Coral and reef fish communities in the thermally extreme Persian/Arabian gulf: Insights into potential climate change effects

Abstract

Coral reefs are facing global challenges, with climate change causing recurrent coral bleaching events at a faster rate than corals may be able to recover from, and leading to an overall decline of coral cover and shifts in communities across the tropics. Scleractinian corals are ecosystem builders that provide a habitat for numerous marine species, and their loss is disrupting a range of ecosystem functions and services that reefs normally provide. Climate change will continue to warm the world's oceans, leading to thermal conditions similar to those already existing in the Persian/Arabian Gulf (hereafter termed "the Gulf"). Indeed, the Gulf is in the summer the world's hottest sea (SST > 36 °C) and thus represents a "natural laboratory" in which to understand how reefs in other regions might respond under increasing temperatures. Recent research has shown that physiological thresholds of Persian/Arabian Gulf corals are higher than elsewhere, allowing them to survive in the Gulf's extreme temperatures. However, these marginal conditions result in coral communities that are low in diversity and comprised mainly of stress-tolerant species that provide limited three-dimensional structure. This low complexity habitat and the environmental extremes are associated with reef fish communities that have lower diversity, abundance, biomass, and size at maturity compared with conspecifics outside of the Gulf, and these fish communities have been shown to function quite differently. As climate change continues, coral reef ecosystems around the world are expected to gradually shift to thermal conditions similar to the present-day Gulf, and as such today's Gulf can provide insights into ecological patterns and processes we can expect in the tropics in the future. However, while Gulf fauna are adapted to extreme temperatures, they live very near their upper thermal threshold each summer. Recent climate change has resulted in recurrent mass bleaching events that have caused widespread loss of coral and knock-on effects on reef-dependent fishes. Thus, paradoxically, on the world's most robust reefs, we may be witnessing the world's first region-wide extirpation of reef fauna as a result of climate change.