Abstract
Tropical coral reefs are among the most biologically diverse and economically important ecosystems on earth. Nevertheless, we found dramatic changes in coral communities on the reef slopes of Curacao and Bonaire since 1973. Cover and abundance declined for virtually all coral species. The data show a shift from communities dominated by framework building species (e.g. Orbicella spp.) to communities consisting of small opportunistic, phenotypically plastic, species, including few remaining structural colonies. Madracis mirabilis, Porites astreoides, Diploria strigosa and Agaricia lamarcki are at present modest winners in the coral assemblage, although overall cover declined also for these species. Increased frequency and intensity of events inducing coral mortality and ongoing reduction in suitable hard substratum, provided by the remnants of large colony building species, could reduce the chance of these species to remain winners in the longer run. The observed loss in coral cover and the shift from larger structural to smaller opportunistic species reduced reef carbonate production by 67% and therewith, in combination with a trend towards smaller coral colonies, reef complexity. Alarmingly, reefs at mesophotic depths (30 - 40 m) did not escape the general degradation of the coral community. The negative effects are larger around densely populated areas where local stressors are adding to degradation caused, for instance, by region wide mass bleaching. Without proper conservation and management this already dramatic degradation will continue and turn more and more coral species into losers.
Highlights
Tropical reefs are among the most biologically rich ecosystems on earth providing income for millions of people through fisheries, tourism and coastal protection (Moberg and Rönnbäck, 2003)
Shifts in Coral Communities of Curaçao and Bonaire change (Glynn, 1996; Aronson et al, 2002a; Wilkinson and Souter, 2008) are generally associated with this ubiquitous decline in coral cover (Jackson et al, 2014). These stressors negatively impact coral resilience, impeding the ability of corals to cope with disease, bleaching or physical disturbance, which in turn affects their competitive position with respect to other reef organisms such as macroalgae or sponges (Vega-Thurber et al, 2014; Zaneveld et al, 2016)
The coral community at the four main sites I-IV has since 1973 developed in a constant uni-directional degrading way (Figure 2, Table 2), which is illustrated in the Non-metric Multi-Dimensional Scaling (nMDS) plot by the generally linear arrangement of points along the y-axis (Figure 2)
Summary
Tropical reefs are among the most biologically rich ecosystems on earth providing income for millions of people through fisheries, tourism and coastal protection (Moberg and Rönnbäck, 2003). Shifts in Coral Communities of Curaçao and Bonaire change (Glynn, 1996; Aronson et al, 2002a; Wilkinson and Souter, 2008) are generally associated with this ubiquitous decline in coral cover (Jackson et al, 2014). These stressors negatively impact coral resilience, impeding the ability of corals to cope with disease, bleaching or physical disturbance, which in turn affects their competitive position with respect to other reef organisms such as macroalgae or sponges (Vega-Thurber et al, 2014; Zaneveld et al, 2016). Aforementioned developments are suggested to cause shifts in coral communities from assemblages dominated by framework building corals to a dominance of more persistent and opportunistic non-framework building species (e.g., Green et al, 2008; Alvarez-Filip et al, 2011a; Darling et al, 2012; Perry et al, 2015)
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