosystems, organism colorations are suggested to assist in behaviors such as camouflaging and communication among schooling fish. The long and slender Atlantic trumpetfish, Aulostomus maculatus, is suggested to take advantage of both bright and neutral color schemes on the Caribbean reefs. This species has three known colorations, or phenotypes, that all exist on the reef of the island Bonaire, Dutch Caribbean: the yellow, blueheaded, and mottled phenotypes. The mottled camouflages easily into their surroundings, the blueheaded can change the color of its head in different surroundings, and the yellow is more brightly colored than most of the substrate on the reef. This study hypothesized that there would be a strong association between bright reef fish and the hunting behaviors of the bright yellow phenotype. To observe how coloration and surroundings may influence the abundance of the yellow A. maculatus in particular, a total of 1600 m2 of abundance surveys for two phenotypes of trumpetfish and associating fish were conducted. Behavioral observations of six fish of each phenotype were conducted to examine links between coloration, surroundings, and behavior. While no links were found between coloration, surroundings and abundance, several significant links were found between coloration, surroundings, and behavior. There are a variety of factors that can affect the behavior of coral reef fishes; the data this study has collected suggests that the relationship between coloration of species and surroundings is one of these factors for the Atlantic trumpetfish

This student research was retrieved from Physis: Journal of Marine Science XVII (Spring 2015)19: 69-79 from CIEE Bonaire.