St. John Underwater Environment: Seagrass

St. John Marine Life: seagrass
Shoal Grass at Maho Bay

seagrass001Local seagrass species include shoal grass, turtle grass and manatee grass. These underwater grasses are commonly found on the sandy bottoms of calm bays and between coral reefs. They reproduce and grow by means of an underground root called a rhizome, which lies down horizontally just beneath the sand. From this rhizome the blades of grass grow up and the roots grow down, forming a mat of root fibers that hold the seagrass to the ocean floor. Seagrass is dependent on sunlight and therefore, cannot tolerate cloudy water for extended periods of time.

Seagrasses control erosion by holding down loose sandy soils with their mat of roots, thus protecting our beautiful beaches. Moreover, they help prevent turbidity, or water cloudiness. This is an important function because cloudy water blocks out sunlight.

Seagrasses control turbidity by trapping sediments washed down from land during rains and ultimately incorporating them into a seabed soil that is held securely by the seagrass roots. The blades of grass also slow down bottom currents and keep loose sediments from getting churned up.

Seagrass beds support a great deal of marine life. They provide nutrition for the green turtle and queen conch, and serve as a habitat for many species of juvenile fish and other sea creatures that are small enough to hide between the blades of grass.

Although not quite as sensitive as corals, seagrasses are also threatened by turbidity. They are currently in grave danger from the exponential increase in residential and commercial development on St. John. The prime turbidity-causing culprit is the failure to pave roads. Other enemies of clear water include unprotected and irresponsible excavation, especially on steep slopes, and improper sewage treatment.

A more immediate threat to seagrass comes from the proliferation of boat anchoring. The act of setting down and then pulling up an anchor tears the seagrass up by the roots and destroys the rhizomes, making recovery slow and difficult. Worse yet, when anchors are set improperly, they may drag, causing widespread damage that often includes injury to nearby coral reefs. Moreover, as an anchored boat swings around in the wind, the anchor chain is dragged over the sea floor in an arc, destroying all the grass in its path.

St. John Sea Creatures: Turtle
Sea Turtle Grazes Seagrass

Years ago, harbors such as Caneel, Maho and Francis Bays had extensive seagrass cover. In those days literally hundreds of conch ambled slowly through the seagrass leaves at the bottom of the bays. With the advent of modern tourism and the great increase in the number of boats anchoring in these picturesque and well-protected harbors, the seagrass has all but disappeared and the conch population has plummeted.

Today a mooring program has been instituted whereby mariners enjoying many of the most popular bays in St. John may secure their vessels to moorings as an alternative to anchoring. The mooring program is a powerful step towards the preservation of seagrass and coral reefs. Unlike anchors, moorings are relatively permanent fixtures. This minimizes the disruption of the seabed. Moreover, moorings do not depend on heavy chains lying on the sea bottom for a secure bite, nor are they subject to dragging.

Excerpted from St. John Off the Beaten Track by Gerald Singer

St. John Weather

Isolated showers. Mostly sunny, with a high near 81. East wind 21 to 24 mph. Chance of precipitation is 20%.

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