Thresholds of hypoxia of two Red Sea coral species (Porites sp. and Galaxea fascicularis)
Jacqueline Victoria Alva Garcia, Shannon G. Klein, Taiba Alamoudi, Silvia Arossa, Anieka J. Parry, Alexandra Steckbauer, Carlos M. Duarte
Abstract
Anthropogenic pressures have driven large-scale declines in coral cover on >50% of tropical coral reefs. Most research efforts have focused on ocean warming, ocean acidification, and overfishing impacts. Despite increasing instances of reef-associated hypoxic events, the role of reduced O 2 in affecting coral reef performance is largely unknown. Here, we assessed the hypoxic thresholds of two Red Sea coral species: Porites sp. and Galaxea fascicularis . We exposed coral fragments of both species to one control treatment (6.8 mg O 2 L −1 ) and three reduced dissolved oxygen treatments (5.25, 3.5, and 1.25 mg O 2 L −1 ) during a 10-day experiment. Across the two species, maximum ( Fv / Fm ) and effective ( F′ / F m ′ ) photochemical efficiency, chlorophyll a , and dark respiration declined under the lowest O 2 treatment (1.25 mg O 2 L −1 ). Porites sp. coral fragments, however, were remarkably resistant and showed no signs of sublethal bleaching after 10 days of exposure to reduced O 2 . Conversely, 17% of G. fascicularis fragments bleached after only three nights of exposure to the lowest O 2 treatment (1.25 mg O 2 L −1 ). Our data show that longer-term hypoxic events (i.e., days to weeks) can induce coral bleaching, but these effects depend on the extent of O 2 reduction and are likely species-specific. Importantly, the levels of O 2 usually defined as hypoxic (~2.0 to 2.8 mg O 2 L −1 ) do not adequately capture the thresholds reported here. Hence, further research is urgently needed to more accurately describe the vulnerability of coral taxa to hypoxic and anoxic events.