Accelerating loss of seagrasses across the globe

threatens coastal ecosystems

Michelle Waycotta,1, Carlos M. Duarteb, Tim J. B. Carruthersc, Robert J. Orthd, William C. Dennisonc, Suzanne Olyarnike,

Ainsley Calladinea, James W. Fourqureanf

, Kenneth L. Heck, Jr.g,h, A. Randall Hughese, Gary A. Kendricki,

W. Judson Kenworthyj

, Frederick T. Shortk, and Susan L. Williamse

aSchool of Marine and Tropical Biology, James Cook University, Townsville, Queensland 4811, Australia; bInstitut Mediterrani d’Estudis Avanc ̧ats, Consejo

Superior de Investigaciones Cientificas (Spain)–Universitat de les Illes Balears, Miquel Marque ́s 21, 017190 Esporles, Spain; c

University of Maryland Center

for Environmental Science, Cambridge, MD 21613; dVirginia Institute of Marine Science, College of William and Mary, Gloucester Point, VA 23062; eBodega Marine Laboratory, University of California at Davis, Bodega Bay, CA 94923; f

Southeast Environmental Research Center, Florida International

University, Miami, FL 33199; gDauphin Island Sea Laboratory, Dauphin Island, AL 36528; hUniversity of South Alabama, Mobile, AL 36608; i

School of Plant

Biology, The University of Western Australia, Crawley, Western Australia 6009, Australia; j

Center for Coastal Fisheries and Habitat Research, National Ocean

Service, National Oceanic and Atmospheric Administration, Beaufort, NC 28516; and kJackson Estuarine Laboratory, University of New Hampshire,

Durham, NH 03824

Communicated by Robert T. Paine, University of Washington, Seattle, WA, May 20, 2009 (received for review October 26, 2008)

Coastal ecosystems and the services they provide are adversely

affected by a wide variety of human activities. In particular,

seagrass meadows are negatively affected by impacts accruing

from the billion or more people who live within 50 km of them.

Seagrass meadows provide important ecosystem services, including an estimated $1.9 trillion per year in the form of nutrient

cycling; an order of magnitude enhancement of coral reef fish

productivity; a habitat for thousands of fish, bird, and invertebrate

species; and a major food source for endangered dugong, manatee, and green turtle. Although individual impacts from coastal

development, degraded water quality, and climate change have

been documented, there has been no quantitative global assessment of seagrass loss until now. Our comprehensive global assessment of 215 studies found that seagrasses have been disappearing

at a rate of 110 km2 yr1 since 1980 and that 29% of the known

areal extent has disappeared since seagrass areas were initially

recorded in 1879. Furthermore, rates of decline have accelerated

from a median of 0.9% yr1 before 1940 to 7% yr1 since 1990.

Seagrass loss rates are comparable to those reported for mangroves, coral reefs, and tropical rainforests and place seagrass

meadows among the most threatened ecosystems on earth.

ecosystem decline  global trajectories  habitat loss  marine habitat

Coastal ecosystems such as salt marshes, coral reefs, mangroves, and seagrasses have declined, leading to growing

concern because they have recognized ecological and economic

values (1–5). Seagrasses, marine flowering plants that include

the widely distributed genera Zostera, Thalassia, and Posidonia,

form some of the most productive ecosystems on earth, rivaling

even crops of corn and sugar cane (6). Further, seagrass meadows provide high-value ecosystem services such as supporting

commercial fisheries worth as much as $3500 ha1 yr1 (7),

subsistence fisheries that support entire communities (8), nutrient cycling (9, 10), sediment stabilization (11), and globally

significant sequestration of carbon (12). Seagrasses and the

services they provide are threatened by the immediate impacts

of coastal development and growing human populations as well

as by the impacts of climate change and ecological degradation

(11, 13). Seagrass losses also disrupt important linkages between

seagrass meadows and other habitats (14), and their ongoing

decline is likely producing much broader and long-lasting impacts than the loss of the meadows themselves.

Previous efforts to assess general trends in seagrass abundance

have been based on a few case studies with limited quantitative

data for the time periods studied (15–19) or on extrapolations

from a few reported regional rates (11, 15, 19). These assessments vary in their conclusions, ranging from those asserting

widespread and abrupt declines, as reported in several recent

studies (16–18), to those reporting less dramatic declines on the

order of 2–5% yr1 (19) and occasional increases at local scales

(20, 21). To expand on these efforts, we synthesized quantitative

data from 215 sites with a total of 1,128 observations around the

world covering the time period 1879–2006, creating the most

comprehensive data set compiled to date (Table S1). Our results

extend previous findings in showing that seagrass areal cover is

declining across the globe and that the rate of loss is accelerating.

Results

Our analysis of the change in areal extent of seagrass populations

demonstrates that, since the earliest records in 1879, seagrass

meadows have declined in all areas of the globe where quantitative data are available, including both high and low latitudes.

Comparing all sites across their total study length, there were

significantly more declines in seagrass meadows than predicted

by chance: 58% of sites declined, 25% increased, and 17%

exhibited no detectable change (Table 1; 2 5.9, P  0.002,

df 2). Over the entire time period of our analysis, there was

a mean decline in seagrass area of 1.5% yr1 (median 0.9%

yr1). Not only are the rates of loss high, but the total seagrass

area lost is large. Overall, the measured area of seagrass loss was

3,370 km2 between 1879 and 2006 (i.e., 27 km2 yr1), representing 29% of the maximum area measured (11,592 km2). In

addition, the difference in area lost among sites that declined was

more than 10 times greater than that among sites that increased

(Table 1). Bootstrap analysis supported the robustness of these

results; subsampling recovered similar overall rates of change

independent of subsample size (Fig. S1). Extrapolation to the

global scale must be qualified by limited seagrass mapping

efforts in turbid water systems and in some geographic regions

that have received less attention from the scientific community.

Thus, global estimates of total seagrass area remain poorly

resolved; however, based on actual mapped areas and inferring

additional unmapped area (19), the current estimate of the total

area of seagrasses is 177,000 km2. Extrapolating our conserAuthor contributions: M.W., C.M.D., T.J.B.C., R.J.O., W.C.D., S.O., A.C., J.W.F., K.L.H.J., R.H.,

G.A.K., W.J.K., F.T.S., and S.L.W. designed research; M.W., C.M.D., T.J.B.C., R.J.O., W.C.D.,

S.O., A.C., J.W.F., K.L.H.J., R.H., G.A.K., W.J.K., F.T.S., and S.L.W. performed research; M.W.,

C.M.D., T.J.B.C., R.J.O., W.C.D., S.O., A.C., J.W.F., K.L.H.J., R.H., G.A.K., W.J.K., F.T.S., and

S.L.W. analyzed data; and M.W., C.M.D., T.J.B.C., R.J.O., W.C.D., S.O., A.C., J.W.F., K.L.H.J.,

R.H., G.A.K., W.J.K., F.T.S., and S.L.W. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Freely available online through the PNAS open access option.

1To whom correspondence should be addressed. E-mail: michelle.waycott@jcu.edu.au.

This article contains supporting information online at www.pnas.org/cgi/content/full/

0905620106/DCSupplemental.

www.pnas.orgcgidoi10.1073pnas.0905620106 PNAS  July 28, 2009  vol. 106  no. 30  12377–12381

ECOLOGY
vative net loss (29%) to this global scale suggests that more than

51,000 km2 of seagrass meadows have been lost during the past

127 years.

In addition, decadal time-course analysis reveals that the rate

of decline in seagrass meadows has accelerated over the past 8

decades (Fig. 1). The median rate of decline was 1% yr1

before 1940 but was 5% yr1 after 1980 (Fig. 1A). The largest

losses occurred after 1980 (Fig. 1B): in total, a loss of 35% of

seagrass area. The acceleration in detected rates of decline

cannot be attributed to increased sampling effort; there was a net

change of 37 km2 site1 decade1 after 1980, twice the rate of

loss before 1980 (18 km2 site1 decade1). Comparing decadal

trends, there was again a significantly greater number of sites

experiencing decreases compared with increases (Wilcoxon

signed pair ranked test, P 0.002) (Fig. S2; 2 23.7, P 

0.0001, df 2). The median rate of change from 1879–2006 for

sites with increased seagrass area was 5.4% yr1 (mean: 11.8 

3.6% yr1), which includes reports of the formation of 23

seagrass meadows where previously absent. As with loss rates,

the rate of increase also accelerated from 4.3% between 1970

and 1980 to 8.4% in the period from 1990 to the 2000s. To date,

however, the observed increase in seagrass area has been small

compared with the total area lost globally (Table 1); details are

available in Table S2.

Evidence of causes of decline was available for 77 of 128

declining sites. Among these, 2 major causes of seagrass loss were

indicated: (i) direct impacts from coastal development and

dredging activities (21 sites) and (ii) indirect impacts from

declining water quality (35 sites). Only 6 sites with decreases

were classified as being caused by natural processes such as storm

damage or biological disturbance. Of the 51 sites with increases,

29 had attributed causes, including 11 increases attributable to

improved water quality and habitat remediation. Among the

remaining increasing sites, recoveries from historical declines

attributable to storm damage or episodes of wasting disease were

the most common explanations.

Discussion

Our analysis included data from all 6 global seagrass bioregions

(22), although sites were not distributed evenly. Europe, North

America, and Australia were well represented (Fig. 2), reflecting

monitoring efforts in these relatively affluent regions and their

strong focus on coastal issues. Major gaps in information exist

for West Africa, northeast South America, and the northwest

Pacific area of the United States, where seagrasses are typically

restricted in distribution. However, the largest data gap exists in

the tropical Indo-Pacific region (from East Africa to Hawaii),

where seagrasses are widespread and abundant. Seagrasses in

this region perform vital ecosystem services for local human

populations, support numerous elements of local economies (8),

and are food for endangered species such as dugong and green

turtle (22). Furthermore, this region has the highest number of

seagrass species, including several endemic species (22). Given

the rapid population growth and development pressures in the

Indo-Pacific, there is a pressing need to acquire more data on

seagrass extent in this important region to aid in evaluating the

status of seagrasses.

Seagrass losses have been attributed to a broad spectrum of

anthropogenic and natural causes (11). Because seagrass meadows are often dominated by a single seagrass species, they are

susceptible to pandemic disease outbreaks like the ‘‘wasting

disease’’ of the 1930s that killed as much as 90% of all eelgrass

Table 1. Percentage rate of change for seagrass meadows globally

Trajectory*

Median % rate

of change,

(N)

Proportion

in category, %

Mean % rate

of change, 

(SE, N)

Net maximum

measured area,

km2

Net change in

study areas, km2

(% of maximum)

Mean study

length, yr

Declining 3.7 (126) 58 6.9 (0.9, 116) 9,147 3,662 (40) 25

Increasing 5.4 (53) 25 11.8 (3.6, 43) 879 314 (36) 20

No detectable change 0.06 (36) 17 0.2 (0.2, 36) 1,565 19 (1) 14

Overall 0.9 (215) 100 1.5 (1.1, 196) 11,592 3,367 (29) 22

Rate of change expressed as , % yr1.

*Meadows were categorized as declining (90% of initial area), increasing (final area 110% of initial area), or having no detectable change (final area within

10% of initial area).

1930s

nd

nd

pre-1930 1940s 1950s 1960s 1970s 1980s 1990s 2000s

increasing

decreasing

no detectable change 

120

80

40

0

C

Number of sites

0

5

-5

10

-10

15

-15

-20

-25

A

Median rate of change (μ, % y-1)

200

100

0

-100

-200

-300

-400

-500

-2000

-1000

-3000

B

Net change in area (km2

)

Fig. 1. Decadal trends in seagrass areal extent. Sites were categorized as

declining in area, as increasing in area, or as having no detectable change (i.e.,

10% of initial area). Values for the 2000s (dotted line) include 2000–2006

data only. nd, not determined because of incomplete data. (A) Median % rate

of change () by decade across sites. Error bars represent 25% and 75%

quartiles. (B) Measured net change in seagrass area, calculated as the net

change across each decade. (C) Number of sites in each category (decreasing,

increasing, or no change) by decade.

12378  www.pnas.orgcgidoi10.1073pnas.0905620106 Waycott et al.
(Zostera marina) in the North Atlantic Ocean (23) or stand

diebacks that killed more than 4,000 ha of turtlegrass (Thalassia

testudinum) in Florida Bay (24). Destructive fishing practices,

boat propellers, coastal engineering, cyclones, and tsunamis also

cause direct and immediate seagrass loss (3, 4, 11). More indirect

and potentially more damaging are the impacts of water quality

degradation resulting from increased nutrient additions and

sediment runoff in human-altered watersheds. In addition, the

indirect effects of aquaculture and invasive species have been

observed to affect seagrasses (25, 26). Other indirect effects

from overfishing have caused the loss of predators, which can

cascade down the food web and lead to the loss of the herbivores

that clean seagrasses of fouling algae, resulting in seagrass loss

(16, 27, 28). Lastly, global climate change is predicted to have

deleterious effects on seagrasses (29) and is emerging as a

pressing challenge for coastal management.

Worldwide, seagrasses are experiencing all 5 of the most

serious threats to marine biodiversity (30); overexploitation,

physical modification, nutrient and sediment pollution, introduction of nonnative species, and global climate change. Seagrass declines have been attributed to all these threats, often in

combination. Managing seagrass meadows requires an integrated approach (31), including efforts to avoid excessive nutrient and organic inputs from agricultural, aquaculture, and urban

sources and to prevent sediment loading, which causes a deterioration in the submarine light climate so critical for seagrass

growth. Best practices should also seek to avoid mechanical

damage through anchors, propellers, and fishing gear. Responsible stewardship that promotes favorable growing conditions

will confer seagrass meadows with resistance and resilience

against pressures that cannot be managed locally, such as those

associated with climate change.

Evidence of outcomes from improved management practices

are emerging. For example, a concerted effort to reduce point

sources of nutrients in Tampa Bay, Florida, over the past 2

decades has resulted in a 50% reduction in total nitrogen loads

and an 50% increase in water clarity, leading to the recovery

of 27 km2 of seagrasses since 1982 (32). Likewise, mitigation

measures adopted in Mondego Bay, a highly eutrophic estuary

in Portugal, reduced nitrogen loads and increased seagrass area

from 0.02 km2 (1997) to 1.6 km2 (2002) by altering estuarine

hydraulics and controlling seagrass habitat destruction by fishing

practices (33). These system-wide management strategies are

improvements on the attempts over past decades to restore

seagrass through transplantation. Numerous transplant projects

have been attempted worldwide as mitigation measures for

seagrass losses (34). However, transplant projects have involved

only a few seagrass species and at spatial scales that have failed

to alter the trajectories of seagrass loss significantly (34). Science-based protection and management approaches supporting

a combination of statutory authorities and consensus planning

must be designed to diminish the cumulative effect of stressors

and accommodate the broad range of impacts on seagrass

meadows to protect them from further losses (35, 36).

Our report of mounting seagrass losses reveals a major global

environmental crisis in coastal ecosystems, for which seagrasses

are sentinels of change (11). Seagrasses are sensitive integrators

of changes in water quality, sediment loading, and other inputs

that accumulate as a result of human modification of watersheds

and receiving coastal water bodies (37). Seagrass meadows signal

the early stages of eutrophication because they give way to faster

growing plant competitors like macroalgae and microalgae as

water quality decreases (38). More importantly, in contrast to

coral reefs, which also herald environmental change but occupy

a relatively small portion of the world’s oceans, seagrasses are

global in extent except for the highest polar regions.

The extent and rate of seagrass losses reported here have had

significant ecological consequences. Losses of seagrass meadows

will continue to reduce the energy subsidies they provide to other

ecosystems such as adjacent coral reefs or distant areas such as

deep-sea bottoms, diminishing the net secondary productivity of

these habitats (14). Seagrass losses also threaten the future of

10

40

1

Number of studies

increase

no detectable

change

decrease

Fig. 2. Global map indicating changes in seagrass area plotted by coastline regions. Changes in seagrass areal extent at each site are defined as declining (red)

or increasing (green) when areal extent changed by 10%, or no detectable change (yellow) when final area was within 10% of the initial area. There were

131 sites in North America, 34 sites in Europe, and 40 sites in Australia.

Waycott et al. PNAS  July 28, 2009  vol. 106  no. 30  12379

ECOLOGY
endangered species such as Chinook salmon (39) and the habitat

for many other organisms. Seagrass losses decrease primary

production, carbon sequestration, and nutrient cycling in the

coastal zone (5). If the current rate of seagrass loss is sustained

or continues to accelerate, the ecological losses will also increase, causing even greater ill-afforded economic losses.

Severe impacts to seagrass meadows have received limited

public attention compared with changes to other coastal (11, 40)

and terrestrial ecosystems, despite the fact that the overall mean

rate of seagrass loss calculated here is similar to that of mangrove

forests (1.8% yr1) (41) and even faster than that of tropical

forests (0.5% yr1) (42). Reported changes in Indo-West Pacific

(43) coral cover are lower, declining at 0.72% yr1 among reefs

repeatedly monitored over the period 1997–2004. Mean decline

rates reported in most coral reef studies (1 to 9% yr1) (44, 45)

are based on changes in percent coral cover, as opposed to the

actual areal extent of the coral reef ecosystem; however, rates of

seagrass meadow and coral reef declines can be considered

roughly equivalent, given that seagrass meadows are expected to

have a concomitant decline in percent cover as total area

declines (46). The cumulative effect of the reported losses in

seagrass, mangrove, coral reef, and coastal wetland habitats

signals a serious deterioration of coastal environments around

the world.

Materials and Methods

We compiled a database that incorporated existing quantitative data on

seagrass areal extent from published studies, reports, web sites, online databases, and unpublished but audited sources (see SI Data Sources). Sources

were identified by conducting a Web of Science search in February 2006 and

then again in October 2006 using the following search term: (seagrass* or SAV

or submerged aquatic vegetation) and (loss* or change* or recovery or

stability or dynamic* or impact* or map* or decline* or increase* or gain*).

This search returned 2,346 references (from which we excluded reports referring to ‘‘freshwater species’’). We also requested relevant data on the Seagrass

Forum listserv in October 2006.

To ensure that reported changes in areal extent were not simply attributable to seasonal variation, we included only studies with at least 2 estimates

of areal extent that covered more than 2 years. If the date of a study was not

specific within a year, it was assigned to the midyear point (i.e., 1980.5). A

known location for each study is referred to as the ‘‘site,’’ and each measurement of seagrass area at a site is referred to as an ‘‘event.’’ The trajectory of

each study was determined as the overall percent rate of change, either

positive (i.e., more seagrass area measured) or negative (i.e., less seagrass area

measured), across the entire time period of each study and across each decade

of the study. At each site, we classified seagrass meadows as declining or

increasing if the areal extent changed by 10% or as no detectable change if

the areal extent changed by 10% [which is typically within the error of

assessment techniques (47)]. Departure from even partitioning of meadow

trajectories was calculated using a 2 test. The final database comprises 215

sites with 1,128 events from 70 sources (Table S1). Several data verification

steps were conducted, including independent checks of 63% of all site entries

(136 sites).

We conducted 2 types of analyses: (i) trajectories were analyzed using the

initial and final observations of seagrass area at each site to represent overall

trends at sites irrespective of the time period, and (ii) trajectories were

analyzed decade by decade to account for trends across decades (‘‘decadal

analysis’’) (see SI Decadal Analysis; Fig. S3). Percentage rates of change (the

trajectory, , % yr1) for sites were calculated over time interval, t, from the

initial to final reported areas (Ao and At, respectively) as[ln(At/Ao)/t]  100.

In addition to the specific rate of change, the net change in area (final area

minus initial area) was calculated for each site and for each site in each decade

that the study traversed. Trajectories and net change in reported area were

calculated across the total time span of each data set and for each decade of

the data set (see SI Decadal Analysis ; Fig. S3). A test of the relative proportion

of sites experiencing decreases as opposed to increases in each decade was

conducted by comparing the departure from a 1:1 relationship between these

increases and decreases using a 2 test and a Wilcoxon signed pair ranked test.

All records of seagrass area before 1930 were grouped for the decade

analysis because of limited sample size. The 215 sites assessing change in area

of seagrass meadows were not distributed randomly because some regions of

the world (the eastern coast of North America, Europe, and southern Australia) have been sampled more intensely than others, irrespective of regional

seagrass abundance. In addition to this geographic bias in available data (i.e.,

developed regions of the world were unavoidably overrepresented), there

was a historical bias. More data were available after 1980 (80% of records),

reflecting recent increased research and monitoring effort (see SI Observational Effort). To address the influence of sample size effects, bootstrap

resampling was used, and we observed the overall trend in , as the median

rate of change, to be independent of sample size. Bootstrap analysis of was

conducted for the overall data by random subsampling of 10–80 records in

steps of 10, for 100 replicate random samples of each subsample size. For each

replicate of a given subsample size, a median value and mean value of were

calculated and plotted with the 25th and 75th percentiles and maximum and

minimum or plus and minus SEs to assess the central tendency for random

subsets of samples taken from the total data set (Fig. S1). Because of the lag

in reporting changes in measured seagrass areal extent (estimated at 5 years

from final date included in the data), the data available for the current decade

should be considered incomplete.

Two global estimates of seagrass area that can be substantiated at present

are (i) the measured global seagrass area, which is the area for which mapping

polygons have been established (124,000 km2; these authors also extrapolate

an estimate of expected total area, including unmapped seagrass as 177,000

km2) (19), and (ii) the potential global seagrass area determined by light

regimen, bathymetry, and seagrass light requirements (4,300,000 km2) (48).

Although these 2 estimates of global seagrass area result in an extremely wide

range (35-fold), further refinement of these global estimates is not possible

without more data. Estimates of global seagrass loss calculated in this study

were based on the estimated total area value of 177,000 km2 (19) that we view

as a minimum value for total global seagrass area. We note that seagrass

meadows grow in turbid, deep, or remote waters in many parts of the world,

making mapping their extent problematic.

ACKNOWLEDGMENTS. Ashley Simmons, Karen McGlathery, Richard Pearson,

and Simon Robson provided comments on the manuscript. This work was

conducted as a part of the Global Seagrass Trajectories Working Group

supported by the National Center for Ecological Analysis and Synthesis, a

center funded by National Science Foundation (Grant DEB-00–72909), the

University of California at Santa Barbara, and the State of California.

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