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The Use of Conceptual Ecological

Landscape Models as Planning Tools for the

South Florida Ecosystem Restoration

Programs

Edited by:

John C. Ogden and Steven M. Davis

Senior Ecologists, Executive Office

South Florida Water Management District

West Palm Beach, Florida

June 29, 1999

EXECUTIVE SUMMARY

An Applied Science Strategy has been developed by south Florida scientists

as a process for organizing current scientific understandings of wetland ecosystems

into formats that can effectively support the emerging ecosystem restoration

programs. A new and more structured science application strategy was needed in

south Florida because of the substantially greater complexity and scales of

ecological issues associated with the regional focus of the restoration goals. This

science strategy is being used to direct much of the planning and evaluation of the

C&SF Restudy Program.

An essential component of the science strategy is a set of conceptual

ecological models developed at landscape scales for the major wetland systems in

south Florida. This report explains the development and application of these

models and describes eight of the models currently in use. The current set of

models are for Lake Okeechobee, the Caloosahatchee and St. Lucie estuaries, the

Everglades ridge and slough landscape, the southern Everglades marl prairies and

rocky glades, the southern Everglades mangrove estuaries, Big Cypress basin,

Biscayne Bay, and Florida Bay. A ninth conceptual model is being developed for the

Total System.

The conceptual model present the consensus opinion of south Florida

scientists and land managers on the causal assumptions that best explain how the

wetlands have been altered by human influences. The Model also identify the

ecological elements that best indicate the health of the system. The C&SF Restudy

planning team is using the information in these models to define the specific

ecological problems that must be resolved and to determine what changes must

occur in the natural system for the Comprehensive Plan to be successful. A Set of

performance measures and restoration targets based on the conceptual models will

lay the foundation for the design of a regionally comprehensive monitoring program

and for the institution of an adaptive assessment strategy for reaching long-term

restoration goals.

TABLE OF CONTENTS

I. OVERVIEW OF THE DEVELOPMENT AND APPLICATION OF THE

CONCEPTUAL MODELS ...............................................................................................1

A. INTRODUCTION ...........................................................................................................................................1

B. METHODS.......................................................................................................................................................2

C. DISCUSSION...................................................................................................................................................4

D. CONCEPTUAL MODELS IN SOUTH FLORIDA......................................................................................5

E. ADAPTIVE ASSESSMENT ...........................................................................................................................7

F. FUTURE TASKS.............................................................................................................................................8

G. REFERENCES...............................................................................................................................................15

II. LAKE OKEECHOBEE CONCEPTUAL MODEL.....................................................16

A. PREPARERS .................................................................................................................................................16

B. INTRODUCTION .........................................................................................................................................16

C. MODEL STRUCTURE.................................................................................................................................16

D. EXTERNAL DRIVERS AND STRESSORS...............................................................................................17

E. ECOLOGICAL ATTRIBUTES ..................................................................................................................18

F. QUANTITATIVE MEASURES...................................................................................................................19

G. ECOSYSTEM EFFECTS .............................................................................................................................20

H. HYDROLOGIC PERFORMANCE MEASURES......................................................................................24

I. REFERENCES...............................................................................................................................................29

III. ST. LUCIE AND CALOOSAHATCHEE CONCEPTUAL MODEL........................33

A. PREPARERS .................................................................................................................................................33

B. ABSTRACT....................................................................................................................................................33

C. INTRODUCTION .........................................................................................................................................33

D. ESTUARY MODEL STRUCTURE.............................................................................................................34

E. ECOLOGICAL & SOCIETAL ATTRIBUTES .........................................................................................34
F. LINKS FROM STRESSORS TO ATTRIBUTES ......................................................................................35

G. SUMMARY....................................................................................................................................................37

H. REFERENCE.................................................................................................................................................40

IV. BIG CYPRESS REGIONAL ECOSYSTEM CONCEPTUAL MODEL ..................41

A. PREPARER....................................................................................................................................................41

B. INTRODUCTION .........................................................................................................................................41

C. ECOLOGICAL STRESSORS......................................................................................................................42

D. ECOLOGICAL ATTRIBUTES AND MEASURES...................................................................................44

E. HYDROLOGIC PERFORMANCE MEASURES FOR RESTORATION ..............................................48

F. REFERENCES...............................................................................................................................................52

V. EVERGLADES RIDGE AND SLOUGH CONCEPTUAL MODEL...........................57

A. PREPARER....................................................................................................................................................57

B. INTRODUCTION .........................................................................................................................................57

C. THE SOCIETAL DRIVERS, ECOLOGICAL STRESSORS, AND ECOLOGICAL EFFECTS .........57

D. CRITICAL ECOLOGICAL PATHWAYS .................................................................................................60

E. ECOLOGICAL ATTRIBUTES AND MEASURES...................................................................................61

F. HYDROLOGICAL PERFORMANCE MEASURES ................................................................................62

G. REFERENCES...............................................................................................................................................66

VI. MARL PRAIRIE/ROCKY GLADES CONCEPTUAL MODEL ..............................71

A. PREPARER....................................................................................................................................................71

B. INTRODUCTION .........................................................................................................................................71

C. EXTERNAL DRIVERS AND ECOLOGICAL STRESSORS ..................................................................71

D. ECOLOGICAL EFFECTS OF WATER MANAGEMENT: CRITICAL LINKAGES BETWEEN

STRESSORS AND ATTRIBUTES..........................................................................................................................74

E. ECOLOGICAL ATTRIBUTES AND MEASURES..................................................................................75

F. HYDROLOGIC PERFORMANCE MEASURES FOR RESTORATION ..............................................80

G. REFERENCES...............................................................................................................................................84
VII. MANGROVE ESTUARY TRANSITION CONCEPTUAL MODEL........................89

A. PREPARER....................................................................................................................................................89

B. INTRODUCTION .........................................................................................................................................89

C. EXTERNAL DRIVERS AND ECOLOGICAL STRESSORS .................................................................89

D. ECOLOGICAL EFFECTS: CRITICAL LINKAGES BETWEEN STRESSORS AND ECOLOGICAL

ATTRIBUTES............................................................................................................................................................90

E. ECOLOGICAL ATTRIBUTES AND MEASURES...................................................................................90

F. HYDROLOGIC PERFORMANCE MEASURES FOR RESTORATION ..............................................93

G. MOVING TARGET ......................................................................................................................................95

H. LITERATURE CITED .................................................................................................................................99

VIII. BISCAYNE BAY CONCEPTUAL ECOLOGICAL MODEL................................103

A. PREPARERS ...............................................................................................................................................103

B. INTRODUCTION .......................................................................................................................................103

C. DRIVERS AND STESSORS.......................................................................................................................104

D. ECOLOGICAL EFFECTS.........................................................................................................................105

E. ECOLOGICAL ATTRIBUTES AND MEASURES.................................................................................107

F. REFERENCES.............................................................................................................................................110

IX. FLORIDA BAY CONCEPTUAL MODEL ...........................................................117

A. PREPARER..................................................................................................................................................117

B. INTRODUCTION .......................................................................................................................................117

C. EXTERNAL DRIVERS AND ECOLOGICAL STRESSORS ................................................................118

D. ESSENTIAL ECOSYSTEM ATTRIBUTES AND LINKS TO STRESSORS.......................................121

E. MEASURES .................................................................................................................................................125

F. REFERENCES.............................................................................................................................................129

I. OVERVIEW OF THE DEVELOPMENT AND APPLICATION OF THE

CONCEPTUAL MODELS

John C. Ogden, South Florida Water Management District, West Palm

Beach, Florida.

A. INTRODUCTION

The Central and South Florida “Restudy” Project team and the Science

Coordination Team of the South Florida Ecosystem Restoration Working Group

have adopted the “Applied Science Strategy” as a process for linking the sciences

and management during the planning and evaluation of the south Florida

ecosystem restoration programs (SCT 1997, Ogden et al. 1997; Figure 1). The

creation of a science application strategy was motivated by the need for a better

focused process for organizing and converting large amounts of existing technical

information into planning and evaluation tools that would directly support the

restoration programs. Some sort of organizing process is clearly required where

restoration planning occurs on such large scales, where information from many

disciplines is widely scattered in time and place, where focused efforts are needed in

order to fill the gaps in our information base with “best professional opinion,” and

where a large degree of consensus regarding the major cause and effect

relationships is necessary. To be successful, it was felt that this strategy must, (1)

lay out a scientifically reviewed sequence of steps and tasks for converting research

and modeling results into planning objectives, performance measures and

evaluation protocols, (2) serve as a strong catalyst for promoting consensus among

scientists and managers regarding the nature of the principle resource issues, and

the probable routes for resolving these issues, and (3) be a process that can

contribute to the objectives and needs of both the scientific and management

communities in the regional restoration programs. The Applied Science Strategy is

a total systems and multi-disciplinary process for determining the most appropriate

restoration targets, and the best measures for each of these targets, during and

following the implementation of the restoration programs. These tasks are

prerequisite to the successful application of “adaptive assessment” during the

implementation of the restoration programs.

An essential step in the Applied Science Strategy is the creation of a set of

conceptual ecological models of the major wetland physiographic regions in south

Florida. These simple, non-quantitative models are an effective means for

developing a consensus regarding a set of causal hypotheses, which explain the

affects that the major anthropogenic stressors have on the wetland ecosystems.

Each model identifies the attributes in the natural systems that are the best

indicators of the changes that have occurred as a result of the stressors. Each

model also delineates the ecological linkages between the stressors and the

attributes and the most appropriate measures for each of the attributes. The
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development of a consensus regarding the components and linkages in the

conceptual models is the first step in the process of reaching agreement on specific

hydrological, ecological, and biological measures of restoration success, and for

designing a regional, performance-based ecological monitoring program.

Conceptual models have been widely used for similar purposes in other regions of

North America (e.g., pp. 31-38 in Gentile 1996; also see Rosen et al. 1995).

This report presents the schematic and narrative descriptions for eight

conceptual ecological models that have been used by the C&SF Restudy team.

These eight models provide a basis for developing performance measures for

evaluating alternative restoration plans, for designing a performance-based

monitoring program, and for adaptive assessment of the south Florida ecosystem

restoration programs. The eight models are for Lake Okeechobee, the

Caloosahatchee and St. Lucie estuaries (northern estuaries), the ridge and slough

Everglades, the Big Cypress basin, the southern Everglades marl prairies, the

southern Shark Slough and Florida Bay mangrove estuaries (southern estuaries),

Biscayne Bay, and Florida Bay. A ninth conceptual for the Total System is being

developed and will be reported soon (Buckingham and Poole, in prep).

B. METHODS

The set of conceptual models was initially developed and reviewed by over

100 scientists and resource managers who participated in a concentrated program

of workshops between October 1996 and July 1997. The list of participating

scientists and managers and a listing of the models that each participant helped to

develop are presented in Table 1. These workshops were open to all interested

participants. Special efforts were made to invite the field scientists in south Florida

who have had considerable hands-on research experience in these landscapes, and

therefore were well qualified to bring knowledgeable professional opinions on these

systems to the modeling discussions. The models were developed at landscape

scales (Figure 2).

The initial steps in the development of each model were to use the informal

format of the workshops to identify and discuss the causal hypotheses which best

explain the major anthropogenically driven alterations in each landscape. From

these discussions, the participants created lists of the appropriate stressors,

ecological effects, and attributes (indicators) in each landscape. The objective was

to identify the physical and biological components and linkages in each landscape

which best characterized the changes explained by the hypotheses. Preparers for

each model used the hypotheses and lists of components to lay out an initial draft of

the model and prepare a supporting narrative document to explain the organization

of the model and the supporting science for the hypotheses. The drafts and
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narratives were reviewed in subsequent workshops, resulting in revisions to the

models.

A schematic diagram and a narrative description are provided for each

conceptual model. The diagrams follow a top-to-bottom hierarchy of information,

which identifies the drivers (external sources), the specific stressors on the natural

systems, the ecological effects resulting from the stressors, and the recommended

ecological attributes (indicators) and measures for each attribute. Each stresssor is

linked to one or more attributes. The symbols used in the models to indicate each of

these model components are as follows.

The major components of the models are defined as follows.

Drivers: The major external driving forces that have large scale influences

on natural systems. Drivers can be natural forces (e.g., sea level rise) or

anthrogenic (e.g., regional land-use programs).

Stressors: The physical or chemical changes that occur within natural

systems that are brought about by the drivers, and which cause significant changes

in the biological components, patterns and relationships in natural systems.

Ecological effects: The biological responses caused by the stressors.

Attributes: Also known as indicators or endpoints. A parsimonious subset of

all potential biological elements or components of natural systems, which are

representative of the overall ecological conditions of the system. Attributes

typically are populations, species, guilds, communities or processes. Attributes are

selected to represent the known or hypothesized effects of the stressors (e.g.,

numbers of nesting wading birds), and the elements of the systems that have

important human values (e.g., endangered species, sports fishing).

Measures: The specific feature(s) of each attribute to be monitored to

determine how well that attribute is responding to projects designed to correct the

adverse effects of the stressors (i.e., to determine the success of the project).

DRIVERS STRESSORS ECOLOGICAL

EFFECTS

ATTRIBUTES MEASURES
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Measures of responses by the stressors and attributes in all models combined, are

recommended as the minimal set of components in any regionally comprehensive

monitoring program for the purpose of determining the success of the restoration

programs.

Critical pathways: The known or hypothesized linkages among stressors,

ecological effects and attributes, which explain a high percentage of the adverse

ecological responses shown in the conceptual model (e.g., the ecologically most

significant pathways leading from a stressor).

Each model narrative includes, (1) a brief introduction to the dynamics and

problems of the landscape, (2) descriptions of specific stressors, ecological attributes

and measures, (3) descriptions of the major ecological linkages affected by the

stressors, and (4) an initial set of recommended performance measures and

restoration targets for the stressors and attributes.

The conceptual models should be viewed as working tools in support of the

restoration programs. The models will undergo regular review and revision as new

information becomes available. Earlier drafts for the four southern Everglades

models (ridge and slough, marl prairies, mangrove transition, and Florida Bay)

were previously reported in Ogden et al. (1997).

The organization and content for each model and the supporting text were

developed by a “preparer” (or preparers), based on the information collected from

the workshops. Each model report in this document is a product of the preparer(s).

C. DISCUSSION

The Applied Science Strategy that is being used to link the sciences and

management in the south Florida ecosystem restoration programs is derived from

the “Ecological Risk Assessment” process of the U.S. Environmental Protection

Agency (EPA 1992, Gentile 1996). Risk assessments are used by the EPA as a

guideline for conducting ecological evaluations of proposed management actions in

both North America and Europe. A similar approach was used by the Man and the

Biosphere Human Dominated Systems program to define sustainability goals and

identify ecological endpoints for a series of restoration scenarios for south Florida

(Harwell and Long 1992, Harwell et al. 1996). Many of the same components and

steps incorporated in the Applied Science Strategy, including conceptual models,

have more recently been described in Margoluis and Salafsky (1998).

Gentile (1996) suggested that risk assessment and recovery of ecological

systems “... can be viewed as opposite sides of the same coin.” Risk assessment “...

is the process of determining the probability (with associated uncertainty) of a
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particular event occurring as a result of the action of a specific agent or stressor... ”

Recovery of ecological systems “... can be viewed as the process for determining the

probability (with associated uncertainty) of a particular event occurring (e.g.,

recovery to a... ecologically desired sustainable state) as the result of mitigating the

action of a specific agent (e.g., canals, berms) or stressor (e.g., phosphorus). Gentile

(1996) listed three principal functions of the risk assessment process: (1)

identification of potential causal relationships between stressors and effects; (2)

selection of endpoints (attributes), indicators, and success criteria; and (3)

development of a scale-dependent conceptual model that describes the interrelationships between multiple stressor pathways and multiple ecological receptors.

The development and application of conceptual models provide benefits to the

scientists who create the models and to the managers and public who use them to

guide and implement resource policy (Gentile 1996, Ogden et al. 1997, SCT 1997).

The process of creating and reviewing conceptual models aids scientific endeavors

by, (1) establishing a forum for open, multi-disciplinary exchanges of ideas and

information pertaining to complex ecological issues; (2) developing scientific

consensus regarding current understandings of ecosystems; (3) creating working

hypotheses which serve to guide both research and management; (4) better defining

(and reducing) areas of scientific uncertainty; and (5) providing a framework for

continuing discussions and revisions as new information becomes available. For

managers and the public, the models serve to (1) de-mystify the science; (2) provide

a means for converting broad policy level goals and objectives into specific,

measurable targets; (3) provide a visual description of the rationale for the

prevailing hypotheses and management priorities; (4) reduce the complexity and

dimensionality of the problems; (5) separate essential from non-essential

information; (6) provide a tool for improved communication.

Gentile (1996) summarized the importance and appropriateness of the risk

assessment and conceptual model process for setting ecological targets and

conducting ecological evaluations in regional ecosystem restoration and

management programs. The points emphasized by Gentile were, 1) that there is a

large body of peer-reviewed literature that describes these methods and processes

for conducting ecological assessments; 2) this literature represents an

internationally accepted framework for structuring assessments; and 3)

considerable effort has been devoted during recent years to formalize the process of

selecting and classifying the endpoints, indicators, and metrics used in assessments.

D. CONCEPTUAL MODELS IN SOUTH FLORIDA

The overall planning strategy for the C&SF Restudy has been to use

conceptual models as a basis for developing performance measures and restoration

targets for the stressors and attributes in each model. These targets, collectively,

describe the physical and biological conditions that will be used to define
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successfully restored natural systems. The rationale for having performance

measures and targets for each stressor is that the stressors are known or

hypothesized to be the immediate causes of the ecological problems in each

landscape. A successful restoration program must remove the adverse affects

created by each stressor. A performance measure identifies which elements of each

stressor must be corrected, how those elements should be measured, and how those

elements must change (i.e., the restoration target) in order to eliminate or reduce

their adverse effects.

Performance measures are also developed for each attribute in the conceptual

models. The attributes have been identified as the biological or ecological elements

that are the best indicators of responses in the natural systems to the adverse

effects of the stressors. The hypotheses used to construct the conceptual models

show how attributes are linked to the stressors which are most responsible for

changes in the attributes. If the hypotheses are correct, reducing the adverse

affects of the stressor will result in a predictable positive response by the attribute.

The performance measure developed for each attribute identifies the element of

that attribute which should respond, how that element should be measured, and

how that element should respond (i.e., improve, thus setting the restoration target

for that attribute) once the affects of the stressor are removed.

The attributes that are selected must provide measures of responses at

appropriate temporal and spatial scales, and from different taxonomic and

hierarchial levels in ecological systems. They must also be measurable, and their

historical patterns, relationships, and functions well enough understood, so that

responses can be correctly determined and interpreted. The model teams also

considered the need for a mix of attributes that can serve either as indicators of

ecological conditions (e.g., diversity and productivity) or of societal priorities (e.g.,

endangered species and high quality sports fishing). Finally, the model teams

considered the most appropriate number of attributes for each model. The desirable

trade-off was to have a sufficient number to adequately reflect the major system

responses, while not having more than would be necessary for this purpose or that

would contribute to an unmanageably large monitoring program.

The modeling teams also identified critical linkages in the conceptual models.

Critical linkages were defined as those ecological links between one or more

stressors and attributes, which seem to explain most of the ecological and biological

changes in the systems. The assumption was that the stressors that are part of

critical linkages should have a priority during restoration planning over other, less

influencial stressors. The Restudy’s Alternative Evaluation Team (AET) generally

placed higher priority on meeting performance targets for stressors with strong

critical linkages than for other stressors in the models. For example, the

Everglades ridge and slough conceptual model suggests that stresses caused by

reductions in the duration of uninterrupted surface water hydroperiods and by
7

compartmentalization may explain much of the adverse changes that have occurred

in the long-hydroperiod slough systems. Correction of these stressors became the

highest priority in evaluating the predicted affects of alternative restoration plans

in the sloughs of the Everglades.

The performance measures and restoration targets from both the stressors

and attributes are used for two primary purposes. Performance measures function

during the planning phases of the restoration projects as assessment tools for

comparisons among alternative plans. What combination of structural and

operational components is most likely to achieve the desired objectives of the

project, as determined by how well a plan is predicted (by modeling) to meet each of

the targets set by the performance measures? In this planning role, the measures

and targets not only are used to determine which plan is most likely to be successful

in achieving its objectives, but also are used to influence the design of the project as

efforts are made to evaluate the combination of features which can best moderate or

eliminate the adverse affects of the stressors.

The second primary use of the measures and targets is in the design of a

system-wide, ecological monitoring program. The purpose of system-wide

monitoring is to measure how elements in the natural system actually respond

following the implementation of each iteration of a restoration program. Because

the design of a restoration program is determined by the features that are predicted

to best meet a suite of regional performance measure targets, the monitoring

program must measure the responses by the same set of performance targets.

Comparisons between the predicted responses by a set of stressors and attributes

and the actual responses by the same stressors and attributes, provide a basis for

making revisions to the causal hypotheses and conceptual models, and a means for

structuring an adaptive assessment strategy throughout the implementation of the

restoration program. The overall focus of the monitoring program must be

performance-based, i.e., the elements of the natural systems to be monitored must

be those that provide actual measures of the stressor and attribute targets.

E. ADAPTIVE ASSESSMENT

The key to successful implementation of a regional ecosystem

restoration program, e.g., maximizing the effectiveness of the program and reducing

uncertainty during its implementation, is an adaptive assessment strategy. The

conceptual ecological models are an integral part of this overall process. The

conceptual models should be revised as new information is collected from research

and monitoring programs. These revisions in the conceptual models influence

predictions of system responses, and the choice of attributes and measures, for

future iterations of the restoration program. As the accuracy of the hypotheses in

the conceptual models improves, the opportunities for making pre-construction

improvements in project design and operations are also enhanced. Greater accuracy
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in the conceptual models leads to improvements in the choice of measures and

targets used to refine the design and judge the predicted performance of each

project design.

F. FUTURE TASKS

The conceptual ecological models, and the planning and evaluation tools

coming from the models, still require several priority improvements. These are: (1)

completion of a conceptual model technical report, and peer review of this report; (2)

new workshops to complete the development of performance measures for the

biological attributes identified by the models; (3) workshops to design a

performance-based monitoring program for the south Florida ecosystem restoration

program; and (4) the development of a consensus among the participating agencies

for a system-wide adaptive assessment strategy for the ecosystem restoration

program, (5) the completion of the Total System and other conceptuals model as

needed.

Table 1. List of the participants and the model that each helped to develop.

Name Affiliation Models

G. Abbott SFWMD

K. Alvarez FDNR BC

C. Anastasiou Univ. FL FB

T. Armentano NPS/EVER R/S, MP, SE, FB

A. Arrington SFWMD

T. Bancroft Arch. Biol. Stat. R/S, MP, SE, BC

I. Barnett FDEP

O. Bass NPS/EVER R/S, MP, SE

S. Bellmund NPS/EVER R/S, MP, SE, BB

R. Best USGS/BRD FB

R. Brock NPS/EVER FB

J. Browder NOAA/NMFS R/S, MP, SE, FB, BB

C. Buckingham FWS BC,

G. Burzycki Dade Co. DERM R/S, MP, SE, BB

D. Busch NPS/EVER R/S, MP, SE

K. Cairns FWS NE

E. Carlson Natl. Audubon Soc. BC

P. Carlson FDEP/FMRI FB

B. Chamberlain SFWMD NE

E. Chipouras Univ. FL FB

J. Colvocoresses FDEP/FMRI FB

W. Cropper U. Miami/RSMAS FB, BB, SC

K. Cummins SFWMD SC

S. Davis SFWMD R/S, MP, SE, BC, FB, SC
9

Name Affiliation Models

R. Day Ind. R. Lagoon NEP NE

D. DeAngelis USGS/BRD SC

P. Doering SFWMD NE

M. Duever The Nature Conserv. BC

M. Durako FDEP/FMRI FB

J. Englehardt U. Miami SC

E. Estevez Mote Marine Lab NE

M. Gaines U. Miami/Biology

D. Gawlik SFWMD R/S, SE

J. Gentile U. Miami/RSMAS SC

C. Goodyear NOAA/NMFS LO

G. Graves FDEP NE

S. Gray SFWMD NE, FB

L. Gunderson U.Florida/Zoology BC, FB

D. Hanisak Harbor Branch Inst. NE

C. Hanlon SFWMD LO

M. Harwell U.Miami/RSMAS SC

D. Haunert SFWMD NE

K. Havens SFWMD LO

L. Heisler FGFWFC LO, R/S, SE

L. Hornung USACOE SC

A. Houser USACOE BC

Soon-Jin Hwang SFWMD LO

B. Irlandi U.Miami/RSMAS FB, BB

T. James SFWMD LO

T. Janicki Coastal Environ. NE

D. Jansen NPS/BICY BC

Kang-Ren Jin SFWMD LO

K. Kibby Lee County NE

W. Kitchens USGS/BRD SC

M. Koch FL Atlantic Univ. SE, SC

J. Koebel SFWMD

B. Kruczynski EPA FB

J. Layne Arch. Biol. Station BC

D. Lirman U.Miami/RSMAS BB

W. Loftus USGS/BRD R/S, MP, SE

J. Lorenz Natl. Audubon Soc. SE, FB

L. McCarthy FDACS LO, R/S, MP, SE

C. Madden SFWMD NE, R/S, MP, SE, FB

L. Manners USACOE LO

S. Markley Dade Co. DERM BB

F. Mazzotti U.Florida/IFAS R/S, MP, SE, BC, FB

J. Meeder FL.Int.Univ./SERP MP, SE
10

Name Affiliation Models

S. Melvin SFWMD

R. Montgomery Coastal Environ. NE

D. Morrison Natl. Audubon Soc. R/S, MP, SE, FB

J. Moulding USACOE R/S, MP, SE

A. Nath SFWMD

S. Newman SFWMD SC

J. Obeysekera SFWMD SC

J. Ogden SFWMD R/S, MP, SE, BC, FB, SC,

S. Olson SFWMD FB

P. Ortner NOAA/AOML FB

R. Pace FWS LO, NE, R/S, MP, SE

T. Pernas NPS/BICY BC

S. Perry NPS/EVER R/S, MP, SE

M. Poole FGFWFC R/S, MP, SE,

R. Punnett USACOE SC

G. Redfield SFWMD LO

C. Regan NPS/EVER FB

L. Richardson FWS BC

J. Rippe SFWMD

M. Robblee USGS/BRD R/S, MP, SE, FB

B. Robertson USGS/BRD R/S, MP, SE, SC

B. Rosen SFWMD LO, NE

D. Rudnick SFWMD NE, MP, SE, FB

P. Sime SFWMD NE

F. Sklar SFWMD R/S, SE

T. Smith USGS/BRD SE

S. Snow NPS/EVER R/S, MP, SE

J. Snyder USGS/BRD BC

M. Steinkamp FWS R/S,MP,SE

A. Steinman SFWMD LO

P. Strayer SFWMD

D. Strom FDEP NE

T. Tisdale SFWMD LO

S. Tosini U.Miami/RSMAS

L. Toth SFWMD

T. Towles FGFWFC R/S,MP,SE

S. Traxler USACOE NE, SE, FB

J. Van Arman SFWMD NE, BC

G. Warren FGFWFC LO

D. Weeks NPS/BICY BC

D. Welcher NOAA/AOML FB

H. Zebuth FDEP LO, NE, R/S, MP

M. Ziminske USACOE LO
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* Models: Lake Okeechobee (LO); northern estuaries, Caloosahatchee & St.

Lucie (NE); Everglades ridge & sloughs (R/S); southern marl prairies (MP);

southern Florida Bay/Shark Slough estuaries (SE); Big Cypress basin (BC); Florida

Bay (FB); Biscayne Bay (BB); model steering committee and overall review (SC).
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Figure 1: APPLIED SCIENCE STRATEGY FOR ECOSYSTEM RESTORATION

Societal

Values

Ecological

research,

models, and

monitoring

Stressors:

Performance

Measures

Comprehensive

Monitoring Program

Ecological Report

Cards

Ecosystem

Responses

Restoration

Plans

Alternative Plans

Evaluation

Conceptual Ecological Models

Attributes:

Performance

Measures

Ecosystem

Restoration

Targets

Goals and

Objectives
13

Figure 2: LOCATION MAP FOR CONCEPTUAL MODELS
14
15

G. REFERENCES

Environmental Protection Agency. 1992. Framework for ecological risk

assessment. EPA/630/R-92/001. Washington, D.C.

Gentile, J.H. 1996. Workshop on “south Florida ecological sustainability criteria.”

Final Report. University Miami, Center for Marine and Environmental

Analysis, Rosenstiel School of Marine and Atmospheric Science, Miami, FL.

54 pp.

Harwell, M.A. and J.F. Long. 1992. U.S. M.A.B. Human-Dominated Systems

Directorate workshop on ecological endpoints and sustainability goals. Univ.

Miami, Rosenstiel School of Marine and Atmospheric Science, Miami, FL.

115 pp.

Harwell, M., J.F. Long, A. Bartuska, J.H. Gentile, C.C. Harwell, V. Myers and J.C.

Ogden. 1996. Ecosystem management to achieve ecological sustainability:

the case of south Florida. Environ. Mgmt. 20: 497-521.

Margoluis, R. and N. Salafsky. 1998. Measures of success. Island Press,

Washington DC. 363 pp.

Ogden, J.C., S.M. Davis, D. Rudnick and L. Gulick. 1997. Natural Systems Team

report to the Southern Everglades Restoration Alliance. Final draft. July

1997. South Florida Water Management District, West Palm Beach, FL. 43

pp.

Rosen, B.H., P. Adamus and H. Lal. 1995. A conceptual model for the assessment

of depressional wetlands in the prairie pothole region. Wetlands Ecology and

Management 3: 195-208.

Science Coordination Team. 1997. Integrated science plan. Report to the South

Florida Ecosystem Restoration Task Force and Working Group. Office of the

Executive Director, SFERTF, Florida International University, Miami, FL.
16

II. LAKE OKEECHOBEE CONCEPTUAL MODEL

A. PREPARERS

Karl E. Havens and Barry H. Rosen, South Florida Water Management

District, West Palm Beach, Florida.

B. INTRODUCTION

Lake Okeechobee is a large (1,730 km2) freshwater lake located at the

center of the interconnected south Florida aquatic ecosystem. The lake is shallow

(average depth <3 m), originated about 6,000 years ago during oceanic recession,

and under natural conditions probably was a slightly eutrophic and had vast

marshes to the west and south. The southern marsh was contiguous with the

Florida Everglades, which received water as a broad sheet flow from the lake during

periods of high rainfall (Gleason 1984).

Modern-day Lake Okeechobee differs in size, range of water depths, and

connections with other parts of the regional ecosystem (Havens et al. 1996a).

Construction of the Herbert Hoover Dike in the early to mid-1900s reduced the size

of the lake's open-water zone by nearly 30%, resulted in a considerable reduction in

average water levels, and produced a new littoral zone within the dike that is only a

fraction of size of the natural one (yet still it is a vast expanse, at over 400 km2).

The lake also has been impacted in recent decades by excessive inputs of nutrients

from agricultural activities in the watershed (Flaig and Havens 1995). These

nutrients have exerted the most dramatic impacts on the open-water region, where

large algal blooms have occurred, along with accumulation of soft organic mud

bottom sediments, which cause the lake water to become highly turbid when they

are resuspended during windy periods (Maceina and Soballe 1991). The littoral zone

has been invaded by 15 species of exotic plants, most notably Melaleuca

quinquenervia and Panicum repens (torpedo grass), which have expanded over large

areas, displacing native plants. Despite these human impacts, and a consensus

that the lake’s overall health has been greatly degraded by human actions, Lake

Okeechobee continues to be a vital aquatic resource of south Florida, with

irreplaceable natural and societal values. These values and their responses to

anthropogenic stressors are described in the following sections.

C. MODEL STRUCTURE

The conceptual model is comprised of a top-to-bottom hierarchy of drivers,

stressors, ecosystem effects, attribures, and measures (refer to figure on 27).

It is a complex model that includes two distinct but highly interconnected

components, the open-water and littoral zones; each zone has different ecological
17

effects and attributes, and different responses to stressors. Stressors in one zone

may have indirect effects that cross the boundary into the other. The Lake

Okeechobee model was developed in the context of the lake's existing spatial extent,

recognizing that the functioning of the modern-day system is drastically different

from that of the historic lake.

D. EXTERNAL DRIVERS AND STRESSORS

Eight in-lake stressors, which exert significant impacts on the lake's natural

and societal attributes, originate from six external sources. Elevated concentrations

of chemical contaminants, including, chloride (Cl-), pesticides, and total dissolved

solids (TDS), are by-products of agriculture or other human activities in the

watershed. A much greater concern, in terms of ecosystem impacts, are the massive

quantities of nutrients, in particular nitrogen (N) and phosphorus (P), which are

discharged to the lake from agriculture. The C&SF Project, which includes deep

canals that allow water to enter the lake with little interaction with natural

wetlands, facilitates the delivery of these chemical stressors to Lake Okeechobee,

and as such, it also is considered a source.

The open-water region experiences elevated concentrations of suspended

sediments, whose source is a region of soft organic mud that covers about 50% of

the lake bottom. When winds mix the shallow water column, the upper few cm of

mud are resuspended. The spatial extent, depth, and nutrient content of this mud

have increased rapidly in the last 100 years, coincident with agricultural

development and increased nutrient inputs from the watershed (Brezonik and

Engstrom, 1997). The Herbert Hoover Dike may facilitate sediment accumulation

by preventing natural flushing that once may have occurred during high water

events.

The C&SF Project, along with natural variations in regional rainfall, have

the potential to cause prolonged or extreme high or low water levels. Because

the lake is constrained within the dike system, these two stressors can cause severe

damage on the lake ecosystem. Extreme high or low lake levels of any duration, or

moderate high or low lake levels of prolonged duration, are considered to cause

significant harm (an adverse change in ecological values that cannot recover under

natural conditions). In contrast to the harmful effects of extremes, a certain degree

of natural variation in lake levels, between 13 and 15 ft NGVD, has been shown to

be benefit the ecosystem (Smith et al. 1995; Smith 1997).

Lake Okeechobee has suffered in recent decades from the expansion of exotic

and nuisance plants and exotic animals. Today there 15 species of exotic plants

in the littoral zone. Species of greatest concern are Melaleuca and torpedo grass,

both of which were purposely introduced to the region, for dike stabilization and

cattle forage, respectively. Other exotic plants that have stressed the lake's values

include Hydrilla sp., water hyacinth (Eichhornia crassipes), and water lettuce
18

(Pistia stratiotes). Exotic animals in the lake now include fish (Tilapia aurea),

mollusks (Corbicula fluminea), and microinvertebrates (Daphnia lumholtzii). Each

of these species exerts different impacts on the ecosystem, as discussed below. The

final major stressor in Lake Okeechobee is increased frequency and extent of

fire, which occurs due to natural causes (e.g., lightening strikes), arson, and

controlled burns conducted by the South Florida Water Management District and

Florida Game and Fresh Water Fish Commission, under permit from the US

Department of Forestry.

E. ECOLOGICAL ATTRIBUTES

Seven natural and societal attributes are associated with the Lake

Okeechobee ecosystem. These values may be broken down into those which are

intrinsic to the lake (fisheries, wildlife habitat, ecotourism) and those which are

extrinsic (water supplies for urban areas, agriculture, and other ecosystems). The

extrinsic values differ in that they are regional in nature, and dependant upon

water supply in general, rather than on Lake Okeechobee per se. It is largely for

this reason that lake hydrologic performance measures (below) are based on the

lake’s intrinsic values, while extrinsic values are addressed separately, by

performance measures ascribed to the Lake Okeechobee Service Area. Nevertheless,

the conceptual model can serve as an integrating framework for these different

attributes.

The lake is an important source of water for urban and agricultural

uses, and environmental water that is discharged from the lake to other

ecosystems, including the east and west coast estuaries, and the Everglades water

conservation areas. For each of these extrinsic values, the quantity, timing, and

quality of water are critical features. The lake is important to the regional economy,

whose income depends in part on use of the ecosystem for ecotourism and

recreation, including bird and wildlife observation, hiking, camping, and

recreational boating. The lake also supports a commercial and recreational

fishery, with an estimated "total economic value" in excess of $480 million US

dollars (Furse and Fox 1994). Some of the most important intrinsic ecological values

of the lake are the wading birds and waterfowl, which include numerous species

and large resident or migratory populations (Smith et al. 1995), and the diverse

mosaic of native vegetation that characterizes the lake's littoral marsh zone

(Richardson and Harris 1995). This diverse vegetation is critical to birds, fish, and

other wildlife that inhabit the zone, and it is an aesthetically pleasing region that is

visited by thousands of persons each year -- thereby supporting the ecotourism,

recreation, and fisheries values mentioned above. The community also is a critical

habitat for the federally-endangered snail kite, whose principal food resource, the

apple snail, occurs at high densities in a large pristine region of the lake's marsh

known locally as Moonshine Bay.
19

F. QUANTITATIVE MEASURES

To quantify the current status of Lake Okeechobee, and its predicted or

realized responses to restoration efforts, a suite of measures were selected that

collectively characterize the condition of each ecological value. Measures were based

on the results of scientific research, or in some cases, best professional judgement

that indicates a strong relationship between measures and the values being

assessed. The measures for Lake Okeechobee ecological values are as follows.

(1) Urban water quality and quantity. Key measures of the quality of water for

urban uses include concentrations of the various Class I numeric water quality

standards (for drinking water use), as well the concentrations of Class IV numeric

standards (for agriculture). Water quantity can be assessed as days per year with

water use restrictions.

(2) Environmental water quality and quantity. Water discharged from Lake

Okeechobee may impact downstream ecosystems, including the east and west coast

estuaries, the WCAs, and the Everglades, if it contains high concentrations of P, N,

toxins, or turbidity. Quantities of water depend on needs of particular ecosystems,

and must include a seasonal component.

(3) Ecotourism and recreation. These ecological values may be best assessed on the

basis of numbers of visitors to the lake region each year and cash flow into the local

economy, associated with designated lake uses, including bird and wildlife

observation, hunting, boating, and hiking.

(4) Sport and commercial fishery. Measures of fishing success may be obtained from

information on catch per unit effort by anglers and commercial fishers, while more

quantitative information can be obtained by FGFWFC studies of population size

and age structure for selected fish (largemouth bass, black crappie, bluegill).

(5) Wading birds and migratory waterfowl. Populations of selected birds may be

surveyed in order to determine population sizes and nesting success.

(6) Diverse mosaic of native vegetation. Geographic Information System (GIS) maps

of the littoral zone may be used to evaluate indices of overall spatial heterogeneity -

several standard ones are available. It also may be important to quantify changes

over time the spatial extent of selected vegetation assemblages, especially those

known to be critical wildlife habitats (e.g., Eleocharis, Scirpus, Salix). The ratio of

exotic+nuisance to native plant coverage also may be a useful metric of overall

vegetation quality. The marsh community is reported to include one threatened

plant species, the Okeechobee gourd, and a regular census of its population density

and distribution should be undertaken.
20

(7) Snail kite. Snail kites are an important endangered species in south Florida, and

there needs to be a continued regular census of their population size and nesting

success in Lake Okeechobee.

G. ECOSYSTEM EFFECTS

The pathways linking ecosystem stressors to the natural and societal

attributes are complex, and have a solid foundation in research and modeling

conducted by scientists at the SFWMD, FGFWFC, and Florida universities (e.g.,

Aumen and Wetzel 1995, Havens et al. 1995, James and Bierman 1995, Reddy et al.

1995, Havens et al. 1996a). As we learn more about how the system functions, it is

likely that additional pathways could be added to the model, or adjustments made

to existing pathways. The model is a flexible tool that at any given time, reflects

the current state of our best available scientific information.

Chemical contaminants in the lake water have the potential to directly

impact all seven of the ecological values. Toxic materials in drinking water are

dangerous to human users, or at a minimum, costly to remove during treatment

operations; toxic chemicals also can kill or harm agricultural crops and the biota

residing in downstream ecosystems that receive environmental water deliveries or

flood control discharges from the lake. Toxic chemicals also may kill or harm native

invertebrates, fish, birds, littoral plants, and other flora and fauna in the lake

ecosystem, including the endangered species. Some of the chemical contaminants,

including Hg and pesticides, undergo biomagnification in the food web, and thereby

indirectly harm animals near the top of the web (e.g., predator birds, fishes, and

other animals such as the American alligator).

Nutrients, in particular N and P, stimulate the growth of phytoplankton,

and at the extreme, can cause noxious surface blooms of blue-green algae (Paerl

1988). When these algal blooms senesce, they can cause oxygen depletion and a

build-up of ammonia and potentially toxic decay products in the water. Depending

on the location where this occurs, these decay products can harm components of the

lake's biota, including littoral macroinvertebrates and fish. This in turn can

secondarily affect the predators that utilize those food resources. To date, the only

documented harm, following a large bloom in summer 1986, was to the lake's apple

snail population (Jones 1987), the primary food item for the endangered snail kite.

The products of algal bloom decay also are detrimental to water quality for urban

and environmental deliveries, and to the ecotourism & recreational uses of a lake

(Heiskary and Walker 1988). The high nutrient inputs that have impacted Lake

Okeechobee since human development began pre-1900 also have left their mark on

the lake sediments (Reddy et al. 1995), which now contain large quantities of N and

especially P. While there is a natural loss of N from the lake, by microbial

degradation in a process called denitrification, there is no such loss for P, except by

gradual burial in deeper sediment layers. High concentrations of P and organic
21

material promote rapid growth of bacteria, which in turn deplete the oxygen from

surface sediments (Hutchinson 1957). Only a small subset of benthic

macroinvertebrates can tolerate low oxygen levels, and it is for this reason that the

Lake Okeechobee community, once dominated by a diverse array of species, now is

strongly dominated by a few species of pollution-tolerant oligochaete worms

(Warren et al. 1995). This change radiates upward through the food web, affecting

fishes and other animals that ultimately depend on the benthic invertebrates as a

food source (Havens et al. 1996b).

Suspended sediments in the water column can cause increased P

concentrations if soluble P is released from the sediment particles. This is a

complex situation that is largely controlled by the concentrations of P on the

particles and in the water column (Reddy et al. 1995). Under most circumstances,

the particles adsorb P, and therefore sediment suspension (followed by sediment

settling to the lake bottom) is a net sink for P. Suspended sediments exert another

important impact. If they are present in the water column at high concentrations,

they greatly reduce the amount of light that reaches the lake bottom. It the

sediments are circulated to shallow near-littoral regions of the lake (this circulation

appears to be enhanced by high water levels), then they may suppress the growth of

submerged plants and algae that naturally occur on the lake bottom. Those plants

and algae are important because they provide food and habitat for

macroinvertebrates and fish (and ultimately wading birds and other animals), and

they help to stabilize the sediments. If the plants are reduced or eliminated, these

functions are lost. One particularly troubling result is a "positive feedback loop"

that may occur, making it very difficult for the lake to recover from a highly turbid

condition: turbid water reduces benthic plants, less benthic plants means more

resuspension, this makes more turbid water, even less plants........etc. This situation

may be occurring today at the south end of Lake Okeechobee, where turbid, P-rich

water and declines in benthic algae (Chara) coincided with the prolonged high lake

stages in 1995 and 1996 (Steinman et al. 1997). Research results also have

documented that benthic algae compete for nutrients with phytoplankton (Havens

et al. 1996c). Thus, when benthic algae are reduced, there is a greater potential for

phytoplankton blooms. Finally, there also is a complex relationship between light

penetration in the water column and phytoplankton biomass. While sediment

resuspension may indirectly promote phytoplankton growth by suppressing benthic

algae, at the extreme, it may be so severe as to cause light limitation of the

phytoplankton too. In this case, which is typical of the mid-lake region during

winter (Phlips et al. 1997), there may be little growth of any algae in the water

column or on the lake bottom. At other times, when a high biomass of

phytoplankton does occur (e.g., during calm mid-summer periods), phytoplankton

cells may bring about a reduction in light penetration, and also contribute to the

decline in benthic algae.
22

Prolonged or extreme high lake levels cause substantial harm to the

ecosystem. When sustained high lake levels (>15 ft for nearly 2 yrs, and exceeding

17 ft at the extreme) occurred in 1995 and 1996, wind and erosion caused emergent

plants to be torn loose from their habitats, resulting in a loss of important fish and

wildlife habitat (e.g., bulrush, a critical spawning habitat for largemouth bass).

These "islands" of dislodged plant material also interfered with recreational uses of

the lake, as they blocked canals and boat trails. During the two years of high lake

levels, District scientists also documented the loss of submerged vegetation at the

lake’s south end, likely due to light limitation caused by the deep, turbid water

(Steinman et al. 1997).

High lake levels also bring about changes in the nature and extent of mixing

within the lake. A Lake Okeechobee hydrodynamic model indicates that when

water levels are high, nutrient and sediment-rich water from mid-lake is mixed

throughout the basin, including into the littoral zone (Sheng and Lee 1991). At low

water levels, the nutrient and sediment-rich water circulation is constrained

towards mid-lake because an extensive rock reef along with south and west edges of

the open-water region restricts water movement into the near-littoral region

(Maceina 1993). When high water levels promote nutrient transport into the marsh

via greater horizontal mixing, those nutrients have the potential to cause changes

in the biomass and composition of both marsh plants and algae, as has been

documented in the Everglades Water Conservation Areas (WCA) by Koch and

Reddy (1992) and McCormick et al. (1996).

 High lake levels also may affect vertical mixing in the mid-lake region.

During summer calm periods, the water column may become thermally stratified

for one to two week-long periods (Jin et al. 1997). When this occurs, bottom waters

may become anoxic, and this in turn allows diffusion of soluble P from lake

sediments. These events may be a precursor to development of mid-lake blooms of

blue-green algae (Havens et al. 1997). Because it takes a greater amount of wind

energy to break down the stratification and mix a deeper water column than a

shallow one, high lake levels might promote longer-lasting mid-summer

stratification, anoxia, and algal blooms (Havens 1996). High lake levels also may

threaten the endangered Okeechobee gourd, which cannot tolerate prolonged

flooding.

Prolonged or extreme low lake levels also cause significant harm to the

ecosystem and its values. Low water levels expose large areas of the marsh

(including native plant communities such as Scirpus and Eleocharis) to desiccation,

and render them useless as habitats for fish and waterfowl. Low water levels also

may promote the expansion of exotic plants. The large Eleocharis habitat found in

Moonshine Bay is an important habitat because it is a primary foraging area for

largemouth bass and other recreational fish, and it supports a thriving population

of snail kites. This habitat now is encircled by torpedo grass, which may readily
23

overtake the region if low water levels suppress the growth and survival of the

native Eleocharis. Torpedo grass grows most rapidly when water levels are at or

near the soil surface, and displays considerable growth suppression when

submerged under >30 cm of water (Thayer and Haller 1990). Eleocharis, on the

other hand, thrives when submersed, even when water depths are near 1 m. There

also is evidence that germination of Melaleuca seeds is inhibited by standing water

(Lockhart 1995). Hence, if large areas of the marsh become dry for long periods of

time, there could be increased expansion of this exotic plant. Indeed, one of the

most rapid Melaleuca increases occurred shortly after the 1990 drought, when lake

levels declined below 11 ft NGVD for just 150 days. Low lake levels also impact

ecotourism and recreation, by making it difficult or impossible to access large

regions of the marsh (many former boat trails become dry land at 11 ft NGVD).

Finally, there is evidence that low water levels directly impact the snail kite, which

requires standing water beneath the macrophytes upon which it constructs its nest.

While it is clearly the case that prolonged or extreme high and low water

levels have negative impacts, these conditions alone do not describe the hydrologic

variation that is required to maintain a healthy ecosystem. There is considerable

evidence that the "optimal" situation would also include a certain degree of

variation. In particular, a January to May recession in lake levels from near 15 ft

NGVD to below 13 ft NGVD has been shown to be beneficial to wading bird and fish

populations. Infrequent (every 5 to 7 years) spring recessions to below 12 ft NGVD

may benefit the entire community, by exposing prey-rich submersed macrophyte

beds to predators, invigorating willow (Salix) stands, and by allowing fires to burn

away Panicum and Typha wrack (Smith et al. 1995). These points must be taken

into account when evaluating the impacts of regional restoration programs or lake

regulation schedules, although they are not readily portrayed as stressors in a

conceptual model of this format.

Exotic and nuisance plants have been discussed in the context of water

levels and other stressors. These plants, which now cover a large portion of the

marsh, exert a variety of direct and indirect impacts on the lake's ecological values.

As they spread and displace native vegetation, they directly alter the vegetation

mosaic. Because of their growth characteristics, they may provide poor habitat for

fish and other wildlife. Torpedo grass, for example, grows in large monocultures

resembling “hayfields,” with little or no standing water. Fish simply cannot swim

through such a habitat, and any wildlife that depend upon those fish as a food

resource are indirectly affected when torpedo grass becomes dominant in a region of

the lake. Hydrilla and other exotic submerged plants have negative impacts on

human uses of the system, by blocking canals and waterways, and often requiring

repeated herbicide treatments. These plants are secondarily responsible for any

adverse impacts that the herbicides have on natural vegetation in the lake or

downstream ecosystems. One of the troubling patterns observed in the lake's marsh

is expansion of cattail (Typha). This plant is a native, but once occurred at
24

considerably lower densities and over a smaller spatial area (Richardson and Harris

1995). Its spread may be related to water level regimes and/or eutrophication of the

lake. In either case, it displaces other native plants and impacts the value of the

marsh as a wildlife habitat. There is some concern that cattail expansion in to

valuable bulrush habitat may be facilitated by low lake levels (particularly those <

11 ft NGVD). Following the drought in 1989, when the lake declined to nearly 10.5

ft NGVD, large areas of bulrush became infested. These events may degrade the

value of the habitat for fish and other wildlife that utilize bulrush for nesting or

foraging.

Exotic animals in Lake Okeechobee can directly displace native animals

through competition and predation, and they also can exert other direct and indirect

effects. Tilapia aurea, for example, is an exotic fish that feeds and burrows at the

benthos, and destroys submerged vegetation stands. It also transforms particulate

nutrients, from sediments, macroinvertebrates, and plants, into soluble nutrients

that are excreted into the water column where they may stimulate localized algal

blooms. Not all impacts of exotic animals are negative, however. The mollusk

Corbicula fluminea is a major food source for diving ducks and shell crackers, and

the microinvertebrate Daphnia lumholtzii grazes phytoplankton, and could

suppress algal blooms if its densities become very high. It also may be a good food

source for plankton-feeding forage fish.

Fire is a natural forcing function in Lake Okeechobee, as well as in other

south Florida ecosystems, often occurring due to natural causes, such as lightening

strikes. However, in recent years, fires have been set in the lake's littoral zone by

humans -- both in acts of arson, and as part of controlled burn management efforts.

Arson fires are reportedly set by hunters and others who desire to visit the littoral

zone for recreational purposes. The focus is most often on regions of dense torpedo

grass, where plant growth has restricted access to the marsh. Interestingly, the

controlled burn programs target similar regions, but for a different reason -- to burn

away the exotic plant material, in order to give native plants a chance to recolonize

the area from a rich buried seed bank that is known to occur in the marsh soil. In

either case, fires impact the vegetation mosaic, as well as the animals that occur

therein.

H. HYDROLOGIC PERFORMANCE MEASURES

A significant feature of the Lake Okeechobee conceptual model is the high

degree of interconnectedness; any given stressor may impact ecological values

directly, or impact those same values indirectly, by exacerbating the effects of other

stressors. Two stressors that display particularly strong effects (direct and indirect)

on the values are high and low water levels. Taken together, they have the potential

to affect the rate of lake eutrophication, the spatial extent and overall health of
25

submerged and emergent plant communities, fisheries, birds, other wildlife, and the

quality of water taken from the lake from human uses.

Water inputs and lake levels also are the variables most directly impacted by

the various Alternatives considered in the C&SF Restudy. Therefore, a suite of

hydrologic performance measures were developed based on water depth (and

seasonal variations in those depths). These measures serve as integrative predictors

of overall lake ecosystem health.

The hydrologic performance indicators and measures for Lake Okeechobee

are concerned with: extreme high lake levels (> 17 ft NGVD) of any duration;

moderate high lake levels (> 15 ft NGVD) of prolonged duration; extreme low lake

levels (< 11 ft NGVD) of any duration; and moderate low lake levels (< 12 ft NGVD)

of prolonged duration; as well as the degree of variability in lake levels, within what

is considered to be an “optimal” window for the lake’s biota (13 to 15 ft NGVD).

Performance Indicator – Daily Stage Hydrographs

Visual inspection of daily stage hydrographs, for the period from January

1965 to 1995, can provide information regarding the frequency of extreme high and

low lake levels. One simply draws horizontal lines across these plots at 17 ft and 11

ft NGVD water levels, and counts the number of cases wherein the hydrograph goes

above or below the lines. The fewer the events, the “better” the result, in terms of

protecting the lake’s values. The restoration goal for the lake is to have no such

events.

Performance Indicator – Stage Duration Curves

Visual inspection of stage duration curves for various alternatives and base

conditions can provide additional information that is directly related to the lake’s

values. By drawing horizontal lines across the plot at 17 ft and 11 ft NGVD, one can

quantify the percent of times that lake stage exceeded or fell below these critical

values. Lower percentages in a given scenario equate to a “better” result. The

restoration goal for the lake is to have zero % for each extreme type of event.

Performance Measure – Similarity in Duration of Stage Events > 15 ft

This performance measure consists of box and whisker plots, which indicate

median, maximum, minimum, and 25th / 75th percentiles, for duration of moderate

high lake stage events. As indicated above, prolonged lake levels above 15 ft NGVD

cause significant harm to the lake’s ecological values, and should be avoided. For

this measure, lower median values and extremes are better. The restoration goal for

the lake is to have a median duration near zero, and a maximal duration no greater

than 1 year.
26

Performance Measure – Similarity in Duration of Stage Events < 12 ft

This performance measure is analogous to the preceding one, except that it is

concerned with the duration of moderate low lake levels. Lower median and

extreme values are better. The restoration goal for the lake is to have a median

duration near zero, and a maximal duration no greater than 1 year.

Performance Measure – Similarity in Duration of Stage Events > 11 ft

When lake stage falls below 11 ft NGVD, significant harm ensues, even if the

duration is short. Nevertheless, during regional droughts, lake levels have declined

to as low as 9.6 ft NGVD. Alternatives that increase the duration of < 11 ft events

are considered especially harmful to the ecosystem. This performance measure uses

box and whisker plots, as in the previous item. Lower medians and extremes are

better. The restoration goal for the lake is to have no such events (i.e., duration

median and extreme = zero).

Performance Measure – Similarity in Duration of Stage Events > 17 ft

This performance measure is analogous to the preceeding one, except that it

is concerned with the duration of extreme high lake levels. Lower median and

extreme values are better. The restoration goal for the lake is to have no such

events (i.e., duration median and extreme = zero).

Performance Measure – Daily Stage Hydrographs with Spring Recession

Windows

In an effort to evaluate the degree to which intra-year variations in lake

levels match the optimal pattern described by Smith et al. (1995), that is, a January

to May recession, without reversal, from near 15 ft to below 13 ft NGVD, “windows”

were drawn over the daily hydrograph plots. Years in which the lake levels recede

smoothly from the upper left to lower right corners of the box are considered “good”

for wading birds and other wildlife in the lake. Years in which lake levels remain

far above or below the box, or those with water levels passing through the box with

no trend or an increasing trend in lake levels are considered “bad.” A greater

number of years with “good” scores is considered a better result. More elaborate

methods may be utilized to score the results, however, it is unclear whether this is

called for, given existing uncertainties about the true relationship between spring

lake level recession and fish / wildlife health. Furthermore, it is uncertain whether

the goal for this measure is to have the designated recession every year, or at a

return frequency corresponding to some selected reference period.


29

I. REFERENCES

Aumen, N.G. and R.G. Wetzel. 1995. Ecological studies on the littoral and pelagic

systems of Lake Okeechobee, Florida (USA). Archiv fur Hydrobiologie,

Advances in Limnology, Volume 45.

Engstrom, D.R. and B.L. Brezonik. 1993. Lake Okeechobee phosphorus dynamics

study Volume V. Phosphorus accumulation rates. Report, South Florida Water

Management District, West Palm Beach, FL.

Flaig, E.G. and K.E. Havens. 1995. Historical trends in the Lake Okeechobee

ecosystem I. Land use and nutrient loading. Archiv fur Hydrobiologie

Monographische Beitrage 107: 1-24

Furse, J.B. and D.D. Fox. 1994. Economic fishery valuation of five vegetation

communities in Lake Okeechobee, Florida. Proceedings of the Annual

Conference of Southeast Association of Fish and Wildlife Agencies 48: 575-591.

Gleason, P.J. 1984. Environments of South Florida Present and Past II. Miami

Geological Society, Coral Gables, FL.

Havens, K.E. 1996. Water levels and total phosphorus in Lake Okeechobee. Lake

and Reservior Management 12: 78-90.

Havens, K.E., V.J. Bierman, Jr., E.G. Flaig, C. Hanlon, R.T. James, B.L. Jones and

V.H. Smith. 1995. Historical trends in the Lake Okeechobee ecosystem VI.

Synthesis. Archiv fur Hydrobiologie Monographische Beitrage 107: 101-111.

Havens, K.E. and B.H. Rosen. 1995. Plan for quantifying long-term ecological

trends in Lake Okeechobee. Report, South Florida Water Management

District, West Palm Beach, FL.

Havens, K.E., N.G. Aumen, R.T. James and V.H. Smith. 1996a. Rapid ecological

changes in a large subtropical lake undergoing cultural eutrophication. Ambio

25: 150-155.

Havens, K.E., L.A. Bull, G.L. Warren, T.L. Crisman, E.J. Phlips and J.P. Smith.

1996b. Food web structure in a subtropical lake ecosystem. Oikos 75: 20-32.

Havens, K.E., T.L. East, R.H. Meeker, W.P. Davis and A.D. Steinman. 1996c.

Phytoplankton and periphyton responses to in situ experimental nutrient

enrichment in a shallow subtropical lake. Journal of Plankton Research 18:

551-566.
30

Havens, K.E., E.J. Phlips, M.F. Cichra, and B-L. Li. 1997. Light availability as a

regulator of cyanobacteria species composition in a shallow subtropical lake.

Freshwater Biology, in press.

Heiskary, S.A. and W.W. Walker, Jr. 1988. Developing phosphorus criteria for

Minnesota lakes. Lake and Reservoir Management 4: 1-9.

Hutchinson, G.E. 1957. A Treatise on Limnology, Volume I Part 2 - The Chemistry

of Lakes. John Wiley and Sons, NY.

James, R.T. and V.J. Bierman, Jr. 1995. A preliminary modeling analysis of water

quality in Lake Okeechobee, Florida: calibration results. Water Research 29:

2755-2766.

Jin, K-R., T. Tisdale and K.E. Havens. 1997. Hydrodynamics and vertical mixing in

Lake Okeechobee, Florida, USA. Water Resources Bulletin, in review.

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Aquatics 9: 21-26.

Koch, M.S. and K.R. Reddy. 1992. Distribution of soil and plant nutrients along a

trophic gradient in the Florida Everglades. Soil Science Society of America

Journal 56: 1492-1496.

Lockhart, C.S. 1995. The effect of water level variation on the growth of Melaleuca

seedlings from the Lake Okeechobee littoral zone. M.Sc. Thesis, Florida

Atlantic University, Boca Raton, FL.

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concentrations in Lake Okeechobee, Florida: the influence of lake levels. Lake

and Reservoir Management 8: 1-11.

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Okeechobee, Florida. Lake and Reservoir Management 6: 93-100.

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phosphorus in the Florida Everglades: a synoptic-experimental approach.

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McCormick, P.V., P.S. Rawlik, K. Lurding, E.P. Smith and F.H. Sklar. 1996.

Periphyton-water quality relationships along a nutrient gradient in the

northern Florida Everglades. Journal of the North American Benthological

Society 15: 433-449.
31

Paerl. H.W. 1988. Nuisance phytoplankton blooms in coastal, estuarine and marine

environments: a review of recent evidence on the effects of enrichment.

Limnology and Oceanography 33: 823-847.

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and P. Hansen. 1997. Relationships between phytoplankton dynamics and the

availability of light and nutrients in a shallow subtropical lake. Journal of

Plankton Research 19: 319-342.

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32
33

III. ST. LUCIE AND CALOOSAHATCHEE CONCEPTUAL MODEL

A. PREPARERS

Susan Gray and Daniel E. Haunert, South Florida Water Management

District, West Palm Beach, Florida.

B. ABSTRACT

A conceptual model of the St. Lucie and Caloosahatchee Estuaries was

developed by scientists working in these systems to identify major stressors,

measurable attributes of the ecosystem that are sensitive to the stressors, and

linkages between the two. Both estuaries have been highly modified. The St. Lucie

Estuary, located on the southeast coast of Florida, discharges into the Indian River

Lagoon and Atlantic Ocean through the St. Lucie Inlet. The Caloosahatchee

Estuary, located on the southwest coast of Florida, discharges into Charlotte

Harbor, and then into the Gulf of Mexico. Due to extensive urban and/or

agricultural drainage projects in both watersheds, the historic watershed area has

been greatly expanded. In addition, both estuaries are linked to Lake Okeechobee

via canals that provide both navigation and releases of floodwaters. In the

Caloosahatchee, the construction of a water control structure downstream of Lake

Okeechobee has also decreased the tidally influenced portion of the Caloosahatchee

Estuary. The major effects of anthropogenic changes in both watersheds are

significant alterations in the timing, distribution, quality, and volume of freshwater

entering the estuaries. Understanding how these systems respond to stress will

provide a basis for well-informed management decisions on restoration activities.

C. INTRODUCTION

The St. Lucie Estuary is located on the southeast coast of Florida, and

discharges into the Indian River Lagoon and Atlantic Ocean at the St. Lucie Inlet.

The estuary encompasses about eight square miles, and the historic watershed was

estimated to be about 1/3 the size of its present configuration. Due to extensive

agricultural and urban drainage projects beginning in the 1910s, the present day

watershed area has been expanded to almost 775 square miles. Major canals in the

watershed include the C-23 and C-24 canals, part of the Central and South Florida

Flood Control Project. In addition, the estuary is linked to Lake Okeechobee by the

C-44 canal that is utilized for both navigation and the release of floodwaters from

the lake.

The Caloosahatchee Estuary is located on the southwest coast of Florida, and
34

discharges into Charlotte Harbor, and then into the Gulf of Mexico. The

Caloosahatchee Estuary is also connected to Lake Okeechobee through the C-43

canal (Caloosahatchee River), and there are a series of smaller drainage works in

association with substantial agricultural development in the watershed. The

construction of a water control structure (Franklin Lock and Dam) downstream of

Lake Okeechobee has decreased the tidally influenced portion of the estuary,

allowing for the convenient use of the C-43 as a potable water supply.

The major effects of anthropogenic changes in both watersheds are significant

alterations in the timing (excess wet season flows, insufficient dry season flows),

distribution, quality, and volume of freshwater entering the estuaries. In addition,

alterations within the estuaries through dredging of navigation channels, filling of

marshes, and construction of bridges and causeways has also contributed to a

number of documented declines in some mollusc species and submerged aquatic

vegetation. Restoration of these important species is considered critical to the

overall health of these systems. Despite these impacts, the estuaries continue to be

an important resource, with significant environmental and economic values.

D. ESTUARY MODEL STRUCTURE

The conceptual model for the St. Lucie and Caloosahatchee Estuaries follows

the format used for other south Florida ecosystem models, with a top-to-bottom

hierarchy of drivers, stressors, ecosystem effects, attributes and measures.

Refer to figure on page 39. The model is similar to those developed for other

ecosystems, including Florida Bay and Lake Okeechobee. One significant difference

is that a single model diagram is proposed for both estuaries. During the

development of this model, it became apparent that one design would serve both

systems, but that the emphasis or relative importance of each segment could vary

between the systems.

E. ECOLOGICAL & SOCIETAL ATTRIBUTES

Nine ecological and societal attributes associated with a healthy estuarine

ecosystem were identified, along with measures that will provide information on

current ecological status, as well as long-term trends. These include the following:

4Mollusc abundance & distribution – Oysters, clams and scallops are natural

components of southern estuaries, and have been documented in the past to be

abundant in these systems. Oyster reefs provide several important functions,

including habitat and food for other species. Scallops, clams and oysters may be a

commercial and/or recreational resource.

4Sediment quality directly affects the occurrence and abundance of benthic and

sessile species. Sediments are a sink for nutrients and other materials, such as
35

pesticides and metals.

4Water quality is a major determinate of the health and condition of aquatic

systems, and changes in water quality are often the linkage between the stressor

and the ecosystem value.

4Submerged aquatic vegetation (SAV) distribution and abundance -

Seagrasses are considered to be a key feature of estuaries due to the functions that

they perform. These functions include sediment stabilization, primary productivity,

and nursery and refugia habitat.

4Habitat structure and function, as considered on a landscape scale, integrates

many of the other values. An example of a landscape-scale attribute is defining a

preferred salinity regime for the estuary. Work on the St. Lucie and

Caloosahatchee Estuaries has identified preliminary salinity ranges and seasonal

and spatial distributions, termed “salinity envelopes” to meet the physiological

requirements for the establishment and productivity of key estuarine species,

including seagrasses and oysters.

4Benthic invertebrate community structure - The composition of invertebrate

species in the benthos serves as an indicator of the nutrient and contaminant

loading to the system. Changes in species diversity and abundance have been

documented in the presence of increased pollutant loadings.

4Manatee population dynamics - Manatees are a very visible and popular

feature of the estuary, and impacted by changes to the ecosystem. Their status as

an endangered species adds considerable importance to their protection and

stabilization of their populations.

4Fisheries population dynamics - Fishes are an integral component of the

estuary food web. In addition, they have a significant societal value for recreation

and commercial harvesting.

4Fish-eating birds population dynamics - Fish eating birds are one of the

more visible keystone species in the estuarine ecosystem. Their dependence on fish

for food serves as an indicator of the health and status of their prey populations.

F. LINKS FROM STRESSORS TO ATTRIBUTES

Seven stressors that exert significant impacts on the estuary’s ecological and

societal attributes were identified for these systems. However, the linkages

between ecosystem stressors to the environmental and societal values are complex,

and considerable uncertainties remain. As more is learned about how these

systems function, these linkages may be changed or new ones included. One aspect
36

of the model is that it is flexible and can be adjusted as new information is

available.

4 Altered hydrology is one of the predominant stressors on the system. This

stress is the result of managing the runoff of the watershed to accommodate

agricultural and urban development, along with flood control releases from Lake

Okeechobee. Accompanying these hydrologic changes are increased sediment

transport, transport of floating freshwater vegetation (Hyacinth and Pistia) into the

estuarine portion of the ecosystem, and alterations in water residence time and

biological flushing of the estuary. Salinity, a key organizing parameter for the

estuary, is directly affected by changes in inflows and tidal exchange. All of these

factors combined have been shown to impact indicator species of estuarine health,

namely seagrasses and oysters. 

4Physical alterations to the ecosystem have also had a significant effect on the

presence and abundance of species normally found within these systems. These

alterations have occurred through a variety of activities, including shoreline

hardening, inlet dredging and stabilization, removal of oyster shell for road bed

material, snagging of navigation hazards, decrease in spatial extent of the estuary

through construction of barriers [S-79], and the impoundment of salt marshes for

mosquito control (on the east coast). Changes in the physical dimensions of the

estuaries, most notably in the Caloosahatchee, have significantly reduced the

euryhaline portion of the estuary, an important nursery area, feeding area and

refugia for juvenile stages of desirable sport and commercial fishes. Construction

and operation of the water control structures have interfered with the migration

patterns of many estuarine species, and have also resulted in the death of manatees

attempting to pass through these structures.

4Nutrients and dissolved organics enter the system as a result of

anthropogenic activities in the watershed, primarily agricultural and water

management, although urban use of fertilizers is also a contributor. In addition,

atmospheric deposition contributes to the nitrogen loading of the system. These

constituents interact with alterations in salinities and sediment loading to reduce

light penetration which, in turn, leads to declines in seagrasses. The shift in the

species composition of benthic invertebrates to more pollution-tolerant organisms is

also linked to increases in nutrients.

4Toxics, which include pesticides, fungicides, herbicides, brine from reverse

osmosis plants, coliforms from sewage treatment plants (STPs), oils, greases, and

mercury are introduced into these systems from urban development, agricultural

practices, and boating. Direct toxic effects have been noted on some zooplankton

and fish. Indirect effects occur through the process of bioaccumulation or

biomagnification through the food web, increasing the toxic load to top predators.

The problem of eggshell thinning in pelicans and osprey was the result of
37

bioaccumulation of DDT.

4Temperature extremes are the result of both rare natural events, such as

freezes, and human activities due to discharges from nuclear power plants and

STPs. Severe freezes have resulted in the direct decline of mangroves and

manatees. In addition, the unnatural congregation of manatees in waters warmed

by plant discharges is thought to contribute to the spread of disease and exposure to

red tide.

4Boating pressure is a significant stress to the estuaries through direct impacts

such as scouring, wake erosion, and construction of boating channels. In addition,

boat collisions are the leading anthropogenic cause of manatee deaths.

4Resource harvesting of sport and commercial fisheries has impacted the

standing stocks of many species where over-fishing has occurred. This in turn

impacts those species that depend on fish as their primary food source.

G. SUMMARY

A conceptual model was developed for the St. Lucie and Caloosahatchee

Estuaries. The model illustrates major linkages between stressors on the system

and ecological and societal values. The purpose of the model is to provide a basis for

management decisions on proposed restoration activities and to assist in the

development of an integrated ecological monitoring program.


40

H. REFERENCE

Havens, K.E. and B.H. Rosen. 1995. Plan for quantifying long-term ecological

trends in Lake Okeechobee. Report, South Florida Water Management

District, West Palm Beach, Florida.
41

IV. BIG CYPRESS REGIONAL ECOSYSTEM CONCEPTUAL MODEL

A. PREPARER

Michael Duever, The Nature Conservancy, Kissimmee, Florida.

B. INTRODUCTION

The Big Cypress region covered by this conceptual model includes the

freshwater portions of the area extending from the southern edge of the

Caloosahatchee River watershed boundary in Lee, Hendry, and northern Collier

Counties, and west of the Everglades, as delimited approximately by L-28 in the

north and a diagonal line running from Forty-Mile Bend on Tamiami Trail

southwest to the coast. In this region historic water flows were primarily south to

the Gulf of Mexico, with minor flows in small creeks that pass through the west

coast ridge to the Gulf and somewhat more significant flows east into the

Everglades (Klein et al. 1970).

There are three distinct subregions within the Big Cypress Region, based on

the kind and degree of development present in each (Lehman 1976). The most

pristine area, because the hydrology is largely rainfall-driven, is located within the

Big Cypress National Preserve in the southeastern portion of the region (Duever et

al. 1986). The Preserve, along with a portion of the adjacent Seminole Indian

Reservation to the north, is the Big Cypress subregion that was included in the

Restudy Alternatives analyses, although only the eastern portion of this area was

affected by components included in the various restoration alternatives. The most

developed portion of the area, including both urban and agricultural development,

is located on and just east of the coastal ridge from Naples north to Ft. Myers. The

rest of the area is a mixture of agricultural lands, suburban and rural communities,

and small-to-large natural areas that have been altered to varying degrees by

upstream and/or adjacent development. Despite the very different degrees of

development in the three subregions, the kinds of stressors and their effects on

ecosystem attributes are similar throughout the Big Cypress Region and they differ

only in the severity of their impacts.

The Big Cypress region is comparable to the freshwater Everglades in terms

of natural community diversity, although the communities tend to form more of a

mosaic as opposed to vast expenses of a single community type. The most extensive

natural communities in the Big Cypress are distributed throughout the region along

very gentle topographic gradients from short-hydroperiod pinelands on the uplands

through marshes to long-hydroperiod cypress forests on lower elevations (Craighead

1971, Davis 1943, Duever 1984, Klein et al. 1970). Small open-water ponds are

found in scattered deeper depressions, and small islands of hardwood forest occur

on elevated sites among or along the edges of deeper herbaceous and forested
42

wetlands. While the Big Cypress is dominated by temperate species, there is a

small but unique tropical component that is most prevalent in forested upland and

swamp communities (Duever et al. 1986). This component is most abundant and

diverse close to the coast, but occurs further inland in association with larger and

deeper wetlands, where it is protected from cold winter weather by their canopy

and/or the moderating effects of standing water. Under natural conditions, the

range of annual fluctuation in water levels above and below ground and the

duration of inundation are the primary factors influencing the distribution of these

communities, although frequent fires exert a secondary influence by controlling the

kinds and structure of their dominant plant species (Craighead 1971, Duever 1984,

Harper 1927, Klein et al. 1970). Cypress and marsh communities tend to be more

common towards the south as elevations approach sea level, while pinelands are

more common in the higher and deeper-sand substrates to the north and on the

west coast ridge along the Gulf of Mexico.

C. ECOLOGICAL STRESSORS

The ultimate source of all ecological stressors in the Big Cypress ecosystem is

development for agriculture and residential use (refer to figure on page 51). The

area’s seasonally high water table required the construction of an extensive system

of small ditches connected to larger and larger canals to assure that water levels

remain below ground throughout the year, in order to make the land suitable for

crops and housing (Lehman 1976). The drainage systems produce lowered water

tables and shortened hydroperiods for considerable distances in otherwise unaltered

lands around and downstream of them (Carter et al. 1973, Klein et al. 1970, Swayze

and McPherson 1977). These drier conditions can facilitate upgradient shifts in the

characteristics of affected plant communities and an associated increased fire

severity (Robertson 1953, Wade et al. 1980). In addition, the water in canals

draining these lands normally has a different chemistry than that found in natural

Big Cypress waters. This is particularly true in terms of undesirably higher

concentrations of nutrients and dissolved minerals (Duever et al. 1986), which can

encourage the spread and potential dominance of sites by nuisance native and

exotic vegetation. The human response to South Florida’s dry season produces an

additional hydrologic stress on natural systems in the Big Cypress in terms of

lowered water tables and shortened hydroperiods. When weather conditions are

dry for extended periods, water use for crops, lawns, etc. goes up substantially,

which results in lower than normal water levels in and for considerable distances

around the wells that provide this water (Lehman 1976, Rochow 1985). Excessive

drawdowns associated with canals and wellfields can produce very significant

impacts on native amphibian, fish, and crayfish populations that depend on the

presence of at least some inundated wetlands for their survival through the dry

season. These drawdowns can also eliminate the standing water that normally

protects tropical components from winter freezes (Duever et al. 1978).
43

Changes in land use associated with agricultural and residential

development not only cause habitat loss on the affected lands, but fragmentation of

the habitat mosaic. Habitat loss directly impacts the availability of resources

required by organisms that utilize these areas. However, the distribution of these

habitats across the landscape is even more important because few organisms utilize

only one habitat type, particularly in a landscape that can fluctuate seasonally from

inundated to completely dry and that may be affected by widespread and sometimes

severe fires. Thus, when wetland connections to adjacent uplands are severed,

innumerable biologically important flows are interrupted and ecosystem diversity

and viability are severely compromised (Forman and Godron 1986, Harris 1984).

Fragmentation and habitat loss also affects populations by reducing the spatial

extent of their resource base to the point where it is no longer able to support viable

populations. Exposure to hazards associated with development also increases

dramatically as habitats become smaller and are more and more fragmented.

As development increases in an area, the seasonality and frequency of fire in

natural communities becomes increasingly unnatural (Wade et al. 1980). Natural

fires were normally most common during the summer wet season as a result of the

high frequency of lightning (Duever 1984). However, they were most widespread at

the end of the dry season, when the region was at its driest and lightning storms

were just beginning to occur. Early in this century, cattlemen greatly increased the

frequency of dormant season fires to provide more forage for their cattle during the

winter months (Ackerman 1976, Alvarez no date, Kennard 1915). More recently,

fire has been suppressed over much of the more developed portions of the area,

which has caused unusually severe fires when they finally occur, as they always do

eventually (Hofstetter 1984). The lack of fire has allowed succession to proceed in

many areas, so that herbaceous communities are being invaded by dense shrub

thickets and pine and cypress forests are being invaded by shrubs and hardwoods

(Hofstetter 1984).

Exotic plants and animals produce drastic alterations in the composition and

structure of natural communities, although many of these changes are poorly

documented, such as the impacts resulting from the spread of feral hogs and exotic

fish (Crowder 1974, Dineen 1984, Duever et al. 1986, Layne 1984). Some that are

well documented include the spread of melaleuca and other exotic plants (Duever et

al. 1986, Myers 1975). Some species, such as melaleuca and feral hogs, can readily

utilize undisturbed natural communities. However, development has facilitated the

invasion of exotics by creating an abundance of disturbed sites that normally would

have been recolonized by native species, but these species are now being

outcompeted by more rapidly invading exotics (Elton 1958, Odum 1971). In

addition, farms and residential areas create new habitats that are more suitable to

new species, which then invade adjacent natural areas and impact native

populations either directly by predation or indirectly through competition for

resources.
44

Bioaccumulation of mercury and other toxins pose a threat to fauna at all

trophic levels in all South Florida ecosystems, including the Big Cypress Region, as

discussed under generic issues.

D. ECOLOGICAL ATTRIBUTES AND MEASURES

Vegetation community and habitat mosaic. This attribute is affected by all of

the stressors except mercury and other toxins, which tend to be more impacting on

animal populations than on plant communities. Hydrologic and fire regimes control

the character and distribution of the major types of Big Cypress plant communities

(Craighead 1971, Davis 1943). The most frequent and widespread hydrologic

alterations typically result in reduced depth and duration of inundation, which

shifts the affected plant communities upgradient towards shallower wetland types,

or with sufficient drainage to upland types (Alexander and Crook 1973, Duever

1984). Fires are more frequent and severe in these drier environments, in some

areas resulting in dense stands of fire-tolerant cabbage palms and woody exotics

(Gunderson and Loope 1982, Tabb et al. 1976). Drainage can also impact tropical

components of the communities by eliminating the moderating effect of standing

water on cold winter temperatures (Duever et al. 1978). Less frequent alterations

involve increasing depths and/or duration of inundation, which can convert uplands

to wetlands and wetlands to aquatic habitats. In some situations, altered hydrology

can produce unnatural permanently disturbed habitats that are at times too deeply

flooded to support upland vegetation, but for too short a period to produce wetland

communities.

Natural communities in South Florida are adapted to surface waters with a

chemical composition that contains very low concentrations of dissolved minerals

and nutrients. Changes in surface water quality typically involve increased

nutrient and mineral concentrations coming from ditch and canal outflows into

natural wetlands (Drew and Schomer 1984). These increased concentrations can

produce dramatic shifts from diverse herbaceous communities to communities

dominated by a few native and exotic nuisance species adapted to high nutrient and

dissolved mineral concentrations, such as cattails and primrose willow (Duever

pers. comm.).

Dormant season fires and reduced fire frequencies can result in shifts from

herbaceous to shrubby communities and from open pinelands and cypress forests to

shrub and hardwood dominated forests (Alexander and Crook 1971, Duever 1984).

As fuels build up in these sites, the inevitable severe wildfire will eventually occur

(Hofstetter 1984), and the sites will most likely be converted to early successional

communities dominated by weedy herbaceous, vine and shrub species, a large

proportion of which are much less abundant, if even present, in natural

communities.
45

Land use changes reduce the area of affected communities and often

eliminate the transitions from one to another, particularly along upland to aquatic

hydrologic gradients. Since wetlands have more legal protection and are more

difficult to develop than uplands, habitat loss is greatest in historical uplands,

although this may not be obvious because new upland habitat is being created by

the drainage occurring throughout much of the area. Upland development is also

resulting in wetlands becoming more and more isolated from each other as well as

from other natural parts of the system, which is in turn affecting their faunal

populations and fire regimes.

All of the ecosystem alterations associated with development are facilitating

the increasingly rapid spread of nuisance and exotic species throughout the Big

Cypress Region landscape (Drew and Schomer 1984, Duever et al. 1986). In many

areas, exotic woody vegetation such as melaleuca, Brazilian pepper, or downy

rosemyrtle completely dominates what were previously herbaceous communities or

open pinelands. Cattails and primrose willow have replaced marsh communities

that once supported diverse herbaceous species assemblages. Feral hogs annually

“plow” thousands of acres, with unknown impacts on what they are foraging on, but

quite visible soil disturbance effects that further exacerbate the spread of exotic

plants (Duever et al. 1986, Layne 1984).

Measurements of changes in the vegetation mosaic in terms of community

distribution, composition, and acreages would provide a basis for assessing habitat

gains and losses in the Big Cypress Region. The above discussion is not meant to be

exhaustive, but provides examples of the kinds of changes that would be useful to

monitor, particularly in any areas that are established as major restoration sites or

are set aside for preservation. Rates of change in these communities in response to

increased or decreased levels of stressors will vary from three to twenty or more

years depending on community type and the management required to accomplish

restoration. Early succession communities tend to respond more rapidly to changes

than do later successional communities. Management that directly affects

vegetation, such as herbiciding or prescribed fire, will produce more rapid responses

than will management actions that indirectly affect it, such as changes in hydrology

or a reduction in fire frequency.

Logging of old growth pines, which occurred in the Big Cypress through the

1950s, has removed many of the prime red-cockaded woodpecker nesting trees

throughout its range (Ligon 1971, Patterson and Robertson 1981). In addition,

conversion to agriculture and housing is eliminating their habitat, and the current

altered fire regime is resulting in the invasion of native and exotic woody vegetation

into remaining pinelands, which is eliminating the open character of these

pinelands that is needed to support this species (Patterson and Robertson 1981,

Jackson 1971). Red-cockaded woodpecker nesting and nesting success is
46

recommended as a measure of pineland condition, since this species is dependent

upon mature, open pinelands. Suitable habitat should be extensive enough so that

it would burn regularly, while having a hydrology that would prevent excessively

severe fires. Sites should not support nuisance exotics that would tend to grow up

and both eliminate the site’s open character and produce fuel loads that would

result in excessively severe fires. Response to restoration that increases the

suitability of habitat would like be expected within five to ten years, where

reestablishing the open character of a pineland is involved, but could be twenty or

more years, where young pine stands need to reach large enough sizes to provide

suitable nest sites.

Native fish, crayfish, and amphibians. These species are affected primarily by

altered hydrology and land use changes, both of which are resulting in habitat loss

and fragmentation over large areas of the Big Cypress. Abundance of marsh fishes

has been correlated with duration of flooding in the Everglades (Loftus and Eklund

1994), suggesting a decline in fish populations in remaining Big Cypress wetlands

where water tables have been lowered and hydroperiods shortened. Habitat

fragmentation is particularly damaging to these species, which in more natural

settings take advantage of the wet season expansion of surface waters into the

surrounding uplands by increasing their numbers and biomass, which then becomes

available to wading birds and other predators during the surface water’s

subsequent retreat during the dry season (Carlson and Duever 1979, Kolopinski

and Higer 1969, Kushlan et al. 1975). A poorly known influence on them are exotic

fish and amphibian species that have invaded the area, which are both preying on

native species and competing with them for resources (Dineen 1984, Duever et al.

1986). Documenting the abundance, distribution, and diversity of these taxa will

provide useful measures of how the stressors affecting them are being minimized in

protected areas and are being ameliorated in areas being restored. Since these

populations show a dramatic annual cycle in their numbers and biomass, responses

to increasing or decreasing levels of stressors should be detectable within three to

five years.

Wood Stork. The Big Cypress region supported the two largest nesting colonies of

wood stork in North America between 1900 and 1965 (Ogden Pers comm.). During

that period, 6,000-10,000 pairs nested annually in the Corkscrew colony, the Sadie

Cypress colony of Okaloacoochee Slough, and smaller subsidiary colonies. Since

1965, wood stork nesting in Big Cypress has declined and has ranged from less than

500 to 1500 pairs since 1990 (Ogden pers. comm.). The subsidiary colonies have

been largely lost. Accompanying the decline in numbers of nesting pairs has been a

reduction in nesting success in the remaining colonies. The decline in success is

attributed to a change in the timing of the initiation of nesting from November to

February (Kushlan et al. 1975, Ogden et al. 1987, Ogden 1994). Delay in nesting

until February puts nesting out of sync with seasonal rainfall patterns. With the

onset of the wet season, rising water levels decrease food availability before the
47

young storks fledge, causing nestling mortality due to starvation. The reduction in

numbers of nesting pairs and nesting success is attributed to the loss of early dry

season foraging habitat of the higher elevation wetlands of the Big Cypress region

that have been drained and developed. Recovery of fish production in the higher

elevation wetlands by raising water tables in preserved areas in the western and

northern portions of the Big Cypress basin is expected to halt the decline in wood

stork nesting in Big Cypress. A trend of increasing numbers of nesting pairs and

nesting success and reestablishment of subsidiary colonies should be evident over a

decade time scale after water table recovery.

Florida Panther. The Big Cypress Region supports the only viable population of

Florida panthers (Logan 1993). The downward spiral of habitat loss and

fragmentation leading to isolation and inbreeding has resulted in a single

population struggling to survive in the largest remaining contiguous block of land

within its historic range. Seventeen years of monitoring indicate a stable but

saturated population in the northern portion of its current range, which

encompasses public lands north of I-75 and adjacent private lands (Jansen pers.

comm.). Habitat vacancies in southern Big Cypress National Preserve and

Everglades National Park have not been filled due to the difficulty of population

increase when there are few individuals, at times only a single sex, and infrequent

ingress. Panther require dry areas for daytime resting but readily travel through

inundated habitats (Jansen 1987). Deer, a preferred prey species, benefit from a

hydroperiod that promotes nutritious wetland vegetation (Loveless 1959), and

prescribed fire improves the nutritional quality of food used by panther prey

species. Stressors for the panther include continued habitat loss, inbreeding, and

mercury toxicosis. Whether the panther can survive on only existing public lands is

unknown, but further habitat reduction will likely increase its vulnerability to

catastrophic events. Efforts to improve genetic diversity have been initiated

through the introduction of another cougar subspecies. The sources of methyl

mercury, reported as the cause of death of at least one panther and known to

compromise normal biological functions, have yet to be confirmed and remedied

(Roelke et al. 1991). Habitat protection and restoration of natural hydroperiods

with clean water would benefit the panther by providing suitable land for

population expansion, as well as an uncontaminated and abundant prey base

(Jansen, pers. comm.). Improvements in population size, reproductive success, and

mercury body burden would likely take decades to document, given the current

small population size, low reproductive rate, and the slow turnover of mercury in

the body, although improvements in the prey base could be apparent in less than a

decade.

Health of Aquatic Fauna. Potential inputs of mercury and other toxins in

agricultural and urban runoff water, which may be needed for hydropattern

restoration in the Big Cypress, could result in reduced health, behavioral and

physical abnormalities, and loss of reproductive vigor unless measures are taken to
48

restrict loads of these toxins in waters flowing into natural areas. Measures of

aquatic faunal health that reflect responses to the mercury and pesticide inputs

include body burdens of mercury and other toxins in representative species.

E. HYDROLOGIC PERFORMANCE MEASURES FOR RESTORATION

Successful protection of undisturbed areas and restoration of disturbed areas

require the establishment of hydrologic targets that define the desired

characteristics of a site’s hydrologic regime, and hydrologic measures that evaluate

the current status of the site relative to the target. In the portions of the Big

Cypress that are included in both the Natural System Model (version NSM4.5F)

and the South Florida Water Management Model (SFWMM), NSM conditions were

defined as the hydrologic target. The following performance measures were used to

evaluate the hydrologic condition of various distinct portions of the area as they

were shown to be affected by proposed restoration Alternatives included in a

modified version of the SFWMM. Plans are underway to create an expanded

version of the SFWMM that will include all of the Big Cypress Region. At the

moment there are no plans to create a NSM for the area, although this would be

more feasible after the SFWMM is available, since the original NSM was created by

modifying the existing SFWMM. Until this model is available, it will be difficult to

accurately utilize the following performance measures for the remaining portion of

the Big Cypress. Information on the hydrologic regimes of the major Big Cypress

freshwater plant community types are described in Duever (1984), and these could

be used as interim hydrologic targets for particular communities.

All aspects of a plant community’s hydrologic regime are significant in

determining its taxonomic and structural characteristics, as well as its distribution

across the Big Cypress landscape. The annual duration of inundation, or

hydroperiod, is the most important aspect of the hydrologic regime (Duever 1984),

largely because of South Florida’s minimal topographic relief and its distinct wet

and dry seasons. Short hydroperiods eliminate wetland species that are intolerant

of the more frequent and severe fires on sites that are dry most of the year, while

long hydroperiods eliminate upland species intolerant of extended inundation in low

areas (Duever et al. 1978). Maximum and minimum water levels are also

important. Wet season water depths can eliminate emergent species that cannot

tolerate extended periods of being submerged. The extent of dry season water level

declines can influence the frequency and severity of fires entering wetlands,

particularly those with organic soils, which maintain a relatively moist substrate

and microclimate as long as they are in contact with the water table (Coultas and

Duever 1984). During the wet season, water naturally moved as sheet flow across

the Big Cypress (Klein et al. 1970). The fact that the water is moving is important,

particularly in terms of the resulting water column mixing, which produces a very

different water quality than is found in ponded water.
49

Duration of inundation was evaluated using a number of performance

measures. One that provided a spatial perspective of hydroperiod was the

percentage of acres within a defined area with a mean hydroperiod for the 31 year

simulation that was within 30 days (longer or shorter) of NSM conditions. Another

compared average hydroperiod for the 31 year simulation in a specified area in

terms of the absolute difference between NSM and current or proposed conditions,

which is divided by 100 and then subtracted from 1. This deviation was almost

always a reduction in hydroperiod, but any deviation is documented by using the

absolute value for the deviation. Another perspective is provided by determining

the absolute difference in the average duration of individual flooding events for

NSM compared to current or proposed conditions in a defined area, dividing this

difference by the NSM duration, and subtracting this value from 1.

Deviations from NSM water levels above and below ground were evaluated

by examining the normalized weekly stage-duration curve from a 31 year

simulation for a defined area. The performance measure involved examining the

current or proposed condition stage-duration curve and determining its maximum

vertical deviation from the NSM curve, and dividing this value by the maximum

vertical range of NSM water level fluctuation at the site. The reason for dividing

the absolute deviation by the maximum NSM water level fluctuation is that a

certain degree of deviation in an area with a large natural fluctuation would be less

significant than the same degree of deviation in an area with a small natural

fluctuation. Typically, the greatest deviation occurred when the water table was

declining through the first foot or two below the ground surface, it was smallest at

its lowest point on the hydrograph, and it was relatively small when the water table

was above ground.

Documenting total flows during the wet and dry season provides a measure of

water movement across a defined boundary and how it changes in response to

proposed hydrologic alterations in a particular portion of the Big Cypress. The

absolute deviation between NSM and current or proposed condition flows was

determined first, this value was divided by NSM flows, and the resulting value

subtracted from 1. Initially flows were evaluated separately for the wet and dry

season, so that the greater volumes of water moving during the wet season wouldn’t

overshadow the smaller dry season flow volumes. Simple additive equations were

then developed for a flow cross-section to combine the wet and dry season

information.

In the final analyses, all of the performance measures for a defined area were

combined in a simple additive equation. Since there could be three measures for

flooding duration and only one each for water level and flows, this automatically

weighted flood duration more strongly than either of the other two components of

the hydrologic regime.
50

It is important to be aware that the above performance measures are based

on long-term averages, and there is a great deal of natural year-to-year variability

in these values. In addition, the same types of communities vary substantially from

one place to another, because of differences in their setting and site history. Even

though extreme environmental events can have major affects in shaping natural

communities, they don’t normally produce shifts from one community type to

another because the overall ecosystem has evolved in the context of these events.

52

F. REFERENCES

Ackerman, J.A., Jr. 1976. Florida cowman, a history of Florida cattle raising.

Florida Cattleman’s Association, Kissimmee. 287 pp.

Alexander, T.R. and A.G. Crook. 1973. Recent and long term vegetation changes

and patterns in South Florida. Appendix G, Part I. South Florida ecological

study. University of Miami, Coral Gables, Florida. 215 pp.

Alvarez, K.C. no date. A brief history of Florida cattle ranching (1521-1900).

Unpublished report. 68 pp.

Carlson, J.E. and M.J. Duever. 1979. Seasonal fish population fluctuations in

south Florida swamp. Proceedings of Annual Conference of Southeastern

Association of Fish and Wildlife Agencies 31: 603-611.

Carter, M.R., L.A. Burns, T.R. Cavinder, K.R. Dugger, P.L. Fore, D.B. Hicks, H.L.

Revells, and T.W. Schmidt. 1973. Ecosystem analysis of the Big Cypress

Swamp and estuaries. United States Environmental Protection Agency.

EPA 904/9-74-002. 375 pp.

Coultas, C.L. and M.J. Duever. 1984. Soils of cypress swamps. Pages 51-59 in H.T.

Odum and K.C. Ewel, eds. Cypress swamps. University of Florida Press,

Gainesville.

Craighead, F.C., Sr. 1971. The trees of South Florida. Volume 1. The natural

environments and their succession. University of Miami Press, Miami,

Florida. 212 pp.

Crowder, J.P. 1974. The exotic vertebrates of South Florida. United States

Department of the Interior, Bureau of Sport Fisheries and Wildlife,

Ecological Report DI-SFEP-74-30. 43 pp.

Davis, J.H., Jr. 1943. The natural features of Southern Florida. Florida Geological

Survey Bulletin 25. 311 pp.

Dineen, J.W. 1984. The fishes of the Everglades. Pages 258-268 in P.J. Gleason,

ed.Environments of South Florida: past and present II. Miami Geological

Society, Miami, Florida.

Drew, R.D. and N.S. Schomer. 1984. An ecological characterization of the

Caloosahatchee River/Big Cypress watershed. United States Department of

the Interior Minerals Management Service and Fish and Wildlife Service.

FWS/OBS-82/58.2. 225 pp.
53

Duever, M.J., J.E. Carlson, L.A. Riopelle, and L.C. Duever. 1978. Ecosystem

analyses at Corkscrew Swamp. Pages 534-570 in H.T. Odum and K.C. Ewel,

eds. Cypress wetlands for water management, recycling, and conservation.

Fourth annual report to National Science Foundation and Rockefeller

Foundation. Center for Wetlands, University of Florida, Gainesville.

Duever, M.J. 1984. Environmental factors controlling plant communities of the Big

Cypress Swamp. Pages 127-137 in P.J. Gleason, ed. Environments of South

Florida: past and present II. Miami Geological Society, Miami, Florida.

Duever, M.J., J.E. Carlson, J.F. Meeder, L.C. Duever, L.H. Gunderson, L.A.

Riopelle, T.R. Alexander, R.L. Myers, and D.P. Spangler. 1986. The Big

Cypress National Preserve. National Audubon Society Research Report No.

8, New York. 455 pp.

Elton, C.S. 1958. The ecology of invasions by animals and plants. John Wiley,

New York. 199 pp.

Forman, R.T.T., and M. Godron. 1986. Landscape Ecology. John Wiley and Sons,

New York.619 pp.

Gunderson, L.H. and L.L. Loope. 1982. A survey and inventory of the plant

communities in the Raccoon Point area, Big Cypress National Preserve.

South Florida Research Center, National Park Service, Everglades National

Park, Homestead, Florida. Report T-665. 36 pp.

Harper, R.M. 1927. Natural resources of southern Florida. 18th Annual Report of

Florida Geological Survey, Tallahassee. 25-206.

Harris, L.D. 1984. The fragmented forest: island biogeography theory and the

preservation of biotic diversity. University of Chicago Press, Chicago,

Illinois. 211 pp.

Hofstetter, R.H. 1984. The effect of fire on the pineland and sawgrass communities

of southern Florida. Pages 465-476 in P.J. Gleason, ed. Environments of

South Florida: past and present II. Miami Geological Society, Miami,

Florida.

Jackson, J.A. 1971. The evolution, taxonomy, distribution, past populations and

current status of the red-cockaded woodpecker. Pages 4-29 in R.L.

Thompson, ed. Proceedings of a Symposium on the Red- cockaded

Woodpecker, Okefenokee National Wildlife Refuge, Folkston, Georgia.
54

Jansen, D.K. 1987. Big Cypress use study. Florida Game and Freshwater Fish

Commission, Tallahassee. 240 pp.

Kennard, F.H. 1915. The Okaloacoochee Slough. Auk 32: 154-166.

Klein, H., W.J. Schneider, B.F. McPherson, and T.J. Buchanan. 1970. Some

hydrologic and biologic aspects of the Big Cypress Swamp drainage area.

United States Geological Survey Open File Report 70003. 94 pp.

Kolopinski, M.C. and A.L. Higer. 1969. Some aspects of the effects of the quantity

and quality of water on biological communities in the Everglades National

Park. United States Geological Survey, Open File Report 69007. 97 pp.

Kushlan, J.A., J.C. Ogden, and A.L. Higer. 1975. Relation of water level and fish

availability to wood stork reproduction in the southern Everglades, Florida.

United States Department of the Interior, Geological Survey, Open File

Report 75-434. 56 pp.

Layne, J.N. 1984. The land mammals of South Florida. Pages 269-296 in P.J.

Gleason, ed. Environments of South Florida: past and present II. Miami

Geological Society, Miami, Florida.

Lehman, M.E. 1976. Collier County: growth pressure in a wetlands wilderness.

Florida Bureau of Comprehensive Planning. Tallahassee. 59 pp.

Ligon, J.D. 1971. Some factors influencing numbers of the red-cockaded

woodpecker. Pages 30-43 in R.L. Thompson, ed. Proceedings of a Symposium

on the Red- cockaded Woodpecker, Okefenokee National Wildlife Refuge,

Folkston, Georgia.

Loftus, W.F. and A. Eklund. 1994. Long-term dynamics of an Everglades smallfish assemblage. Pages 461-483 in S.M. Davis and J.C. Ogden, eds.

Everglades: the ecosystem and its restoration. St. Lucie Press, Delray Beach,

Florida.

Logan, T., A.C. Eller, Jr., R. Morrell, D. Ruffner, J. Sewell. 1993. Florida panther

habitat preservation plan: south Florida population. Florida Panther

Interagency Committee. 105 pp.

Loveless, C.M. 1959. The Everglades deer herd, life history and management.

Florida Game and Freshwater Fish Commission Technical Bulletin 6. 104 pp.
55

Myers, R.L. 1975. The relationship of site conditions to the invading capability of

Melaleuca quinquenervia in southwest Florida. Master’s Thesis, University

of Florida, Gainesville. 151 pp.

Odum, H.T. 1971. Environment, power, and society. John Wiley, New York. 331 pp.

Ogden, J.C., D.A. McCrimmon, Jr., G.T. Bancroft, and B.W. Patty. 1987. Breeding

populations of the wood stork in the southeastern United States. The Condor

89: 752-759.

Ogden, J.C. 1994. A comparison of wading bird nesting colony dynamics (1931-

1946 and 1974- 1989) as an indication of ecosystem conditions in the

southern Everglades. Pages 533-570 in S.M. Davis and J.C. Ogden, eds.

Everglades: the ecosystem and its restoration. St. Lucie Press, Delray Beach,

Florida.

Patterson, G.A., and W.B. Robertson, Jr. 1981. Distribution and habitat of the redcockaded woodpecker in the Big Cypress National Preserve. South Florida

Research Center, National Park Service, Homestead, Florida. 137 pp.

Robertson, W.B., Jr. 1953. A survey of the effects of fire in Everglades National

Park. Unpublished Report. United States Department of the Interior,

National Park Service, Homestead, Florida. 169 pp.

Rochow, T.F. 1985. Hydrologic and vegetational changes resulting from

underground pumping at the Cypress Creek well field, Pasco County, Florida.

Florida Scientist 48: 65-80.

Roelke, M.E., D.P. Schultz, C.F. Facemire, S.F. Sundlof, and H.E Royals. 1991.

Mercury contamination in Florida panthers. Florida Panther Technical

Subcommittee. 55 pp.

Swayze, L.J. and B.F. McPherson. 1977. The effect of the Faka Union canal system

on water levels in the Fakahatchee Strand, Collier County, Florida. United

States Geological Survey Water Resources Investigation. 77-61.

Tabb, D.C., E.J. Heald, T.R. Alexander, M.A. Roessler, and G.L. Beardsley. 1976.

An ecological and hydrological assessment of the Golden Gate Estates

drainage basin, with recommendations for future land use and water

management strategies. Tropical Bioindustries Development Company.

Miami, Florida. 178 pp.
56

Wade, D., J. Ewel, R. Hofstetter. 1980. Fire in south Florida ecosystems. United

States Department of Agriculture, Forest Service General Technical Report

SE-17. 125 pp.
57

V. EVERGLADES RIDGE AND SLOUGH CONCEPTUAL MODEL

A. PREPARER

John C. Ogden, South Florida Water Management District, West Palm

Beach, Florida.

B. INTRODUCTION

As natural systems, the Ridge and Slough landscape features of the

Everglades were extensive, long-hydroperiod, freshwater marshes, characterized by

low-velocity "sheet flow", moderate to deep organic soils and sawgrass “ridges

(Davis et al. 1994, Gunderson 1994, Gunderson and Loftus 1993, McVoy, Park &

Obeysekera in prep.)”. The ridge and slough systems were dominated by sawgrass

strand (Cladium), water lily, and wet prairie (Eleocharis) communities. A spatially

variable mosaic of discreet tree islands occurred throughout, ranging from widely

spaced in the central Everglades to more densely distributed islands in the

Loxahatchee basin and Shark Slough. The Ridge and Slough systems modeled here

are defined by the portion of the Everglades basin where there were Loxahatchee

Peat soils (McVoy, Park and Obeysekera, in prep.; the combined “wet prairie/slough,

tree island, sawgrass mosaic” and “sawgrass dominated mosaic” in Davis et al.

1994). These sloughs made up the deeper central portions of the total Everglades

basin, and occurred to a more limited extent in a series of much smaller scale flow

systems located on both the eastern and western flanks of the Shark Slough in the

southern Everglades (Taylor, East, Middle, Lostman's sloughs). The ridge and

sloughs were principal centers for primary and secondary production and interannual survival of aquatic organisms in the freshwater wetlands of southern

Florida.

The major, known alterations in the ridge and slough systems have been

caused by changes in land use, drainage and water management practices in south

Florida (Davis and Ogden 1994, Science Subgroup 1993). The effects from these

regional “drivers” include compartmentalization and reduced spatial extent of the

natural system, loss of organic soils, altered water depth, distribution and flow

patterns, altered water chemistry, and the introduction of exotic species. It is the

effects from these “stressors” which are modeled in the conceptual ecological model

for the ridge and slough systems, which is presented here.

C. THE SOCIETAL DRIVERS, ECOLOGICAL STRESSORS, AND

ECOLOGICAL EFFECTS

The workshops of the SERA Natural Systems Team and the C&SF Restudy

Alternative Evaluation Team have identified four major categories of external

societal influence (drivers) on the Everglades Ridge and Slough systems, leading to
58

five major ecological stressors. The combined effects from these stressors explain

much of the ecological changes which have occurred in the ridge and slough systems

(refer to figure on page 65). The four societal drivers and their respective stressors,

and the ecological effects resulting from the stressors, are as follows.

Urban and Agricultural Expansion. Drainage of wetlands, and the subsequent

conversion of slough habitat into agricultural and urban uses has reduced the total

spatial extent of the ridge and slough system in the main Everglades from 490,000

ha in the pre-drainage system to 365,000 ha in the current system (Davis et al.

1994). The biological and physical effects from this 26% loss in spatial extent

include, (a) a substantial reduction in habitat options for wildlife, and (b) a

reduction in the system-wide levels of primary and secondary production.

Space was the one physical characteristic of the south Florida wetlands that was

necessary for all other physical and ecological components of these systems to be in

place. The broad spatial extent of the pre-drainage sloughs was essential for

collecting and storing the amount of regional rainfall that was required to maintain

the ecological vigor of these systems. Large spatial scale provided the foundation

for the processes that created and maintained a mosaic of habitats in a low profile

terrain (Craighead 1971, DeAngelis & White 1994). Disturbances, such as fire,

freezes, tropical storms, and animal related impacts (e.g., an abundance of alligatorcreated ponds and trails) served to increase the complexity of habitats, as well as

the range of habitat choices for species of wildlife with both small and large home

ranges (Craighead 1968, DeAngelis 1994, Duever et al. 1994, Gunderson & Snyder

1994, Davis & Ogden 1994). Extensive space was necessary for supporting robust

numbers of animals requiring extensive feeding or hunting ranges throughout

different seasons and a range of hydrological conditions For example, the location

of feeding and nesting sites for white ibis and other wading birds, snail kites, and

other common aquatic birds shifted across large spatial scales and across seasons

and years, in response to variability in hydrological patterns (Kushlan 1979,

Bennetts et al. 1994, Hoffman et al. 1994, Ogden 1994). The large number of ponds

and depressions occurring over an extensive region allowed for comparatively

sedentary species such as the alligator to maintain very large, regional numbers

(Craighead 1968). Greater space enabled the system-wide aquatic production in a

nutrient-poor ecosystem necessary to support large numbers of wading birds and

alligators (Browder 1976, Mazzotti and Brandt 1994).

(2.) Influences from Industrial and Agricultural Practices. Increased

loads of Phosphorus (P) and Mercury (Hg), originating as bi-products from an array

of agricultural and urban industrial practices, were identified as the two alterations

in water quality which have been demonstrated or hypothesized, respectively, to

have had the greatest ecological significance in the slough systems (Roelke et al.

1991, Spalding & Forrester 1991, Lange et al. 1994, Browder et al. 1994, Davis

1994, Frederick et al. 1997). Increased phosphorus loading in a low-nutrient system
59

has, (a) caused a substantial expansion of "nutrient-loving" plants (monocultures of

cattail) into other communities, (b) contributed to shifts in the species composition

in periphyton mats, from communities dominated by green algae/diatoms to

communities dominated by blue-green algae, and (c) contributed to reductions or

alterations in primary and secondary production patterns. Increased levels of

environmental mercury, and the conversion to the toxic methyl mercury form, has

considerable potential to substantially reduce survival rates, and alter behavior

patterns, for omnivorous and carnivorous animals dependent on wetland food

chains.

(3.) Water Management Practices. All five of the major ecological

stressors acting on the slough systems (reduced spatial extent, the introduction and

spread of degraded water, reduced water storage capacity,

compartmentalization, and the introduction and spread of exotic species) have

had part or all of their origins in water management practices. Major objectives of

water management have included water supply and flood control, which have been

achieved by means of a complex system of structural and operational modifications

to the natural system. These modifications have, (a) contributed to the substantial

reduction in spatial extent, (b) provided a network of canals and levees which have

accelerated the spread of degraded water and exotic species, (c) greatly reduced the

water storage capacity within the remaining natural system, (d) altered sheet flow

velocities and directions, and (e) created an unnatural mosaic of impounded and

over-drained marshes in the Water Conservation Areas (Science subgroup 1993,

Davis and Ogden 1994, Light and Dineen 1994, Fennema et al. 1994).

Ecological effects from (a) and (b) are summarized in (1) and (2) above. The

combined effects from reduced water storage capacity, altered sheet flow patterns

and compartmentalization have, paradoxically, included both a reduction in the

duration of annual and multi-year hydroperiods and an increase in the frequency of

unnatural high water events.

Shortened hydroperiods (i.e., an increase in the frequency and duration of

surface water dryouts) have resulted in a substantial increase in the frequency and

extent of over-drained marshes, primarily during dry season months. Over-drained

marshes have contributed to a broad array of adverse ecological impacts, including

the spread of woody species of plants into marshes, reduced alligator nesting effort,

reduced levels of aquatic production, reduced survival by aquatic organisms over

multi-year periods, and considerable loss of peat soils due to increased frequencies

and intensities of fire and increased levels of soil oxidation (e.g., Craighead 1971,

Davis and Ogden 1994, Fleming 1991, Gunderson and Loftus 1993, Loftus et al.

1990, Loftus et al. 1992, Loftus and Eklund 1994, Mazzotti and Brandt 1994, Myers

1983, Stober et al. 1996 ).
60

The unnaturally distributed mosaics of impounded water in the southern

portions of the WCAs, caused by compartmentalization, have contributed to the

flooding of tree islands, have altered marsh community structure, and have caused

an artificial re-distribution of the centers of aquatic production and animal survival

throughout the WCA portion of the ridge and slough system (Armentano 1996,

Bancroft et al. 1994, Davis et al. 1994, Fennema et al. 1994, Frederick and Spalding

1994). The break-up of the large, traditional wading bird nesting colonies and

relocation of smaller, less successful colonies to the WCAs, are hypothesized to be

responses by wading birds to the combined affects of marsh drainage and the

relocation and fragmentation of the region's aquatic production and survival centers

(Ogden 1994).

Societal influences on the species composition of regional floras/faunas.

The introduction, both intended and unintended, of large numbers of non-native

species of plants and animals that have been brought into south Florida because

they have been considered to be desirable additions to the region, from the

perspective of a wide range of human values, has resulted in substantial alterations

to the Everglades sloughs. The most blatant examples affecting the sloughs have

been, (a) the invasions by Melaleuca, and the conversion of extensive marshes into

woody swamps (Bodle et al. 1994, Laroche and Ferriter 1992), and (b) the spread by

15-20 species of introduced fishes throughout much of the region of the ridge and

slough systems (Courtenay 1994, Robertson and Frederick 1994). Although the

ecological consequences of these fishes has been largely unmeasured, it must be

assumed that their presence has substantially altered the characteristics of marsh

fish communities.

D. CRITICAL ECOLOGICAL PATHWAYS

The principal, broad-scale ecological responses from the combined affects of

the five major ecological stressors acting on the ridge and slough systems have been,

(a) a substantial alteration and degradation in the natural patterns of plant

community composition and structure, and (b) substantial changes in the

distribution, and reductions in abundance, among many of the native animals that

are dependant on aquatic habitats and food chains. The relative contribution made

by each of the stressors, along the different pathways shown in the model, to these

two primary responses is not well known. For example, the presence of physical

and biological thresholds, some of which may have been crossed as a result of the

affects of the stressors, have yet to be demonstrated for most functions and

relationships in the sloughs (an exception might be the 10 ppb value for Ph). Neverthe-less, current understandings of the ridge and slough systems, as expressed by

the NST and AET workshop participants, suggest that four ecological pathways

have been most important for explaining how these two principle, broad-scale,

ecological responses have been linked to the stressors. These four pathways are:
61

(1.) The pathway that links the reduction in water storage capacity and the

substantial increase in the extent and frequency of over-drained marshes

(shortened hydroperiods), with an array of ecological effects characterized by a

spread in woody vegetation, reduced numbers of nesting alligators, reduced marsh

production, reduced dry season survival by aquatic organisms, and the loss of peat

soils.

(2.) The pathway that links compartmentalization, and the adverse affects

that levees have on water depth/distribution patterns and sheet flow, with altered

patterns of nutrient cycling and reduced marsh production, and relocated animal

distribution, production and survival centers.

(3.) The pathway that links the reduction in water storage capacity,

compartmentalization and the adverse affects of regulatory releases, with flooded

tree islands and flooded alligator nests.

(4.) The duel pathways that link degraded water quality and the affects of

canals with increased phosphorus loading, resulting in the spread of cattail

monocultures, degraded periphyton communities, and reduced marsh production.

E. ECOLOGICAL ATTRIBUTES AND MEASURES

The NST and AET workshops recommended seven ecological attributes of the

Everglades ridge and slough systems, which, collectively, will provide the best

measure of system responses to restoration projects (refer to figure on page 65).

The model shows how each attribute is linked to one or more stressor. The

recommended measures for each attribute should be treated as priority components

for a comprehensive ecological monitoring program.

(1.) Peat Soils. A recovery of more natural patterns in the duration of

uninterrupted surface flooding, and in water depths, should result in the return of

pre-drainage rates of accretion for peat soils. Conversely, unnatural rates of loss of

organic soil due to oxidation and excessive fires should be curtailed.

(2.) Community of Marsh Fishes, Invertebrates & Herps. Improved water

quality, longer hydroperiods, improved patterns of nutrient cycling and primary

production, and reductions in the influences of exotic fishes, should be reflected by

measures of the composition, size structure and abundance of these animals.

(3.) Tree Islands. Reductions in the adverse affects of flooding, fire, and

exotic plants should be reflected by measures of tree island distribution, and by the

species composition and structure of woody plants on the islands.
62

(4.) Marsh Plant Communities. Elimination of the adverse impacts due to

excessive fire, soil oxidation, shortened hydroperiods, unnaturally deep

impoundments, overdrained marshes, the spread of exotic plants, and degraded

water quality should be reflected by measures of the distribution, composition and

structure of the dominant marsh communities in the sloughs.

(5.) Alligators. An increase in the abundance and density of alligator nests

and active alligator ponds is an expected response from lengthened hydroperiods,

increased marsh production rates, and reductions in wet season regulatory releases

and the unnatural affects of impoundments.

(6.) Periphyton. Improvements in water quality and in the depth and

duration of flooding should be reflected by shifts in species composition and

production rates in the periphyton communities.

(7.) Wading Birds. The size, timing and location of wading bird nesting

colonies, as well as the occurrence and frequency of "super colonies", should reflect

improvements in foraging habitat options under a range of climatological

conditions, in marsh production rates, survival rates of marsh organisms during dry

periods, and in the location of the major production and survival centers in the

slough systems.

F. HYDROLOGICAL PERFORMANCE MEASURES

The four critical pathways (above) suggested by the Everglades ridge and

slough model are driven by hydrological stressors (over-drained marshes due to

reduced storage capacity; unnatural depth-distribution patterns due to levees; wet

season flooding due to regulatory releases and the affects of canals and levees;

increased phosphorus levels due to agricultural practices and canals). These

pathways suggest that the most important hydrological targets for meeting the

ecological objectives of the SERA and Restudy projects are, (a) reduction in the

frequency, duration, extent and magnitude of marsh dryouts during dry seasons, (b)

reduction in the unnatural spatial mosaics of deep impoundments and over-drained

marshes, reductions in the duration and unseasonable timing of unnatural high

water events during wet seasons, and (d) reductions in the input and rapid

distribution of phosphorus. The shortened dry-season hydroperiods characteristic of

the managed system are a response to substantially reduced water storage capacity

in the system, coupled with the affects of compartmentalization. The unnatural

mosaics of deep and shallow water are caused by compartmentalization; more

specifically, by the network of levees and the operational schedules associated with

the WCAs. The unnatural wet season depth patterns have been due to the

combined affects of water storage in WCA-3A, and regulatory releases through the

S-12s. These three stressors working in concert have caused a loss of natural

patterns of inter-seasonal and inter-annual water depth distribution and depth,
63

which is thought to have substantially disrupted the pulses in marsh production

that once characterized the natural system. The increased phosphorus loading has

been due to the rapid transport of water with elevated phosphorus levels into much

of the Everglades ridge and slough system.

Of these four priority hydrological targets, three are characterized by

measures of water depth, and the duration and timing of surface and ground water

flooding. For these three, hydrological performance measure targets can be

established, based on regional hydrological patterns predicted by the Natural

Systems Model. These performance targets, and a suggested order of priority

relative to the magnitude of the ecological issues associated with each, are as

follows.

Priority 1. The highest priority for ecological restoration of the Everglades

ridge and slough systems is the establishment of annual and interannual patterns

in the duration of uninterrupted surface flooding, which are generally consistent

with predictions of the NSM, in Everglades National Park and the WCAs. Included

in this priority is the increase in duration of flooding in the overdrained, northern

ends of the WCAs without an increase in flooding in the southern WCAs, and a

recovery of the pre-drainage patterns of multi-year hydroperiods in Shark Slough.

Priority 1 Measure. For the 30 year period of record, compare patterns of

uninterrupted surface water flooding for cells at key indicator regions. The

performance criterion is the difference between the duration patterns shown by the

NSM and the 2X2 simulations for each plan alternative. The target is a mean

difference of zero. Examine annual hydrographs to insure trends are correct and

that the annual drydown in any given year does not occur before the time indicated

by the NSM.

Priority 2. Elimination of compartmentalization effects, characterized by

unnatural mosaics of deep and shallow water across levee alignments, and by loss of

sheet flow.

Priority 2 Measure. Compare monthly surface and ground water depth

patterns for selected sets of cells representing opposite sides of levees, for all major

structural divides within the WCAs, and between WCA 3 and Everglades National

Park. The performance criterion is for water depths between paired sets of cells to

not differ more than 0.2 foot during any given month, and for these depth patterns

to be consistent with NSM predictions. For sheet flow, compare numbers of miles of

levees and canals internal to the ridge and slough systems, and separating the ridge

and sloughs from adjacent wetlands (i.e., sawgrass plain; Big Cypress).

Priority 3. A third priority in the ecological restoration of the Everglades

sloughs is the elimination of regulatory schedules and other operational practices,
64

and internal levees, which cause unnaturally deep water patterns, primarily during

wet seasons.

Priority 3 Measure. For the 30 year period of record, compare monthly

patterns of surface water depth for cells at key indicator regions. The performance

criterion is the difference between the depth patterns shown by the NSM and the

2X2 simulations for each plan alternative. The target is a mean difference of zero.

Examine monthly depth pattern graphs to insure trends are correct for timing and

location, relative to NSM patterns.

66

G. REFERENCES

Armentano, T.V (ed.). 1996. Proceedings of the conference: Ecological assessment

of the 1994-1995 high water conditions in the southern Everglades.

Everglades National Park, Homestead, Fl. 210 pp.

Bancroft, G.T., A.M. Strong, R.J. Sawicki, W. Hoffman and S.D. Jewell. 1994.

Relationships among wading bird foraging patterns, colony locations, and

hydrology in the Everglades. Pp. 615-657 in, Everglades. The ecosystem and

its restoration (S.M. Davis and J.C. Ogden, eds.). St. Lucie Press, Delray

Beach, FL.

Bennetts, R.E., M.C. Collopy and J.A. Rodgers, Jr. 1994. The snail kite in the

Florida Everglades: a food specialist. Pp. 507-532 in, Everglades. The

ecosystem and its restoration (S.M. Davis and J.C. Ogden, eds.). St. Lucie

Press, Delray Beach, FL.

Bodle, M.J., A.P. Ferriter and D.D. Thayer. 1994. The biology, distribution, and

ecological consequences of Melaleuca quinquenervia in the Everglades. Pp.

341-355 in, Everglades. The ecosystem and its restoration (S.M. Davis and

J.C. Ogden, eds.). St. Lucie Press, Delray Beach, FL.

Browder, J.A. 1976. Water, wetlands, and Wood Storks in southwest Florida.

Ph.D. dissertation, Univ. Florida. Gainesville, FL. 406 pp.

Browder, J.A., P.J. Gleason, and D.R. Swift. 1994. Periphyton in the Everglades:

spatial variation, environmental correlates, and ecological implications. Pp.

379-416 in, Everglades. The ecosystem and its restoration (S.M. Davis and

J.C. Ogden, eds.). St. Lucie Press, Delray Beach, FL.

Courtenay, W.R., Jr. 1994. Non-indigenous fishes in Florida. Pp. 57-63 in, An

assessment of invasive non-indigenous species in Florida’s public lands (D.C.

Schmidt and T.C. Brown, eds.). Tech. Report TSS-94-100, Bureau of Aquatic

Plant Management, FDEP, Tallahassee, FL

Craighead, F.C., Sr. 1968. The role of the alligator in shaping plant communities

and maintaining wildlife in the southern Everglades. Florida Naturalist 41:

2-7, 69-74, 94.

Craighead, F.C., Sr. 1971. The trees of south Florida. Vol. 1. The natural

environments and their succession. Univ. Miami Press, Coral Gables, FL
67

Davis, S.M. 1994. Phosphorus inputs and vegetation sensitivity in the Everglades.

Pp. 357-378 in, Everglades. The ecosystem and its restoration (S.M. Davis

and J.C. Ogden, eds.). St. Lucie Press, Delray Beach, FL.

Davis, S.M. and J.C. Ogden. 1994. Towards ecosystem restoration. Pp. 769-796 in,

Everglades. The ecosystem and its restoration (S.M. Davis and J.C. Ogden,

eds.). St. Lucie Press, Delray Beach, FL.

Davis, S.M., L.H. Gunderson, W.A. Park, J. Richardson, and J. Mattson. 1994.

Landscape dimension, composition, and function in a changing Everglades

ecosystem. Pp. 419-444 in, Everglades. The ecosystem and its restoration

(S.M. Davis and J.C. Ogden, eds.). St. Lucie Press, Delray Beach, FL.

DeAngelis, D.L. 1994. Synthesis: Spatial and temporal characteristics of the

environment. Pp. 307-320 in, Everglades. The ecosystem and its restoration

(S.M. Davis and J.C. Ogden, eds.). St. Lucie Press, Delray Beach, FL.

DeAngelis, D.L. and P.S. White. 1994. Ecosystems as products of spatially and

temporally varying driving forces, ecological processes, and landscapes. Pp.

9-27 in, Everglades. The ecosystem and its restoration (S.M. Davis and J.C.

Ogden, rds.). St. Lucie Press, Delray Beach, FL.

Duever, M.J., J.F. Meeder, L.C. Meeder and J.M. McCollom. 1994. The climate of

south Florida and its role in shaping the Everglades ecosystem. Pp. 225-248

in, Everglades. The ecosystem and its restoration (S.M. Davis and J.C.

Ogden, eds.). St. Lucie Press, Delray Beach, FL.

Fennema, R.J., C.J. Neidrauer, R.A. Johnson, W.A. Perkins, and T.K. MacVicar.

1994. A computer model to simulate natural south Florida hydrology. In:

S.M. Davis and J.C. Ogden (eds.), Everglades. The ecosystem and its

restoration. St. Lucie Press, Delray Beach, FL.

Fleming, D.M. 1991. Wildlife ecology studies. 1991 annual reports. South Florida

Research Center, Everglades National Park, Homestead, FL. Vol. 1, sect. 5.

Frederick, P.C. and M.G. Spalding. 1994. Factors affecting reproductive success of

wading birds (Ciconiiformes) in the Everglades ecosystem. Pp. 659-691 in,

Everglades. The ecosystem and its restoration (S.M. Davis and J.C. Ogden,

eds.). St. Lucie Press, Delray Beach, FL.

Frederick, P.C., M.G. Spalding, M.S. Sepulveda, G. Williams, S. Bouton, H. Lynch,

J. Arrecis, S. Loerzel and D. Hoffman. 1997. Effects of environmental

mercury exposure on reproduction, health and survival of wading birds in the
68

Florida Everglades. Final Report. Department Wildlife Ecology and

Conservation. Univ. Florida, Gainesville, FL 126 pp.

Gunderson, L.H. 1994. Vegetation of the Everglades: determinants of community

composition. Pp. 323-340 in, Everglades. The ecosystem and its restoration

(S.M. Davis and J.C. Ogden, eds.). St. Lucie Press, Delray Beach, FL

Gunderson, L.H. and W.F. Loftus. 1993. The Everglades. Pp. 199-255 in, Biotic

communities of the southeastern United States (W.H. Martin, S.G. Boyce and

A.C. Echternacht, eds.). John Wiley & Sons, New York, NY.

Gunderson, L.H. and J.R. Snyder. 1994. Fire patterns in the southern Everglades.

Pp. 291-305 in, Everglades. The ecosystem and its restoration (S.M. Davis

and J.C. Ogden, eds.). St. Lucie Press, Delray Beach, FL.

Hoffman, W., G.T. Bancroft and R. J. Sawicki. 1994. Foraging habitat of wading

birds in the water conservation areas of the Everglades. Pp. 585-614 in,

Everglades. The ecosystem and its restoration (S.M. Davis and J.C. Ogden,

eds.). St. Lucie Press, Delray Beach, FL.

Kushlan, J.A. 1979. Feeding ecology and prey selection in the white ibis. Condor

81: 376-389.

Lange, T.R., H.E. Royals and L.L. Connor. 1994. Mercury accumulation in

largemouth bass (Micropterus salmoides) in a Florida lake. Arch. Environ.

Contam. Toxicol. 27: 466-471.

LaRoche, F.B. and A.P. Ferriter. 1992. The rate of expansion of melaleuca in south

Florida. Jour. Aquatic Plant Management 30: 62-65.

Light, S.S. and J.W. Dineen. 1994. Water control in the Everglades: a historical

perspective. Pp. 47-84 in, Everglades. The ecosystem and its restoration

(S.M. Davis and J.C. Ogden, eds.). St. Lucie Press, Delray Beach, FL.

Loftus, W.F. and A. Eklund. 1994. Long-term dynamics of an Everglades smallfish assemblage. Pp. 461-483 in, Everglades. The ecosystem and its

restoration (S.M. Davis and J.C. Ogden, eds.). St. Lucie Press, Delray Beach,

FL.

Loftus, W.F., J.D. Chapman and R. Conrow. 1990. Hydrological effects on

Everglades marsh food webs, with relation to marsh restoration efforts. Pp.

1-22 in, Fisheries and coastal wetlands research (G.Larson and M. Soukup,

eds.). Proceedings of a 1986 conference on science in National Parks. Ft.

Collins, CO.
69

Loftus, W.F., R.A. Johnson and G. Anderson. 1992. Ecological impacts of the

reduction of groundwater levels in the rocklands. Pp. 199-208 in,

Proceedings of the First International Conference of Groundwater Ecology

(J.A. Stafford and J.J. Simon, eds.). American Water Resources Association,

Bethesda, MD.

McVoy, C., W. Park and J. Obeysekera. In Prep.

Mazzotti, F.J. and L. A. Brandt. 1994. Ecology of the American Alligator in a

seasonally fluctuating environment. Pp. 485-505 in Everglades. The

ecosystem and its restoration (S.M. Davis and J.C. Ogden, eds.). St. Lucie

Press, Delray Beach, FL.

Myers, R.L. 1983. Site susceptibility to invasion by the exotic tree Melaleuca

quinquenerva in south Florida. Jour. Applied Ecology, 20: 645-658.

Ogden, J.C. 1994. A comparison of wading bird nesting colony dynamics (1931-

1946 and 1974-1989) as an indication of ecosystem conditions in the southern

Everglades. Pp. 533-570 in, Everglades. The ecosystem and its restoration

(S.M. Davis and J.C. Ogden, eds.). St. Lucie Press, Delray Beach, FL.

Robertson, W.B., Jr. and P.C. Frederick. 1994. The faunal chapters: context,

synthesis, and departures. Pp. 709-737 in, Everglades. The ecosystem and its

restoration (S.M. Davis and J.C. Ogden, eds.). St. Lucie Press, Delray Beach,

FL

Roelke, M.E., D.P. Schultz, C.F. Facemire and H.E. Royals. 1991. Mercury

contamination in Florida panthers. Report of the Florida Panther Technical

Subcommittee to the Florida Panther Interagency Committee.

Science Sub-group. 1993. Federal objectives for the south Florida restoration. The

Science Sub-group of The South Florida Management and Coordination

Working Group.

Spalding, M.G. and D.J. Forrester. 1991. Effects of parasitism and disease on the

nesting success of colonial wading birds (Ciconiiformes) in southern Florida.

Florida Game and Fresh Water Fish Commission, Nongame Wildlife

Program. Report NG88-008. Tallahassee, FL. 106 pp.

Stober, J., D. Scheidt, R. Jones, K. Thornton, R. Ambrose and D. France. 1996.

South Florida ecosystem assessment. Monitoring for adaptive management:

Implications for ecosystem restoration. Interim report. Environmental

Protection Agency EPA 904-R-96-008.

71

VI. MARL PRAIRIE/ROCKY GLADES CONCEPTUAL MODEL

A. PREPARER

Steven M. Davis, South Florida Water Management District, West Palm

Beach, Florida.

B. INTRODUCTION

In 190,000 ha of higher-elevation, freshwater marshes on either side of Shark

River Slough, water levels typically drop below the ground surface each year (Davis

et al., 1994). Consequently peat accretion is inhibited, and substrates consist either

of marl produced by periphyton mats, or exposed limestone bedrock with solution

holes (Gleason, 1972). The landscape consists of a mosaic of wet prairie, sawgrass,

tree island, and tropical hammock communities that support a high diversity of

plant species (Olmstead and Loope, 1984). The ephemeral characteristics of the

hydrology of marl prairies and rocky glades pose stresses to the wetland animal

communities regarding survival through the dry season when standing water is

usually absent (Loftus and Eklund, 1994; Loftus et al., 1990). In that respect, the

American alligator represents a keystone species in the marl prairie/rocky glades

because alligator holes provide important refugia for aquatic fauna during dry

periods (Craighead, 1968).

C. EXTERNAL DRIVERS AND ECOLOGICAL STRESSORS

The dominant sources of stressors on the marl prairie/rocky glades ecosystem

are water management practices and agricultural and urban development (refer to

the figures on pages 82 and 83). The water management system has altered

hydropatterns of remaining wetlands primarily through the lowering of water

tables and through the unseasonable regulatory release of water from upstream

(Van Lent et al., 1993). A network of canals and levees, particularly the C-111

system, has resulted in the compartmentalization of the remaining marl

prairie/rocky glades wetland system. The canals and pump stations are conduits for

inputs of nutrients, mercury, other toxins, and exotic fishes into the remaining

wetland system

Agricultural and urban development represents a direct loss of spatial extent

of the marl prairie/rocky glades wetland system through conversion of marshes to

farmland and housing (Davis et al., 1994). Development provides potential sources

of nutrients, mercury and other toxins to the natural system via runoff water that

may need to be captured and stored for hydropattern restoration.

 Shortened hydroperiod (duration of uninterrupted flooding). Much of the marl

prairie/rocky glades remains dry most of an average year under present conditions,
72

compared to a duration of uninterrupted flooding averaging nine months predrainage as indicated by the Natural System Model (Van Lent et al., 1993;

Fennema et al., 1994; VanZee, in preparation). Shortened hydroperiod directly

affects ecosystem food webs through the 1) decrease in diatoms, desmids, and other

green algae associated with the periphyton mat and the related reduction of the

primary production food base (Browder et al., 1994), 2) reduced secondary

production and altered species composition of fish based on comparisons of long and

short hydroperiod sites in Shark River Slough (Loftus and Eklund, 1994; Loftus et

al., 1990; Turner et al., 1999 in press) and possibly crayfish, 3) reduced pupolation

density and altered species composition of small herpetofauna, based on abundance

in short-hydroperiod habitats in relation to rainfall and water levels on Big Pine

Key (Diffendorfer et al., in preparation; Dalrymple, 1987; Dalrymple, personal

communication), 4) loss of the potential for hypothetical wading bird and wood stork

foraging during the early dry season (Fleming, personal communication in Ogden,

1994), and 5) decline in the American alligator population in the landscape where it

was once abundant (Craighead, 1968; Mazzotti and Brandt, 1994). Shortened

hydroperiod directly affects plant community and habitat structure, in addition to

the already noted alteration of the periphyton mat and filling in of alligator holes,

by enabling the invasion of woody plants such as willow and exotics such as

Brazilian pepper and melaleuca in a naturally herbaceous landscape (Armentano et

al., 1995; Jones and Doren, 1997), and by altering the plant species composition of

macrophyte vegetation toward more terrestrial plant species (Hofstetter and

Hilsenbeck, 1980; Hilsenbeck et al., 1979). Shortened hydroperiod indirectly affects

plant community and habitat structure through altered fire regime: hotter and

more severe fires due to the prolonged dry season (Gunderson and Snyder, 1994)

further alter herbaceous plant species composition and reduce plant community

heterogeneity by burning out hardwood hammock tree islands (Loope and Urban,

1980). Shortened hydroperiod, in combination with increased drought severity,

threatens Cape Sable sparrow reproductive success and survival in the marl

prairies to the east of Shark River Slough by increasing the frequency and severity

of nest-destroying fires, or by enabling the invasion of woody plants in the absence

of fire (Pimm, 1995)

Increase in drought severity. In a system where water levels typically fall below the

ground surface each year, remnant pools of standing water in solution holes and

alligator holes during the dry season are critical to the survival of aquatic and

amphibious fauna such as fish, crayfish, herpetofauna, and alligators (Loftus et al.,

1992; Mazzotti and Brandt, 1994; Diffendorfer et al., in prep.; Dalrymple, 1987;

Dalrymple, personal communication). When dry conditions are prolonged, and

minimum water levels during a dry season fall below the bottoms of the solution

holes and alligator holes, the function of these dry season refugia is diminished or

eliminated.
73

Wet season water level reversals. Field observations indicate that when water

depth drops to less than 0.2 feet during a period of flooding, aquatic fauna

population densities decline, survivors retreat to refugia in solution holes or

alligator holes, and population recovery is slowed (Loftus and Eklund, 1994; Loftus,

personal communication).

Drying pattern reversals due to upstream regulatory releases. Reversals in the

seasonal timing of water level recessions due to regulatory releases from WCA3A

detrimentally affect nesting density and success of the endangered Cape Sable

sparrow (Pimm, 1995). Drying pattern reversals have been particularly

detrimental to sparrow nesting in the marl prairies to the west of Shark River

Slough, in an area that is affected by water releases from the S-10 A and B outflow

gates from Water conservation Area 3A. Unnaturally high flow volumes to the west

of Shark River Slough, rather than via Northeast Shark River Slough under NSM

conditions, exacerbates the problem of the sparrow. The Cape Sable sparrow

depends on predictably dry conditions during its spring nesting period which

corresponds to the late dry season (Pimm, 1995). Drying pattern reversals and

their effects on the sparrow are analyzed as an endangered species issue.

Increased nutrient inputs. Agricultural and urban runoff water from the lower east

coast that may be needed for hydropattern restoration is anticipated to bring with it

increased inputs of phosphorus, nitrogen and other nutrients. Cattail has already

dominated portions of Taylor Slough downstream of the Park road since pump

station S332 commenced to deliver canal water to Taylor Slough (Davis, personal

observation). A probable effect of nutrient additions to the marl prairie/rocky

glades would be the loss in the periphyton mat and the dominance of blue-green

algae, with resulting reductions in the primary production food base and reductions

in marl substrate accretion (Browder, 1994).

Compartmentalization. The levees that bound and dissect the marl prairie/rocky

glades on the east side of the Everglades, and the C-111 system in particular, may

create barriers that genetically isolate populations of aquatic fauna, as

demonstrated by the genetic structure of mosquitofish and grass shrimp in the

Water Conservation Areas compared to Everglades National Park (Trexler et al., in

review). Canals adjacent to the levees provide corridors of permanently flooded,

deep-water habitat that would not otherwise occur in the marl prairie/rocky glades.

These corridors allow the expansion of exotic and higher trophic level fishes into

areas where they could not survive naturally (Howard et al., 1995). The prevalence

of higher trophic fishes in the canals diminishes the value that this habitat might

otherwise serve as a dry season refugium for aquatic and amphibious fauna.

Loss of spatial extent. The conversion of eastern portions of the marl prairie/rocky

glades to agriculture and housing has reduced the spatial extent of primary

production that provides the base of the food webs of fish, crayfish, herpetofauna,
74

alligators, and wood storks and wading birds. The spatial loss of the aquatic

habitat has also directly reduced populations of these organisms. The extent of loss

in production and aquatic habitat is directly proportional to the spatial extent of the

area that has literally been taken out of production as wetland (Davis et al., 1994).

Loss of spatial extent may have reduced the range of habitat options that are

available at any given time for faunal populations (DeAngelis and White, 1994),

including nesting by the Cape Sable sparrow nesting (Pimm, 1995) and potential

foraging by wood storks and wading birds (Fleming, personal communication in

Ogden, 1994).

Introduction of exotic plants. Melaleuca, Brazilian pepper and Australian pine are

the three most widespread exotic plant species that have spread into native plant

communities of the marl prairie/rocky glades, due to their introduction combined

with shortened hydroperiod which allows their expansion (Olmstead and Loope,

1984; Armentano et al., 1995; Jones and Doren, 1997).

Burning practices. Burning practices, particularly from illegally set fires, have

shifted the seasonal timing of fires from the May-June early wet season, due to

lightning strikes, to the winter-spring dry (Gunderson and Snyder, 1994). Fire

severity has been exacerbated by shortened hydroperiod and increased drought

severity. The road access provided by levees contributes to fire ignition by humans

by allowing arsonists access to the marshes and by creating a source of ignition

from vehicle sparks (Burzycki, personal communication). Intense, dry season fires

have burned out hardwood hammock tree islands in the marl prairies and rocky

glades to the east of Shark River Slough. Fire hazard due to over-drainage also

poses a threat to Cape Sable sparrow nesting success and survival in the marl

prairies to the east of Shark River Slough.

D. ECOLOGICAL EFFECTS OF WATER MANAGEMENT: CRITICAL

LINKAGES BETWEEN STRESSORS AND ATTRIBUTES

Four major causal pathways appear to account for the decline in the ecological

attributes of the marl prairies and rocky glades as a result of water managementinduced stressors. Each pathway, other than that of the Cape Sable sparrow, has

been most severely impacted in the marl prairies and rocky glades to the east of

Shark River Slough.

1) The periphyton mat has been altered in ways that reduce its contribution to

aquatic food chains and marl formation. Diatom, desmid and other green algal

components of the periphyton mat have decreased as a result of severely

shortened hydroperiod. The mat is lost entirely and replaced by non-matforming bluegreen algae as a result of nutrient additions in pumped inflow

water.
75

2) The American alligator has been mostly eliminated, due to severely shortened

hydroperiod and lowered water table, from the landscape where it was once most

abundant. Alligator holes in depressions in the limestone bedrock have silted in

with organic debris and no longer function as dry season refugia for aquatic

fauna. Their loss has decreased the heterogeneity of vegetation habitat and

micro-topography.

3) The marsh fishes, herpetofauna and probably other aquatic fauna of the marl

prairies and rocky glades can maintain only diminished population density and

secondary production given the shortened hydroperiod and lowered water table,

the altered periphyton mat, the loss of dry season refugia in alligator and

solution holes, and the physiological stress due to wet season water level

reversals.

4) The vegetation community mosaic has lost heterogeneity and changed in species

composition due to community shift due to shortened hydroperiod and lowered

water table, the loss of alligator holes, tropical hammock tree island burn-out

due to altered fire regime, woody plant invasion, and the spread of exotic plants.

5) The Cape Sable sparrow is limited in reproductive success and population size

by drying pattern reversals from upstream water releases to the west of Shark

River Slough. To the east of the Slough, the sparrow is threatened by shortened

hydroperiod and lowered water table, which result either in increased fire

frequency and severity, or in woody plant invasion in the absence of fire.

E. ECOLOGICAL ATTRIBUTES AND MEASURES

Periphyton mat. The periphyton mat of attached algae plays a key role as a food

chain base (Browder et al., 1994), as habitat structure (Loftus, personal

communication), and as the source of marl sediments (Gleason, 1972) in the marl

prairies and rocky glades. Species composition of the periphyton is a sensitive

indicator of both hydrologic conditions and water quality (Browder et al., 1994).

Severely reduced hydroperiod in the marl prairies and rocky glades results in a

decrease in the diatom, desmid and other green algal species in the periphyton mat,

leaving mostly mat-forming blue-green algae that are less valuable as food sources

for consumers (Browder et al., 1994). Nutrient enrichment, that may accompany

the inflow of canal water, results in the loss of mat-forming species altogether and

replacement by non-mat-forming blue-green algae (Browder et al., 1994). Measures

of periphyton communities that indicate recovery due to lengthened hydroperiod

and potential impacts of nutrient enrichment include aerial coverage, species

composition, biomass, and primary productivity rates of periphyton mats of the

marl prairies and rocky glades.
76

Marl substrate. Mat-forming calcitic algae deposit the marl substrate that is

characteristic of the marl prairies (Gleason, 1972; Browder et al., 1994), where

hydroperiod is too short for the accretion of peat soils. Most of the plant

communities described by Olmstead and Loope (1984) and Gunderson (1994) for

this region are supported by the marl soils. The dependence of marl deposition on

the productivity of the calcitic periphyton mat indicates that the negative impacts of

shortened hydroperiod and nutrient enrichment that are noted above for the

periphyton mat would also adversely affect the rate of marl accretion. Measures of

the marl substrate that should reflect responses to hydroperiod, and nutrients and

restoration efforts include marl distribution and depth and marl accretion rate.

Vegetation mosaic and diversity. A unique feature of the marl prairies and rocky

glades is the extremely high species richness of their flora. Within the mosaic of

low-stature graminoid communities, sawgrass and muhly grass dominate, although

more than 100 species of mostly herbaceous plants have been reported (Olmstead

and Loope, 1984). The higher-elevation tropical hammock and pine forests that

occur as islands within the prairie landscape support flora of West Indian origin

that are unique to south Florida, other than the Keys, and that contain the highest

number of rare and threatened plant species in south Florida (Gunderson, 1994).

The plant communities of the marl prairies and rocky glades have been impacted by

man through drainage, exotic invasion and burning practices. Tree island burn-out

(Loope and Urban, 1980), shifts to more terrestrial plant associations in the

graminoid communities (Hofstetter and Hilsenbeck, 1980; Hilsenbeck et al., 1979),

loss of the vegetation heterogeneity associated with the micro-topography of

alligator holes (Craighead, 1968), and woody plant invasion of the graminoid

communities (Armentano, et al., 1995) have accompanied shortened hydroperiods

and lowered water tables. These effects are exacerbated by the spread of exotic

trees (Olmstead and Loope, 1984; Armentano et al., 1995; Jones and Doren, 1997)

and by dry season burning practices (Gunderson and Snyder, 1994), particularly

from illegally-set fires. Measures of vegetation responses to stressors and

restoration efforts in the marl prairies and rocky glades include the spatial cover

and species composition of graminoid and tree island communities .

Cape Sable sparrow. The Cape Sable seaside sparrow is endemic to the marl

marshes of the southern Everglades, where it is listed as an endangered species.

The three major breeding colonies of the sparrow include the western subpopulation in marl marshes to the west of Shark River Slough, the eastern subpopulation to the east of mid Shark River Slough, and the central sub-population to

the east of the southwest Slough (Pimm, 1995). The sparrow nests during the dry

season months, mostly during April–June, in sites free of standing shrubs or trees.

The construction of nests approximately 10 cm above the ground at the bases of

tussocks of muhly grass and other marsh graminoids makes the sparrow vulnerable

both to spring wildfires and unseasonable water depth increases (dry season water

level reversals). Successful nesting requires approximately 45 days of nearly dry
77

conditions without fire. Dry season water level reversals have been a particular

problem to the nesting success of the western sub-population, where regulatory

water releases through the S-12 A and B outflow gates of Water Conservation Area

3A contribute to the reversals and subsequent nest drowning (Pimm, 1995). The

eastern sub-population occupies an over-drained region of marl prairie subject to

spring wildfires or to invasion of woody vegetation. The central population is the

most stable because of its remote location, although it is vulnerable to infrequent,

naturally-occurring dry season fires. Measures of sparrow populations that reflect

responses to stressors and restoration measures include nesting distribution,

density and success.

Aquatic fauna. The aquatic fauna of freshwater Everglades marshes include the

myriad of small fishes, amphibians, reptiles, crayfish, grass shrimp, snails,

amphipods and other invertebrates that must play enormously important roles in

food webs, nutrient cycles and energy transfers from primary consumers to the

highest trophic levels in the ecosystem. Considering their prevalence, little is

known about the life histories, population dynamics and ecosystem roles of the

aquatic fauna, particularly regarding their ecology in the marl prairies and rocky

glades.

Population density of small marsh fishes in the Everglades is directly related

to the duration of uninterrupted flooding, and maximum densities are reached only

after several years of continual surface water (Loftus and Eklund, 1994; Loftus et

al., 1990; Turner et al., 1999 in press). The herpetofauna of the short-hydroperiod

wetlands reach higher abundance in habitats with longer hydroperiods and higher

water tables (Diffendorfer et al., in preparation.;Dalrymple, 1987; Dalrymple,

personal communication). These studies provide evidence that the marl prairies

and rocky glades represent stressful environments for aquatic fauna. These

marshes typically dried for an average of three months each year under predrainage conditions, and extensive areas of this landscape are only wet for an

average of three months each year under present conditions (Van Lent et al., 1993;

Fennema et al., 1994; VanZee, in preparation). Survival of aquatic fauna during

dry periods in the marl marshes and rocky glades depends in the availability of

refugia in solution holes, alligator holes, and under periphyton mats which may

retain aquatic or moist environments during times when surface water is absent

(Loftus et al., 1990; Loftus et al., 1992; Dalrymple, 1987; Diffendorfer et al., in

preparation; Loftus, personnal communication; Dalrymple, personal

communication).

The small marsh fishes provide the major prey base for the wading birds,

wood storks and roseate spoonbills that forage and nest in the Everglades (Ogden,

1994; Bjork and Powell, 1994). Crayfish are particularly important in the diets of

white ibis. The marl marshes may provide foraging opportunities for wading birds

early in the dry season when deeper water depths in the lower-elevation sloughs are
78

unsuitable for efficient feeding (Fleming, personal communication in Ogden, 1994).

However, the relationship of such foraging opportunities to the advent of nesting

and the subsequent reproductive success of wading birds is still hypothetical.

Reduced production and altered species composition of fishes and

herpetofauna in the marl prairies and rocky glades result from 1) the direct impact

of shortened hydroperiod (Loftus and Eklund, 1994; Turner et al., 1999 in press;

Diffendorfer et al., in preparation; Dalrymple, 1987; Dalrymple, personnal

communication), 2) indirect impacts of shortened hydroperiod via the loss of the

periphyton mat and a reduced primary production food base (Browder et al., 1994),

3) the prolongation of dry conditions and reduction in dry season minimum water

levels via the diminished dry season refugium function of solution holes, alligator

holes and periphyton mats (Loftus et al., 1992; Diffendorfer et al, in preparation;

Dalrymple, 1987; Loftus, personal communication; Dalrymple, personal

communication), 4) reduction in spatial extent of wetland habitat (Davis et al.

1994), 5) increased nutrient inputs via the loss of the periphyton mat and a

resulting reduction in the primary production food base (Browder et al., 1994), and

6) compartmentalization via levees as potential genetic barriers (Trexler et al., in

review) and via canals as corridors for the spread of exotic and higher trophic level

fishes (Howard et al., 1995). Measures of aquatic fauna populations that reflect

responses to the above stressors include density and species composition of fishes

and density of representative herpetofauna and invertebrate species.

American alligator. Prior to drainage, the American alligator was more abundant

in the marl marshes and rocky glades than in the deeper central marshes of the

Everglades(Craighead, 1968; Mazzotti and Brandt, 1994). By excavating sediments

from large solution holes in the limestone bedrock, the alligator created aquatic

habitats in which it, and many other groups of aquatic fauna, survived the few

months of dry conditions that occurred most years (Craighead, 1968). In the

excavation of holes and the creation of nest mounds, the alligator created a microtopography that supported aquatic as well as terrestrial flora and fauna (Craighead,

1971; Kushlan, 1974). The mounds provided nesting sites not only for the alligator,

but also for other marsh reptiles (Deitz and Jackson, 1979; Kushlan and Kushlan,

1980). In these ways, the alligator was a keystone species in the marl prairies and

rocky glades because of the aquatic habitats and dry season refugia it created. The

high abundance of alligators in the marl marshes and rocky glades, in comparison

to lower densities in Shark River Slough, was attributed to high water depths in the

Slough that would have been less suitable for alligators prior to drainage (Ogden,

1976; Mazzotti and Brandt, 1994). Given the shortened hydroperiods and lowered

water tables in the Everglades due to drainage (Van Lent et al., 1993;Fennema et

al., 1994; VanZee, in preparation), the alligator has mostly abandoned the marl

marshes and rocky glades, and today the distribution of the alligator in the

southern Everglades has shifted to Shark River Slough (Craighead, 1968; Mazzotti

and Brandt, 1994). With the loss of alligators from the marl prairies and rocky
79

glades, the habitats and dry season refugia that were provided by its holes and nest

mounds have also diminished.

Abandonment by the alligator of the marl prairie/rocky glades has resulted

from shortened hydroperiod and increased drought duration and severity, in that

the alligator does not occupy habitat that is predominantly dry. Cumulative effects

leading to the reduced production of aquatic fauna at all trophic levels probably

have also contributed to the demise of the alligator in the mark prairies, since the

alligator depends upon these organisms for food during various stages of its life

history. The reduced population of alligators has in turn allowed depressions that

were previously alligator holes to fill in, further diminishing the populations of

aquatic fauna upon which upon which the alligator fed. The restoration of

hydroperiods and water tables to those approaching pre-drainage conditions in the

marl prairies and rocky glades is anticipated to allow the alligator to re-colonize

this region, as measured by density and distribution by age class and density and

distribution of alligator holes.

Wading bird and wood stork early dry season foraging. Prior to drainage, the marl

prairie/rocky glades may have provided shallow-water foraging opportunities for

wading birds and wood storks early in the dry season, at a time when water was too

deep to allow successful foraging in Shark River Slough (Fleming, personal

communication in Ogden, 1994). The early dry season foraging opportunities may

have contributed to the early formation of nesting colonies, and thereby the nesting

success of the birds, but this would have been dependent upon an adequate prey

base of aquatic fauna in the marl prairie/rocky glades. The relationship of such

foraging opportunities to the advent of nesting and subsequent reporductive success

of wading birds and wood storks is hypothetical. Under present conditions, both

shortened hydroperiod (Van Lent et al.,1993; Fennema et al., 1994; VanZee, in

preparation)and loss of spatial extent Davis et al., 1994) have directly reduced the

area of feeding habitat and the range of feeding options available to the wading

birds and wood storks in the marl prairie/rocky glades (Ogden 1994). Cumulative

effects leading to the reduced production of fish and other aquatic fauna, as noted

above, have further reduced the prey base for wading birds and wood storks. As a

result, the value of the marl prairies and rocky glades for wading bird and wood

stork foraging appears to be very limited today in comparison to pre-drainage

conditions (Fleming, personal communication in Ogden, 1994). Measures of wading

bird and wood stork utilization of the marl prairie/rocky glades that reflect

responses to the above stressors include the seasonal distribution and abundance of

foraging birds.
80

F. HYDROLOGIC PERFORMANCE MEASURES FOR RESTORATION

Recovery of the ecological values of the marl prairie/rocky glades is

dependent upon the reversal of the cumulative effects of decades of drainage,

detrimental fire regimes, and exotic plant invasion. The key role of the C&SF

Comprehensive plan in the restoration of the marl prairie/rocky glades ecosystem is

to provide for the quantity, timing, and distribution of water deliveries that contain

low levels of nutrients and toxins. Three priority hydrologic performance measures

for the ecological restoration of the marl prairie/rocky glades, in order of priority,

are the duration of uninterrupted flooding, drought severity as measured by the

duration of dry conditions, and the number of wet season depth reversals.

The duration of a uninterrupted flood events is the highest priority

hydrologic performance measure for ecological restoration in the marl prairie/rocky

glades. It is identified as the single hydrologic parameter that affects all the

ecological values of that region. Only flood events when the mean water depth

equals or exceeds 0.2 feet are measured because depths less than 0.2 feet have been

observed to impair the establishment of populations of aquatic organisms. The

mean duration of all such flood events during the 31-year period of record is the

performance measure. The target is the mean duration of flooding indicated by

NSM45F. Duration of uninterrupted flooding given a weighting of 3 when averaged

with the other performance measures.

Drought severity is the second most important hydrologic variable for

ecological restoration in the marl prairie/rocky glades. Drought severity is most

clearly indicated by 2X2 model output as the duration of dry conditions. Dry events

are defined as times when the water level drops either to zero or a negative value

below the ground surface. Two dry events that are separated by a flood event when

the water level rises to less than 0.2 feet above the ground surface are grouped as

one dry event because such a minor flood event has been observed to not support

the establishment of populations of aquatic organisms. In that case, the duration of

the dry event is calculated as the sum of the two dry events and the intermittent

flood event. The performance measure is the mean duration of all dry events during

the 31-year period of record. The target is the mean duration of dry events

indicated by NSM45F. The mean duration of dry events is given a weighting of two

when averaged with the other performance measures.

Wet season water level reversal is the third most important hydrologic

variable for ecological restoration in the marl prairie/rocky glades. A wet season

water level reversal is defined as an incident during a period of flooding when water

depth recedes to less than 0.2 feet, but then rebounds to greater than 0.2 feet,

without the marsh drying completely. A reversal is distinguished from a dry period

in that during a reversal, water depth does not drop to or below the ground surface.

The performance measure is the total number of reversals during the 31-year period
81

of record. The target is not to exceed the number of reversals indicated by NSM45F.

The number of wet season reversals is given a weighting of one when averaged with

the other performance measures.

Ecological attributes and indicators of restoration success in the rockland

marl marsh that are linked to the above hydrologic performance measures in the

conceptual model include 1) re-colonization and population resurgence by American

alligators and a subsequent increase in the number of occupied alligator holes to

serve as dry season refugia for aquatic fauna and to increase habitat heterogeneity,

2) increased population density of aquatic fauna, 3) increased seasonal abundance

and foraging activity of wading birds and wood storks, 4) enhanced production and

community composition of periphyton, 5) accelerated accretion of marl substrate, 6)

increased nesting success and population size of Cape Sable seaside sparrows, and

7) persistence and resilience of highly diverse macrophyte and tree island plant

communities.

A performance measure that is generic to the conceptual models of all

physiographic regions of south Florida is the input and bio-accumulation of mercury

and other toxins. Potential inputs of mercury and pesticides in agricultural and

urban runoff water that may be needed for hydropattern restoration in the marl

prairie/rocky glades might result in reduced health, behavioral and physical

abnormalities, and loss of reproductive vigor of the fauna unless measures are

taken to restrict loads of these toxins in inflow water. Measures of faunal health

that reflect responses to the mercury and pesticide inputs include body burdens of

mercury and other toxins in representative species of fauna and the incidence of

physical and behavioral abnormalities in fauna.


84

G. REFERENCES

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Andrew on coastal and interior forests of southern Florida: Overview and

synthesis. Journal of Coastal Research Special Issue 2: 111-114.

Bjork, R.D. and G.V.N. Powell. 1994. Relations between hydrologic conditions and

quality and quantity of foraging habitat for roseate spoonbills and other

wading birds in the C-111 basin. National Audubon Society Final Report to

South Florida Research Center, Everglades National Park.

Browder, J.A., P.J. Gleason and D.R. Swift. 1994. Periphyton in the Everglades:

Spatial variation, environmental correlates, and ecological implications. In:

S.M. Davis and J.C. Ogden, eds. Everglades: The Ecosystem and Its

Restoration, St. Lucie Press, Delray Beach FL: 379-418.

Burzycki, G. Personal communication, Dade County Department of Environmental

Resource Management, Miami FL.

Busch, David E., William F. Loftus, and Oron L. Bass, Jr. 1998. Long-term

hydrologic effects on marsh plant community structure in the southern

Everglades. Wetlands 18: 230-241.

Craighead, F.C. 1968. The role of the alligator in shaping plant communities and

maintaining wildlife in the southern Everglades. Florida Naturalist 41: 2-7,

69-74, 94.

Craighead, F.C. 1971. The Trees of South Florida, Volume 1. University of Miami

Press, Coral Gables FL: 212pp.

Dalrymple, G.H. 1987. The herpetofauna of Long Pine Key in relation to

vegetation and hydrology. In: C. Szaro et al., tech. coordinators.

Management of Amphibians, Reptiles, and Small Mammals in North

America, USDA Forest Service General Technical Report RM-166R, Ft.

Collins, CO.

Dalrymple, G. Personal communication, Everglades Research Group, Inc.,

Homestead, FL.

Davis, S.M., L.H. Gunderson, W.A. Park, J.R. Richardson and J.E. Mattson. 1994.

Landscape dimension, composition, and function in a changing Everglades

ecosystem. In: S.M. Davis and J.C. Ogden, eds. Everglades: The Ecosystem

and Its Restoration, St. Lucie Press, Delray Beach FL: 419-444.
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Davis, S.M. personal observation, South Florida Water Management District, West

Palm Beach FL.

DeAngelis, D.L., and P.S. White. 1994. Ecosystems as products of spatially and

temporally varying driving forces, ecological processes, and landscapes: A

theoretical perspective. In: S.M. Davis and J.C. Ogden, eds. Everglades:

The Ecosystem and Its Restoration, St. Lucie Press, Delray Beach FL: 9-27.

Deitz, D.C. and D.R. Jackson. 1979. Use of alligator nests by nesting turtles.

Journal of Herpetology 13: 510-512.

Diffendorfer, J.E., P.M. Richards, G.H. Dalrymple and D.L. DeAngelis. In

preparation. Using ecosystem and food webs in conservation; an example

from the herpetological assemblage.

Fennema, R.J., C.J. Neidrauer, R.A. Johnson, T.K. MacVicar and W.A. Perkins. A

computer model to simulate natural Everglades hydrology. In: Davis, S.M.

and J.C Ogden, eds. Everglades: The Ecosystem and Its Restoration, St.

Lucie Press, Delray FL: 249-289.

Gleason, 1972. The origin, sedimentation, and stratigraphy of a calcitic mud

located in the southern fresh-water Everglades. Ph.D. thesis, The

Pennsylvania State University, University Park PA.

Gunderson, L.H. 1994. Vegetation of the Everglades: Determinants of community

composition. In: S.M. Davis and J.C. Ogden, eds. Everglades: The

Ecosystem and Its Restoration, St.Lucie Press, Delray Beach FL: 323-340.

Gunderson, L.H. and J.R. Snyder. 1994. Fire patterns in the southern Everglades.

In: S.M. Davis and J.C. Ogden, eds. Everglades: The Ecosystem and Its

Restoration, St. Lucie Press, Delray Beach FL: 291-305.

Hilsenbeck, C.E., R.H. Hofstetter and T.R. Alexander. 1979. Preliminary synopsis

of major plant communities in the east Everglades area, vegetational maps

supplement, University of Miami Report, University of Miami, Miami FL:

36pp.

Hofstetter, R.H. and C.E. Hilsenbeck. 1980. Vegetational studies of the east

Everglades. Final Report to Dade County Planning Department, East

Everglades Resources Planning Project: 109pp.

Howard, K. S., W. F. Loftus, and J. C. Trexler. 1995. Seasonal dynamics of fishes

in artificial culvert pools in the C-111 basin, Dade County, Florida. Final
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Report to the U. S Army Corps of Engineers as Everglades N. P. Cooperative

Agreement #CA5280- 2-9024.

Jones, D.B. and R.F. Doren. 1997. Distribution, biology and control of Schinus

terebinthifolius in southern Florida with special reference to Everglades

National Park. In: J.H. Brock, M.Wade, P. Tysek and D. Green, eds. Plant

Invasions: Studies From North America and Europe, Backhuys Publishers,

Lyden, Netherlands: 81-93.

Kushlan, J.A. 1974. Observations on the role of the American alligator (Alligator

mississippiensis) in the southern Florida wetlands. Copeia 1974: 993-996.

Kushlan, J.A. and M.S. Kushlan. 1980. Everglades alligator nests: Nesting sites

for marsh reptiles. Copeia 1980: 930-932.

Loftus, W.F. Personal communication.

Loftus, W. F. and A. M. Eklund. 1994. Long-term dynamics of an Everglades fish

community. In: S.M. Davis and J.C. Ogden, eds. Everglades: The Ecosystem

and Its Restoration, St. Lucie Press, Delray Beach, FL: 461-483.

Loftus, W. F. and James A. Kushlan. 1987. Freshwater Fishes of Southern Florida.

Bulletin of the Florida State Museum, Biological Sciences 31: 147-344.

Loftus, W.F., J.D. Chapman and R. Conrow. 1990. Hydroperiod effects on

Everglades marsh food webs, with relation to marsh restoration efforts. In:

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The National Parks, July 13-18, 1986, U.S. National Park Service and the

George Wright Society: 1-22.

Loftus, W.F., R.A. Johnson, and G.H. Anderson. 1992. Ecological impacts of the

reduction of ground water levels in short-hydroperiod marshes of the

Everglades. In: J.A. Stanford and J.J. Simons eds. Proceedings of the First

International Conference on Ground Water Ecology. American Water

Resources Association, Bethesda, MA: 199-207.

Loope, L.L. and N.H. Urban. 1980. A survey of fire history and impact in tropical

hardwood hammocks in the east Everglades and adjacent portions of

Everglades National Park. Report T-592, South Florida Research Center,

Everglades National Park: 48pp.

Mazzotti, F.J. and L.A. Brandt. 1994. Ecology of the American alligator in a

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Ogden, J.C. 1976. Crocodilian ecology in southern Florida. In: Research in the

Parks: Transactions of the National Park Centennial Symposium, 1971.

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Ogden, J.C. 1994. A comparison of wading bird nesting colony dynamics (1931-

1946 and 1974-1989) as an indication of ecosystem conditions in the southern

Everglades. In: S.M. Davis and J.C. Ogden, eds. Everglades: The

Ecosystem and Its Restoration, St. Lucie Press, Delray Beach FL: 533-570.

Olmstead, I.C. and L.L. Loope. 1984. Plant communities of Everglades National

Park. In: P.J. Gleason, ed. Environments of South Florida: Past and

Present II, Miami Geological Society, Coral Gables FL.

Pimm, S.L. 1995. Population ecology of the Cape Sable Sparrow (Ammodramus

maritima mirabilis). Annual report 1995. Department of Ecology and

Evolutionary Biology, University of Tennessee, Knoxville TN: 57pp + 35

figures.

Trexler, J.C., W. F. Loftus, and O. L. Bass. 1996. Documenting the effects of

Hurricane Andrew on Everglades aquatic communities, Part I. In: J. C.

Trexler, L. Richardson, and K. Spitze eds. Effects of Hurricane Andrew on

the structure and function of Everglades aquatic communities. Final

Contract Report CA5280-3-9014 to Everglades National Park, Homestead,

FL: 6-136.

Trexler, J. C., W. F. Loftus, C. F. Jordan, J. Lorenz, and J. Chick. In Prep.

Empirical assessment of fish introductions in southern Florida: an evaluation

of contrasting views. Biological Invasions.

Trexler, J. C., K. L. Kand, and C. F. Jordan. In review. Metapopulations or patchy

populations: population structure of three species of aquatic animals from the

Everglades. Evolution.

Turner, A., J. C. Trexler, F. Jordan, S. J. Slack, P. Geddes, J. Chick, and W. F.

Loftus. In Press. Conservation of an ecological feature of the Florida

Everglades: pattern of standing crops. Conservation Biology.

Van Lent, T.A., R.A. Johnson and R.J. Fennema. 1993. Water management in

Taylor Slough and effects on Florida Bay. South florida Natural resources

Center Technical report 93-3: 97pp.
88

VanZee, R. In preparation. Natural system model version 4.5 documentation

report. Draft report, South Florida Water Management District, West Palm

Beach FL.
89

VII. MANGROVE ESTUARY TRANSITION CONCEPTUAL MODEL

A. PREPARER

Steven M. Davis, South Florida Water Management District, West Palm Beach, Fl.

B. INTRODUCTION

A brackish water ecotone of coastal bays and lakes, mangrove and

buttonwood forests, salt marshes, and tidal creeks separates Florida Bay from the

freshwater Everglades. The 24 km-wide ecotone adjoins the north shoreline of

Florida Bay between Highway Creek (US1) and Whitewater Bay, which delineate

the eastern and western boundaries of the mangrove estuary transition model.

Whitewater Bay is included in the model because it is influenced by the Shark River

drainage basin originating in the Everglades. The mangrove estuary transition is

characterized by a salinity gradient and mosaic that vary spatially with topography

and that vary seasonally and inter-annually with rainfall and freshwater flow from

the Everglades. Because of its location at the lower end of the Everglades drainage

basin, the mangrove estuary transition zone is potentially affected by upstream

water management practices that alter the freshwater heads and flows that drive

salinity gradients.

C. EXTERNAL DRIVERS AND ECOLOGICAL STRESSORS

Stressors on the mangrove estuary transition ecosystem and the drivers that

create them fall into five categories (refer to figures on pages 97 and 98). Sea level

rise is an important non-societal driver (Wanless et al., 1994) that is causing the

inland movement of marine conditions into the estuary transition zone (Meeder et

al., 1996). The inland movement of marine conditions due to sea level rise is

happening independent from other societal-driven stressors. The societal-driven

water management operations of the C&SF Project stress the transition zone

through reductions in the volume and duration of freshwater flow entering the zone

(McIvor et al., 1994) The introductions of exotic fishes and plants are other societaldriven drivers that have resulted in the dominance of the Mayan cichlid east of

Taylor Slough (Trexler et al., in prep.) and the invasion of Schinus and Colubrina

into mangrove forests (Armentano et al., 1995). The societal input and

bioaccumulation of mercury and other toxins pose a threat to faunal health at all

trophic levels in all south Florida ecosystems, including the mangrove estuary

transition, as discussed under generic issues.
90

D. ECOLOGICAL EFFECTS: CRITICAL LINKAGES BETWEEN STRESSORS

AND ECOLOGICAL ATTRIBUTES

The inland movement of marine conditions due to sea level rise and the

reduced input of freshwater due to water management work together to alter and

compress the salinity gradient of the mangrove estuary transition (Browder and

Moore, 1981). They both result in the landward movement of salinity, the loss of a

seasonal shift from moderate saline to oligohaline conditions, and occurrence of

hypersalinity within the transition zone during dry years. Both directly change

habitat structure by contributing to the siltation and mangrove encroachment of

tidal creeks (Meeder et al., 1996), to the extent that open water courses that were

described earlier this century are no longer recognizable (Glen Simmons, personal

communication).

The alteration and compression of the salinity gradient results in the loss of

the spatial overlap of salinity zones with shoreline habitat and nursery grounds

(Browder and Moore, 1981). Ecological values of the mangrove estuary transition

that depend on the overlap of salinity and habitat include the mangrove/salt marsh

vegetation mosaic, the resident mangrove fish assemblage, the wood stork and

roseate spoonbill, the American crocodile, spotted seatrout nursery grounds, pink

shrimp nursery grounds, and coastal lake vegetation and waterfowl.

E. ECOLOGICAL ATTRIBUTES AND MEASURES

Mangrove/salt marsh vegetation mosaic. The alteration and compression of the

salinity gradient potentially can affect the community cover, distribution, and

production of the mangrove forests (Rhizophora, Avicennia, Laguncularia, and

Conocarpus), salt marshes and tidal creeks of the mangrove estuary transition that

are documented by Welsh et al. (1995). Some mangrove forests in the transition

zone have experienced invasion by the exotic trees Schinus and Colubrina

(Armentano et al., 1995). Tidal creeks and adjacent salt marshes have been

encroached by red mangrove as described above. The invasion of the freshwater

marl marshes at the upstream end of the salinity gradient by red mangrove

(Meeder et al., 1996) corresponds to an accelerated rate of sea level rise. The

vegetation mosaic defines the habitats of the mangrove estuary transition zone.

The spatial distribution of those habitats, in combination with the salinity gradient

that overlays them, may determine the suitability of this region to sustain its

ecological attributes (Browder and Moore, 1981). The importance of this habitat

mosaic warrants the monitoring of the distribution and cover of vegetation

communities and tidal creeks in the mangrove estuary transition zone as efforts

proceed to restore freshwater inputs and salinity regimes.

Resident mangrove fish community. The resident fish community of sheepshead,

sailfin mollies, topminnows, rainwater killifish, and sunfish thrives under low
91

salinity, decreasing in production and increasing in mortality when salinity exceeds

5-8 ppt (Lorenz, 1997 and in press). The exotic Mayan cichlid has become

established in this fish community to the extent that it presently is the dominant

species from Taylor River east to Highway Creek (Trexler et al., in prep.). Lowered

salinity regimes due to increased freshwater inputs are expected to result in

community recovery as measured by increased production and abundance of

resident mangrove fishes. The dependence of wood storks on larger resident

mangrove fishes above approximately 10 cm in length (Ogden et al., 1978) provides

an additional measure of increased survival of topminnows and sunfish to year class

one (into their second year of life). Consistently lower salinities at the upstream

end of the salinity zonation are expected to reduce mortality and allow survivorship

of these species to the larger size classes that are available to wood storks.

Wood stork and roseate spoonbill. The collapse of the coastal nesting colonies of

wood storks and great egrets is attributed largely to a decline in the production and

density of the resident mangrove fishes (Ogden, 1994), particularly topminnows and

sunfish that survive past their first year to a size that wood storks can capture

(Ogden et al., 1978). The decline in roseate spoonbill nesting and the shift of

nesting distribution from eastern to western Florida Bay (Powell et al., 1989) are

also attributed to the reduction in populations of resident mangrove fishes upon

which they feed (Bjork and Powell, 1994). Small fishes have been reported to be the

primary part of the diet of roseate spoonbills in Florida Bay (Allen, 1942; Powell

and Bjork, 1990). Increased density of resident mangrove fishes and increased fish

survival to year class one, as a result of consistently lower salinity patterns at the

upstream end of the gradient, are expected to contribute to the re-establishment of

wood stork coastal nesting colonies, the re-establishment of roseate spoonbill

Florida Bay nesting colonies east of Seven Palm Lake, and an increase in number of

nesting pairs and nesting success of both species.

American crocodile. The American crocodile dwells in the ponds and creeks of the

mangrove estuaries of Florida Bay (Ogden, 1976; Mazzotti, 1983). American

crocodiles are tolerant of a wide salinity range as adults because of their ability to

osmoregulate (Mazzotti, 1989). Juvenile crocodiles lack this ability, however,

(Mazzotti, 1989) and their growth and survival decline at salinities exceeding 20 ppt

(Mazzotti et al., 1988; Mazzotti and Dunson, 1984; Moler, 1991). Juvenile crocodiles

tend to seek freshwater pockets such as black mangrove stands when those choices

are available. Re-establishment of a salinity gradient with levels below 20 ppt in

shoreline and tidal creek habitats, which would indicate a gradient and mosaic of

lower salinities upstream, is expected to benefit the crocodile as measured by

increased growth and survival of juveniles.

Spotted sea trout nursery grounds. Post larval spotted sea trout utilize the coastal

basins of the Florida Bay mangrove estuary as nursery grounds from Terrapin Bay

west to Whitewater Bay. Densities of post larvae in those basins are highest at an
92

intermediate salinity range of 20-30 ppt, and densities drop when salinity exceeds

that of seawater (35 ppt) (Thayer et al., 1998; Schmidt, 1993). Restoration of a

salinity gradient with a persistent zone of <35 ppt in the coastal basins, as a result

of freshwater input from upstream, is expected to result in an increase in the post

larval density and thereby an enhancement of the nursery ground value for spotted

sea trout and possibly other sport fish species in the coastal basins.

Pink shrimp. Mangrove estuaries in Everglades National Park, along with Florida

Bay, are nursery grounds for pink shrimp, an ecologically and economically

important species in south Florida. Pink shrimp are harvested commercially on the

Tortugas grounds, and the pink shrimp fishery is one of south Florida’s most

valuable fisheries in terms of ex-vessel value. Pink shrimp are also a food source for

many recreationally and commercially important estuarine and marine species such

as mangrove snapper and spotted seatrout (Higher Trophic Levels Working Group,

1998).

Pink Shrimp spawning occurs in the Dry Tortugas area, and eggs and larvae

are carried inshore by currents and tides (Jones et al., 1970; Hughes, 1969).

Browder (1985) and Sheridan (1996) have found positive relationships between

indices of freshwater inflow to the coast and Tortugas pink shrimp landings.

Sheridan’s annually updated statistical model based on various freshwater inflow

indices has successfully predicted annual pink shrimp landings in most of the past

decade (Sheridan 1996 and unpublished). The salinity gradient associated with

coastal runoff may provide navigational directions to immigrating young pink

shrimp (Hughes, 1969). Survival rates of juvenile pink shrimp are sensitive to

salinity and decrease markedly under extreme hypersaline conditions (Browder, in

press). Optimal salinities for survival are not fully determined, but probably are

somewhat below that of seawater (35 ppt). Tabb et al. (1962), Rice (1997), and

others have documented that the mangrove estuaries in the Whitewater Bay system

of Everglades National Park are pink shrimp nursery grounds.

Coastal lake vegetation and waterfowl. Compression of the salinity gradient has

changed the coastal lakes and basins of the mangrove ecotone from estuarine to

predominantly marine systems. Coastal lakes such as Seven Palm Lake, Cuthburt

Lake, Long Lake, West Lake, Lake Monroe and the Taylor River ponds are

contained within the mangrove forest and are connected to Florida Bay only by tidal

creeks. Coastal basins such as Joe Bay, Little Madeira Bay, Terrapin Bay, Garfield

Bight, and Whitewater Bay open directly to Florida Bay or the Gulf of Mexico. The

coastal lake and basin estuary ecosystems require seasonal salinity variations from

oligohaline (wet season) to mesohaline (dry season) conditions, in contrast to the

mesohaline to marine conditions that presently occur during most years. Prolonged

periods of salinity concentrations near that of seawater (35 ppt) in the coastal lakes

and basins appear to have contributed to the near-elimination the once-abundant

beds of the submerged aquatic plants Ruppia, Chara, and Utricularia (Ogden,
93

personnal communication) which require oligohaline to mesohaline conditions

(Morrison and Bean, 1997). Utricularia tolerates only oligohaline salinities with an

upper limit of 5-8 ppt. Chara also thrives under freshwater conditions but tolerates

mesohaline salinities up to 15-20 ppt. Ruppia grows under a mesohaline salinity

range of 10-25 ppt. Waterfowl species including coot, scaup, widgeon and pintail

feed on the Ruppia, Chara, and Utricularia. The reduction in beds of these plants

apparently has contributed to the precipitous decline in numbers of seasonally

abundant waterfowl that formerly utilized the coastal lakes and basins (Kushlan et

al., 1982). Recent high-rainfall years have witnessed an increase in coot numbers

on the West Lake to approximately 2000 during winter 96-97 (Bass, personal

communication), but not to the population size of approximately 50,000 that overwintered there until the 1960's (Kushlan et al., 1982). Re-establishment of a

salinity gradient that restores seasonal variation from oligohaline to mesohaline

conditions in the coastal lakes and basins is expected to result in an increase in the

aerial cover of Ruppia, Chara, and Utricularia and the return of winter waterfowl

populations of coot, scaup, widgeon and pintail to the lakes and basins.

F. HYDROLOGIC PERFORMANCE MEASURES FOR RESTORATION

Ecological restoration of the mangrove estuary transition requires a

reduction in the frequency of high salinity events that have been identified for each

coastal basin through the conceptual model process. Another restoration measure is

to increase the frequency of low salinity events that have been identified for each

coastal basin. The high and low salinity levels represent the best professional

judgement of those scientists working in the mangrove estuary, based on the

existing information on the biological requirements and distributions of the

estuarine organisms that are described above, available salinity data, and field

observation.

Table 1 displays the lower and upper salinity levels identified for coastal

basins. It is desirable to decrease the frequency that salinity exceeds upper levels,

and to increase the frequency that salinity drops below lower levels.

Table 1

Salinity Values

Basin Lower Level Upper Level

Joe bay 5 ppt 15 ppt

Little Madeira Bay 15 ppt 25 ppt

Terrapin Bay 25 ppt 35 ppt

Garfield Bight 25 ppt 35 ppt

North River Mouth 5 ppt 15 ppt

The strategy for ecological restoration of the mangrove estuary transition is

to maintain freshwater heads and flows in the Everglades at the upstream end of

the salinity gradient in order to achieve desirable salinity regimes in the Florida
94

Bay coastal basins at the downstream end of the salinity gradient. Regression

analyses demonstrated inverse relationships of salinity in the coastal basins to

water level upstream in the Everglades (Davis, 1997). The regressions indicated

that stages of 7.3 and 6.3 feet msl at the P33 gage in central Shark River Slough

produce the lower and upper salinity levels for Joe Bay, Little Madeira Bay,

Terrapin Bay, Garfield Bight, and North River Mouth. Four performance measures

for the ecological restoration of the Florida Bay mangrove estuary and coastal

basins are derived from the simulated stages at the P33 gage and salinity levels in

the coastal basins.

The frequency of stages of 6.3+ at P33 is applied as a performance measure

for the Florida Bay coastal basins. The performance measure is the number of

months during the 31-year period of record when stages at P33 rose to, or above,

6.3. The target is the number of months that NSM45F provided stages of 6.3 or

above. A reduced frequency of high salinity events is given a high priority in the

ecological restoration of the coastal basins, thus the frequency of 6.3+ stages is

given a weighting of two when averaged with the other performance measures.

The frequency of stages of 7.3+ at P33 is applied as a performance measure to

the Florida Bay coastal basins. The performance measure is the number of months

during the 31-year period of record when stages at P33 rose to, or above, 7.3. The

target is the number of months that NSM45F provided stages of 7.3 or above. An

increased frequency of low salinity events is given a lower priority than a reduced

frequency of high events, thus the frequency of 7.3+ stages is given a weighting of

one when averaged with the other performance measures for the coastal basins.

The transition from the late dry season to the early wet season during March

through June is a critical period to estuarine organisms in the Florida Bay coastal

basins regarding the frequency and duration of high salinity events. Salinity is

estimated based on relationships between mean monthly salinity in the coastal

basins and water stage at the P33 gage in mid Shark River Slough. The cumulative

salinity difference (ppt) from the high salinity levels that have been identified for

Florida Bay coastal basins is summed during the dry/wet season transition months

of March-June. Differences are summed over five coastal basins (Joe Bay, Little

Madeira Bay, Terrapin Bay, Garfield Bight and North River Mouth) and over the

31-year period of record. Differences above the specified high salinity levels are

given a positive value, and differences below the high salinity levels are given a

negative value. The target is to reduce the cumulative salinity difference to a value

that does not exceed the cumulative difference produced by NSM45F. The

cumulative March-June salinity difference from high levels is given a weighting of

one when averaged with the other performance measures for the coastal basins

During the August-October transition from the late wet season to the early

dry season, it is important to achieve low salinity levels in the Florida Bay coastal
95

basins to provide the seasonal environment for low-salinity estuarine organisms

and to postpone the onset of high salinity events further into the dry season.

Salinity is estimated based on relationships between mean monthly salinity in the

coastal basins and water stage at the P33 gage in mid Shark River Slough. The

cumulative salinity difference (ppt) from the low salinity levels that have been

identified for the Florida Bay coastal basins is summed during the wet/dry season

transition months of August-October. Differences are summed over the five coastal

basins and over the 31-year period of record. Differences above the specified low

salinity levels are given a positive value, and differences below the low salinity

levels are given a negative value. The target is to reduce the cumulative salinity

difference to a value that does not exceed the cumulative difference produced by

NSM45F. The cumulative August-October salinity difference is given a weighting

of one when averaged with the other performance measures for the coastal basins.

Ecological attributes and indicators of restoration success in the Florida Bay

mangrove estuary and coastal basins that are linked to the above hydrology/salinity

performance measures in the conceptual model include 1) increased production of

low-salinity mangrove fishes, 2) re-establishment of coastal nesting colonies of wood

storks/great egrets and eastern Florida Bay colonies of roseate spoonbill, 3) earlier

timing of coastal colony formation by wood storks/great egrets and of Florida Bay

colony formation by roseate spoonbills, 4) increased growth and survival of juvenile

American crocodiles, 5) increased cover of low-to-moderate salinity aquatic

macrophyte communities in coastal lakes and basins, 6) return of seasonal

waterfowl aggregations to coastal lakes and basins, 7) enhanced nursery ground

value for spotted seatrout and pink shrimp in coastal basins, and 8) persistence and

resilience of the mangrove, salt marsh and tidal creek vegetation mosaic.

A performance measure that is generic to the conceptual models of all

physiographic regions of south Florida is the input and bio-accumulation of mercury

and other toxins. Potential inputs of mercury and pesticides in agricultural and

urban runoff water that may be needed for freshwater input into the mangrove

estuary transition might result in reduced health, behavioral and physical

abnormalities, and loss of reproductive vigor of the fauna unless measures are

taken to restrict loads of these toxins in inflow water. Measures of faunal health

that reflect responses to mercury and pesticide inputs include body burdens and the

incidence of physical and behavioral abnormalities in representative species.

G. MOVING TARGET

Predicted rises in sea level require re-evaluation of relationships between

Everglades stage and mangrove estuary transition salinity during the next century.

However, the strategy for the maintenance of salinity at the lower end of the

gradient by adjusting upstream water stage at key Everglades gages will continue
96

to apply. Maintaining Everglades stages based on presently derived stage/salinity

relationships provides one potential strategy to support a salinity gradient, but with

a landward shift in response to rising sea level. Raising Everglades stages based

upon revised stage/salinity relationships provides another potential strategy to

offset sea level rise and maintain the mangrove estuary transition in its present

location. Regardless of rising sea level, however, a salinity gradient supportive of an

ecologically functional mangrove estuary transition zone will be required to

maintain the integrity of the south Florida ecosystem.


99

H. LITERATURE CITED

Allen, R.P. 1942. The Roseate Spoonbill. Dover Publications, New York: 142pp.

Armentano, T.V., R.F. Doren, W.J. Platt and T. Mullins. 1995. Effects of Hurricane

Andrew on coastal and interior forests of southern Florida: Overview and

synthesis. Journal of Coastal Research Special Issue 2: 111-114.

Bass, O.L., Jr. Personal communication. Everglades National Park, Homestead

FL.

Bjork, R.D. and G.V.N. Powell. 1994. Relations between hydrologic conditions and

quality and quantity of foraging habitat for roseate spoonbills and other

wading bird in the C-111 basin. National Audubon Society Final Report to

South Florida Research Center, Everglades National Park.

Browder, J.A. 1985. Relationship between pink shrimp production on the Tortugas

grounds and water flow patterns in the Florida Everglades. Bulletin of

Marine Science 37: 830-856.

Browder, J.A. In press. Environmental influences on potential recruitment of pink

shrimp, Penaeus duorarum, from Florida Bay nursery grounds. Estuaries

1999.

Browder, J.A. and D. Moore. 1981. A new approach to determining the

quantitative relationship between fishery production and the flow of fresh

water to estuaries. In: R. Cross and D. Williams, eds. Proceedings of the

National Symposium on Freshwater Inflow To Estuaries,Volume 1,

FWS/OBS-81/04, Office of Biological Services, U.S. Fish and Wildlife Service,

Washington D.C.: 403-430.

Davis, S.M. 1997. Salinity in coastal basins estimated from upstream water stages.

In: Central and Southern Florida Project Comprehensive review Study,

http://www.restudy.org, Comprehensive Plan Evaluation, Evaluation of

Alternative Plans, About the P.M.’s.

Higher Trophic Level Working Group. 1998. Draft Report of Higher Trophic Level

Working Group of Florida Bay Program Management Committee, South

Florida Ecosystem restoration Task Force.

Hughes, D.A.. 1969. Responses to salinity changes as a tidal transport mechanism

of pink shrimp Penaeus duorarum. Biological Bulletin 136: 43-53.
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Jones, A.C., D.E. Di itriou, J.J. Ewald and J.H. Tweedy. 1970. Distribution of early

developmental stages of pink shrimp, Penaeus duorarum, in Florida waters.

Bulletin of Marine Science 20: 634-661.

Kushlan, J.D., O.L. Bass Jr. and L.C. McEwan. 1982. Wintering waterfowl in

Everglades National Park. South Florida Research Center Report T-670,

Everglades National Park, Homestead FL: 26pp.

Lorenz, J.J. 1997. The effects of hydrology on resident fishes of the Everglades

mangrove zone. National Audubon Society Final Report to South Florida

Research Center, Everglades National Park, Homestead FL: 193pp.

Lorenz, J.J. In press. The response of fishes to physical-chemical changes in the

mangroves of northeast Florida Bay. Estuaries 1999.

Mazzotti, F.J. 1983. The ecology of Crocodylus acutus in Florida. Ph.D.

dissertation, The Pennsylvania State University, University Park PA: 161pp.

Mazzotti, F.J. and W.A. Dunson. 1984. Adaptations of Crocodylus acutus and

Alligator for life in saline water. Comp. Biochem. Physiol. 79A: 641-646.

Mazzotti, F.J. and W.A. Dunson. 1989. Osmoregulation in crocodilians. Am. Zool.

29: 903-920.

Mazzotti, F.J., A. Dunbar-Cooper and J.A. Kushlan. 1988. Desiccation and cryptic

nest flooding as probable causes of embryonic mortality in the American

crocodile, Crocodylus acutus, in Everglades National Park, Florida. Florida

Scientist 52: 65-72.

McIvor, C.C., J.A. Ley and R.D. Bjork. 1994. Changes in freshwater inflow from

the Everglades to Florida Bay including effects on biota and biotic processes.

In: S.M. Davis and J.C. Ogden, eds. Everglades: The Ecosystem and Its

Restoration, St. Lucie Press, Delray Beach FL: 117-146.

Meeder, J.F., M.S. Ross, G. Telesnicki, P.L. Ruiz and J.P. Sah. 1996. Vegetation

analysis in the C-111/Taylor Slough Basin. Document 1: The Southeast

Saline Everglades revisited: a half-century of coastal vegetation change.

Document 2: Marine transgression in the Southeast Saline Everglades,

Florida: rates, causes and plant sediment responses. Final Report. Contract

C-4244. Southeast Environmental Research Program. Florida International

University, Miami FL

Moler, P.E. 1991. American crocodile population dynamics. Final report, Florida

Game and Freshwater Fish Commission, Tallahassee FL: 23pp.
101

Morrison, D. and D.L. Bean. 1997. Benthic macrophyte and invertebrate

distribution and seasonality in the Everglades-Florida Bay ecotone. National

Audubon Society Final Report to South Florida Research Center, Everglades

National Park, Homestead FL: 25pp+figures and tables.

Ogden, J.C. Personal communication. South Florida Water Management District,

West Palm Beach FL.

Ogden, J.C. 1976. Crocodilian ecology in southern Florida. In: Research in the

Parks: Transactions of the National Park Centennial Symposium, 1971,

National Park Service Symposium Series No. 1, U.S. Department of the

Interior, Washington D.C.

Ogden, J.C. 1994. A comparison of wading bird nesting colony dynamics (1931-

1946 and 1974-1989) as an indication of ecosystem conditions in the southern

Everglades. In: S.M. Davis and J.C. Ogden, eds. Everglades: The

Ecosystem and Its Restoration. St. Lucie Press, Delray Beach Fl: 533-570.

Ogden, J.C., J.A. Kushlan and J.A. Tilmont. 1978. The Food Habits and Nesting

Success of Wood Storks in the Everglades National Park in 1974. Natural

Resources Report 16, U.S. National Park Service, Washington D.C.: 25pp.

Powell, G.V.N. and R.D. Bjork. 1990. Relationships between hydrologic conditions

and quality and quantity of foraging habitat for roseate spoonbills and other

wading birds in the C-111 basin. National Audubon Society Second Annual

Report to South Florida Research Center, Everglades National Park,

September 1990.

Powell, G.V.N., R.D. Bjork, J.C. Ogden, R.T. Paul, A.H. Powell and W.B. Robertson,

Jr. 1989. Population trends of some south Florida wading birds. Wilson

Bulletin 101: 436-457.

Rice, J.K. 1997. An analysis of environmental factors influencing juvenile pink

shrimp (Penaeus duorarum) abundance in southwest Florida. Masters

Thesis. University of Miami, Coral Gables FL.

Schmidt, T.W. 1993. Community characteristics of dominant forage fishes and

decapods in the Whitewater Bay-Shark River Estuary, Everglades National

Park. Technical Report NPS/SEREVER/NRTR-93/12.

Sheridan, P.F. 1996. Forecasting the fishery for pink shrimp, Penaeus duorarum,

on the Tortugas grounds, Florida. Fishery Bulletin 94: 743-755.
102

Simmons, Glen. Personal communication. Homestead FL.

Tabb, D.C., D.L. Dubrow and A.E. Jones. 1962. Studies on the biology of the pink

shrimp Penaeus duorarum Burkenroad, in Everglades National Park,

Florida. Technical Series 37: 1-30, Florida State Board of Conservation,

University of Miami, Miami Laboratory, Miami Florida.

Thayer, G.W., A.B. Powell, and D.E. Hess. 1998. Response of larval, juvenile and

small adult fishes to changes in environmental conditions in Florida Bay: A

decadal comparison. Proceedings of the 1998 Florida Bay Science

Conference, May 12-14, 1998.

Trexler, J.C., W.F. Loftus, F. Jordan, J.J. Lorenz and J. Chick. In prep. Empirical

assessment of fish introductions in southern Florida: An evaluation of

contrasting views.

Wanless, H.R., R.W. Parkinson and L.P. Tedesco. 1994. Sea level control on

stability of Everglades wetlands. In: S.M. Davis and J.C. Ogden, eds.

Everglades: The Ecosystem and Its Restoration. St. Lucie Press, Delray

Beach FL: 199-223.

Welsh, R., M. Remillard and R.F. Doren. 1995. GIS data base development for

south Florida’s national parks and preserves. Photogrammetric Engineering

and Remote Sensing 61(1): 1371-1381.
103

VIII. BISCAYNE BAY CONCEPTUAL ECOLOGICAL MODEL

A. PREPARERS

Diego Lirman, Elizabeth A. Irlandi, and Wendell P. Cropper, Jr., Center for Marine

and Environmental Analyses, University of Miami, Miami, Florida.

B. INTRODUCTION

Biscayne Bay is a subtropical lagoon located on the southeast coast of Florida.

The Bay is generally subdivided into three sections with unique biological and physical

characteristics: a) North Bay, north of the Rickenbacker Causeway, b) Central Bay,

from the Rickenbacker Causeway to Card Sound, and 3) South Bay, an area that

includes Card Sound and Barnes Sound (VanArman et al., 1989).

The North Bay, an area heavily influenced by urban development activities, has

reduced water clarity, high concentrations of toxicants and nutrients, and chronic

sewage pollution (e.g., Wanless, 1976; Harlem, 1979; Judge et al., 1979; Wanless et al.,

1984; Corcoran et al., 1987). The main sources of pollution for this heavily urbanized

watershed are the Miami River, the Port of Miami, and various canals (e.g., Buck,

1976; Corcoran et al., 1984; Dade County , 1987; Markley et al., 1990). High levels of

toxic metals, organic contaminants and pesticides have been found in the water and

sediments of discharge sites. Altered salinity regimes, increased nutrient and

contaminant loadings, and high water turbidity have all contributed to the decreased

health and extent of biological communities within the northern section of Biscayne

Bay.

The central portion of the Bay is better flushed than the North Bay and has

extensive, healthy seagrass beds and other benthic communities. Nevertheless,

industrial and agricultural discharges, as well as leachete from the South Dade

landfill, are important sources of pollution to this area (McKenzie, 1983; Shinn and

Corcoran, 1987). A salinity gradient is found in the Central Bay, with lower, variable

salinity occurring on the western margin of the Bay due to freshwater inflow from

canal discharge and runoff, and higher, more stable salinities in the eastern margin,

where oceanic influences prevail (Wang et al., 1978; Chin Fatt and Wang, 1987).

The southern portion of the Bay (Card Sound and Barnes Sound) is influenced

primarily by agricultural development. Freshwater inflow into the area is mostly

through canal discharge, and flushing is limited as there are no direct connections to

the ocean. Accordingly, salinity can vary from relatively low values during the wet

season (10-20 ppt) to potentially hypersaline values (40-45 ppt.) during the dry season

(Lee and Rooth, 1975). Southern Biscayne Bay has extensive, healthy seagrass beds,
104

and is critical habitat for a number of threatened species such as the Cape Sable

seaside sparrow, roseate spoonbills, as well as crocodiles, manatees, and snail kites

(Kushlan and White, 1977; Kushlan and Mazzotti, 1989).

The benthic habitats of Biscayne Bay are dominated by extensive beds of

seagrass that cover most of the Bay bottom (Thorhaug, 1976). Thalassia testudinum is

the dominant seagrass species, but Halodule wrightii and Syringodium filiforme are

also present and may dominate locally. The seagrass beds also have associated

rhizophytic (e.g., Halimeda spp., Penicillus spp., and Udotea spp.) and drift-algal (e.g.,

Laurencia spp., Polysiphonia spp.) components. Hardbottom areas with sponges, soft

corals, hard corals, and macroalgae are less extensive and occur primarily in the

eastern and central portions of the Bay (DiResta et al., 1995; Cropper and DiResta,

1999).

Both commercial and recreational fishing are important activities within

Biscayne Bay (Berkeley et al., 1985). Sport-fishing activities are concentrated mainly

on fish species such as tarpon, snook, bonefish, and permit (deSylva, 1969). The groups

of organisms harvested commercially within the Bay include: spotted seatrout, the

snapper-grouper fish complex, bait fish, bait shrimp, stone and blue crabs, lobsters and

sponges (Ault et al., 1997).

C. DRIVERS AND STESSORS

The anthropogenic and natural drivers creating potential stressors to the

Biscayne Bay ecosystem include urban, industrial, and agricultural development,

water management (C&SF project), commercial and recreational fishing and boating

activities, and natural disturbances such as storms (refer to figure on page 109).

Prior to the extensive human development in the area, inputs of fresh water into

Biscayne Bay were mainly from the Miami River, Little River, Arch Creek, and

Snapper Creek. Sheet flow from the adjacent Everglades system, artesian flow from

the underlying limestone bedrock, and runoff after storms or heavy rains were

important additional sources of fresh water before development (Harlem, 1979;

Wanless et al., 1984).

However, since the early 1900s, the hydrology of the Bay has been significantly

modified due to coastal construction and the development of the extensive water

management system now in place (Wanless et al., 1984). Fresh water input patterns

have been highly modified from pre-development patterns, limiting both surface and

groundwater inflow into the Bay. At present, the main sources of fresh water are local

rainfall and runoff, as well as canal discharges (Wanless et al., 1984).
105

During the rainy season, tremendous volumes of water can be released through

flood control structures creating large and rapid fluctuations in salinity in the vicinity

of canal mouths and over-all reduced salinity on the western fringe of the Bay (Wang et

al., 1978). Increases in freshwater inflow from runoff as well as from point sources

have also contributed to a significant decrease in water quality within Biscayne Bay.

Major issues of concern include increased sedimentation and turbidity as well as

increased nutrient loading and chemical pollution (VanArman et al., 1989).

Both commercial and recreational boating activities can cause significant

physical damage to seagrass beds, hardbottom communities, and manatees (O'Shea et

al., 1985; Sargent et al., 1995; Ault et al., 1997). Major storms can also have significant

effects on water quality and benthic habitats. After Hurricane Andrew, high turbidity,

increased nutrient and pollutant loading, high dissolved organic carbon, and persistent

plankton blooms were observed within Biscayne Bay (Tilmant et al., 1994). Benthic

communities also experienced high erosion, and burial and removal of organisms

during Andrew and previous hurricanes (Thomas et al., 1962; Tilmant et al., 1994).

D. ECOLOGICAL EFFECTS

Salinity: Alterations in salinity can impact species composition and productivity of

seagrass communities. Of the three most abundant seagrass species that occur in the

Bay, Halodule is the most tolerant of low-salinities, and tends to be dominant in areas

adjacent to canal discharge sites. Thalassia and Syringodium are also found near

canals, but growth and productivity of these species is reduced at sites influenced by

freshwater discharge (Conover, 1964; Lewis et al., 1985; Montague, 1989). Salinity

also affects other benthic primary producers associated with seagrass communities.

Pulses of reduced-salinity water can have negative effects on the growth of the drift

alga Laurencia and rhizophytic algae such as Penicillus spp., Udotea spp., and

Halimeda spp. (Conover, 1964; Irlandi et al., unpublished data).

Similarly, salinity fluctuations can affect faunal composition and abundance as

some species are more tolerant to low salinity than others (Serafy et al., 1997).

Salinity effects on the distribution of grazers (both fish and invertebrate) may influence

the abundance of phytoplankton, epiphytes and macroalgae that can potentially shade

seagrasses (Irlandi et al., 1997). Negative effects of low salinity have also been

detected in sponges (Smith, 1973; Storr, 1976).

Pollution: Urban and agricultural development, as well as boating activities in the

Bay, contribute to the introduction of contaminants and toxins into the environment.

The use of herbicides in farming may potentially influence seagrass growth and

production while heavy metals, mercury, and other toxic materials can be detrimental

to the health and survival of fish populations and higher trophic levels that feed on

them (e.g. bioaccumulation of toxins). Several studies have detected a high number of
106

abnormalities in several fish species within Biscayne Bay, specially around heavily

polluted areas such as marinas and canal outflows (e.g., Browder et al., 1993; Gassman

et al., 1994)

Nutrients: Agricultural runoff, urban runoff, and stormwater release can result in an

increase in both water-column and sediment nutrients, promoting growth of

phytoplankton, seagrass epiphytes, and macroalgae (e.g., Lapointe and Clark, 1992;

Frankovich and Fourqurean, 1997). Phytoplankton blooms, epiphyte growth, and

growth of attached and drift macroalgae can reduce light availability to seagrasses and

reduce their growth and production. Similarly, high concentrations of phytoplankton

may be deleterious to filter-feeding sponges in hardbottom habitats due to a potential

clogging effect.

Nutrient gradients caused by point sources may have a major influence on the

abundance, composition, and distribution of benthic communities. Mats of drift algae

are common along the western boundary of the Bay where several canals drain

agricultural fields, and their growth and distribution may be the direct result of the

increased nutrients. Changes in nutrient levels can also affect the community

composition of seagrass meadows by influencing the competitive interactions among

the dominant seagrass species (Williams, 1987; Harwell and Fong, 1994; Fong et al.,

1997), producing shifts in dominance from Thalassia to Halodule (Fourqurean et al.,

1995) as well as influencing competitive interactions between seagrasses and algae

(Williams, 1990; Holmquist, 1992).

Sedimentation: Increased sediment loads associated with canal discharge, runoff, and

resuspension due to storms and boating activities can increase water turbidity, shading

benthic plants and decreasing productivity (Hall et al., 1991; Onuf, 1991; Tilmant et

al., 1994; Sargent et al., 1995). If sediment concentrations are high, there is also the

potential for burial and smothering of benthic-communities components.

Harvesting and Poaching: Commercial and recreational fishing can influence the biotic

communities of Biscayne Bay. Fishing pressure, species targeted, and size of fish

captured can alter the species composition and trophic structure of fish communities.

For example, the removal of omnivorous bait fish in large quantities (e.g., pinfish) may

result in decreased grazing rates that may facilitate algal and epiphyte overgrowth of

seagrasses and hardbottom-community components.

Poaching can also have significant impacts on the populations targeted. The

removal of undersized lobsters as well as the removal of large, reproductive sponges

can have significant long-term effects on the survival of these populations (Cropper and

DiResta, 1999).

Many of the commercially and recreationally important fish and

macroinvertebrates found in the Bay rely on seagrass and hardbottom habitats for food
107

and/or shelter, therefore, factors influencing the distribution, productivity, and

abundance of these benthic habitats will have an effect on the distribution and

abundance of fishery resources in the Bay.

Physical Damage: Direct physical damage to the biotic communities of Biscayne Bay

can be caused by boating and harvesting activities as well as by intense storms.

Shrimp trawlers actively target seagrass habitats but may venture into hardbottom

areas removing and damaging sponges and gorgonians with their gear. Ault et al.

(1997) found that considerable damage to both seagrass and hardbottom communities

can result from trawl and trap fishing practices. Significant by-catch of juvenile fish

that suffer physical damage and/or mortality can also occur.

Most of the seagrass beds within Biscayne bay show some signs of boat scarring.

Boat scars result in the loss of essential habitat for associated fauna, sediment erosion,

and increased water turbidity (Sargent et al., 1995). Furthermore, several studies

have shown that the recovery of deep scars by seagrasses can be a slow process taking

anywhere from a few months to several years to recuperate original seagrass shoot

densities (Zieman, 1976; Durako et al., 1992). Lastly, the scouring effects of sediments

suspended during intense storms may cause significant mortality of benthic organisms

and affect their distribution within the Bay (Tedesco et al., 1995).

E. ECOLOGICAL ATTRIBUTES AND MEASURES

A set of endpoints or attributes can serve as indicators of the health of the

Biscayne Bay ecosystem. Most of these attributes are biological components of the

system such as the health of birds, macroinvertebrates (e.g., lobsters, shrimp, scallops),

fishes, manatees, and seagrass and hardbottom communities.

Faunal Communities: This endpoint considers the abundance, size, and health of Bay

species or species groups that are of commercial or recreational importance such as bait

shrimp, crabs, lobsters, tarpons, bonefish, snappers and groupers, birds, dolphins,

crocodiles and manatees. Measurements and indicators for this endpoint include

distribution, abundance, growth, recruitment, and population size structure of these

organisms, nest distribution, as well as incidence of disease and abnormalities.

Information about the health of commercial stocks could be obtained from landings and

fishing effort statistics.

Seagrass Communities: Seagrasses compose a keystone community within Biscayne

Bay; they not only dominate spatially but also serve as essential habitat (e.g., food and

refuge) for a productive food web. Their presence also plays a role in maintaining

water quality (e.g., sediment stabilization, nutrient uptake), another major endpoint

for the Bay. The spatial extent, density, and productivity of seagrass beds are key

attributes of these communities and can be estimated directly.
108

Hardbottom Communities: Hardbottom communities within Biscayne Bay contain high

densities of sponges, soft corals, and macroalgae. Hard corals can also be important

components locally. Just like seagrass communities, hardbottom communities provide

essential habitat for a large number of organisms. Juvenile lobsters are often found

within sponges (Butler et al., 1994) and large stands of the attached alga Sargassum

provide both food and shelter to juvenile fishes seasonally.

Also, water and sediment quality are important attributes of the system as the

health of the biological components is often tightly linked to them. Direct indicators

and measurements of water and sediment quality include turbidity, salinity, dissolved

oxygen, bacterial counts, chlorophyll concentrations, and the concentration of nutrients

and pollutants. Secondary indicators of reduced water and sediment quality may be

the distribution and composition of invertebrate communities (e.g., meiofauna) that

respond differently to different environmental conditions.

110

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Butler, M.J., W.F. Herrnkind, and J.H. Hunt,. 1994. Sponge mass mortality and

Hurricane Andrew: catastrophe for juvenile spiny lobsters in south Florida? Bull

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chlorinated pesticides, polychlorinated biphenyls and phthalic acid esters in

sediments of Biscayne Bay. Dade County Environmental Resource

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Cropper, W.P., Jr. and D. DiResta 1999. Simulation of a Biscayne Bay, Florida

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DiResta, D., B. Lockwood, and R. Curry. 1995. Monitoring of the recruitment, growth

and mortality of commercial sponges in Biscayne National Park. Final report for

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beds: an assessment and experimental study of recolonization in Weedon Island

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Fong, P. and M.A. Harwell. 1994. Modeling seagrass communities in tropical and

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hypotheses. Bull. Mar. Sci. 54: 757-781.

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analysis and experiments. Ecol. Appl. 7: 300-315.

Fourqurean, J.W., G.V.N. Powell, W.J. Kenworthy, and J.C. Zieman. 1995. The effects

of long-term manipulation of nutrient supply on competition between the

seagrasses Thalassia testudinum and Halodule wrightii in Florida Bay. Oikos

72: 349-358.

Frankovich, T. A. and J.W. Fourqurean. 1997. Seagrass epiphyte loads along a

nutrient availability gradient, Florida Bay, USA. Mar. Ecol. Prog. Ser. 159: 37-

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Gassman, N.J. L.B. Nye, and M.C. Schmale. 1994. Distribution of abnormal biota and

sediment contaminants in Biscayne Bay, Florida. Bull. Mar. Sci. 54: 929-943.
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Hall, M.O., D.A. Tomasko, and F.X. Courtney. 1991. Responses of Thalassia

testudinum to in situ light reduction. In: Kenworthy, W.J. and D. Haunert

(eds.), The capability of water quality criteria, standards and monitoring

programs to protect seagrasses from deteriorating water transparency. NOAA

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Harlem,P.W . 1979. Aerial photographic interpretation of the historical changes in

northern Biscayne Bay, Florida: 1925 to 1976. University of Miami Sea Grant

technical bulletin 40. 150 pp.

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benthic assemblage: influence of a mobile habitat on seagrasses and associated

fauna. Ph.D. Dissertation. Florida State University, Tallahassee, Florida. 183

pp.

Irlandi, E.A., S. Macia, and J. Serafy. 1997. Salinity reduction from freshwater canal

discharge: effects on mortality and feeding of an urchin (Lytechinus variegatus)

and a gastropod (Lithopoma tectum). Bull. Mar. Sci. 61: 869-879.

Johnson, D.R. and W. Seaman. 1986. Species profiles: life histories and environmental

requirements of coastal fishes and invertebrates (South Florida) - spotted

seatrout. U.S. Fish and Wildlife Service biological report 82(11.43). Slidell,

Louisiana. 18 pp.

Judge,R.M. and F.W. Curtis. 1979. Heavy metal distribution in Biscayne Bay

sediments. Florida Scientist 42: 242-248.

Kushlan, J.A. and D.A. White. 1977. Nesting wading bird populations in southern

Florida. Florida Scientist 40: 65-72.

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American crocodile in Florida. Journal of Herpetology 23: 1-7.

Lapointe, B.E. and M.W. Clark. 1992. Nutrient inputs from the watershed and coastal

eutrophication in the Florida Keys. Estuaries 15: 465-476.

Lee, T.N. and C. Rooth. 1976. Circulation and exchange processes in southeast

Florida's coastal lagoons. In: Thorhaug, A. and A. Volke (eds.), Biscayne Bay:

past/present/future; papers prepared for Biscayne Bay Symposium I. University

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Bay - a review. In: Treat, S.A.F., J.L. Simon, R.R. Lewis, III, and R.L. Whitman,
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Jr. (eds.). Proceedings Tampa Bay Area Scientific Information Symposium.

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Markley, S.M., D.K. Valdes, and R. Menge. 1990. Sanitary sewer contamination of the

Miami River. Metro Dade DERM technical report 90-9. Miami, Florida.

McKenzie, D.J. 1983. Water quality at and adjacent to the south Dade County solidwaste disposal facility. U.S. Geological Survey. Water resources investigations

report 83-4003. Tallahassee, Florida. 37 pp.

Montague, C. 1989. The distribution and dynamics of submerged vegetation along

gradients of salinity in northeast Florida Bay. Bull. Mar. Sci. 44: 521.

O'Shea, T.J., C.A. Beck, R.K. Bonde, H.I. Kochman, and D.K. Odell. 1985. An analysis

of manatee mortality patterns in Florida, 1976-81. Journal of Wildlife

Management 49: 1-11.

Onuf, C.P. 1991. Light requirements of Halodule wrightii, Syringodium filiforme, and

Halophila engelmanni in a heterogeneous and variable environment inferred

from long-term monitoring. In: Kenworthy, W.J. and D. Haunert (eds.), The

capability of water quality criteria, standards and monitoring programs to

protect seagrasses from deteriorating water transparency. NOAA Coast. Ocean

Prog. est. Habitat Stud. 151 pp.

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assessment and management options. Florida Department of Environmental

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canal discharge on fish assemblages in a subtropical bay: field and laboratory

observations. Mar. Ecol. Prog. Ser. 160: 161-172.

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Biscayne Bay, Florida. Unpublished report. 8 pp.

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Press, New York. pp. 277-282.

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Hurricane Andrew on south Florida's sandy coastlines. In: Stone,G.W. and C.

W. (eds.), Impacts of Hurricane Andrew on the coastal zones of Florida and

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Foundation. Journal of Coastal Research Special Issue no. 21. pp. 59-82.

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turtle grass beds of Biscayne Bay, Florida. Bull. Mar. Sci. 11: 191-197.

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116
117

IX. FLORIDA BAY CONCEPTUAL MODEL

A. PREPARER

David T. Rudnick, South Florida Water Management District, West Palm

Beach Florida.

B. INTRODUCTION

A simple conceptual model of the Florida Bay ecosystem is presented here.

This model is consistent with our effort to assess the current understanding of south

Florida’s ecosystems, identify the most important human effects on these

ecosystems, identify restoration goals and success criteria, and identify the

minimum measurements required to determine whether these criteria are being

met. The structure of the model is largely based on the expert opinions of scientists

who have focused their attention on Florida Bay during the past several years.

During this time, detailed reviews of our understanding of the Florida Bay have

been presented (Boesch et al. 1993, Boesch et al. 1995, Boesch et al. 1997,

Fourqurean and Robblee 1999). Detailed plans that identify quantitative

information needs for environmental management decision making, as well as

strategies to provide this information, have also been presented (Armentano et al.

1994, Armentano et al. 1997). While the conceptual model presented here is largely

consistent with the body of knowledge described in the reviews and plans noted

above, some details or omissions of this model may not be consistent will the

opinions of every contributor listed above or every contributor to the larger Florida

Bay research effort.

Florida Bay primer. Florida Bay is a triangularly shaped estuary, with an area of

about 850 square miles, that lies between the southern tip of the Florida mainland

and the Florida Keys. About 80% of this estuary is within the boundaries of

Everglades National Park. A defining feature of the bay is it’s shallow depth, with

a mean depth of about 1 meter (Schomer and Drew 1981). This shallowness allows

light to penetrate through the water to the sediment surface in almost all areas of

the bay and results in the potential for the bay to sustain seagrass beds as a

dominant habitat and source of productivity. The shallowness of the bay also

affects the circulation and salinity regime of the bay; with a complex network of

shallow mud banks, water exchange among the bay’s basins and between these

basins and the Gulf of Mexico is restricted (Smith 1994, Wang et al. 1994). With a

long residence time and shallow depth, the salinity of Florida Bay water can rapidly

rise during drought periods. Salinity levels as high as twice that of seawater have

been measured (McIvor et al. 1994). Another defining feature of the bay is that the

sediments are primarily composed of carbonate mud, which can scavenge inorganic

phosphorus from bay waters (DeKanel and Morse 1978).
118

Until the 1980s, Florida Bay was perceived by the public and environmental

managers as being a healthy estuary, with clear water, lush seagrass beds, and

productive fish and shrimp populations. By the mid 1980s, however, catches of pink

shrimp had declined dramatically (Browder et al. 1999) and in 1987, the mass

mortality of turtle grass (Thalassia) beds began (Robblee et al. 1991). By 1992, the

ecosystem appeared to shift from a clear water system, dominated by primary

production on the sediment (benthic production) to a turbid water system,

dominated by algae blooms in the water column and resuspended sediment. The

conceptual model focuses on these changes in seagrasses and water quality as the

central issues to be considered by environmental managers.

Reality check. The simple model presented below does not address the spatial

complexity of Florida Bay. Florida Bay is, indeed, not so much a singular estuary,

but a complex array of basins, banks, and islands that differ across a set of regions.

The mosaic of seagrass habitat and mangrove habitat, as well as water quality and

ecosystem processes, vary distinctly with this spatial variation. Nevertheless, only

a single, generic model is described and this model is intended to summarize the

main characteristics and trends of the bay. While the structure of this model is

appropriate for most areas of the bay, the relative importance of the model’s

components differ considerably among the bay’s sub-regions. Any application of

this model (for example, recommendations for a specific set of monitoring

parameters and guidelines) must take the spatial variability of the bay into account.

C. EXTERNAL DRIVERS AND ECOLOGICAL STRESSORS

It has often been assumed that a direct cause of Florida Bay’s ecological

changes is a long-term increase in the Bay’s salinity that resulted from the

diversion of freshwater away from Florida Bay via SFWMD canals. However,

recent research has indicated that the Bay’s changes are not attributable to a single

cause - while decreased freshwater inflow and resultant increased salinity have

been part of the problem, it appears that other human activities, as well as natural

forces, have also contributed to the problem (Armentano et al. 1997, Boesch et al.

1993, Boesch et al. 1995, Boesch et al. 1997, Fourqurean and Robblee 1999). The

conceptual model thus includes both natural and human derived sources of stress

(refer to figure on page 127).

Altered salinity regime. The salinity regime of an estuary is a primary

determinant of the species composition of communities, as well as strongly

influencing functions of these communities (Sklar and Browder 1998). Salinity is a

direct stress on biota; all estuarine biota have adapted to a given salinity range and

a given degree of salinity variability. For a given organism, changing salinity

beyond this range or too quickly within this range can result in poor health or
119

death. Thus long-term changes in salinity level or variability are detrimental to

some species, but favorable for other species.

Florida Bay’s salinity regime varies greatly over time and space. This

variation ranges from coastal areas that can be nearly fresh during the wet season,

to large areas of the central bay that can have salinity levels near 70 ppt during

prolonged droughts, to nearly stable marine conditions (about 35 ppt) on the

western boundary of the bay. The main forces that determine salinity regime in the

bay are the inflow of freshwater from the Everglades, rainfall over the bay,

evaporation from the bay, and exchange with seawater from the Gulf of Mexico and

the Atlantic Ocean. Both freshwater inflow and seawater exchange have changed

drastically in the past hundred years, resulting in an alteration of the bay’s salinity

regime.

Freshwater inflow to Florida Bay decreased in volume and changed in timing

and distribution during this century because of water management. Hydrologic

alteration began in the late 1800s, but accelerated with the construction of drainage

canals by 1920, the Tamiami Trail by 1930, and the C&SF Project and South Dade

Conveyance System from the early 1950s through 1980 (Light and Dineen 1994).

With the diversion of freshwater to the Atlantic coast and Gulf of Mexico coast, the

bay’s mean salinity inevitably increased. The extent of this increase and how the

variability of salinity changed is not known, but is the subject of current research.

Results from this research indicate that another important development that

altered the salinity regime of Florida Bay was construction of the Flagler railway

across the Keys from 1905 to 1912 (Swart et al. 1996, Swart et al. 1999). It appears

that in the last century, prior to railway construction and water management,

Florida Bay had a lower mean salinity and more frequent periods of low (10 ppt - 20

ppt) salinity than during this century. The extent and frequency of high salinity

events does not appear to have changed between centuries. The bay’s salinity

regime changed abruptly around 1910 because passes between the Keys were filled

to support the railway. Thus, water exchange between Florida Bay and the Atlantic

Ocean was decreased and water circulation throughout the bay was probably

altered.

Two important natural controls of salinity, sea level rise and the frequency of

major hurricanes must also be considered. Florida Bay is a very young estuary, the

product of sea level rising over the shallow slope of the Everglades during the past

4000 years. With rising sea level, the bay not only became larger but also became

deeper. With greater depth, exchange of water between the sea and the bay

probably increased, resulting in a more stable salinity regime with salinity levels

increasingly similar to the sea. However, a factor that has counteracted the rising

sea is the accumulation of sediment, which makes the bay more shallow. Most

sediment that accumulates in Florida Bay is carbonate that is precipitated from
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water by organisms that live in the Bay. The extent to which these sediments

accumulate is a function of the biology of these organisms, the chemistry of the

water, and the physical energy available to transport these sediments from the Bay.

Major hurricanes are thought to be important high energy events that can flush the

bay of these sediments. However, since 1965, no major hurricane has directly

affected Florida Bay. Florida Bay’s ecological changes during the past decade may

thus be indirectly influenced by changing circulation patterns and resultant

changing salinity regimes because of changing water depth in the bay.

Nitrogen and phosphorus inputs. The productivity and food web structure of all

ecosystems is strongly influenced by patterns of nutrient cycling and the import and

export of these nutrients. Throughout the world, estuarine ecosystems have

undergone dramatic ecological changes because they have been enriched by

nutrients derived from human activity. These changes have often been

catastrophic, with the loss of seagrasses and the occurrence of algal blooms and

lethal low oxygen or anoxic events. The input of nitrogen and phosphorus (N and P)

to estuaries is thus a potentially important stressor of estuaries.

The importance of N and P as stressors in Florida Bay is unclear. In general,

the bay is rich in N and poor in P, especially towards the eastern region of the bay

(Boyer et al. 1997). There is little evidence that nutrient inputs to the bay have

increased during this century, but with expanding agriculture and residential

development in south Florida through this century, and particularly development of

the Keys, nutrient enrichment almost certainly has occurred (Lapointe and Clark

1992, Orem et al. 1998). Anthropogenic nutrients that enter Florida Bay are

derived not only from such local sources (fertilizer and wastes from agriculture and

residential areas), but also from remote sources. It is likely that remote

contributions to the Gulf of Mexico, such as from the phosphate fertilizer industry of

the Tampa-Port Charlotte area and residential development from Tampa to Naples,

are the most important external sources of nutients (Rudnick et al. 1999). This

enrichment from external sources, however, may not less important to the bay’s

ecology than it’s own internal sources and cycling. It is, nevertheless, a reasonable

hypothesis that a chronic increase in nutrient inputs has occurred in Florida Bay in

this century and this increase has contributed to ecological changes. Ongoing

research will provide information to test this hypothesis. Development of a water

quality model will also help us understand the effects of past nutrient inputs and

predict the effects of future management scenarios.

In the conceptual model, water management is listed as a source of stress

because the canal system can transport nutrients through the wetlands toward the

bay, decreasing nutrient retention by the wetlands and possibly increasing nutrient

inputs to the bay. Nutrient inputs from the Everglades and the Gulf of Mexico are

affected not only by changes of freshwater flowing from Taylor Slough and Shark

River Slough, but also by changes in bay circulation. Nutrient retention within the
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Bay is certainly sensitive to these changes in circulation, which have been caused by

Flagler railway construction and the balance of sea level rise and sedimentation or

sediment removal by major hurricanes. The influence of hurricanes may be

particularly important, as nutrients (particularly P) accumulate in the bay’s

carbonate sediment and the absence of major hurricanes may have resulted in an

accumulation of nutrients during the past few decades.

Pesticides and mercury. With the widespread agriculture and residential

development of south Florida, the application and release of pesticides and other

toxic materials has increased. Mercury is of particular concern because of high

concentrations of methylmercury in upper trophic level species. However, it is

unclear whether anthropogenic mercury inputs to the Everglades or Florida Bay

have increased or whether mercury cycling and methylation rates have changed.

Pesticides and mercury are of concern because they can affect human health after

the consumption of fish or other biota with high concentrations of these toxins, and

because other species may be adversely affected by these compounds. To date, there

is no evidence the main ecological changes in Florida Bay are in any way linked to

inputs of toxic compounds. Water management affects the distribution of these

toxic materials and potentially their transport to Florida Bay. Controlling water

levels in wetlands may also influence the decomposition of pesticides and mercury

methylation rates because both of these processes are sensitive to the presence of

oxygen in soils, which is affected by water levels.

Fishing pressure. For any species that is the target of recreational or commercial

fisherman, fishing pressure directly affects population dynamics and community

structure. Within Everglades National Park, commercial fishing has been

prohibited since 1985, but populations that live outside of ENP boundaries for at

least part of their life cycle, which includes most of Florida Bay’s sport fish species,

are nevertheless affected by fisheries (Tilmant 1989).

D. ESSENTIAL ECOSYSTEM ATTRIBUTES AND LINKS TO STRESSORS

A set of Florida Bay’s attributes that are either indicators of the health of the

ecosystem or intrinsically important to society are given in the conceptual model.

These attributes in most cases are biological components of the ecosystem, including

seagrass, molluscs, shrimp, fish and birds, but also an aggregated attribute of the

chemical and physical condition of the bay, termed “water quality condition.” While

the list of biological components is broad, it is clear from the links to stressors that

are presented that these attributes are not equally weighted within the model; the

central attribute of this conceptual model of Florida Bay is the seagrass community.

Details of each attribute and linkage are given below.
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Seagrass community. The keystone of the Florida Bay ecosystem is its

seagrasses (Zieman et al. 1989, Fourqurean and Robblee 1999). These plants are

not only a highly productive foundation of the food web, but are also the main

habitat of higher trophic levels and a controller of the bay’s water quality.

Understanding how seagrasses affect water quality is essential for understanding

the current status and fate of the bay.

Seagrasses affect water quality by three mechanisms: nutrient uptake and

storage, binding of sediments by their roots, and trapping of particles within their

leaf canopy. With the growth of lush seagrass beds, these mechanisms drive the

bay towards a condition of clear water, with low nutrients for algae growth in the

water and low concentrations of suspended sediment in the water. During the

1970s through the mid-1980s, lush Thalassia beds grew throughout central and

western Florida Bay and the water was reported to be crystal clear. We hypothesize

that with the onset of a Thalassia mass-mortality event in 1987 (Robblee et al.

1991), these mechanisms reversed, initiating a cycle that causes continued seagrass

habitat loss and propagates persistent turbid water with algae blooms (Stumpf et

al. 1999).

The cause of the 1987 mass-mortality event is not known, but thought to be

related to earlier changes in two stressors, the salinity regime and nutrient

availability. These changes caused Thalassia beds to grow to an unsustainable

density by the mid 1980s. It is also likely that a decrease in shoal grass and

widgeon grass (Halodule and Ruppia) occurred with the Thalassia increase.

Thalassia “overgrowth” may have occurred because the species thrived when the

salinity regime of the bay was stabilized, with few periods of low salinity. Nutrient

enrichment also may have played a role, with a chronic accumulation of nutrients

caused by increased inputs over decades or decreased outputs because of the

absence of major hurricanes or closure of Keys’ passes. The factors that conspired

to initiate the mass-mortality event in 1987 are also unknown, but thought to be

related to the high respiratory demands of the dense grass beds and accumulated

organic matter. During the summer of 1987, with high temperatures, sulfide levels

may risen to lethal concentrations.

Regardless of the cause of the mass-mortality event, once this event was

initiated, the ecology of Florida Bay changed. The cycle causing continued seagrass

habitat loss, which characterizes the present Florida Bay, is illustrated in the

model. Continued seagrass mortality results in increased sediment suspension and

increased nutrient release from the sediments (>N & P), stimulating the growth of

algae in the water column. The presence of both these algae and suspended

sediment result in decreased light penetration to the seagrass bed. In this cycle, it

is this decreased light that stresses the seagrasses and sustains the feedback loop.

Light penetration is thus an essential aspect of the attribute, water quality.
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The dynamics of this feedback loop are probably not independent of the

salinity regime. A disease of seagrass, caused by a slime mold infection, seems to be

more common at salinities near or greater than seawater (= 35 ppt) than at low (15

to 20 ppt) salinities (Landsberg et al. 1996). This may have played a role in either

the initial seagrass mass mortality event, but more likely has served to continue

seagrass mortality since that event. The incidence of this disease may be directly

affected by water management actions.

If the state of the seagrass community is to be used as a criterion to decide

the success of environmental restoration efforts, environmental managers must

specify the desirability of alternative states. The consensus among scientists is that

the Florida Bay of the 1970s and early 1980s, with lush Thalassia and clear water,

was probably a temporary and atypical condition. From an ecological perspective,

restoration should probably strive for a more diverse seagrass community, less

dominated by Thalassia than during that period.

Water quality condition. Water quality condition reflects not only obvious

characteristics, such as salinity, but also the light field, algae in the water column,

and the availability of nutrients in the ecosystem. All of these characteristics are

closely related to the condition of seagrasses and the food web structure and

dynamics of the bay. While these characteristics have been monitored and

researched since the early 1990s, earlier information is scarce for salinity and even

less available for other characteristics. Thus, at the present time, we do not know

whether nutrient inputs to the bay have actually increased in recent decades or

whether periods with sustained algal blooms and high turbidity occurred in the

past.

Salinity has frequently been suggested as a primary restoration target.

However, establishing salinity success criteria, such as those used in the Restudy’s

evaluation of the effects of hydrological alternatives on coastal salinity, depends on

the development of a model of the “natural” salinity distribution of Florida Bay in

time and space. This requires a both a water budget for the bay (monitoring

rainfall, evaporation, and freshwater flow, water level, and salinity) and a

hydrodynamic model, which is now under development. With modeled salinity

variability for a wide variety of target sites in the bay, the fit of observed salinity

fields to modeled fields could serve as the basis of deciding levels of success.

The magnitude of nutrient inputs to the bay, and their relationship to

freshwater inputs is under investigation. Success criteria based on water column

nutrient concentrations are probably less meaningful than criteria based on

nutrient loading. Preliminary results indicate that phosphorus loads to the bay do

not greatly increase with increased freshwater inputs (Rudnick et al. 1999), but

Florida Bay is probably very sensitive to any increase in P availability. Unlike

phosphorus, nitrogen loads probably do increase with more freshwater flow and
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algae blooms in western and central Florida Bay appear to be stimulated by

increased N (Tomas 1996).

Finally, as emphasized earlier, the penetration of light through Florida Bay

waters is a key to the health of seagrasses. An important success criterion should

be light penetration, which is largely a function of turbidity from algae and

suspended sediment. Light penetration should be sufficient to support a viable

seagrass habitat. Such light-based criteria have been used successfully in other

estuaries.

Molluscs. Because of our ability to assess historical community structure, molluscs

are a good indicators for the entire ecosystem. The composition and activity of the

molluscan community is a function of salinity, seagrass and other habitat

availability, and food supply. Studies of long-term changes in the composition of

this community (by analyzing shells in the sediment) have indeed found changes

that reflect the large-scale changes of the bay’s salinity regime. Furthermore,

molluscs are likely to be important as grazers of algae in bay waters; the trophic

status of the bay is reflected by molluscan community composition.

Pink shrimp. Pink shrimp are intrinsically important to society as an economic

asset. They are also ecologically important, serving as a major component of the

diet of game fish and wading birds; pink shrimp are an indicator of the bay’s

productivity. Florida Bay and nearby coastal areas are a primary nursery ground

for pink shrimp - a nursery that supports the shrimp fishery of the Tortugas

Grounds (Costello and Allen 1966). Hydrological and ecological changes in the

Everglades and Florida Bay may have impacted this fishery, which experienced a

decline in annual harvest from about 10 million pounds per year in the 1960s and

1970s to as little as 2 million pounds per year in the late 1980s (Ehrhardt and

Legault 1999). This decline may have been associated with seagrass habitat loss or

high salinity (50 to 70 ppt) during the 1989-1990 drought; experiments have shown

that pink shrimp mortality rates increase with salinities above 40 ppt (Browder et

al. 1999). Shrimp harvest statistics indicate that shrimp productivity increases

with increasing freshwater flow from the Everglades (Browder 1985).

Fish populations. The health of Florida Bay’s fish populations is of great

importance to the public; the sport fishing is a major economic asset to the region.

It is clear from recent studies that seagrass beds and the mangrove zone are

important habitats for fish, but no dramatic bay-wide decreases in total fish

abundance have been observed along with seagrass mass-mortality (Thayer et al.

1999). Rather, a shift in the species composition of this upper trophic level has

occurred as a result of the cycle of seagrass habitat loss and sustained algae blooms.

While some fish species have declined, fish that eat algae in the water, such as the

bay anchovy, are thriving. Thus the stressors, such as altered salinity, not only
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affect upper trophic level animals directly, but also affect them indirectly through

food web changes.

Another important stressor that needs to be considered with regard to fish

populations is the impact of pesticides and mercury. As concentrations of mercury

and some pesticides greatly increase in upper trophic level animals, such as sport

fish, (via the process of bioaccumulation), and people eat such fish, a human health

issue potentially exists. Pesticides and mercury can also have ecological impacts by

physiologically stressing organisms (particularly reproductive functions). The

extent of any existing problem with these toxic compounds in Florida Bay is being

investigated, but they currently do not appear to significantly impact human health

or ecological health in the bay. The possible impact of future restoration efforts on

these issues, however, must still be considered.

Among the many fish species that could be used as indicators of the health of

the ecosystem’s upper trophic level, there is consensus among scientists that spotted

sea trout is a key species. This is the only major sport fish species that spends its

entire life-span in the bay. Population changes and toxic residues in this species

thus reflect the specific problems of the bay and should also reflect the restoration

actions that we take. For northeastern Florida Bay, the abundance of snook,

tarpon, and crevalle jack should also be considered.

Water Birds. Florida Bay and its mangrove coastline is an important feeding

ground and breeding ground for water fowl and wading birds. Conceptual models

for other regions of the Everglades, particularly the mangrove - estuarine transition

zone conceptual model, present more detailed descriptions of the use of bird

populations as ecological indicators and consider a wide variety of birds. For the

Florida Bay conceptual model, we consider only fish-eating birds, such as osprey,

brown pelicans, and cormorants. These birds are important predators of fish in the

bay and are potentially impacted by any stressors that affect their prey base,

including salinity changes, nutrient inputs, toxic compounds, and fishing pressure.

As with other top predators, these bird species are the most vulnerable members of

the ecosystem with regard to pesticide and mercury effects.

E. MEASURES

A list of fundamental measures associated with each of the model’s ecosystem

attributes is given. This list should be considered minimal; interpretation of many

of these measures requires a set of associated measures. The list includes not only

“structural” variables (for example, pink shrimp abundance), but also dynamic,

process variables (for example nutrient fluxes). Note that this list does not reflect

the temporal or spatial time scale at which measurements are necessary, but

temporal patterns, such as seasonality and interannual variability, and spatial
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patterns are a central aspect of ecological dynamics. Also note that the power to

predict the fate of any ecosystem requires more than monitoring; research and

modeling are also essential components of sound environmental management.

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129

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