Discovering the Secret Gardens in the Mangroves of St. John, U.S. Virgin Islands
When scientists peered into the secret world of mangrove forests fringing the protected coastlines of the Virgin Islands Coral Reef National Monument, in St. John, U.S. Virgin Islands, they discovered vibrant marine gardens growing there. Tucked among the roots and shade of the red mangrove trees is a stunning and colorful array of corals, sponges, anemones, and fish. The communities are remarkably diverse, rich in texture, color, and number of species. The diversity of corals may be unique among mangroves of the Caribbean.
In 2001, approximately 50 km2 was designated as the Virgin Islands Coral Reef National Monument to protect a wide array of marine habitats, such as coral reefs, seagrass beds, and mangrove forests. An area within the monument known as Hurricane Hole includes some of the least disturbed mangrove ecosystems remaining in the U.S. Virgin Islands. Hurricane Hole is made up of a series of shallow-marine bays with a narrow zone of red mangrove trees fringing the shorelines. The mangroves use long, branching prop roots to extend offshore and anchor themselves to the seafloor. These roots create shelter, providing a safe haven and nursery areas for small fish and many invertebrates. Very little research has been done in Hurricane Hole, and not much was known about the marine communities there. It was not until 2009 that Caroline Rogers, a scientist with the U.S. Geological Survey (USGS), discovered the secret coral gardens growing among the prop roots of the red mangrove trees.
"The discovery of all of the corals in the mangroves is very exciting," said Rogers. With more than 30 years working in the Caribbean as a coral-reef ecologist, she realized the area was special. "Within Hurricane Hole, there are at least 30 coral species, some of which are rarely seen even in the nearby coral reefs," she said. No other similar mangrove ecosystems, with such a high diversity of corals, are known to exist in the Caribbean.
"There are about 45 coral species identified on coral reefs around St. John, and to date we've identified 30 in the mangrove areas. The diversity is remarkable and is not unique to the corals. We're seeing great diversity in the sponges as well. Many of the sponges are more typically found in coral reefs than in mangroves," said Rogers.
It is not clear why the prop-root communities are so diverse, or why the individual bays within Hurricane Hole differ so much from each other with respect to coral abundance and diversity, but the unique assembly of marine creatures offers up a visual feast of subtle textures and rich colors. "There is always something new to see with each visit," said Rogers. (For a list of many of the coral species, see the short article by Rogers in Coral Reefs, v. 28, no. 4, p. 909, http://dx.doi.org/10.1007/s00338-009-0526-4.)
Scientists in the National Oceanic and Atmospheric Administration (NOAA) Coral Reef Conservation Program have identified critical goals that need to be commonly addressed by all regulatory and management strategies to save and sustain coral reefs. The goals are to help coral-reef ecosystems cope with climate change. Scientific research can identify areas that are vulnerable as well as areas that are more resistant to environmental stressors, including stressors associated with climate change. The biological community in the mangroves of Hurricane Hole is important for research because of its richness and its possibly greater tolerance to adverse conditions, such as higher seawater temperatures.
In 2005, a massive coral-bleaching event in the northeast Caribbean and a subsequent severe disease outbreak caused a 60-percent decline in corals in the U.S. Virgin Islands (see publication by National Park Service biologist Jeff Miller and others in Coral Reefs, v. 28, no. 4, p. 925-937, http://dx.doi.org/10.1007/s00338-009-0531-7). Surprisingly, the corals living among the mangrove roots seem to be in better condition than many corals on the reefs. Some of the colonies are so large that they clearly survived the 2005 bleaching event and disease outbreak. Many others are small enough that they may have settled and recruited to the mangrove roots since 2005.
The name "Hurricane Hole" describes the protective function the mangrove-lined bays provide during hurricanes. When a hurricane threatens the area, boaters seek shelter in the protected waters. Sometimes in the past, before the monument was established, they even tied their boats directly to the mangrove tree trunks and roots. This practice can significantly injure the mangroves and their associated ecological communities. The boats can break loose and run aground in shallow water, damaging the mangroves and destroying the fragile communities that grow on them.
In 2005, the National Park Service and the Friends of Virgin Islands National Park began installing a storm-mooring system in these bays to give boaters a secure alternative that would reduce damage to the mangroves. Other protective regulations include bans on fishing, water skiing, and jet skiing; these bans will help reduce wave action that stirs up sediment and dislodges organisms from the submerged prop roots. The improved mooring method and the added protective measures may be having a positive effect on the coral communities by reducing disturbance to the overall ecosystem.
Scientists are just beginning to understand and characterize the physical and chemical parameters that afford resilience to coral reefs in the face of climate change. Although it is not clear why corals in the mangroves are thriving and those on the coral reefs are not, such critical areas as Hurricane Hole may ultimately preserve coral species while more vulnerable reef habitats succumb to the effects of climate change. Understanding what factors contribute to the health and diversity of corals in these areas will help us develop strategies to protect other coral communities that are more vulnerable.
Rogers hopes to conduct future research in these fascinating bays, with a particular focus on the roles that seawater chemistry and patterns of water circulation may play in maintaining the high species richness of the corals and in their relative resistance to bleaching and disease.
Scientific understanding will give us the tools to tend our ecosystem gardens for the future. The coral communities thriving in the mangroves in Virgin Islands Coral Reef National Monument show excellent potential for helping scientists unlock their secrets.
(The photographs shown here are among the many that Rogers has taken while exploring the mangrove ecosystems around St. John. These and more can be viewed in an online slide show at http://fl.biology.usgs.gov/Science_Feature_Archive/2010/mangrove_secret/mangrove_secret_slideshow.html.)
in this issue: