Tropical scleractinian corals are particularly vulnerable to global warming as elevated sea surface temperatures (SSTs) disrupt the delicate balance between the coral host and their algal endosymbionts, leading to symbiont expulsion, mass bleaching and mortality. While satellite sensing of SST has proved a reliable predictor of coral bleaching at the regional scale, there are large deviations in bleaching severity and mortality on the local scale that are poorly understood. Here, we show that internal waves play a major role in explaining local coral bleaching and mortality patterns in the Andaman Sea. Despite a severe region-wide SST anomaly in May 2010, frequent upslope intrusions of cold sub-pycnocline waters due to breaking large-amplitude internal waves (LAIW) mitigated coral bleaching and mortality in shallow waters. In LAIW-sheltered waters, by contrast, bleaching-susceptible species suffered severe bleaching and total mortality. These findings suggest that LAIW benefit coral reefs during thermal stress and provide local refugia for bleaching-susceptible corals. LAIW are ubiquitous in tropical stratified waters and their swash zones may thus be important conservation areas for the maintenance of coral diversity in a warming climate. Taking LAIW into account can significantly improve coral bleaching predictions and provide a valuable tool for coral reef conservation and management.

We independently manipulated mixing intensity (strong artificial mixing vs. background turbulence) and water-column depth (2 m, 4 m, 8 m, and 12 m) in order to explore their separate and combined effects in a field enclosure experiment. To accentuate the vertical light gradient, enclosures had black walls, resulting in a euphotic depth of only 3.7 m. All enclosures were placed in a well-mixed water bath to equalize temperature across treatments. Phytoplankton responded to an initial phosphorus pulse with a transient increase in biomass, which was highest in the shallowest, least light-limited water columns where dissolved mineral phosphorus subsequently became strongly limiting. As a consequence, the depth-averaged mineral phosphorus concentration increased and the seston carbon (C) : phosphorous (P) ratio decreased with increasing water-column depth. Low turbulence enclosures became quickly dominated by motile taxa (flagellates) in the upper water column, whereas mixed enclosures became gradually dominated by pennate diatoms, which resulted in higher average sedimentation rates in the mixed enclosures over the 35-d experimental period. Low turbulence enclosures showed pronounced vertical structure in water columns .4 m, where diversity was higher than in mixed enclosures, suggesting vertical niche partitioning. This interpretation is supported by a primary production assay, where phytoplankton originating from different water depths in low-turbulence treatments had the relatively highest primary productivity when incubated at their respective depths of origin.

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The trophic response of the scleractinian coral Pocillopora meandrina (Dana, 1846) to large amplitude internal waves (LAIW) was investigated in the Andaman Sea. Corals living on the western sides of the Similan Islands (Thailand) exposed to LAIW showed significantly higher biomass and protein content than sheltered corals on the eastern sides. LAIW-exposed corals were also more heterotrophic, displaying lower δ 13 C ratios in their tissues and higher rates of survival in artificial darkness compared to sheltered counterparts. Heterotrophic nutrition in concert with photosynthesis leads to higher energy reserves in corals from LAIW-exposed reefs, making them more resilient to disturbance. As these differences in trophic status are due to LAIW-enhanced fluxes of organic matter, LAIW may play an important role in supporting coral metabolism and survival in these monsoon beaten reefs.KEY WORDS: Large amplitude internal waves · Corals · Heterotrophic plasticity · Current regime · Pocillopora meandrina · Andaman Sea Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 412: [113][114][115][116][117][118][119][120][121][122][123][124][125][126][127][128] 2010 solved organic materials (Muscatine et al. 1989, Grottoli 2002.Along with photosynthesis, heterotrophy enhances skeletal (Houlbrèque et al. 2003) and tissue growth by building up energy stores including lipids (Anthony 2006, Treignier et al. 2008 and proteins , Houlbrèque et al. 2003. Heterotrophy has been shown to support coral photosynthesis (Grottoli 2002, Borell et al. 2008) and resilience to stresses such as turbidity (Anthony 2006), warming (Borell et al. 2008) and bleaching , Palardy et al. 2008. Although active feeding does not generally constitute the dominant carbon source for zooxanthellate corals, it may reduce temporary energy deficits (Anthony 2000) so that corals with a high capability to heterotrophically assimilate carbon may be more effective in surviving multiple bleaching events and become dominant in future reefs .The relative proportion of heterotrophy in coral metabolism may vary markedly between species , and has been documented in several studies. For example, Wellington (1982) observed that the branching coral Pocillopora damicornis grew independent of zooplankton supply, and was more markedly affected by shading than the massive coral Pavona clavus. Sebens & Johnson (1991) documented higher zooplankton capture rates by Madracis decactis with increasing current strength, but not by Meandrina meandrites. showed that the δ 13 C ratios of Montipora capitata host tissue decreased when bleached because of increased heterotrophic feeding, while Porites compressa did not alter its nutrition. Moreover, Palardy et al. (2008) observed that the feeding response to one disturbance may vary significantly between different coral species.The importance of heterotrophic feeding in coral metabolism may further vary between environments (Palardy et al. 2005). Decreasing light and photosynthesis (Mus...

The resilience of tropical corals to ocean acidification depends on their ability to regulate the pH within their calcifying fluid (pHcf). Recent work suggests pHcf homeostasis under short-term exposure to pCO2 conditions predicted for 2100, but it is still unclear if pHcf homeostasis can be maintained throughout a corals lifetime. At CO2 seeps in Papua New Guinea, massive Porites corals have grown along a natural seawater pH gradient for decades. This natural gradient, ranging from pH 8.1–7.4, provides an ideal platform to determine corals’ pHcf (using boron isotopes). Porites maintained a similar pHcf (~8.24) at both a control (pH 8.1) and seep-influenced site (pH 7.9). Internal pHcf was slightly reduced (8.12) at seawater pH 7.6, and decreased to 7.94 at a site with a seawater pH of 7.4. A growth response model based on pHcf mirrors the observed distribution patterns of this species in the field. We suggest Porites has the capacity to acclimate after long-time exposure to end-of-century reduced seawater pH conditions and that strong control over pHcf represents a key mechanism to persist in future oceans. Only beyond end-of-century pCO2 conditions do they face their current physiological limit of pH homeostasis and pHcf begins to decrease.