Tropical dead zones and mass mortalities on coral reefs

Abstract

Degradation of coastal water quality in the form of low dissolved oxygen levels (hypoxia) can harm biodiversity, ecosystem function, and human wellbeing. Extreme hypoxic conditions along the coast, leading to what are often referred to as "dead zones," are known primarily from temperate regions. However, little is known about the potential threat of hypoxia in the tropics, even though the known risk factors, including eutrophication and elevated temperatures, are common. Here we document an unprecedented hypoxic event on the Caribbean coast of Panama and assess the risk of dead zones to coral reefs worldwide. The event caused coral bleaching and massive mortality of corals and other reef-associated organisms, but observed shifts in community structure combined with laboratory experiments revealed that not all coral species are equally sensitive to hypoxia. Analyses of global databases showed that coral reefs are associated with more than half of the known tropical dead zones worldwide, with >10% of all coral reefs at elevated risk for hypoxia based on local and global risk factors. Hypoxic events in the tropics and associated mortality events have likely been underreported, perhaps by an order of magnitude, because of the lack of local scientific capacity for their detection. Monitoring and management plans for coral reef resilience should incorporate the growing threat of coastal hypoxia and include support for increased detection and research capacity.biodiversity | coral bleaching | dissolved oxygen | hypoxia | water quality

Oxygen‐mediated plasticity confers hypoxia tolerance in a corallivorous polychaete

“…However, our understanding of the extent of both the physical changes in ocean deoxygenation and the corresponding biological responses are still in their infancy (Pörtner et al ). This is especially true for tropical systems (Altieri et al, ).…”

The perfect calm: Reoccurring mass die-offs on a remote coral atoll

“…Predicting these events will be important in forecasting coral reef futures, but doing so requires an understanding of the causes of dead zones. Our observations contribute to a relatively small but growing database of dead zones potentially related to hypoxia [3]. Future studies characterizing the physical and chemical drivers of specific events, and meta-analyses of the common drivers of these events globally will be important for advancing our knowledge of coral reef responses to rapidly changing ocean habitats.…”

“…Hypoxia-driven mortality events are increasingly recognized as an important component of the effect of climate change on coral reef ecosystems [1] [2], yet such events are underreported because they are not targeted in standard monitoring programs [3]. Here, we report mass die-offs of reef-associated fauna in 2014 and 2015 on Dongsha Atoll, a remote coral reef ecosystem in the South China Sea [4].…”

Hypoxia shapes coral reefs

“…The third reason is that, because the decay of dead organisms by aerobic bacteria consumes oxygen, any reef mortality event in a low-flow environment, whether initiated by oxygen depletion or not, has the potential to become an oxygen crisis through runaway feedback, as observed in staghorn-dominated lagoons by Simpson et al (1993) and Hobbs & Macrae (2012). These two observations are part of a wider set of suspected hypoxia-related mass mortality events involving a variety of reef communities and occurring in a variety of reef settings, the common attribute being stagnation of the water column, generally induced by warm, calm weather (Kinsey 1979;Delesalle 1985 In reef environments prone to stagnation, hypoxia is potentially a dominant ecological influence, as recognised by Altieri et al (2017), and the other researchers cited above, but also much earlier by Verwey (1931). In addition to his oxygen consumption experiments in corals and algae, Verwey (1931) undertook comprehensive dissolved oxygen measurements on multiple reefs in Batavia (Djakarta) Bay, …”