Water bodies, watersheds and storm water
Docks, such as the one depicted in this artist’s rendering, block sunlight from reaching grasses below the water. Less aquatic grass means less food and habitat for aquatic animals and fish, such as the juvenile fish at right in the illustration.
Aquatic grasses, like these at Lake Apopka in central Florida, help filter pollutants entering the waterway.
At Lake Jesup, the District attached young grass plants to netting. The netting was attached to the lake bottom and the area blocked off to allow the grass to grow.
The District monitors aquatic grass beds in the Indian River Lagoon. Information about these grass beds is critical in determining the water body’s health.
Dean Dobberfuhl, a District environmental scientist, attaches fiberglass panels to a dock in the St. Johns River. Scientists hope that enough light will shine through these panels to allow aquatic grasses to grow. Aquatic grasses generally cannot grow beneath docks because the docks shade the underlying water.
Aquatic grasses are a vital part
of the water world
Life in most waterways depends on the survival of underwater grasses, which have been imperiled by pollution, lack of sunlight, dredging and development.
The temptation may be strong to make the shorelines of rivers, such as the St. Johns River, feel more like a sandy-bottomed lake. Underwater grasses feel squishy underfoot, and one might be a bit jumpy if a stringy plant brushes an ankle or leg.
But when underwater grasses are removed or destroyed, a vital life-giving element of a waterway is taken away. A closer look underwater reveals a thriving micro-community.
The 310-mile-long St. Johns River is home to many plant species and marine animals — manatees, many types of fish, crabs, shrimp and other shellfish, river otters, waterfowl, and alligators and other reptiles.
Underwater plants here and in other waterways are food for some animals and nurseries for others. They act as surfaces for organisms — snails, algae and insects — to hold on to.
Submersed vegetation is also necessary because it adds dissolved oxygen to the water so aquatic animals can breathe. Without aquatic plants, the St. Johns River would not be as full of life as it is today.
Threatened grasses
Sixty seagrass species are found throughout the world. Of the many types of sea grass found within the St. Johns River Water Management District’s 18-county service area, one type of sea grass became a threatened species in the late 1990s under the Endangered Species Act — the first sea grass to be listed under the act.
Johnson’s sea grass (Halophilia johnsonii) is found nowhere in the world except in the Indian River Lagoon, at the District’s southern end. Its roots help stabilize sediments, and its blades provide shelter for invertebrates (small spineless animals) and small fish. The grass also produces food for lagoon marine animals and microorganisms. But it is vulnerable to disturbance and is unable to colonize new areas, as no male flowers and thus no seeds have ever been found for the species.
By mapping sea grasses in the lagoon and other waterways, scientists can measure a waterway’s health by monitoring plant distribution and abundance from year to year.
Grasses indicate waterway health
In the lower St. Johns River, the most common plants found are eelgrass or tapegrass (Vallisneria americana), water naiad (Najas guadalupensis), and widgeon grass (Ruppia maritima), which is a salt-tolerant grass.
Information about underwater grasses is a critical tool in determining the river’s health, and the river’s water quality must be maintained at a level that supports the growth of underwater grasses.
Monitoring grass beds
District staff regularly collect scientific data about aquatic grass beds in the St. Johns River and the Indian River Lagoon.
This allows researchers to assess characteristics and differences in grass beds at locations throughout the District, and detect changes that occur only in certain areas.
Illuminating aquatic grass beds
Though poor water quality has the greatest effect on underwater plants, thousands of boat docks have also had a significant impact, particularly by fragmenting grass beds and causing grasses under docks to dwindle. Just as lawns need sunlight to grow, underwater grasses depend upon light to survive.
Since February 2000, the District has been conducting studies on SAV-friendly dock designs. District scientists built several docks at an experimental site in the St. Johns River, off the banks of the District’s 10,320-acre Bayard Conservation Area property.
In late 2003, scientists replaced several panels of the experimental wooden docks with 3-by-5-foot fiberglass panels. Several docks were left with wooden panels for comparison, and one area was left without a dock as a “control plot,” to measure how the grass grows with no interference whatsoever.
Researchers hope the heavy-duty panels, designed for boat docks and other high-traffic surfaces, will allow enough sunlight through to permit grasses to grow beneath the docks. Also, the fiberglass panels won’t leach toxic compounds into the water, unlike treated wood.
District scientists attached fiberglass panels to six dock-type platforms near Bayard Point in Clay County to test how well the panels allow light to penetrate the water. Three docks have panels in them; the other three do not.
This study comes at the end of a three-year study that considered the use of cone-shaped glass prisms built into docks to redirect light downward to the grasses. Though initially hopeful, District scientists found that the prisms allowed too little ambient light below, and the grasses beneath the docks virtually disappeared.
Light measurements conducted at District headquarters have indicated, however, that the new translucent panels allow about 56 percent of ambient light below, compared to the 3 to 4 percent allowed by the prisms.
Many hope that the study, if successful, will offer an alternative to current federal regulations that require boat docks to be built no more than 4 feet wide and 5 feet above mean high tide.
Updated on 5-21-2010
