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MARINFORSK-Marine ressurser og miljø

In the heat of the night: The role of hypoxia on coral reefs

Alternative title: Hete netter: Rollen til hypoksi på korallrev

Awarded: NOK 12.0 mill.

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Project Period:

2021 - 2024


A tropical coral reef is a colony of tiny animals that secrete a calcified skeleton they live within. They filter the water and feed on small zooplankton, but they also have an alga, called a zooxanthellae, living inside, as a symbiont. This alga receives nutrients from the food the coral has eaten, and in return uses sunlight to make sugars the coral can use. But it is dark during night, and then both the coral and its alga breathe and use the oxygen, and if the water is warm and there is little current then the water can be completely void of oxygen, especially early in the morning. This is critical for the coral, but also for all the fish that shelter for the night within the reef. With life history models, we have seen that this can explain why typical coral reef fishes grow only 2-3 cm per year, whereas cod in the ice cold Barents Sea grows 10 cm per year and Dolphinfish 50 cm in just six months. In this project, we will investigate the consequences of late-night hypoxia for corals and other animals on tropical reefs. We will combine models to formulate hypotheses, physiological laboratory experiments to test some of the expectations, and night-time filming at natural reefs with light-sensitive cameras to record natural behaviours. We will study fish, because they have behaviours that can easily be observed, but also corals. One hypothesis is that coral bleaching, i.e. that the coral expels the zooxanthellae and thereby lose an important source of energy, first and foremost is a response to hypoxia and therefore is initiated during early morning, especially during warm and long nights. We are also expecting that hypoxia becomes more severe as one moves from the outskirts of the reef towards its interior, and that this can gradient can explain why different species of fish and corals are found along the gradient from reef edge to reef interior.

We focus on progressive hypoxia during night, which we argue has extensive impact on tropical corals and reef inhabitants. Corals have symbiotic algae (zooxanthella) inside that provide ~80% of the coral’s energy, but at night all organisms on the reef, including zooxanthellae, use oxygen. The result is a daily rhythm with hypoxia building up during night (typically down to 20-40% but commonly more severe). We argue this is an uncompromising bottleneck that all reef organisms are adapted to. We will use modelling, experiments, and data analyses of physiology, behaviour, and life history traits. Our plan: - We will document late-night hypoxia and its consequences by filming in the field with a night-camera, analyse models for life history and behaviour, perform controlled physiological experiments, and analyse published data. - We argue that the coral-zooxanthellae symbiotic relationship undergoes a stress test every dawn when oxygen levels are at minimum. Should the coral stick with its zooxanthellae and hope that they both survive despite little oxygen, or should the coral kick it out to survive alone? - Small fish hiding in crevices also become hypoxic, which impairs senses and locomotion. How much light should it wait for before it leaves the coral to reach well-oxygenated waters outside? - A spatial gradient in hypoxia from reef edge to reef interior predicts variation in coral growth rate and vulnerability to bleaching. We will model and analyse how adaptations to hypoxia can be the missing gradient that explains the enormous biodiversity at coral reefs. - Effect of late-night hypoxia are most severe during the warm season, and more so later when nights are longer. We will analyse whether long nights explain peak season for coral bleaching. Fish need to keep energy consumption low to survive long hypoxic nights. We will observe, experiment, and model annual routines for fish and corals. The project will employ two postdocs and one researcher.


MARINFORSK-Marine ressurser og miljø