Use of Mangroves by Corals as Habitats and Potential Climate Refugia in the Florida Keys (USA)

Abstract:

Mangrove forests and coral reefs are two tropical ecosystems that occur along a continuum of the land-ocean interface. Both systems have experienced significant ecosystem degradation due to many synergistic stressors. A growing number of recent studies has reported corals occupying non-reef habitats that may provide a supportive environment or refugium, sheltering them by reducing stressors such as oxidative light stress, temperature stress, and low pH. The Florida Keys host significant areas of coastal mangrove forests along the shoreline, and large numbers of coral colonies offshore, however no systematic studies have previously been conducted to describe mangrove habitat utilization by corals in the region. This study used satellite imagery and manual ground-truth surveys to search for mangrove-coral habitats in the Florida Keys National Marine Sanctuary and collected basic environmental parameters (temperature, salinity, dissolved oxygen, pH, turbidity) at sites were corals grew in mangrove habitats. Two types of mangrove-coral habitats were found and described in both the Upper and Lower Florida Keys: (1) prop-root corals, where coral colonies were growing directly on (and around) mangrove prop roots, and (2) channel corals, where coral colonies were growing in mangrove channels under the shade of the mangrove canopy, at deeper depths and not in as close proximity to the mangroves. Coral species found growing on and directly adjacent to prop roots included Porites porites (multiple morphs), Siderastrea radians, and Favia fragum. Channel coral habitats predominantly hosted S. radians and a few S. siderea, although single colonies of Solenastrea bournoni and Stephanocoenia intersepta were observed. Although clear, low-turbidity water was a consistent feature of these mangrove-coral habitats, the specific combination of environmental factors that determine which mangrove habitats are favorable for coral recruitment remains to be defined. Circumstantial evidence suggests additional coral communities existed on mangrove shorelines of oceanside and backcountry islands until destroyed, likely by Hurricane Irma. These mangrove-coral habitats may be climate refugia for corals and could be included in ecosystem management plans and considered for their applications in coral restoration.