Life on the Edge: Can Corals Thriving in Mangroves Provide Insights into Climate Change?

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On an island in the U.S. Virgin Islands, USGS scientists discover corals are seeking refuge from climate change in mangroves.

Life on the Edge: Can Corals Thriving in Mangroves Provide Insights into Climate Change?

Coral often bleaches in response to changes in water temperature. 

The Science Issue and Relevance: Over 30 species of scleractinian corals are growing on or near red mangrove prop roots in Hurricane Hole, a portion of Virgin Islands Coral Reef National Monument, St. John, US Virgin Islands (USVI). This high diversity of corals in the shallow, mangrove-lined bays of Hurricane Hole appears to be unique within mangrove ecosystems in the Caribbean. Four of the coral species have been listed as threatened under the US Endangered Species Act. Many of the larger, intact corals clearly survived the 2005/2006 bleaching and disease event that caused an average loss of 60% of the coral cover on reefs within the USVI including Virgin Islands National Park. Although seawater temperatures would have been higher in these shallow waters than on the reefs, these colonies proved to be resistant. In general, the corals in this area have fewer signs of disease and less partial mortality than those on the “true” reefs surrounding St. John. Species-specific and intra-specific differences in response to thermal stress provide clues as to how reefs might respond to climate change. Climate change is expected to bring warmer temperatures associated with coral bleaching that may be linked to disease, as well as lower pH levels that hinder calcification and reduce coral growth rates. In 2010, water temperature exceeded 30°C for several weeks and most of the corals in the mangroves bleached. However, two major reef-builders varied greatly in their response, although most recovered. Diploria labyrinthiformis bleached more severely and exhibited more mortality than C. natans a species that declined substantially on the reefs in 2006 because of disease. Corals shaded by red mangroves bleached less overall. Major bleaching has not occurred since 2010. Biological and sedimentological processes appear to buffer declines in pH and carbonate saturation states. Research by USGS scientists has led to the identification of Hurricane Hole as a previously unconsidered refuge from changing climate and ocean acidification. 

Methodology for Addressing the IssueWe are collaborating with USGS scientists from the St. Petersburg Coastal and Marine Science Center and other scientists outside USGS to better understand the role of Hurricane Hole as a refuge from changing climate and ocean acidification. We hypothesize that hydrodynamic conditions in Hurricane Hole partially isolate bay waters from open ocean waters, allowing this system to remain out of equilibrium with the open ocean, creating refuge conditions. These conditions may place this refuge at risk from proposed development of two marinas in nearby Coral Bay. Whether or not this area is a source of coral larvae to nearby reefs also depends on water mass residence time in refuge bays and transport conditions from bays to open water. We are examining carbonate seawater chemistry as well as temperature and irradiance. The corals in Hurricane Hole appear to be more resilient to climate change, in particular high seawater temperatures and ocean acidification. A major objective is to correlate coral bleaching severity and recovery from bleaching with detailed water temperature records. Coral status is evaluated using photographs and in situ observations. Several Hobo Temperature Loggers are collecting data in each of the 3 major bays.  

Future Steps: Research is planned in 2016 to characterize water circulation patterns in Hurricane Hole and to collect data on indicator biogeochemical parameters. In addition, we hope to have the opportunity to collaborate with scientists at Mote Marine Laboratory and others to determine the genotypes of resistant and susceptible corals and to characterize their symbiotic zooxanthellae and associated microbial communities.