Abstract
Abstract. Ocean acidification, the assimilation of atmospheric CO2 by the oceans that decreases the pH and CaCO3 saturation state (Ω) of seawater, is projected to have severe adverse consequences for calcifying organisms. While strong evidence suggests calcification by tropical reef-building corals containing algal symbionts (zooxanthellae) will decline over the next century, likely responses of azooxanthellate corals to ocean acidification are less well understood. Because azooxanthellate corals do not obtain photosynthetic energy from symbionts, they provide a system for studying the direct effects of acidification on energy available for calcification. The solitary azooxanthellate orange cup coral Balanophyllia elegans often lives in low-pH, upwelled waters along the California coast. In an 8-month factorial experiment, we measured the effects of three pCO2 treatments (410, 770, and 1220 μatm) and two feeding frequencies (3-day and 21-day intervals) on "planulation" (larval release) by adult B. elegans, and on the survival, skeletal growth, and calcification of newly settled juveniles. Planulation rates were affected by food level but not pCO2. Juvenile mortality was highest under high pCO2 (1220 μatm) and low food (21-day intervals). Feeding rate had a greater impact on calcification of B. elegans than pCO2. While net calcification was positive even at 1220 μatm (~3 times current atmospheric pCO2), overall calcification declined by ~25–45%, and skeletal density declined by ~35–45% as pCO2 increased from 410 to 1220 μatm. Aragonite crystal morphology changed at high pCO2, becoming significantly shorter but not wider at 1220 μatm. We conclude that food abundance is critical for azooxanthellate coral calcification, and that B. elegans may be partially protected from adverse consequences of ocean acidification in habitats with abundant heterotrophic food.
Highlights
As aqueous CO2 concentrations continue to rise over the century, pH of oceanic surface waters will decline in the process known as ocean acidification (Caldeira and Wicket, 2003, 2005; Sabine et al, 2004)
B. elegans planula larvae were collected as they emerged from adults every 3 days for 3 months (6 November 2011 to 15 January 2012)
Responses of calcifying organisms to ocean acidification are likely to vary at different stages of their life cycles, and several studies provide evidence that early stages of many marine taxa are sensitive to acidification (Kroeker et al, 2010, 2013; Hettinger et al, 2012)
Summary
As aqueous CO2 concentrations continue to rise over the century, pH of oceanic surface waters will decline in the process known as ocean acidification (Caldeira and Wicket, 2003, 2005; Sabine et al, 2004). In many laboratory and field investigations, calcification rates of tropical corals declined as pH and aragonite saturation state ( arag, a measure of ease of CaCO3 formation) decreased (Fine and Tchernov, 2007; Anthony et al, 2008; Jokiel et al, 2008; Krief et al, 2010). Almost all tropical corals have algal symbionts (zooxanthellae) whose photosynthesis contributes to the host’s nutrition and increases calcification rates. Most deep and cold-water corals lack zooxanthellae, but their potential responses to ocean acidification are largely unknown; as the saturation state of seawater decreases, their calcification rates are likely to decline and their geographical. Because azooxanthellate corals lack symbionts and rely solely on heterotrophy for energy, they provide a simplified system for exploring the roles of nutrition (and energy) in coral calcification
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