http://sanctuaries.noaa.gov

Seagrass Monitoring Project

Seagrass Is a Vital Link in the Ecosystem 

Seagrass meadows cover much of the seafloor in the 

Florida Keys National Marine Sanctuary and are a vital 

link in the coral reef ecosystem of the Keys. They serve as 

nursery and feeding grounds for manatees, sea turtles, 

bottle-nose dolphins, snook, pink shrimp and a variety 

of reef fish and invertebrates. Any significant loss of this 

productive habitat could result in negative impacts on 

the ecosystem, including reduced invertebrate and fish 

populations at the coral reef. 

Since 1995, scientists from Florida International 

University have been studying seagrass in the Florida 

Keys as part of the Water Quality Protection Program 

for the Florida Keys National Marine Sanctuary. A 

major goal of the seagrass monitoring project is to track 

changes in grassbeds over time. Scientists are particularly 

interested in detecting changes in seagrass cover 

caused by nutrient enrichment, or eutrophication, in nearshore waters. Using a variety of sampling methods and data analyses, the 

project produces numerical indicators that reflect environmental conditions in the field and can be used by resource managers to 

detect trends in seagrass meadows before there has been any significant loss of this vital habitat.

Seagrass Species Are Sensitive to Nutrients 

To detect potential changes in seagrass communities, project 

scientists developed a conceptual computer model that 

uses field data to determine whether nutrients, specifically 

phosphorus and nitrogen, are having negative effects on the 

sanctuary’s seagrass beds. This “eutrophication” model is 

based on the concept that higher nutrient conditions favor 

faster-growing plants over those that grow more slowly. If 

nutrient enrichment is occurring in sanctuary waters, 

faster-growing plants will replace slower-growing ones over time. 

Such changes can be tracked using visual surveys to record 

the abundance of seagrass and other plant species at fixed 

sampling sites throughout the sanctuary. 

The model shows that as more nutrients become available, 

predictable changes will occur in the most common, or 

dominant, plant species. Turtle grass (Thalassia testudinum) is 

the dominant species in much of the sanctuary. Highly 

productive turtle grass meadows are found in Florida Bay and on 

the ocean side. This slow-growing species thrives in low-nutrient 

conditions. 

Scientists use a square quadrat to record species and their abundance. 

Photo: Florida International University 

Eutrophication Model shows changes in species with increasing nutrients. 

Image: Florida International University

Florida Keys National Marine Sanctuary
To learn more about these and other accomplishments, 

visit sanctuaries.noaa.gov

Principal Investigator: Dr. Jim Fourqurean, Florida International University

Written by: Nancy Diersing, Florida Keys National Marine Sanctuary

Published: August 2011 

Turtle Grass Disappears Under High Nutrient Conditions

Manatee grass (Syrinogodium filiformes) and shoal grass (Halodule wrightii) are faster-growing seagrass species that prefer higher 

nutrient conditions. As nutrients increase or become more vailable, turtle grass will gradually disappear and be replaced by manatee and/or shoal grass. With even more nutrient enrichment, the faster-growing seagrass species will 

disappear altogether and be replaced by seaweed 

and other macroalgae. Finally, with very high levels 

of nutrients, macroalgae will be replaced by microalgae—microscopic plants living in the water column 

that can have undesirable effects on water clarity 

and quality when experiencing rapid population explosions, or blooms. 

Significant changes in the dominant plants brought

about by nutrient enrichment can have profound

impacts on the seagrass-based food web. Species shifts away from seagrass and toward more 

macroalgae or microalgae leads to changes in the 

kinds and availability of invertebrates, fish, birds and 

other animals in that food web. In time, the species 

like shrimp, crabs and certain fish will disappear and 

be replaced by a new food web made of organisms that 

thrive in the new nutrient-rich environment. 

Indicators Detect Higher Nutrient Availability in Sanctuary Waters

Seagrass scientists have recorded thousands of observations of species composition in the field and fed these data into the eutrophication model. So far, they have not found any significant overall loss of seagrass coverage in the sanctuary, but significant 

changes have occurred in the composition of species in plant communities. In 19 of the 30 monitoring sites, the dominant species 

had shifted from turtle grass to another species, suggesting higher availability of nutrients. In most of these sites, turtle grass decreased, but sites with increases in turtle grass density were seen as well. Both trends indicate that nutrients are increasing in that 

location. As nutrient levels rise, an increase in turtle grass density occurs first, and then with more nutrients this dense grass dies 

and is replaced with faster-growing species. 

Scientists have also developed the Species Composition 

Index (SCI), a measure of the relative abundance of turtle 

grass based on one year’s sampling results. If the SCI shows 

that the abundance of turtle grass compared to other species is declining, then nutrients are increasing and nutrientloving plants are being favored. This one-year SCI value can 

be compared to the baseline SCI value, which was calculated using data collected over a 10-year period between 1995 

and 2005. In 2006, the annual SCI was slightly higher than 

the 10-year baseline, suggesting lower nutrient availability. 

In 2007 and 2008, the annual SCI was slightly lower, indicating high nutrients and declining water quality. It should 

be noted, though, that none of the values were statistically 

significant from the baseline SCI.

Good Water Quality Is Essential to Grassbeds 

Good water quality is an essential ingredient for healthy seagrass meadows and the marine ecosystem as a whole. The seagrass 

monitoring program and other research efforts in the Water Quality Protection Program will continue to track the status of seagrass 

meadows, coral reefs and water quality in the Florida Keys National Marine Sanctuary. For reports, graphs and more information 

about seagrass monitoring, visit http://www.fiu.edu/~seagrass. For more information on preventing degradation of nearshore waters, visit http://floridakeys.noaa.gov/wqpp/welcome.html.

Sites shown in red and green exhibited changes that point to increasing nutrients.

Image: Florida International University

Healthy turtle grass beds thrive in clear, low-nutrient waters.

Photo: Florida Keys National Marine Sanctuary