Final – 2 October 2006 

Seagrass Monitoring in the Florida Keys National Marine Sanctuary 

James W. Fourqurean (Southeast Environmental Research Center and Department of Biology, 

Florida International University, Miami, FL) 

Michael J. Durako (Center for Marine Science and Department of Biology, University of North 

Carolina at Wilmington, Wilmington, NC) 

Joseph C. Zieman (Department of Environmental Science, University of Virginia,

Charlottesville, VA) 

Susie P. Escorcia and Leanne M. Rutten (Southeast Environmental Research Center and 

Department of Biology, Florida International University, Miami, FL) 

Goals 

The general objective of seagrass monitoring in the Florida Keys National Marine Sanctuary 

(FKNMS) (part of the Water Quality Protection Program) is to measure the status and trends of 

seagrass communities to evaluate progress toward protecting and restoring the living marine 

resources of the Sanctuary. The scope and depth of this monitoring effort are without precedent 

or peer for seagrass ecosystems throughout the world. Specific objectives are: 1) To provide data

needed to make unbiased, statistically rigorous statements about the status and temporal trends of 

seagrass communities in the Sanctuary as a whole and within defined strata; 2) To help define

reference conditions in order to develop resource-based water quality standards; and 3) To 

provide a framework for testing hypothesized pollutant fate/effect relationships through processoriented research and monitoring. In order to meet these objectives, we have developed these

goals for the project: 

• Define the present distribution of seagrasses within the FKNMS 

• Provide high-quality, quantitative data on the status of the seagrasses within the FKNMS 

• Quantify the importance of seagrass primary production in the FKNMS 

• Define the baseline conditions for the seagrass communities 

• Determine relationships between water quality and seagrass status 

• Detect trends in the distribution and status of the seagrass communities 

Methods 

To reach these goals, four kinds of data are being collected in seagrass beds in the FKNMS: 

• Distribution and abundance of seagrasses using rapid assessment Braun-Blanquet surveys 

• Demographics of the seagrass communities using leaf-scar counting and population

demographics techniques 

• Seagrass productivity of the dominant species of seagrass in the FKNMS (Thalassia 

testudinum) using the leaf-mark and harvest method 

• Seagrass nutrient availability using tissue concentration assays

These data are being collected at three different types of sites within the FKNMS: 

Level 1 Stations: Sampled quarterly for seagrass abundance, demographics, productivity, and 

nutrient availability. These stations are all co-located with Water Quality Monitoring Project 

stations (Fig. 1). 

57
Final – 2 October 2006 

Level 2 Stations: Randomly selected locations 

within the FKNMS, sampled annually for 

seagrass abundance, demographics, and 

nutrient availability. Each year, new

locations for Level 2 stations are chosen. 

Level 3 Stations: Randomly selected locations 

within the FKNMS, sampled annually for 

seagrass abundance. Each year, new 

locations for Level 3 stations are chosen. 

We are assessing both interannual and 

seasonal trends in seagrass communities. The 

mix of site types is intended to monitor trends

through quarterly sampling at a few permanent

locations (Level 1 sites) and to annually 

characterize the broader seagrass population

through less intensive, one-time sampling at 

more locations (Level 2 and 3 sites). 

Figure 1. Map of the Florida Keys showing the 

location of Level 1 seagrass status and trends 

monitoring sites. Site numbers correspond to 

stations of the Water Quality Monitoring 

Findings to Date 

The significant changes in seagrass communities at the permanent Level 1 stations that we

reported last fiscal year continue to be present after an additional year of sampling. These 

changes are consistent with model predictions of nutrient-induced changes of these systems. 

There may be reasons for these observations that are unrelated to human activities in the region,

but the spatial pattern of changes and the agreement of the changes with models of the system

suggest that there is regional-scale change in nutrient availability that is causing changes in 

seagrass beds over a wide portion of the FKNMS. 

In 2003, we resurveyed 202 Level 2 and Level 3 stations that were last visited during the summer

of 1996. The data collected during this resurvey are still being assessed, but preliminary analyses

indicate that there are no large-spatial-scale trends in the abundance of the dominant benthic 

plant types over these seven years. 

In general, nutrient addition to aquatic environments shifts the competitive balance to fastergrowing primary producers. The consequence of this generality in seagrass-dominated

environments is that seagrasses are the dominant primary producers in oligotrophic conditions. 

As nutrient availability increases, there is an increase in the importance of macroalgae, both freeliving and epiphytic, with a concomitant decrease in seagrasses because of competition for light.

Macroalgae lose out to even faster-growing microalgae as nutrient availability continues to

increase. First, epiphytic microalgae replace epiphytic macroalgae on seagrasses; then planktonic 

microalgae bloom and deprive all benthic plants of light under the most eutrophic conditions. 

The South Florida case is more complicated than the general case described above because there 

are six common seagrass species in South Florida. These species have different nutrient and light 

requirements, and have differing responses to eutrophication. Large expanses of the shallow 

marine environments in South Florida are so oligotrophic that biomass and growth of even the

slowest-growing local seagrass species, Thalassia testudinum, are nutrient-limited; at this very 

oligotrophic end of the spectrum, increases in nutrient availability actually cause increases in 

58
Final – 2 October 2006 

seagrass biomass and growth rate. As nutrient availability increases beyond what is required by a 

dense stand of T. testudinum, there are other seagrass species that will out-compete it (Fig. 2). 

The relative importance of the various primary producers, then, can be used to assess the trophic 

state of the community. 

Eutrophication model

Trophic state

Relative Im

p

orta

nce

Thalassia testudinum

Syringodium filiforme

Halodule wrightii

Ruppia maritima

Macroalgae

Microalgae 

Oligotrophic Eutrophic

Figure 2. Conceptual model showing the change in 

importance of primary producers as nutrient availability 

increases from low (oligotrophic) to high (eutrophic). 

Each species in the species 

dominance-eutrophication gradient 

model (Fig. 2) can potentially 

dominate over a range of nutrient 

availability and the model predicts a 

change in species dominance as 

nutrient availability changes. These

changes are not instantaneous, 

however. Field evidence suggests 

that species replacements may take 

place on a time scale of a decade or 

more. It is desirable that we be able 

to predict the tendency of the system 

to undergo these changes in species 

dominance before they occur, so that 

management actions can be taken. 

Tissue nutrient concentrations can be

monitored to assess the relative

availability of nutrients to the plants. 

For phytoplankton communities, this 

idea is captured in the interpretation

of elemental ratios compared to the 

familiar “Redfield ratio” of 

106C:16N:1P. For the seagrass T.

testudinum, the critical ratio of N:P 

in green leaves that indicates a 

balance in the availability of N and P 

is Figure 2. Conceptual model showing the change in importance of primary producers as

nutrient availability increases from low (oligotrophic) to high (eutrophic).approximately 30:1, 

and monitoring deviations from this ratio can be used to infer whether N or P availabilities are 

limiting this species’ growth. Hence, T. testudinum is likely to be replaced by faster-growing 

competitors if nutrient availability is such that the N:P of its leaves is ca. 30:1. A change in the 

N:P in time to a value closer to 30:1 is indicative of eutrophication (Fig. 3). 

These models lead directly to a definition of trends likely to be encountered in the seagrass 

communities of South Florida if humans are causing regional changes in nutrient availability 

because of alterations to quantity and quality of freshwater inputs to the marine ecosystem: 

1) regional eutrophication will cause N:P ratios of seagrasses to approach 30:1 from higher 

or lower values indicative of oligotrophic conditions; and 

59
Final – 2 October 2006 

3.0

0.05 0.10 0.15 0.20 0.25

0.5

1.0

1.5

2.0

2.5

N u tr ient-r ep lete

P-limited

Oligotrophic

N-limited

Eu troph ic ation E utro p hicatio n

N:P =

30:1

Leaf tissue P content (% of dry weight)

L e a f tiss

Oligotrophic

on t e n t ( f d ry w e ig h t)

2) regional eutrophication will cause a shift in

species dominance in South Florida seagrass

beds. 

u e N c % o

The first responses to eutrophication will be 

evidenced by an increase in the relative abundance

of fast-growing seagrass species (H. wrightii and S. 

filiforme) at the expense of the now dominant, slowgrowing T. testudinum. At later stages of 

eutrophication, macroalgae and microalgae will 

become the dominant primary producers. 

At four nearshore Level 1 sites in the Florida Keys, 

there has been an increase in the relative abundance 

of macroalgae over the period 1995-2003 that is 

consistent with an increase in nutrient availability. 

At none of these sites has there yet been a decrease 

in seagrass abundance, but our conceptual model 

predicts that increases in fast-growing macroalgae should precede decreases in seagrass

abundance (Fig. 2). One example, from site 235 offshore of Lower Matecumbe Key, shows how 

macroalgae have steadily increased in abundance over the monitoring period (Fig. 4). In addition 

to these sites where relative abundance of primary producers has changed, at four more Level 1 

sites there have been long-term shifts in the ratio of nitrogen to phosphorus in seagrass leaves 

that are consistent with increases in nutrient availability (Fig. 5). 

Figure 3. Conceptual model indicating how 

elemental ratios of seagrasses responds to 

increasing nutrient availability. 

1995 1996 1997 1998 1999 2000 2001 2002

A

b

u

n

d

a

n

c

e

0

1

2

3

4

5

Figure 4. At Level 1 station 235 (see Fig. 1 for location) there has been a slow

and consistent shift in species abundance, with faster-growing macroalgae 

becoming more abundant over the seven-year period. This change is consistent 

with model predictions 

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Final – 2 October 2006 

The sites that showed changes consistent with increased nutrient availability were not randomly 

distributed across the Sanctuary. All of these sites were relatively close to shore in the Middle 

and Lower Keys (Fig. 6). The lack of any such changes in the Upper Keys suggests that the 

factor driving the observed changes is not present across the entire Sanctuary, so factors acting at 

the global scale (like global warming or coastal overfishing) are not likely responsible for the 

observations. In addition to Level 1 sites that are exhibiting changes that are consistent with

long-term increase in nutrient supply, two additional sites were severely impacted by hurricanes 

over the course of the monitoring period. 

Change category, 1995-2003

None

Severe Hurricane Impact

Increase in Macroalgae

Shift in N:P

Figure 6. Long-term changes in seagrass beds at Level 1 sites. 

Resurveying the Level 2 and Level 3 sites revealed no spatially consistent patterns in changes in

relative abundance of seagrass communities throughout the Sanctuary. The mean changes in 

Braun-Blanquet density for the major taxa for the period 1996 and 2003 were not significantly 

different from zero (Fig. 7), but there were some locations that had large differences between

1996 and 2003. Whether these changes were real changes in the benthic communities or artifacts 

caused by small scale spatial heterogeneity is currently being investigated. There were some

areas, such as around Islamorada, that showed declines in Thalassia in a large area contiguous 

with Level 1 permanent sites that exhibited changes consistent with eutrophication (see Fig. 8 for 

spatial pattern of change and Fig. 6 for Level 1 site summary). However, other regions of

apparent change were not consistent with the patterns seen at the permanent sites. In FY 2004, 

we will resample an additional 200 sites that were surveyed in 1997. 

61
Final – 2 October 2006 

Our surveys have provided clear 

documentation of the distribution and 

importance of seagrasses in the FKNMS. 

The seagrass bed that carpets 80% of the 

FKNMS is part of the largest documented 

contiguous seagrass bed on earth. These 

extensive meadows are vital for the 

ecological health of the FKNMS and the 

marine ecosystems of all of South Florida. 

Maps of spatial distributions can be found 

at (http://www.fiu.edu/~seagrass/) or 

DVD. 

-3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3

Change in Thalassia density 1996-2003

Our permanent monitoring sites have 

provided valuable data on the interannual 

and seasonal variability of seagrass cover 

and abundance. Time series of species 

composition, seagrass productivity, 

nutrient availability, and physical

parameters can be found for each 

permanent monitoring site on the web site 

or the DVD. There have been some

striking trends in the seagrass communities

at these permanent sites: seagrasses were 

lost completely at 3 of the 30 sites during

hurricanes over the last four years. At the 

remaining 27 sites, the benthic 

communities are relatively stable. There 

are no common trends across the sites in

seagrass cover or community composition.

This can be interpreted

to mean that there are no

regional trends in the 

health of the seagrass

beds represented by the 

permanent monitoring 

sites that can be detected with the six years of monitoring data. However, manipulative 

experiments in seagrass beds in south Florida demonstrate that the time course of the response of 

Change from 1996-2003

-5 -4 -3 -2 -1 0 1 2 3 4 5

N

u

m

b

er of Statio

n

s

0

20

40

60

80

100

120

140

Change from 1996-2003

-5 -4 -3 -2 -1 0 1 2 3 4 5

N

u

m

b

er o

atio

n

s f St

0

20

40

60

80

100

120

140

Change from 1996-2003

-5 -4 -3 -2 -1 0 1 2 3 4 5

N

u

m

b

atio

n

s

er of St

0

20

40

60

80

100

120

140

Thalassia testudinum

Calcareous Green Algae

Syringodium filiforme

Figure 8. Spatial

pattern in the changes 

in the Braun-Blanquet 

density of Thalassia 

testudinum at Level 2 

and 3 sites surveyed in 

1996 and revisited in 

2003. 

Figure 7. Frequency distributions of changes in

Braun-Blanquet densities for the three most common

taxa based on revisiting 202 sites in 2003 that were 

originally surveyed in 1996. The mean change in 

density for all three taxa is not significantly different 

from zero. 

62
Final – 2 October 2006 

seagrass beds to eutrophication is on the order of decades, and we do not understand completely

the interaction humans have with the natural dynamics of these systems. These 30 sites should 

continue to be monitored on a quarterly basis. 

Detailed analyses of the monitoring data have led to 17 peer-reviewed publications in the peerreviewed scientific literature. These publications address aspects of the functioning, status and 

trends of benthic communities as well as lay the groundwork for forecasting future

anthropogenic impacts on this ecosystem.

Acknowledgments 

This data report represents the dedicated work of many people. Craig Rose, Alan Willsie, Brad

Peterson, and Leanne Rutten led the field collection efforts and spearheaded the compilation of 

the data report; Meredith Ferdie, Dottie Byron, Virginia Cornett, Sean Meehan, and Kevin 

Cunniff collectively put in the thousands of hours in the field and laboratory that went into the 

collection of these data. The fieldwork was conducted under permits FKNMS-2003-036 from the 

FKNMS, FDEP permit number 1587, FDEP Parks and Recreation Permit number 5-04-09, and 

permit number DRTO-2003-SCI-0009 from the National Park Service. 

Publications Resulting from the Project to Date (Chronological Order)

Rose, C.D., W.C. Sharp, W.J. Kenworthy, J.H. Hunt, W.G. Lyons, E.J. Prager, J.F. Valentine, 

M.O. Hall, P. Whitfield, and J.W. Fourqurean. 1999. Sea urchin overgrazing of a large 

seagrass bed in outer Florida Bay. Marine Ecology Progress Series 190: 211-222. 

Fourqurean, J.W., A. Willsie, C.D. Rose, and L.M. Rutten. 2001. Spatial and temporal pattern in 

seagrass community composition and productivity in south Florida. Marine Biology 138: 

341-354. 

Davis, B.C., and J.W. Fourqurean. 2001. Determining species interactions between the tropical 

alga, Halimeda incrassata, and the seagrass, Thalassia testudinum. Aquatic Botany 71(3): 

217-232. 

Peterson, B.J., and J.W. Fourqurean. 2001. Spatial variability of Thalassia testudinum population 

demographics in south Florida. Limnology and Oceanography 46(5): 1077-1090. 

Fourqurean, J.W., M.J. Durako, M.O. Hall, and L.N. Hefty. 2002. Seagrass distribution in south 

Florida: a multi-agency coordinated monitoring program. Pp. 497-522 in: Porter, J.W. and 

K.G. Porter, eds. The Everglades, Florida Bay, and the coral reefs of the Florida Keys. CRC 

Press LLC, Boca Raton, FL. 1000 pp. 

Peterson, B.J., C.D. Rose, L.M. Rutten, and J.W. Fourqurean. 2002. Disturbance and recovery 

following catastrophic grazing: studies of a successional chronosequence in a seagrass bed.

Oikos 97: 361-370. 

Fourqurean, J.W., and J.C. Zieman. 2002. Seagrass nutrient content reveals regional patterns of 

relative availability of nitrogen and phosphorus in the Florida Keys, FL, USA. 

Biogeochemistry 61: 229-245. 

Fourqurean, J.W., and L.M. Rutten. 2003. Monitoring of soft-bottom marine habitat on the 

regional scale: the competing goals of spatial and temporal resolution. Pp. 257-288 in: Busch, 

D. and J.C. Trexler, eds. Ecological monitoring of ecosystem initiatives. Island Press,

Washington, D.C. 

Fourqurean, J.W., J.N. Boyer, M.J. Durako, L.N. Hefty, and B.J. Peterson. 2003. The response of 

seagrass distribution to changing water quality: predictive models from monitoring data. 

Ecological Applications 13(2): 474-489. 

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Final – 2 October 2006 

Anderson, W.T. and J.W. Fourqurean. 2003. Intra- and interannual variability in seagrass carbon

and nitrogen stable isotopes from south Florida, a preliminary study. Organic Geochemistry 

34: 185-194. 

Fourqurean, J.W., S.P. Escorcia, W.T. Anderson, and J.C. Zieman. Spatial and seasonal 

variability in elemental content, δ13C and δ15N of Thalassia testudinum from south Florida. 

Submitted to Estuaries. 

Rutten, L.M. 2002. An assessment of nearshore benthic communities of the Florida Keys. 

Masters of Science Thesis, Florida International University, Biology Department. 

Ferdie, M. 2002. Nutrient limitation of benthic primary production in the upper Florida Keys. 

Masters of Science Thesis, Florida International University, Biology Department. 

Fourqurean, J.W., and L.M. Rutten. In press. The impact of Hurricane Georges on soft-bottom,

backreef communities: site- and species-specific effects in South Florida seagrass beds. 

Bulletin of Marine Science. 

Fourqurean, J.W., N. Marbà and C.M. Duarte. 2003. Elucidating seagrass population dynamics: 

theory, constraints and practice. Limnology and Oceanography 48(5): 2070-2074. 

Collado-Vides, L., L.M. Rutten, and J.W. Fourqurean. In review. Spatiotemporal variation of the 

abundance of calcareous green macroalgae in the Florida Keys: A study of synchrony within 

a macroalgal functional-form/group. Journal of Phycology. 

Ferdie, M. and J. W. Fourqurean. In review. Responses of seagrass communities to fertilization 

along a gradient of relative availability of nitrogen and phosphorus in a carbonate 

environment. Limnology and Oceanography. 

64