Reef structure regulates small-scale spatial variation in coral bleaching

Abstract

Coral bleaching is often characterized by high spatial variation across reef systems. Using a field survey and manipulative experiment, we tested whether the physical structure of coral reefs modifies environmental conditions that, in turn, influence spatial variation in bleaching in 3 scleractinian corals, Pocillopora verrucosa, Acropora elseyi, and Porites rus. Corals inhabit mainly the hard-bottom seafloor, or dead or partially dead coral heads ('bommies'). Bommies (0.10 to 3.0 m tall) position corals at different water depths and expose them to differences in light, temperature, hydrodynamics, and sedimentation, factors that can influence patterns of bleaching. We conducted our study in association with a 14 d warming event that caused bleaching in lagoons of Moorea, French Polynesia. Bleaching in naturally occurring colonies of Pocillopora spp. and Acopora spp. was greater on the seafloor (0 m tall) than on short (0.35 to 0.40 m tall) and tall bommies (1.0 to 1.2 m tall). Bleaching in P. verrucosa and A. elseyi transplanted to reef structures in the experiment generally decreased with increasing reef height (seafloor > short bommies > tall bommies). P. rus did not bleach under any conditions observed. Regression analyses revealed that reef structure controlled current speed and sedimentation at the microhabitat scale (from centimeters to meters), and that these factors regulated bleaching and mortality in P. verrucosa and A. elseyi. Our results imply that the physical structure of shallow water reef habitat influences the performance of coral colonies by modifying environmental stress, and that accounting for this structure is important in managing coral reef systems.KEY WORDS: Bleaching · Coral · Environmental stress · Hydrodynamics · Habitat structure · Light · Sedimentation · Temperature Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 370: [127][128][129][130][131][132][133][134][135][136][137][138][139][140][141] 2008 conditions, thereby reducing environmental stress and regulating coral performance.Decline in coral populations and associated degradation of reef communities are being reported worldwide with increasing frequency (Hughes et al. 2003). Natural catastrophes, including storms, predator outbreaks, and disease, kill corals and damage reefs (Connell et al. 1997, Hughes et al. 2003, as do many anthropogenic disturbances (Hughes 1994, Jackson et al. 2001, Fox & Caldwell 2006. Coral bleaching is both a natural and human-induced phenomenon that causes complex spatial heterogeneity in coral mortality and resulting changes in coral community composition (Lang et al. 1988, Glynn 1996, Brown et al. 2002, Coles & Brown 2003. Corals bleach when they lose symbiotic dinoflagellate algae (zooxanthellae), or when symbionts lose photosynthetic pigment. Without energy from symbiont photosynthesis, primary production is reduced and coral mortality increases (Glynn 1993). Corals bleach in response to many environmental stressors (Brown 1997), but a com...

Hydrodynamics influence coral performance through simultaneous direct and indirect effects

“…A previous study at this site, designed to test whether total zooplankton flux influenced corals, found no relationship between skeletal growth and particulate food flux (Alldredge et al 2013), thereby suggesting that hydrodynamicallyenhanced autotrophy, rather than heterotrophy, was the more important factor in our study. We also observed no differences in temperature across the sites during the assay, and saw no differences in light levels in a previous study (Lenihan et al 2008), so we cannot attribute growth differences to these often-considered factors. Differences in coral growth among treatments were also closely associated with corallivore abundance and feeding behavior.…”

Near-surface enrichment of zooplankton over a shallow back reef: implications for coral reef food webs

“…Coral bleaching also decreased with increasing coral height above the bottom and was correlated with current speed and sedimentation rate near our study site (Lenihan et al 2008). But greater availability of zooplankton prey may also mitigate bleaching Palardy et al 2008) suggesting that taller corals may have bleached less because they captured more zooplankton higher in the water column.…”

Water-flow mediated oxygen dynamics within massive Porites-algal turf interactions

“…Regions of the backreef that are most likely to experience low flow (0 to 7.7 cm s −1 ) are consistently the downstream sides of coral bommies (Brown 2012) and on short bommies (30 to 40 cm tall; Lenihan et al 2008), as well as within gaps or crevices. This study highlights how water flow mitigates potential chemical effects of algae on corals by decreasing the thickness of the DBL over the zone of interaction, likely rendering the chemical mechanisms underlying competition between corals and algae ineffective.…”