Benthic communities at two remote Pacific coral reefs: effects of reef habitat, depth, and wave energy gradients on spatial patterns

Abstract

Kingman Reef and Palmyra Atoll in the central Pacific are among the most remote coral reefs on the planet. Here we describe spatial patterns in their benthic communities across reef habitats and depths, and consider these in the context of oceanographic gradients. Benthic communities at both locations were dominated by calcifying organisms (54–86% cover), namely hard corals (20–74%) and crustose coralline algae (CCA) (10–36%). While turf algae were relatively common at both locations (8–22%), larger fleshy macroalgae were virtually absent at Kingman (<1%) and rare at Palmyra (0.7–9.3%). Hard coral cover was higher, but with low diversity, in more sheltered habitats such as Palmyra’s backreef and Kingman’s patch reefs. Almost exclusive dominance by slow-growing Porites on Kingman’s patch reefs provides indirect evidence of competitive exclusion, probably late in a successional sequence. In contrast, the more exposed forereef habitats at both Kingman and Palmyra had higher coral diversity and were characterized by fast-growing corals (e.g., Acropora and Pocillopora), indicative of more dynamic environments. In general at both locations, soft coral cover increased with depth, likely reflecting increasingly efficient heterotrophic abilities. CCA and fleshy macroalgae cover decreased with depth, likely due to reduced light. Cover of other calcified macroalgae, predominantly Halimeda, increased with depth. This likely reflects the ability of many calcifying macroalgae to efficiently harvest light at deeper depths, in combination with an increased nutrient supply from upwelling promoting growth. At Palmyra, patterns of hard coral cover with depth were inconsistent, but cover peaked at mid-depths at Kingman. On Kingman’s forereef, benthic community composition was strongly related to wave energy, with hard coral cover decreasing and becoming more spatially clustered with increased wave energy, likely as a result of physical damage leading to patches of coral in localized shelter. In contrast, the cover of turf algae at Kingman was positively related to wave energy, reflecting their ability to rapidly colonize newly available space. No significant patterns with wave energy were observed on Palmyra’s forereef, suggesting that a more detailed model is required to study biophysical coupling there. Kingman, Palmyra, and other remote oceanic reefs provide interesting case studies to explore biophysical influences on benthic ecology and dynamics.

Local genomic adaptation of coral reef-associated microbiomes to gradients of natural variability and anthropogenic stressors

“…Coral cover varied markedly from 2.2% at one site on Kiritimati to 86.7% at one site on Malden (mean = 44.4%; Table S2). In general, the uninhabited atolls were dominated by reefbuilding calcifiers including coral, crustose coralline algae, and calcified macroalgae (24), whereas fleshy algae, such as turf and fleshy macroalgae, dominate the inhabited atolls (14). Table S2.…”

Ocean warming and acidification have complex interactive effects on the dynamics of a marine fungal disease

“…However, within Palmyra's forereef habitat, CFD occurrence appeared to be independent of host abundance. Moreover, CCA cover peaks on Palmyra's shallow (less than 5 m) western reef terrace [10], where CFD was virtually absent; previous surveys at Palmyra have documented CFD on the terrace habitat, but again at deeper (approx. 15 m) depths (B. Vargas-Á ngel 2013, personal communication) where host cover is lower [10].…”

“…Crustose coralline algae (CCA) serve essential functional roles in coral reef ecosystems, including facilitating reef accretion and consolidation [7], providing a settlement substrate for coral larvae [8] and forming a key successional state promoting reef recovery following acute disturbance [9]. While CCA can occupy up to 50% of the living reef benthos [7,10], relatively little is known about their biology and ecology [11], particularly their susceptibility to and subsequent impacts from disease [12].…”

Field observations of wave‐driven circulation over spur and groove formations on a coral reef

“…Palmyra Atoll (5 52 0 N, 162 05 0 W) is part of the Northern Line Islands of the central equatorial Pacific ( Figure 1a) and largely because of the absence of acute anthropogenic stress on the ecosystems, its reefs contain abundant calcifiers, namely hard corals and crustose coralline algae [Williams et al, 2013] and high growth rates [Koweek et al, 2014]. Two separate field experiments were conducted on the atoll to characterize SAG circulation cells (Figure 1b).…”