Abstract
Abstract. In recent decades, extensive mortality of reef-building corals throughout the Caribbean region has led to the erosion of reef frameworks and declines in biodiversity. Using field observations, structural models, and high-precision U–Th dating methods, we quantify changes in structural complexity in the major framework-building coral Orbicella annularis over a 20-year period at Long Caye (Belize). Despite extensive mortality following the mass coral bleaching event of 1998, the structural complexity of frameworks remained largely unchanged between 1998 (rugosity index, R, of 2.35±0.1) and 2018 (R of 2.29±0.1). Colony-scale structural complexity was maintained, as the rapid growth of surviving ramets (0.69±0.1 cm yr−1) offset the slower bioerosion of dead ramets (-0.11±0.16 cm yr−1). Despite the apparent stability of the structural complexity at colony scales, bioerosion of individual dead ramets over 2 decades led to declines in microhabitat complexity, with an overall reduction in the depth of microhabitats within frameworks. Altered microhabitat complexity appears to have negative effects on cryptic fauna, with the grazing urchin Echinometra viridis declining from 1.5±0.4 individuals m−2 in 1998 to 0.02± individuals m−2 in 2018. Changes in microhabitat complexity have the potential to alter ecological interactions that can impact recovery dynamics on coral reefs in ways that are undetectable using traditional rugosity metrics of structural complexity.
Highlights
As an ecosystem engineer, Orbicella annularis (Ellis and Solander, 1786) plays a critical ecosystem role as a framework-building coral in the Caribbean (Geister, 1977) providing reef-scale structural complexity that supports a diverse range of fish (Alvarez-Filip et al, 2011) and invertebrate (Idjadi and Edmunds, 2006) assemblages
O. annularis first appeared in the late Pliocene to early Pleistocene (∼ 2–1.5 Myr ago, Budd and Klaus, 2001) and linear extension of ∼ 10 mm yr−1 resulted in the accretion of modern day Caribbean coral reefs at a rate of ∼ 3.3 m kyr−1 (Gischler, 2008)
By June 2000, cover of O. annularis had declined to 20 ± 5 %, and it remained at ∼ 15 % in the decades following the mass bleaching event despite repeated minor hurricane disturbances (Fig. 1c)
Summary
Orbicella annularis (Ellis and Solander, 1786) plays a critical ecosystem role as a framework-building coral in the Caribbean (Geister, 1977) providing reef-scale structural complexity that supports a diverse range of fish (Alvarez-Filip et al, 2011) and invertebrate (Idjadi and Edmunds, 2006) assemblages. The erosion of reef frameworks in the Caribbean over the past few decades has resulted in a loss of reef-scale structural complexity, a process described as “flattening” (Alvarez-Filip et al, 2009) This region-wide loss of structural complexity has led to substantial declines in ecosystem functioning, including reductions in biodiversity (Newman et al, 2015) and decreases in fisheries’ productivity (Rogers et al, 2018). Projections of future thermal stress events indicate that such mass bleaching events may become an annual occurrence by 2040 (van Hooidonk et al, 2015) These events are predicted to have a strong negative impact on carbonate budgets and reef accretion (Kennedy et al, 2013; Perry et al, 2013), yet the landscape-scale impacts on the structure and function of coral frameworks are not well understood. We quantify multi-decadal changes in O. annularis frameworks at Long Caye (Glovers Reef) and quantify long-term changes in structural complexity and microhabitat complexity
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