Writing Sample Seagrass

B. L. Marchant

WRITING SAMPLE

Core Samples Reveal Secrets of Seagrass

first appeared October 1992
Sebastian Sun

At the October 20 meeting of the Sebastian Inlet Tax District (SITD) Commission, Dr. Randy Parkinson of Florida Institute of Technology (FIT) supplied commissioners with a bit of hands-on observation of just what does make up the river bottom in the 1/4 mile region between the existing inlet channel and the Intracoastal Waterway (ICW).

An actual core sample of the river bottom spanned across almost the entire length of the table behind which the commissioners sat inquisitively.

Parkinson explained information gathered from the nine vibracore samples and 10 surface soil samples taken in the proposed linking channel area, as well as the effect on area seagrasses the proposed channel could have.

"As you can observe in this sample on the table, we have a layer of fine sand, with three per cent organic materials, then a shelly layer with one per cent organic materials, a mundane-looking section, and shelly sand once again."

Limestone, Parkinson said, can be found beneath this last shelly layer, and could pose a problem for dredgers should the machine's teeth contact the hard surface, "But at the depth you are discussing (eight-foot overall depth), I feel there should not be a problem, although you should be aware of its existence down there."

"The sand," Parkinson added, "is of marginal beach quality," which means it is possible the dredged material could be used for beach renourishment, according to present standards set by the Department of Environmental Regulation (DER).

Seagrasses have also been a major concern for inlet officials, especially within the area of the proposed channel linking the ICW and the Sebastian inlet at marker 20.

Out of approximately 52 species of marine seagrasses existing worldwide, only seven are found in Florida waters, and five of those seven are found in the area of the Sebastian inlet. As a matter of fact, the Indian River is the northern limit of distribution on Florida's east coast for these five seagrass species, and two of these species occur no farther north than the Sebastian inlet.

These two species are called Thalassia testudinum, known as turtle grass, which is the most common of the Florida seagrasses; and Halophila johnsonii, which is also called Johnson's seagrass, a small fragile variety.

Halophila decipiens, or paddle grass, grows to a length of two to three feet, and occurs no farther north than the Ft. Pierce inlet. This variety remains undetectable in conventional aerial photgraphs.

Only Halodule wrightii, known as shoal grass (which grows to one foot in length) and Ruppia maritima, also called widgeon grass (which grows in both salt and fresh water), occur farther north than the Indian River lagoon system, and then not until North Carolina. Seagrasses do not occur in north Florida, Georgia or South Carolina.

Manatees feed on a variety called Syringodium filiforme, which is easily recognizable by its cylindrical leaves. This variety also grows to a height of about three feet.
Seagrasses evolved from rooted land plants that migrated back to the sea, producing one of the most productive ecosystems on earth, according to research and studies collected by the Marine Resources Council in Brevard county.

Because of the grasses' sediment-trapping ability, the protection they provide from erosion, the shelter and food source they offer area marine life, the importance of their protection has increased with time.

The destruction of seagrasses has become an element of concern for both their natural qualities and the fact that vegetative spread is the rule for reproduction, except for the Halophila varieties, which reproduce by seed.

The Department of Natural Resources (DNR) estimates that since 1950, Sebastian area seagrasses have declined by 34 per cent in 1970 and 42 per cent in 1986.

According to FIT's studies, the area affected by the proposed channel shows to have one to six per cent seagrass coverage on the west 1/2 of the area, with the east half having "a good bit higher" coverage.

With the proposed channel's path following a course due west, FIT studies show that of approximately 95 acres of seagrasses present in the area, 2.3 acres would be subject to dredging at a channel width of 150 feet. At a channel width of 200 feet, 3.3 acres would be affected, and at a 300-foot width, 5.8 acres would be plowed from the river's bottom.

An alternate route, traveling first south, and then dog-legging to the west, would destroy markedly less of the pre-existing seagrasses.

With this route, a channel width of 150 feet would place 1.9 acres of seagrasses in the dredging path. At a width of 200 feet, 2.7 acres would be affected, and at the 300-foot width, the area increases to 4.0, still 1.8 acres less than the same width on the due west plan.

One of the seagrasses in the proposed channel's path, taking the south dog-leg route, is the Halophilia johnsonii, which is listed in the Florida Natural Areas Inventory as carrying a G1G2 rating. This rating translates to say that the species is critically imperiled globally because of its extreme rarity. The johnsonii seagrass, which is found only from the Sebastian Inlet to Virginia Key, Fla., is not, however, considered endangered.

The study concludes that the proposed channel, as presented by the SITD, "is not necessarily a negative thing to benthic (bottom-dwelling) animals in the proposed area."
FIT's study also recommends the proposed channel as a logical solution to lower the occurrence of damage observed on the river bottom in the proposed area, which was described as "going from cropped grass to a few propeller scars, to exposure of seagrass root systems.

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BL Corcoran