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Mangrove lagoons of the Great Barrier Reef support coral populations persisting under extreme environmental conditions

Emma F. Camp*, John Edmondson, Annabelle Doheny, John Rumney, Amanda J. Grima, Alfredo Huete, David J. Suggett

*Corresponding author:

ABSTRACT: Global degradation of coral reefs has increased the urgency of identifying stress-tolerant coral populations, to enhance our understanding of the biology driving stress tolerance, as well as identifying stocks of stress-hardened populations to aid reef rehabilitation. Surprisingly, we are continually discovering that naturally extreme environments house established coral populations adapted to grow within extreme abiotic conditions comparable to seawater conditions predicted over the coming century. Such environments include inshore mangrove lagoons that carry previously unrecognised ecosystem service value for corals, spanning from refuge to stress preconditioning. However, the existence of such hot-spots of resilience on the Great Barrier Reef (GBR), remains entirely unknown. Here, we describe for the first time, two (Woody Isles and Howick Island) extreme GBR mangrove lagoons, exposing taxonomically diverse coral communities (34 species, 7 growth morphologies) to regular extreme low pH (<7.6), low oxygen (<1 mg l–1) and highly variable temperature (>7°C) conditions. Coral cover was typically low (<5%), but highly patchy and included established colonies (>0.5 m), with net photosynthesis and calcification rates of two dominant coral species (Acropora millepora, Porites lutea) reduced (20–30%), and respiration enhanced (11–35%), in the mangrove lagoon relative to adjacent reefs. Further analysis revealed that physiological plasticity (photosynthetic 'strategy') and flexibility of Symbiodiniacea taxa associations appear crucial in supporting corals capacity to thrive from reef to lagoon. Prevalence of corals within these extreme conditions on the GBR (and elsewhere) increasingly challenge our understanding of coral resilience to stressors, and highlight the need to study unfavourable coral environments to better resolve mechanisms of stress-tolerance.