A Connection between Colony Biomass and Death in Caribbean Reef-Building Corals

Abstract

Increased sea-surface temperatures linked to warming climate threaten coral reef ecosystems globally. To better understand how corals and their endosymbiotic dinoflagellates (Symbiodinium spp.) respond to environmental change, tissue biomass and Symbiodinium density of seven coral species were measured on various reefs approximately every four months for up to thirteen years in the Upper Florida Keys, United States (1994–2007), eleven years in the Exuma Cays, Bahamas (1995–2006), and four years in Puerto Morelos, Mexico (2003–2007). For six out of seven coral species, tissue biomass correlated with Symbiodinium density. Within a particular coral species, tissue biomasses and Symbiodinium densities varied regionally according to the following trends: Mexico≥Florida Keys≥Bahamas. Average tissue biomasses and symbiont cell densities were generally higher in shallow habitats (1–4 m) compared to deeper-dwelling conspecifics (12–15 m). Most colonies that were sampled displayed seasonal fluctuations in biomass and endosymbiont density related to annual temperature variations. During the bleaching episodes of 1998 and 2005, five out of seven species that were exposed to unusually high temperatures exhibited significant decreases in symbiotic algae that, in certain cases, preceded further decreases in tissue biomass. Following bleaching, Montastraea spp. colonies with low relative biomass levels died, whereas colonies with higher biomass levels survived. Bleaching- or disease-associated mortality was also observed in Acropora cervicornis colonies; compared to A. palmata, all A. cervicornis colonies experienced low biomass values. Such patterns suggest that Montastraea spp. and possibly other coral species with relatively low biomass experience increased susceptibility to death following bleaching or other stressors than do conspecifics with higher tissue biomass levels.

The role of light in mediating the effects of ocean acidification on coral calcification

“…Following our 2011 research, it was clear that the P CO2 -light interaction relied heavily on the two calcification rates recorded at 70μmol photonsm −2 s −1 (Fig.1), and therefore the experiment was repeated in order to evaluate the possibility that the results could be attributed to chance alone. Although calcification rates were lower in June 2012 than in March 2011 -most likely reflecting seasonality in coral performance (Fitt et al, 2000;Thornhill et al, 2011) -the results of the two experiments were similar for ambient P CO2 and deviated under high P CO2 when light was >56μmol quantam . The altered response of corals at high irradiances under high P CO2 in the second experiment may be the result of seasonal changes to holobiont physiology caused by increased temperature during the second experiment (27.5 versus 24.0°C), or by changes to seawater quality caused by heavy rainfall just prior to the start of the second experiment.…”

Convergent mortality responses of Caribbean coral species to seawater warming

“…In contrast to host susceptibility to thermal stress, the susceptibility of coral symbiosis to bleaching is highly related to numerous biological traits of corals (see Gates and Edmunds 1999, Baker et al 2008, van Woesik et al 2012). Since bleaching is typically a light mediated response to thermal stress (Lesser 1997), factors that reduce light or heat stress, or increase resistance to light and heat stress can buffer the effects of v www.esajournals.org thermal stress.…”

“…Since bleaching is typically a light mediated response to thermal stress (Lesser 1997), factors that reduce light or heat stress, or increase resistance to light and heat stress can buffer the effects of v www.esajournals.org thermal stress. Both Baker et al (2008) and van Woesik et al (2012) summarized known colony traits that imparted higher tolerance in a thermally fluctuating environment, including massive morphology (versus branching morphology), thick tissues, large inter-corallite spacing, low growth rates, large colony size, large corallite size, association with Symbiodinium Clade D, and a porous skeletal structure. Each of these characteristics can impart resistance to bleaching and also resistance to thermally induced mortality.…”

“…Lowered densities of zooxanthellae lead to decreasing photosynthesis to respiration ratios and the possibility of declining energy reserves to maintain metabolism (Porter et al 1989, Anthony et al 2007, Rodrigues and Grottoli 2007. For some Western Atlantic coral species there is a positive relationship between colony energy and resistance to mortality among conspecifics (Thornhill et al 2011), and this may apply more broadly between coral species (van Woesik et al 2012). If the latter is true, then species that can maintain higher energy stores may be more successful at resisting mortality if bleached, while those species that tend to fall into negative energy states may be at an increased risk of starvation (Grottoli et al 2004).…”

Modelling coral calcification accounting for the impacts of coral bleaching and ocean acidification

“…There are a wide range of mechanisms and strategies that corals can use to mitigate the damage from bleaching. For example, a coral with significant stored energy reserves (lipids and proteins) and greater tissue biomass has increased survivorship following a bleaching event (Anthony et al, 2009;Thornhill et al, 2011). The coral and the symbiont may also employ anti-oxidants to deal with the increase in reactive oxygen species or express heat shock proteins to deal with the increased temperature (Baird et al, 2009), or corals may switch to feeding to meet the temporary decrease in autotrophic energy from photosynthesis (Houlbreque and Ferrier-Pages, 2009).…”