Hypoxia on tropical coral reefs

This is a project about the role of progressive hypoxia during night on tropical corals and reef inhabitants. Coral is a colony-living animal with a symbiotic alga (zooxanthella) inside its tissue that provides ca. 80% of the coral’s energy. Sugars and oxygen are produced during daytime, but at night all organisms on the reef, including corals and zooxanthellae, use oxygen. The result is a daily rhythm of hyperoxia during day and progressive hypoxia build-up during night. We argue this is an uncompromising bottleneck that all reef organisms are adapted to.

This project will embrace theoretical and empirical work integrating ecology, physiology and behaviour within life history to better our understanding of what is happening in the heat of the night in these valuable ecosystems.

Background

Coral reefs are now the first major ecosystem to suffer extensive damage from global change, threatened by rising temperatures, ocean acidification and hypoxia. It is well-known that temperature and hypoxia affect coral bleaching. Within the realm of eco-physiology, impacts of warming on fish performance have been studied extensively. However, there is an important knowledge gap on how and when hypoxia causes coral bleaching and how oxygen availability affects life history strategies of reef organisms.

Hypoxia occurs naturally on reefs at night and the coral-zooxanthellae symbiotic relationship undergoes a natural stress test every dawn when oxygen levels are at minimum. Also, small fish hiding within the coral at night suffer daily hypoxic conditions. Yet even under these extreme conditions the fish remain within the coral. An important ecological trade-off and/or physiological constraint must therefore exist. In this project we argue that hypoxic conditions during night and the ensuing trade-offs with predation risk must be critical in explaining life strategies for reef inhabitants.

Aim and objectives

Our aim is to test, document, and refine our key hypothesis that late-night hypoxia is a daily bottleneck that structures physiology, life history, behaviour, and biodiversity in corals and reef-dwelling fish.

Our research questions:

1: How does late-night hypoxia act as a bottleneck for adaptations in corals and reef-dwelling fish?

2: Can the spatial gradient in night-time hypoxia from reef edge to reef interior explain a pace-of-life gradient and sustain biodiversity?

3: How does hypoxia during the warm season structure reef biology via phenology and opportunity?

Conceptual scheme of some hypothesis about how hypoxia during night and the ensuing trade-offs with predation may determine slow growth and behavioural strategies in coral-dwelling fish

Our approach

This project will combine theory and modelling, laboratory experiments, field observations, and mining and analysis of published data.

A modelling-testing-modelling cycle enables strong inference: modelling and theory development will be used to derive alternative hypotheses. The predictions from these will be tested in experiments and challenged with analyses of published data and new field observations to refine the hypothesis, revise the models, and make new predictions.

Working flow involving a cycle of induction and duduction thinking.

• Modelling and theory: We will develop evolutionary life history models for fish and corals by detailing the role of progressive hypoxia overnight.

• Physiological and behavioural methods: We will measure sensory performance, metabolism and hypoxia tolerance in a broad species range. Experiments in Norway will be used to test hypotheses and fine-tune methods for physiological experimentation in the original tropical system at the Lizard Island Research Station.

• Field observations during night: What happens on the reef at night? Our understanding of behavioural ecology is biased towards daytime. Here, we will film with new technology at night to establish nocturnal biology as a field that merits its own discipline.

•Data mining and analysis: We will take theoretical predictions further by challenging them with useful and significant data. We will compile a dataset for growth, life history and metabolism for a broad species range, and reanalyse for interactions between bleaching events, temperature, daylength, metabolism, etc.