VI Marine Environment
The waters surrounding the U.S. Virgin Islands (St. Thomas, St. John and St. Croix) contain coral reefs and other coral habitats, as well as associated mangroves and seagrass beds. Like in many other areas around the world, coral habitats in our region are threatened by human activities and natural events such as hurricanes. Recreational activities in areas of coral reefs are a common source of adverse impacts to the ecosystem as is coastal development.
This website is meant for students, teachers, fishers, and the general public and visitors to the U.S. Virgin Islands with the objective of raising awareness regarding the importance of our marine ecosystem and the need to protect it. The ultimate goal is to be able to enjoy our natural resources and at the same time guarantee their continued existence for future generations.
What is a Coral Reef?
Corals are tiny animals which belong to the group cnidaria (the “c” is silent). Corals are sessile animals, meaning they are not mobile but stay fixed in one place. They feed by reaching out with tentacles to catch prey such as small fish and planktonic animals.
Most corals live in colonies consisting of many individuals, each of which is called polyp. Polyps secrete a hard calcium carbonate skeleton, which serves as a uniform base or substrate for the colony. The skeleton also provides protection, as the polyps can contract into their skeleton if predators approach. It is these hard skeletal structures that build up coral reefs over time. The calcium carbonate is secreted at the base of the polyps, so the living coral colony occurs at the surface of the skeletal structure, completely covering it. Growth of these structures varies greatly, depending on the species of coral and environmental conditions, ranging from 0.3 to 10 centimeters per year.
Besides corals, there are many other species of animals and plants that inhabit the spaces created by the skeletons of the corals, forming the coral reef community. For instance, anemones, which are related to corals; and have a very similar body shape except they do not have a rigid external skeleton. Different species of fish inhabit the coral reef. As a matter of fact, most of the Caribbean fishery industry is based on reef fish species such as grouper, snapper and parrotfish. Lobster, conch and octopus are also reef dwellers. Other members of the coral reef community include sea urchins, sea stars, and different species of crabs, shrimps and worms.
Coral reefs are part of the larger marine ecosystem and are often found associated with seagrass beds and mangroves that are important habitats for reef species, especially when young.
What is Seagrass?
Did you know that seagrass beds are an essential part of marine ecosystems? Seagrass is a marine flowering plant found in coastal waters throughout the globe. It provides crucial habitats for the development of juvenile organisms such as fish and lobster. Seagrass beds are also the primary food source of green sea turtles native to the Virgin Islands. During intense storms deep rooted seagrass beds can aid in sediment retention maintaining the beaches we have come to love. Seagrass beds also play an essential role in keeping our seawater clean.
The USVI is home to three major seagrass species; shoal grass (Halodule wrightii), manatee grass (Syringodium filiforme) and turtle grass (Thalassia testudinum). In recent years the existence of these species has become jeopardized due to specific human actions. Construction and vegetation clearance increases sedimentation into the ocean that smothers local seagrass communities. Boat anchoring in seagrass beds can cause direct seagrass mortality.
What is a Wetland?
Wetlands are very common land features found almost everywhere in the world and are the dominant ecosystems along the Caribbean coast, in addition to coral reefs. There are many different kinds of wetlands and many ways to categorize them. NOAA classifies wetlands into five general types: marine, estuarine (estuary), river, lake, and marsh. Some common names for wetlands include marshes, mangroves, mudflats, ponds, fens, swamps, deltas, coral reefs, lagoons, shallow seas, bogs, lakes, and floodplains.
Wetland habitats serve essential functions in an ecosystem, including acting as water filters, providing flood and erosion control, and furnishing food and homes for fish and wildlife. They do more than sustain plants and animals in the watershed, however. During periods of excessive rain, wetlands absorb and slow floodwaters, which helps to alleviate property damage and may even save lives. Wetlands also absorb excess nutrients, sediments, and other pollutants before they reach rivers, lakes, and other bodies of water.
For more information about Wetlands and their importance in the Caribbean: Wetlands & Fish
What is a Mangrove Forest?
Mangroves are flowering trees that live in saltwater or brackish water in mudflats near shorelines. There are about 80 different species of mangrove trees. All of these trees grow in areas with low-oxygen soil, where slow-moving waters allow fine sediments to accumulate. Mangrove forests only grow at tropical and subtropical latitudes near the equator because they cannot withstand freezing temperatures.
Many mangrove forests can be recognized by their dense tangle of prop roots that make the trees appear to be standing on stilts above the water. This tangle of roots allows the trees to handle the daily rise and fall of tides, which means that most mangroves get flooded at least twice per day.
In the Virgin Islands there are three types of mangroves; red, black and white. Red mangroves are the most distinctive, with their complex aerial prop roots. These root systems, when submerged, support a diverse community of sponges, ascidians, algae, corals, and crabs. They provide crucial habitat for juvenile reef and pelagic fish as well as lobsters. The roots also trap sediment and associated pollutants to improve offshore water quality and slowly build more land. The trees also provide roosts, nesting habitat, and feeding areas for many bird species. Red managroves grow in the wettest environment of the three types, typically growing directly in water.
White and black mangroves grow in the soil upland from the red mangroves. They are often found in salt ponds. Black mangroves grow in dry areas where flooding may occur only during high tides or storms, while using snorkels to breathe! These snorkels are called ‘Pneumatophores’, which are special roots that allow the mangrove to breathe. These black mangrove roots project above the soil surface or water level to allow oxygen exchange.
White mangroves are usually found more inland in moist sandy areas of lower salinity. They prefer drier soil and more fresh water areas and sometimes produce ‘snorkels’ like the black mangrove. Another trait they share with the black mangrove is the ability to excrete excess salt through glands on their leaves. The white mangrove has two small pores on the stem which it uses in addition to the leaf surface for ridding itself of salt.