http://sanctuaries.noaa.gov

Seagrass Meadows and Nutrients

Seagrass Meadows Are Everywhere! 

The greater South Florida area has the largest 

documented seagrass bed in the world, covering 

approximately 5,900 square miles. Much of this 

grassbed lies within the Florida Keys National 

Marine Sanctuary and nearby Florida Bay, where 

it serves as shelter and a nursery ground for a 

myriad of fish and invertebrate species. Along with 

mangrove forests, meadows of seagrass are vital to 

supporting the coral reefs of the Florida Keys. 

In 1995, scientists from Florida International 

University began tracking seagrass as part of the 

Water Quality Protection Program for the Florida 

Keys National Marine Sanctuary. The program, 

implemented jointly by the U.S. Environmental 

Protection Agency and the state of Florida, was established to protect the coral reef from impacts due to nutrient pollution. 

Since its inception, the program has supported critical ecosystem monitoring and research projects designed to help sanctuary 

managers make informed decisions about resource issues.

Loss of Meadows Means Loss of Valuable Habitat

Turtle grass (Thalassia testudinum) is the most common, or dominant, seagrass species in the sanctuary. It thrives 

in shallow waters with relatively low-nutrient levels. When nutrient levels are high, turtle grass is outcompeted by plants that are 

adapted to the nutrient-rich environment. Under very high nutrient levels, tiny plants known as microalgae flourish in the water 

column. Sometimes, under the right conditions, the microalgae become so dense that they block sunlight reaching the seafloor and 

promote the growth of small plants that grow directly on the grass blades. Both situations make it difficult for the seagrass to absorb 

the sunlight needed for photosynthesis, the process of making its own food. 

The long-term result of nutrient enrichment, or eutrophication, can be the loss of seagrass meadows, which are critical nursery 

and feeding grounds for many invertebrates, fish, birds, reptiles and marine mammals, including many species that are important 

to the local fisheries. If microalgae thrive at the expense of seagrass, animals like crabs, shrimp, clams, urchins and other 

seagrass-dependent species are likely to disappear due to lack of 

habitat. Instead, animals that feed and live in the water column as part 

of the microalgae-based food web will predominate. This shift from a 

seagrass-based food web to one that is water column-based represents 

a significant change in the area’s ecology and can negatively affect 

recreational and commercial fisheries, which depend upon seagrass 

meadows as nurseries and feeding grounds. 

Turtle Grass Leaves Offer Clues about Nutrient Levels 

One method developed by scientists to assess the nutrient levels 

in sanctuary waters involves comparing the concentrations of two 

very important nutrients found in the blades of turtle grass: nitrogen 

and phosphorus. These nutrients are absorbed by seagrass from the 

environment, and concentrations of these nutrients in the leaf tissue 

reflect the surrounding environment of the plant. 

Grassbed with turtle grass (wide blade) and manatee grass. 

Photo: Florida Keys National Marine Sanctuary

Green macroalgae growing near turtle grass shoots.

Photo: Florida International University 

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

visit sanctuaries.noaa.gov

Ecosystem Model Uses N:P Ratio To Assess Nutrient Conditions

In nature, it is the ratio of nitrogen to phosphorus that is important in determining the dominant plant community. For turtle grass, 

when the ratio of N:P reaches 30:1, nutrient conditions are such that seagrass will be lost and replaced by nutrient-loving plants 

like seaweeds and microalgae. This ratio, called the Redfield Ratio, forms the basis for this “ecosystem behavior” model. Scientists 

designed the ecosystem behavior model 

to assess whether or not seagrass beds 

are in danger of disappearing due to 

increasing nutrient conditions. 

To obtain the N:P ratio data needed to 

feed the model, leaf tissue samples were 

taken from 30 permanent monitoring 

sites throughout the sanctuary in 2008. 

The N:P ratio was determined for each 

sample. Of the 30 sites, 13 exhibited 

trends toward the 30:1 Redfield Ratio. 

That is, increasing nutrient levels at 

these sites were approaching those 

conditions that may eventually result in 

the loss of turtle grass. 

Two Different Nutrient Environments Exist Naturally in the Florida Keys

The ecosystem behavior model depicts two environments that exist naturally in the Florida Keys. One environment has high nitrogen and low phosphorus concentrations. In this phosphate-limited environment, plant growth will take place with the addition of 

more phosphorus, but without it, growth will be limited. Florida Bay is an example of a phosphate-limited environment. Even the 

addition of a small amount of phosphorus can promote excessive plant growth, which is normally held in check by the lack of this 

nutrient. 

The second environment is the opposite in terms of the nutrient balance—naturally low in nitrogen, but rich in phosphorus. Reef 

waters on the ocean side are an example of a nitrogen-limited environment. The basic differences in nutrient conditions in the bay 

and ocean reef tract, along with differences in other physical characteristics, play a role in determining the kinds of marine animals 

and plants inhabiting each area. Even small changes in a limiting nutrient can translate to shifts in the kinds of plants inhabiting 

an area. For example, at the reef, the addition of nitrogen can stimulate the growth of seaweed, which can compete with coral for 

space. The full range of nutrient conditions in the Florida Keys is shown in the map above.

Index Tracks Nutrient Conditions by Year

To better track the direction of changes in the future, scientists 

developed a measure of the deviation from the Redfield Ratio, called 

the Elemental Index (EI). The index, which is calculated for each year 

based on that year’s data, can be compared with the 10-year index 

baseline. Thus far, the EIs have been determined for 2006, 2007 and 

2008 and all numbers were within the confidence intervals for the 

10-year average of 8.28 (see figure on right). A general decline in the 

EI values is noted, though, and could indicate a shift away from an 

environment in which plant growth is controlled by nutrients toward one 

in which plant growth is limited by lack of sunlight. 

Monitoring is Critical to Informed Management 

Continued monitoring is critical to understanding the effects of 

changing environmental factors on seagrass beds of the Florida Keys. 

Indicators are important for determining when these communities are 

being impacted by high-nutrient waters. Indicators in this program point 

toward broad-scale changes that suggest seagrass beds have more 

nutrients today than in 1995. For reports, graphs and more information 

about seagrass monitoring, visit http://www.fiu.edu/~seagrass. 

Principal Investigator: Dr. Jim Fourqurean, Florida International University

Written by: Nancy Diersing, Florida Keys National Marine Sanctuary

Published: August 2011 

EI values are compared to the 10-year average (solid gray line).

Image: Florida International University

Most of Florida Bay is phosphate-limited and the reef tract is nitrogen-limited. 

Image: Florida International University