U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2010–3047

June 2010

Estuaries of the Greater Everglades Ecosystem:

Laboratories of Long-term Change

Introduction

Restoring the greater Everglades ecosystem of south 

Florida is arguably the largest ecosystem restoration effort 

to date and is guided by the Comprehensive Everglades 

Restoration Plan (CERP) (U.S. Army Corps of Engineers, 

1999), which was authorized by Congress in the Water 

Resources Development Act of 2000. A critical goal of the 

restoration plan is to return more natural patterns of flow 

through south Florida wetlands and into the estuaries. Resource 

managers are developing performance measures and targets 

for various components of the system to assess the progress 

of restoration—an effort that will continue for the next few 

decades.

Development of realistic targets requires acknowledgement 

that ecosystems are constantly evolving and changing in 

response to a variety of natural and human-driven stressors. 

What did the ecosystem look like prior to substantial water 

management and land-use changes introduced in the 20th 

century? How did the system respond to different modes of 

climate variability? How accurately can we predict responses 

to natural climate variability, changes in resource management, 

and anticipated changes in sea level and 

temperature over the coming decades? The 

answers to these questions rely on examination 

of the longer term history of the ecosystem over 

decadal and centennial scales to understand 

the historical responses to past climatic and 

environmental change.

The Greater Everglades Ecosystem as a 

Laboratory of Change

Ecosystem changes operate on time scales 

ranging from instantaneous to thousands of years 

or longer. Humans can study and monitor these 

changes over minutes, days, weeks, and months, 

and, if persistent, over years and decades. Over 

these short periods of record, the ecosystem 

acts as a living laboratory to study the dynamic 

interaction of the physical, chemical, and 

biological components of the system.

Examination of ecosystems over longer 

periods of time requires analysis of sedimentary 

records, such as those deposited in wetlands 

and estuaries of south Florida. As sediment 

accumulates, it preserves information about the 

animals and plants that lived in the environment 

and the physical, chemical, and climatic conditions 

present. One of the methods used to interpret this 

information is paleoecology—the study of the 

ecology of previously living organisms. A key to 

paleoecology is to first understand the ecology of 

the living organisms. How does a snail species live today? At 

what water depths does it live? At what salinity does it survive? 

Where do you find a particular plant species? Do you find it 

in freshwater wetlands or the mangrove fringe environment? 

Such types of information help scientists interpret the longer 

term history of the ecosystem when we find the same species 

preserved in sediment cores collected throughout a region.

Significance of the Estuaries

The southern estuaries of the greater Everglades ecosystem 

include Biscayne Bay on the east, Florida Bay to the south, 

and the channeled mangrove environment of the southwest 

coast (fig. 1). These areas are transition zones between the 

freshwater flows from the wetlands of south Florida and 

the marine environments of the Gulf of Mexico, the Florida 

Keys reef tract, and the Atlantic Ocean. They provide critical 

habitat for many commercially important species, such as pink 

shrimp (Farfantepenaeus duorarum), Caribbean spiny lobsters 

(Panulirus argus), and stone crabs (Menippe mercenaria).

The die-off of vast amounts of seagrass in Florida Bay 

in the late 1980s served as one of the initial motivations for 

EVERGLADES

NATIONAL PARK

BIG CYPRESS

NATIONAL

PRESERVE

BISCAYNE

NATIONAL PARK

Florida Bay

Shark River Slough

Florida Keys

Florida Reef Tract

Ten Thousand

Islands

Miami

Gulf of Mexico

Biscayne Bay

Atlantic Ocean

Cape

Sable

0 N

0 25 Kilometers

25 MIles

FLORIDA

Taylor Slough

Figure 1. Satellite image of south Florida showing the key features of the greater 

Everglades ecosystem. The southern estuaries of the ecosystem include (1) Biscayne 

Bay on the east coast, including Biscayne National Park, (2) Florida Bay, which is 

part of Everglades National Park, and (3) the southwestern mangrove coastal margin 

between Ten Thousand Islands and Cape Sable, which falls within Everglades National 

Park. The image is modified (cropped) with permission from Jones and others (2001). 
0

10

20

30

40

50

60

70

80

0 20 40

Polymesoda maritima (southern marshclam)

0 20 40 60

Hydrobiidae (freshwater snails)

0 20 40

Anomalocardia auberiana

(pointed venus clam)

0

Transennella spp.

(transennella clam)

0 20

Cerithium muscarum (flyspeck cerith snail)

0

Laevicardium spp.

(eggcockle)

0 20

Brachidontes exustus (scorched mussel)

0

Crepidula spp.

(slippersnail)

Percent abundance of selected species in core

Calendar year (A.D.)depth in core, in cm

Wide salinity

range

Freshwater Low salinity Intermediate salinity (< marine)

≈1960

≈

≈

1900

400

Figure 2. Changes in abundance of mollusk groups in a sediment core collected 

near the mouth of Taylor Slough (fig. 1) reflect changes in estuarine salinity 

through time. Calendar year dates are based on radiocarbon dating, pollen 

biostratigraphy, and short-lived radioisotopes. Blue arrows show declines in 

species preferring freshwater and low salinities during the last 1,500 years. Red 

arrows highlight increased abundance of species that tolerate broad and rapid 

fluctuations in salinity. The red dashed line indicates the approximate depth of 

sediments deposited A.D. ≈1900.

Figure 3. Salinity patterns over time from five sediment cores obtained along the transition zone between the wetlands and the estuaries. Locations of the 

cores are illustrated in the inset satellite image. Information for this diagram is compiled from individual core analyses such as those portrayed in figure 2 

(data for the Taylor Slough core). This figure illustrates the gradual increases in salinity that occur between ≈1 B.C. and A.D. ≈1900 in the transition zone. After 

A.D. 1900, most of the mollusk assemblages in the cores shift to species that tolerate rapidly and widely fluctuating salinities.

0 cm

50

100

150

0 cm

50

100

150

?

mc ni , er oc ni ht pe D

Roberts

River

core

(RR)

Taylor

Creek

core

(TC)

Mud

Creek

core

(MC)

Manatee

Bay

core

(MB)

Middle

Key

core

(MK)

A.D. ≈1900

≈1 B.C.

EXPLANATION

Wide-ranging salinities

Mixed salinities (freshwater and low salinity)

Freshwater, with minor estuarine influence

Freshwater

RR

TC

MC MB

MK

restoration of The Everglades. Resource managers were 

concerned that the changes in Florida Bay resulted from a 

reduction in freshwater flow through Everglades National 

Park to the southern estuaries. The two systems—the wetlands 

and the estuaries—are interdependent, and estuarine salinity 

depends largely on the volume of freshwater delivered 

from the wetlands to the estuary. One way to estimate past 

hydrologic conditions in The Everglades wetlands is by 

reconstructing past salinity in the estuaries.

Methods of Reconstructing Past Salinity Patterns

The abundance and composition of estuarine animal 

communities is dependent on salinity, nutrient content, 

temperature, and other environmental conditions. Figure 2 

shows the changes in mollusk assemblages that occurred 

during a 1,600-year period, as recorded in a sediment core 

collected near the mouth of Taylor Slough, one of the two 

principal flow-ways through The Everglades (fig. 1). Our 

studies of the living mollusks in south Florida indicate that 

salinity is an important control on composition of mollusk 

communities, and we can interpret past salinity patterns from 

changes we see in the abundance of different species in the 

sediment cores. We integrate data from multiple locations to 

reconstruct changes in salinity patterns over time (fig. 3) and 

space (fig. 4).

Descriptive assessments of salinity patterns are useful for 

a qualitative examination of the changes that have occurred, 

but hydrologic models that link freshwater flow from the 

wetlands to the estuaries require quantitative salinity data. 

We used data on salinity preferences of living mollusks 

from south Florida to calculate an average salinity value for 

each species. A weighted averaging technique that integrates 

salinity requirements of all species in an assemblage (see page 

4) provides a single estimated salinity value for each sample 

in a core.
Figure 4. Satellite images of Florida and Biscayne Bays with 

superimposed salinity patterns for three time intervals. Salinity patterns 

were derived from paleoecological data on mollusks from multiple sites in 

the estuaries. A, Reconstructed scenario for minimum extent of freshwater 

environments indicates freshwater habitats extending into northern Florida 

Bay ≈2000 years ago. B, Typical estuarine salinity patterns characterized 

Florida Bay A.D. ≈1900. C, Present-day mollusk assemblages indicate loss 

of typical estuarine salinity zones and a shift to species that tolerate rapid 

and widely ranging salinity changes.

Freshwater

Minimum

extent of

freshwater

Nearly

 marine

Outer 

estuarine

 salinity

Inner

estuarine

 salinity

Outer 

estuarine

 salinity

Inner

estuarine

 salinity

Nearly

 marine

Wideranging

salinities

Mixed 

salinity 

A

B

C

1 B.C.

A.D. 1900

A.D. 2000

Estimates of past salinity provide the input to statistical 

models that reconstruct past freshwater stage and flow in the natural 

Everglades wetlands. The first step in the process (described in 

Marshall and others, 2009) derives salinity estimates from the 

sediment cores based on the species present, as described above. In 

the second step, equations are developed that relate stage and flow 

of freshwater in the wetlands of The Everglades to salinity in the 

estuaries; these equations are based on instrumental measurements 

from water monitoring stations and gages in Everglades National 

Park. The third step links the first two by introducing salinity 

estimates from samples dated just prior to A.D. 1900 into the 

equations developed from the current hydrologic conditions. This 

combination of data types allows us to calculate the stage and flow 

necessary to produce the salinities estimated for the beginning of 

the 20th century (fig. 5).

Past Changes in Hydrology and Salinity

Salinity fluctuations during the last 2000 years (figs. 3 and 4) 

reflect changes in sea level and freshwater flow. Before A.D. 1900, 

the gradual increases in salinity were consistent with late Holocene 

sea level rise. The estuaries exhibited a typical zoned pattern of 

lower salinities nearshore and higher, more marine-like salinities 

farther out in the estuaries. During the 20th century, however, the 

mollusks present in the cores indicated that typical estuarine zones 

disappeared. Species that can tolerate wide and rapidly changing 

salinities predominated in the 20th century portions of the cores. 

The implication drawn from these results is that there was a 

reduction in freshwater entering the estuaries from overland and 

tidal creek flow, groundwater, and/or rainfall. Reduced 20th century 

freshwater flow to The Everglades wetlands has been documented 

in other paleoecologic studies (for example, Willard and Cronin, 

2007; Bernhardt and Willard, 2009), and the timing of that reduced 

flow corresponds to emplacement of water control structures that 

divert water from the wetlands to the ocean.

Application of the modeling method described herein 

allows us to quantify the differences between freshwater flow 

conditions now and those prior to the 20th century. Initial 

findings indicate that flow through Shark River Slough prior to 

the 20th century was about 2–2.5 times greater than now, and 

flow through Taylor Slough was 3–4 times greater (fig. 5). Stage 

in The Everglades wetlands was 0.5 m higher on average (fig. 

5) around A.D. 1900. The estimated salinities from Florida Bay 

cores indicate reduced flow volumes during the 20th century. 

Stations from the nearshore transition zone show the most 

significant changes in salinity after A.D. 1900, compared to the 

previous centuries. The impact of freshwater flow on Florida Bay 

salinity decreases with increased distance from shore because of 

increased water exchange between the Gulf of Mexico and the 

Atlantic Ocean with Florida Bay in open water areas.

Significance to Restoration

The south Florida ecosystem presents a laboratory of 

change—a series of environmental experiments over many time 

scales, which provide a long-term record of the combined effects 

of natural climate variability, land use, and water management 

practices on the greater Everglades ecosystem. Paleoecologic 

investigations from south Florida estuaries provide quantitative data 

on historical variability of salinity and trends that may be applied to 

statistical models to estimate historical freshwater flow. These data 

provide a unique context to estimate future ecosystem response to 

changes related to restoration activities and predicted changes in sea 

level and temperature, thus increasing the likelihood of successful 

and sustainable ecosystem restoration.
Figure 5. Comparison of averaged instrumental data collected between 

1991 and 2003 with model-derived estimates for A.D. ≈1900. Data include 

salinity (parts per thousand [ppt]), flow rate (cubic meters per second 

[m3/sec], and stage depth (meters [m]). Model results shown here are 

based on the analysis of one sediment core. (Adapted from Marshall and 

others, 2009.)

25.33 ppt 

Nearshore 9.87 ppt

transition zone

salinity

34.63 ppt

27.40 ppt Outer estuary

salinity

Taylor Slough flow

at bridge on Park Rd.

Shark River flow

at Tamiami Trail

0.39 m

Taylor Slough 0.99 m

stage at Craighead

Pond

1.93 m

2.48 m Shark River stage

at station P33

Modeled average for A.D. 1900 ≈

Present-day observed average 

115.8 m3/sec

42.4 m3

/sec

8.9 m3

/sec

2.23 m3

/sec

Living mollusk dataset Application

Species 1

Species 2

Species 3

25%

25%

50%

Sample 1

25%

50%

25%

Sample 2

50%

25%

25%

Sample 3

Species 1 10

Species 2 20

Species 3 30

Average salinity

Collect

data

and

samples

Calculations

of historical,

single salinity values

for each core sample

Species 1

Species 2

Species 3

Sum

22.5 17.5

250

1,000

750

2,000

20

Sample 2

500

500

750

1,750

Sample 1 Sample 3

250

500

1,500

2,250

Output:

Estimates of

historical salinity values

for each core sample Estimates of historical salinity values (mean, median, minimum, and maximum) for each 

core sample. In radiometrically dated piston cores, the output provides estimates of salinity 

change over time for periods where instrumental salinity records do not exist.

Divided by 100

Collect data on mollusks 

and the salinities where 

they are found alive

Compile simple descriptive statistics from the 

field observations

Salinity 

measured in 

the field

Instrumental

salinity

measurements

do not exist

Collect sediment cores in 

the estuaries to determine 

past salinity patterns in the 

absence of measured 

salinity data

Present-day average salinity 

value for each species

Percent abundance of each 

species in core samples

A B

C

A X B = C

The cumulative, weightedaverage salinity value for a 

0-2-cm core sample is 22.5, 

not the mathematical 

average of 20 ppt, because it 

is weighted by abundance.

Count species in core samples

Steps to determine estimates of historical, single salinity values for each core sample. These estimates are produced from data on the salinity 

preferences of species in the present-day environment (step A) and measurements of the abundance of those same species in sediment-core 

samples collected from the estuaries in south Florida (step B). These two values are multiplied (step C), summed for each core sample, and then 

divided by 100 to produce a single salinity estimate for each core sample. These salinity estimates allow managers to set salinity targets for 

restoration of the estuaries that reflect natural patterns of change, prior to human alteration of the environment.

References

Bernhardt, C.E., and Willard, D.A., 2009, Response of the Everglades 

ridge and slough landscape to climate variability and 20th century 

water management: Ecological Applications, v. 19, p. 1,723–1,738.

Jones, J.W., Thomas, J.C., and Desmond, G.B., 2001, south 

Florida Everglades satellite image map: U.S. Geological Survey 

Miscellaneous Investigations Series Map I-2742. 

Marshall, F.E., Wingard G.L., and Pitts, P.A., 2009, A simulation 

of historic hydrology and salinity in Everglades National Park—

coupling paleoecologic assemblage data with regression models: 

Estuaries and Coasts, v. 32, p. 37–53. 

U.S. Army Corps of Engineers, 1999, Central and southern Florida 

comprehensive review study—final integrated feasibility report 

and programmatic environmental impact statement—April 1999, 

accessed June 7, 2010, at http://www.evergladesplan.org/pub/

restudy_eis.aspx#mainreport.

Willard, D.A., and Cronin, T.C., 2007, Paleoecology and ecosystem 

restoration—case studies from Chesapeake Bay and the Florida 

Everglades: Frontiers in Ecology and the Environment, v. 5, p. 

491–498.

Publishing support provided by the Lafayette Publishing Service Center

By G. Lynn Wingard, Joel W. Hudley, and Frank E. Marshall

For more information

G. Lynn Wingard

U.S. Geological Survey

lwingard@usgs.gov

Web site: http://sofia.usgs.gov/exchange/flaecohist/