Friday, March 27, 2009

Mangroves: Back to Bermuda’s Roots

By: Jessie Hallett - BZS



Two views across The Lagoon at Ireland Island showing the fringing mangroves and neighbouring seagrass habitats. These natural areas are critical habitat for juvenile snappers and grunts which eventually leave and live as adults out on the coral reefs.

While we tend to think of marine habitat restoration as something modern – I have found two instances where healthy mangrove forest were created over decades and centuries ago.

During the last few months, I have been comparing extant marine and coastal habitats around Bermuda to those that were present two hundred years ago. I have been doing this by comparing recent aerial photos of the coast and reefs to the same areas charted by Thomas Hurd between 1788 and 1797. Throughout my work, I have been striving to determine what changes have occurred to these habitats, whether the changes have benefited the environment or not, and whether they are natural or anthropogenic. While in many cases, the environmental changes that have occurred over the past two centuries have been negative, two positive changes have occurred inadvertently due to human perturbations. Both these cases involve the creation of mangrove swamps where none existed before.

With a thicket of roots that hang like slender fingers into the water, mangrove trees may look ominous, but they are an important habitat to many fish species, as well as aquatic and terrestrial invertebrates, and birds. These trees have adapted to live in one of the most extreme and variable habitats in tropical marine systems: brackish marshes where salinity, temperature, nutrients, and sunlight can all be highly erratic. Unlike most plants, mangroves are halophytes, which means they are specially adapted to thrive in salty conditions. Mangroves are important because their roots help to stabilize coastlines, preventing erosion by expediting sediment deposition, and they also help recycle nutrients throughout seagrass and reef communities. The complex structure created by their roots also provides an ideal nursery habitat for juvenile reef fish, including many of the commercial fish species we like to eat.

Many mangrove habitats in Bermuda have disappeared entirely in the past two centuries. Many of Bermuda’s wetlands were in-filled as garbage dumps, or drained in the 19th Century when marsh-breeding mosquitoes spread deadly diseases such as yellow-fever. Since we have already lost so many mangrove areas, any remaining habitat where mangroves are thriving should be protected.

Over the past two centuries, human modifications to the coast has inadvertently created thriving mangrove marshes – at Morgan’s Point, and The Lagoon on Ireland Island.

Morgan’s Point was originally a group of islands called “The Brothers”, each surrounded by seagrass and patches of coral reefs. When the islands and surrounding area was reclaimed to create the US Naval Base during the 1940’s, a small inlet on the north side of the point was also formed, in order to provide a safe berth for small boats. Today the coastline of this man-made inlet is dominated by mangroves, and acts as a miniature estuary that provides habitat for herons and juvenile fishes.

An aerial image of the North Inlet at Morgans Point

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Another “accidental” mangrove swamp was created in the lagoon located on the south side of Ireland Island. Two hundred years ago, this lagoon was an larger inlet and open completely to the sea. When the Dockyard was built in the early 1800’s, the lagoon was enclosed by boulders except for a small cut through the rock out to the western side of the island, and a second pass between the boulders that connected to the Great Sound on the east side. These connections to the sea were excavated to prevent the lagoon waters going stagnant. By making the lagoon so sheltered - the workers who created it inadvertently also created a shallow sheltered area which is ideal for mangroves.

An aerial image of The Lagoon

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Presently, the Dockyard Lagoon and the inlet at Morgan’s Point both have healthy populations of mangroves, and the adjacent waters are dotted with propagules - the seeds with which mangroves reproduce. Both these areas are currently facing the possibility of development: a resort hotel on Morgan’s Point, and housing near Dockyard. Hopefully, these habitats will be spared if the regions are developed, as without protection Bermuda’s remaining mangrove forests may disappear.

Since local research has show reef fishes start their life in mangrove forests, losing these critical intertidal areas will have negative consequences that will resonate across the entire reef platform. These two "manufactured" habitats provide Bermuda’s mangrove population a chance to re-establish itself, and to restore the island with a healthy abundance of this vital coastal habitat.