Dry Tortugas National Park is known for its

coral reefs, colonial seabirds, and Spanish-American

fort, Fort Jefferson. Mangrove forest and seabird

surveys were conducted on Bush and Long Keys.

History and Ecology of Mangroves in the Dry Tortugas

Introduction

Dry Tortugas National Park, which

includes Bush, Long, Loggerhead,

Garden, and Bird Keys, is a cluster of

islands and coral reefs approximately

112.9 km (70 miles) west of Key West,

Florida (fig. 1). These islands were

explored in 1513 by Ponce de León, who

named them for the abundance of sea

turtles, “tortugas,” and the lack of fresh

water in the area. Historically, the

Tortugas shoals have been valued as a

military outpost, and the area is now

additionally recognized as nesting grounds

for diverse seabirds. The Dry Tortugas

were declared a national treasure and bird

sanctuary as early as 1908 and were

incorporated into the National Park

Service in 1935. These islands have been

the setting for the U.S. Geological

Survey’s National Wetlands Research

Center (NWRC) research into mangroves

and their relationship to bird life.

Historical and Ecological Survey

The Carnegie Institute maintained a

remote marine laboratory in the Dry

Tortugas at the turn of the 20th century.

Throughout historical biological investigations in the Dry Tortugas, mangrove trees,

which create habitats for birds and marine

life, were rarely noted. Naturalist John

Henry Davis mapped the vegetation of the

Dry Tortugas in 1935 and observed the

conspicuous absence of mangroves that

are dominant in the nearby Marquesas

Atoll and Florida Keys. Consequently, in

the protected shoals of Long Key in the

Dry Tortugas, Davis planted thousands of

red mangroves that survived several years

before being washed away by hurricane

strikes in the 1940’s. Davis wanted to

provide a chronological baseline for

monitoring the growth, succession, and

disturbance of mangroves in the Dry

Tortugas.

A recent historical and ecological

survey completed by NWRC scientists

revealed that current populations of

mangroves emerged on Bush and Long

Keys after Davis’s population was

destroyed (fig. 2). The survey included a

thorough review of photographs of

mangrove colonization as well as anecdotal evidence consisting of historical

accounts from early explorers, lighthouse

keepers, and military personnel who refer

to the presence of mangroves dating back

to the mid-1700’s. The presence of robust

mangrove sites, or stands, on Bush and

Long Keys in the Dry Tortugas demonstrates that propagules of all three mangrove species (Avicennia germinans, black

mangrove; Laguncularia racemosa, white

mangrove; and Rhizophora mangle, red

mangrove) drift from the Florida keys and

mainland and maintain their viability by

recolonizing on these low relief islands.

Hurricane Strikes

Plant and animal surveys of the

reefs and keys of the Dry Tortugas

document the history of shoal development and colonization dating back two

centuries. Historical evidence shows that

the Tortugas Atoll has supported up to 11

low relief islands over this time period

with only 8 in existence today. Some of

the islands have disappeared and reappeared multiple times as a result of

hurricane impact. Historical accounts

noting the presence of mangroves coincide

with decades of suspended hurricane

activity, while the absence of mangroves is

associated with periods of hurricane

strikes. Exposure to sea conditions and

hurricane impact may explain the historical evidence of recurring mangrove

colonization on the Dry Tortugas prior to

and since Davis’s plantings. Because

hurricane strikes have not affected the Dry

Tortugas since the mid-1960’s, 30 years of

relative calm has fostered mangrove

colonization.

U.S. Department of the Interior USGS Fact Sheet FS-047-02

U.S. Geological Survey April 2002

 Historical photos and contemporary aerial

images of the Keys helped USGS researchers

reconstruct the history of mangrove colonization in

the Dry Tortugas.

FLORIDA

Gulf of Mexico

 Dry Tortugas

National Park

Loggerhead Key

Garden Key Bush Key

Bird Key Long Key

Fort Jefferson
Recent NWRC Field Inventory Findings

Although mangroves are a salttolerant species with salt-filtering mechanisms in their roots and leaves that allow

them to thrive in a saltwater environment,

the flourishing, robust mangroves of the

Dry Tortugas are anomalies because they

grow on coral islands without organic

matter. In 1995, 1997, and 2001 researchers conducted field inventories to identify

the structure and composition of healthy

mangrove stands and conditions that

account for their current colonization and

stature. Tree size and density by species

were measured in fixed circular plots at

Bush and Long Keys. Surface and

interstitial water samples were collected to

characterize the salinity, pH, and nutrient

concentrations for each site.

Mangrove Populations on the Keys

 When researchers compared

historical maps and photos taken by Davis

in 1942 to recent aerial photography, they

found that the north end of Bush Key has

undergone much shoreline erosion since

the 1940’s. Beach erosion may be

hastening the death of the red and white

mangroves that have grown to 20-25 cm

(8-10 inches) diameter at breast height

(dbh) over the last 50 years. While

emergent red mangrove saplings were

found inside the beach dune, many were

dying from an excess input of nutrients

from pelican guano, and others were

extremely chlorotic, or experiencing a loss

of chlorophyll (fig. 3).

Researchers found a mixed mangrove stand containing all three species,

black, white, and red, on the east end of

Bush Key. The largest canopy trees

approached 14 cm dbh and 7.5 m high

(6 inches dbh and 25 ft high) and are

estimated to be 25 years old. Researchers

also established plots on the north and

south ends of Long Key in a fairly large

stand consisting of mature mangroves of

all species, the largest and tallest of which

were 25 cm dbh and 15 m high (10 inches

dbh and 49 ft high). Personal accounts and

dated aerial photography confirm a stand

age of 50 years or younger.

Frigatebird Association

The period between major hurricane

events in the Dry Tortugas may have

provided the opportunity for mangroves to

develop sufficient size and density to

attract nesting seabirds. A colony of

magnificent frigatebirds (Fregata

magnificens) that is thought to have

migrated westerly from previous nesting

grounds in the Marquesas Atoll circa 1989

occupies the most developed mangrove

stands, those approaching 15 m (49 ft) in

height along the extended shoal of Long

Key (fig. 4). Frigates have been observed

in the Dry Tortugas area for most of the

last century either roosting or feeding, and

recently, researchers confirmed that they

have nested in the area as well. The

current population number of frigates is

estimated at 100-150 breeding pairs. The

nutrient input of nearby colonies of

frigatebirds provides fertile growing

conditions for the mangroves.

Scientists determined age estimates

and growth rates of young red mangrove

saplings from Bush Key by measuring

internodes, or stem lengths between leaf

scars. They based mangrove growth

potential and seasonal elongation patterns

on growth rates from 1995 to 1997 to

calculate tree age and yearly productivity.

Age estimates based on seasonal growth

cycles indicated that the red mangrove

saplings were 10-12 years old by 1997.

Increased internode elongation patterns

coincided with the nesting history and

relocation of a frigatebird colony to the

island.

Laboratory results demonstrated

orders of magnitude differences in nitrate

and phosphate concentrations in the

vicinity of frigate roosts. These findings

indicate that seabirds dependent on

mangroves for nesting and roosting

increase nutrient loads in surrounding

 Large mangroves on the north end of Bush

Key are succumbing to a combination of beach

erosion and increased concentrations of phosphate

and nitrate from the accumulated droppings of

roosting pelicans.

soils and waters. The added nutrients

improve mangrove photosynthesis and

water-use efficiency, resulting in enhanced

growth. While mangrove-dependent

seabirds elevate nutrient levels of surrounding seawater and sustain largely

favorable growth conditions for mangroves, excessive nutrient loading can

cause overgrowth stress and kill them.

The relationship between breeding bird

populations and mangrove community

development and decline poses an

important research and management

question for the Dry Tortugas National

Park.

Risk of Destruction

If mangroves on the Dry Tortugas

withstand hurricanes and the influence of

humans, they can achieve stand stature

and density comparable to mainland

populations, despite less than favorable

conditions. The current risk of destruction to mangrove habitat and nesting sites

remains high, however, depending on the

recurrence and intensity of future hurricanes. NWRC researchers hope to

monitor the health and resilience of

mangroves of known stand age in order to

develop and validate simulation models of

mangrove growth and colonization in the

Dry Tortugas.

For more information, contact

T. W. Doyle, T. C. Michot, R. H. Day, or

C. J. Wells

U.S. Geological Survey

National Wetlands Research Center

700 Cajundome Blvd.

Lafayette, LA 70506

337-266-8500

thomas_doyle@usgs.gov

http://www.nwrc.gov

Frigatebirds have claimed the flourishing

mangroves on Long Key as their current nesting site.