Abundance, size, and survival of recruits of the reef coral Pocillopora acuta under ocean warming and acidification

Abstract

Ocean warming and acidification are among the greatest threats to coral reefs. Massive coral bleaching events are becoming increasingly common and are predicted to be more severe and frequent in the near future, putting corals reefs in danger of ecological collapse. This study quantified the abundance, size, and survival of the coral Pocillopora acuta under future projections of ocean warming and acidification. Flow-through mesocosms were exposed to current and future projections of ocean warming and acidification in a factorial design for 22 months. Neither ocean warming or acidification, nor their combination, influenced the size or abundance of P. acuta recruits, but heating impacted subsequent health and survival of the recruits. During annual maximum temperatures, coral recruits in heated tanks experienced higher levels of bleaching and subsequent mortality. Results of this study indicate that P. acuta is able to recruit under projected levels of ocean warming and acidification but are susceptible to bleaching and mortality during the warmest months.

Fig 1. Mean mid-day (~12:00 h) temperatures (°C) (a.) and pH_T (b.) in experimental treatments of control (AT ACO2; blue solid line), acidified (AT HCO2; black dotted line), heated (HT ACO2; red solid line), and heated acidified (HT HCO2; purple dotted line) throughout the 22-month experimental period.

Survey periods (i.e., July 2017, October 2017, and January 2018) are shaded. Error bars represented SE of the mean (n = 10).

Fig 2. Total (a.) and mean (b.) number of Pocillopora acuta recruits within each treatment: Control (AT ACO2), acidified (AT, HCO2), heated (HT, ACO2), and heated acidified (HT, HCO2) pooled acrossed tanks in July (black), October (grey), and January (white).

Numbers above bar indicate number of tanks coral recruits were found in (total number of tanks per treatment is 10). Error bars are SE of the mean.

Fig 3. Mean maximum diameter Pocillopora acuta recruit size (cm) pooled across tank means within treatments of control (AT ACO2, n = 8), acidified (AT, HCO2, n = 9), heated (HT, ACO2, n = 6), and heated acidified (HT, HCO2, n = 6) during initial (July 2017, black), bleaching (October 2017, grey), and recovery assessment (January 2018, white).

Error bars are SE of the mean.

Fig 4. Mean maximum diameter Pocillopora acuta recruit size (cm) by tank in control (a.), acidified (b.), heated (c.) and heated acidified (d.) conditions during the July (black), October (grey), and January (white) assessments.

Fig 5. Mean coral recruit health assessment of each treatment (pooled across tanks) (control, acidified, heated, heated acidified) during each survey (July 2017, October 2017, and January 2018).

Coral health assessment is denoted by pigmentation of normal (red), pale (yellow), bleached (white), and dead (black).