Seagrass Ecology

Overgrazing in a Seagrass Meadow by Urchins

Florida Bay Project Profile - 11

---

Question: What is the short term impact of overgrazing by variegated urchins on a manatee seagrass meadow located in the Gulf of Mexico south of Florida Bay?

Background/Project: In August 1997, fishermen reported sighting a large group of variegated urchins (Lytechinus variegatus) in a seagrass meadow located about 10 miles north of Marathon in the Florida Keys. This 500 square mile grassbed, which is mostly manatee grass (Syringodium filiformes), is in Outer Florida Bay. An inspection of the site using SCUBA gear revealed high densities of urchins and long grazed channels within the tall grassbed. These herbivorous animals were feeding on the manatee grass and leaving channels of sand behind. The highest concentrations of urchins were found at the grazed-ungrazed boundaries of the channels. Based on an aerial survey, scientists estimate that the entire defoliated area is approximately five square miles.

Periodic monitoring of the urchin/manatee grass populations began in the fall of 1997 and included measuring the short-term impact of urchin grazing on seagrasses, sediment and molluscan community structure. Five channels were randomly selected for sampling and transects are set up across the channels from the ungrazed to the grazed zone. Urchin counts, urchin size measurements, seagrass shoot density and biomass were made along the transect. Urchins were collected and measured to determine their average size and reproductive condition.

Findings to Date: Examination of the size and frequency data revealed that all urchins at the site were adults that were not reproductively active. The urchin densities were highest along the boundary between the grazed and ungrazed interface where the average density was 167 per square meter. Seagrass biomass, the amount of living manatee grass, Syringodium filiformes, was significantly reduced when compared with ungrazed areas. In grazed areas, nearly all of the above-ground plant material was lost and approximately 50 percent of the below-ground plant material was lost. It is not known whether or not the urchins actually consumed the below-ground rhizome tissue or whether consumption of the leaves causes the rhizomes to die from lack of nourishment. Mollusk species richness was significantly lower in grazed areas than in ungrazed areas. As of April 1998, densities of urchins remained high and continued overgrazing of the seagrass beds was evident. In grazed areas, there were no signs of recolonization by the manatee grass. It appears that concentrations of urchins present in grazed spots were high enough to keep recolonization from taking place.

Status: This study is ongoing. Additional permanent study sites have been selected to monitor urchin and seagrass populations and to measure the potential for manatee grass recovery.

Restoration Impacts: This extensive seagrass meadow stabilizes bottom sediments and consequently affects the biological, chemical, and optical properties of the water flowing through the area. The continued degradation of this seagrass meadow may impact other seagrasses throughout the Florida Keys ecosystem.

Funding Sources: Florida Marine Research Institute; National Marine Fisheries Service; Florida International University

Related Protiles: • FBPP 1 • FBPP 6 • FBPP 19