Benthic foraminifera as bioindicators of coral condition near mangrove environments

Climate change stressors such as ocean warming, acidification and deoxygenation are severely threatening coral reefs and the vital ecosystem services they provide. Corals found to survive in mangrove habitats that naturally possess stressful conditions, are being increasingly studied to investigate the impact of multiple co-occurring stressors on coral growth. However, the water quality within mangrove-coral habitats and how this changes with varying distance from the mangroves remains largely unknown. We used the Foraminifera in Reef Assessment and Monitoring Index (FORAM Index) to assess the suitability of environments for coral growth across a mangrove to reef gradient. Sediment samples were collected from five sites at varying proximity to the Low Isles mangroves in the northern Great Barrier Reef. Sites were located: amongst the inner mangroves, the mangrove fringe, the adjacent leeward and windward reef margins and at Opal Reef (control) 38 km away from the mangroves. Foraminiferal community assemblages were used to determine the environmental suitability for coral growth and recovery. The poorest seawater conditions for coral occurred at the inner mangrove site with water quality improving outside the mangroves. Leeward and windward margin sites differed in their suitability for coral growth despite being a similar distance from the mangroves, indicating that water conditions were not necessarily constrained by proximity to mangroves alone. These findings have important implications for studies investigating mangrove-coral habitats as refugia and for assessing linkages between coral reefs and mangroves - two highly threatened tropical ecosystems.