JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

VOL. 36, NO. 1 AMERICAN WATER RESOURCES ASSOCIATION FEBRUARY 2000

ESTABLISHING POLLUTANT LOAD REDUCTION TARGETS

FOR THE INIIAN RWER LAGOON, FLORIDA1

Gilbert C. Sigua and Joel S. Steward2

ABSTRACT. Establishment of seagrass-based pollutant load reduction targets is a major aim of water quality management in the

Indian River Lagoon (IRL) estuary. It is believed that light and

water clarity are the limiting factors affecting seagrass abundance

and distribution in the Lagoon. Thus, targeted reductions of nutrients, dissolved organic matter andlor suspended sediments should

achieve desired seagrass coverage goals. On an annual average, the

Lagoon is receiving external loadings of 5,346, 771, and 54,408 metric tons per year of nitrogen (N), phosphorus (P), and total suspended solids (TSS), respectively. Most of the loadings are stormwater

generated; thus, implementation of reduction targets will key in on

major urban and agricultural stormwater systems. The process of

developing targets requires the establishment of light requirements

for seagrass. Two methods are being employed to develop such targets. One method is the application of a predictive, 3-D model that

accounts for the essential interactive processes of the Lagoon:

hydrology, hydrodynamics, water quality, nutrient uptake/release,

seagrass growth rates, and light attenuation. The other method is

based simply on the comparison of loading rates between a developed sub-basin with associated seagrass impacts and an undeveloped sub-basin with adjacent healthy seagrass coverages. The

latter method will provide the initial and preliminary targets that

can be tested, confirmed, or modified by application of the 3-D

model.

(KEY TERMS: nutrient; N; P; TSS; loading; estuary; IRL; seagrass;

runoff.)

INTRODUCTION

The IRL of East Central Florida is a biogeographic

transition zone, rich in habitats and species, with the

highest species diversity of any estuary in North America. Habitat and species diversity in the Lagoon

are believed to be affected by the decline in water

quality. There is growing evidence that the ecological

and biological integrity of the Lagoon has declined

during the last 50 years. There are three major types

of impacts responsible for this decline: (1) pollution

from point and nonpoint sources; (2) disruption in the

natural patterns of water circulation in the Lagoon;

and (3) alterations in freshwater inflows, especially

augmentation of wet season discharges.

Increased anthropogenic activities have altered

hydrologic and hydrodynamic patterns and increased

the loading of pollutants, especially of nutrients

and suspended matter (Steward et al., 1994). These

pollutants are transforming the Lagoon from a

macrophyte-based system to a phytoplankton or algalbased system. The indicator of this transformation is

the loss of seagrass near urbanized areas, where

water quality has declined. To reverse this transformation, the establishment of pollutant load reduction

targets (PLRTs) must be pursued with reductions

based on the water and sediment quality requirements of selected living resources of the IRL basin,

such as seagrass and clams. These targets will allow

managers to develop goal-oriented strategies aimed at

specific pollutants and their sources.

The St. Johns River Water Management District

(SJRWMD) is in the process of developing PLRTs

based on the environmental requirements for seagrass growth and expansion. In the IRL, seagrasses are indicators of water quality and ecological integrity. Seagrass habitats are the foundation for a billion

dollar per year coastal fisheries industry in east central Florida (Steward et al., 1996).

Reductions in nutrients, dissolved andlor suspended matter loadings will aid sufficient penetration of

sunlight for the expansion of seagrass to targeted

depths and areal coverages (Steward et al., 1996). It is

1Paper No. 98074 of the Journal of the American Water Resources Association. Discussions are open until October 1, 2000.

2Respectively, Environmental Scientist and Technical Program Manager, Division of Environmental Sciences, Department of Water

Resources, St. Johns River Water Management District, P.O. Box 1429, Palatka, FL 32178-1429 (E-MaiL/Sigua:gilbert_sigua@ district.sjrwmd.state.fl.us).

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 123 JAWRA
Sigua and Steward

the consensus of many investigators that the primary

factor controlling the growth of seagrass is the

amount of sunlight that reaches seagrass blades

(Kenworthy and Haunert, 1991; Dennison et al., 1993;

Morris and Tomasko, 1993). Light penetration is

strongly affected by water clarity, which can be monitored using indicators such as suspended matter,

color, and phytoplankton density. Excessive nutrients

in the water column can induce the growth of phytoplankton and seagrass blade epiphytes. Dense epiphytic algae growth has been shown to be detrimental

to seagrass growth (Hanisak, 1995; Short et al., 1995;

Virnstein et al., 1987).

The purpose of this paper is to describe two methods for the development of PLRTs in the Indian River

Lagoon, Florida.

The Study Area

The SJRWMD's jurisdiction within the IRL system

extends about 248 km from Ponce de Leon Inlet to Ft.

Pierce Inlet along the east coast of Florida. The IRL

system is composed of three inter-connected estuarine

lagoons: Mosquito Lagoon, Indian River Lagoon, and

the Banana River Lagoon (Figure 1). The study area

is relatively flat with relief averaging less than 1 m

km-1 in many areas. The mean water depth within

the Lagoon is 0.84 m. The effect of tides, surface

runoff, and wind mixing are complex and vary locally

within the estuary. Little flushing action exists at the

northern end of the estuary as tidal influence is minimal and overshadowed by wind. In areas close to the

inlets, tidal elevations and currents are more pronounced and flushing rates increase.

Rainfall in the Lagoon basin averages about 125

cm annually. Runoff enters the Lagoon as diffuse

overland flow and through numerous creeks and several man-made canals (Rao, 1987). Most of the soils in

the basin are moderately to poorly drained sandy soils

with loamy or organic stained layers, known as

argillic or spodic horizons. The eastern mainland

landscape is interspersed by flatwood marshes, mixed

hardwoods, and wet/dry prairies (USDA-SCS, 1974;

USDA-SCS, 1978; USDA-SCS, 1980).

The IRL can be divided hydrologically and hydrodynamically into six segments (Figure 1). The three

northern segments are characterized by wide expanses of water, small watersheds, low tidal ranges, and

low tidal flushing or exchange (Segment 1A —

Mosquito Lagoon, ML; Segment lB — Banana River,

BRL; and Segment 1C —North IRL, NIRL). Segment 2 includes the remaining areas north of Sebastian

Inlet and has large watersheds and greater influx

of freshwater runoff and streamfiow from tributaries

(North Central IRL, NCIRL). The two southern seg

ments represent stretches, where the drainage basin

has been artificially expanded through interbasin

diversion projects (Segment 3 — South Central IRL,

SCIRL; and Segment 4 — South IRL, SIRL). Tidal

exchange in Segments 3 and 4 is high due to the pres

ence of three inlets (Steward and VanArman, 1987).

Water Resources Issues and Problems

Sources and Nutrient Levels. The increasing

levels for N, P, and TSS may be a result of increasing

loadings of nutrients and soils to the Lagoon. More

tributary streams and canals (and much larger

drainage areas as a result of interbasin diversion pro

jects) are found in the central/southern IRL (e.g., IRL

SUS, IRLVNC, IRLVMC, IRLVSC) than in the

northern (e.g. IRLAUS, IRLHUS) sub-basin (Table 1).

Anthropogenic land uses also intensify (urban and

agriculture) as one travels from north to south in the

IRL basin. Flushing rates are quite slow throughout

much of the IRL except within a few miles of the

inlets where complete flushing may occur in a matter

of days. Therefore, water quality and seagrasses in

the ML, BRL and the northern/central IRL (north of

Titusville to Melbourne) would be more sensitive to

increases in pollutant loadings than in the southern

IRL near Sebastian Inlet.

Mosquito Lagoon generally exhibits "fair to good" water quality relative to the State of Florida stan

dards (FDEP, 1994). Except for occasional cloudy

water conditions caused by wind resuspending bottom

sediments, most of the area in the ML is considered

pristine habitat. The present water quality conditioni

of ML are associated with the lack of urbanization

and the relatively small amount of agricultural discharges from nearby citrus groves. Water quality in the BRL is correlated with the

amount of urban development and the history of

wastewater treatment plant discharges. Pollutanth

from these sources have resulted in organic enrichment of sediments (muck) in urban canals and the

deeper areas of the Lagoon. Water quality in the mini

mally developed northern portion of the BRL, near

the Kennedy Space Center, is good and seagrass coverage is excellent (Virnstein and Morris, 1996). However, south of Cape Canaveral, the BRL basin i

highly urbanized showing degraded water quality and

lower seagrass coverage with losses of about 50 percent since the 1940s (Steward et al., 1994). One of the

most highly urbanized and poorest water quality

areas in the Banana River is the Sykes Creek! Newfound Harbor sub-basin (FDEP, 1994).

JAWRA 124 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
Establishing Pollutant Load Reduction Targets for the Indian River Lagoon, Florida

Figure 1. Indian River Lagoon's Basin Boundary and Segments.

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 125 JAWRA
Sigua and Steward

TABLE 1. Estimated Rate of Tributary Loadings for TSS, P, and N to the IRL System.

Drainage Basin!

Tributaries

Drainage Area

(ha)

Latitude!

Longitude

(degrees)

Average Flow

(m3yr1)

TSS

(kg halyrl)

P

(kg ha1yr1)

N

(kg ha1yr1)

Addison Creek (IRLAUS) 809.4 28.54N 80.79W 2,810,474.2 33.5 0.43 5.82

Horse Creek (IRLHUS) 709.0 28.17N 80.64W 3,657,011.6 22.8 0.35 5.72

Eau Gallie River (IRLEGU) 2507.1 28.12N 80.63W 24,856,165.7 148.7 1.81 16.93

Crane Creek (IRLCCU) 4725.3 28.08N 80.60W 49,442,334.7 132.4 1.75 14.70

Thrkey Creek (IRLTPM) 28505.4 28.02N 80.60W 116,992,332.4 27.5 0.40 6.78

Goat Creek (IRLGUS) 4146.2 27.97N 80.58W 12,275,869.2 20.9 0.22 3.82

Trout Creek (IRLTRU) 2386.9 27.95N 80.58W 3,987,873.5 7.1 0.20 2.50

Sebastian River (IRLSUS) 43899.8 27.85N 80.49W 246,518,257.1 93.9 0.84 19.58

Vero North Canal (IRLVNC) 5296.3 27.69N 80.41W 30,045,116.2 73.6 1.56 8.88

Vero Main Canal (IRLVMC) 8784.0 27.65N 80.40W 67,439,123.5 107.7 2.27 13.69

Vero South Canal (IRLVSC) 6706.3 27.60N 80.38W 39,591,001.7 68.4 1.47 9.31

Water quality in the NIRL and SIRL is influenced

both by urban and agricultural development and

proximity to inlets. In the NIRL, development is currently limited and water quality is good to excellent.

However, water quality along the developed western

side between Titusville and Cocoa is somewhat poor

due to urban runoff and past wastewater treatment

plant effluent. In the Melbourne area, water quality is

good except for the immediate vicinities of Turkey

Creek, Crane Creek, and the Eau Gallie River. These

tributaries receive significant interbasin flows, and

consequently discharge large volumes to the Lagoon.

Urban runoff contributes huge amounts of nutrients

and TSS (organic matter and soils) to the IRL (Table

1). The history of wastewater treatment plant discharge into these tributaries, now reduced, contributed to the deposition of muck in the tributaries and

some IRL bottom areas (NEP, 1996). Seagrass loss in

the Melbourne segment has been considerable, with

about 80 percent loss in seagrass acreage over the last

50 years (Steward et al., 1994). Water quality near

Sebastian Inlet meets minimum state water quality

standards (FDEP, 1994), because of the rapid flushing

rate through Sebastian Inlet.

Population growth and land use intensification

have been appreciable in the IRL basin, particularly

in the central and to a lesser degree in the south. The

population throughout the basin has more than doubled since 1970, from about 302,000 to 679,000 in

1990 (NEP, 1996). Since 1970, the greatest population

densities have existed in the central IRL, from southern BRL southward to Sebastian, with a current density of roughly 450 people per km2. From Sebastian

southward, the land use is predominantly agricultural, although residential land use is expected to

increase after 2010 (Woodward-Clyde, 1994).

As part of this rapid increase in human population

and activities, the IRL basin has experienced profound drainage improvements. In the SCIRL and

SIRL segments, extensive canal systems have been

constructed since the 1920s, to accelerate drainage of

over 1800 km2. Most of these lands historically

drained west to the Upper St. Johns or Okeechobee

basins (Steward and VanArman, 1987). These interand intra-basin drainage improvements augment the

pre-development rates and volumes of freshwater discharges, and, consequently, increase the loadings of nutrients and soils to the IRL. In the NIRL and ML

sub-basins, drainage hydrology should mimic historical patterns because drainage features have not been

altered, and land use intensification is comparatively

low.

In the SIRL, water quality and seagrass coverages

have improved, especially near Sebastian Inlet, probably because of strong oceanic flushing through the

inlet. However, there is also an enormous amount of

nutrient and TSS loading with extremely high phosphorus loadings and concentrations. Large concentrations of discharge-related P in the SIRL (Sebastian to

Vero Beach) are significantly and strongly correlated

JAWRA 126 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
Establishing Pollutant Load Reduction Targets for the Indian River Lagoon, Florida

with the high chlorophyll a production in that segment (Sigua et al., 1999).

The projection of nutrient loadings into the Lagoon

clearly indicate that stormwater (urban + agricultural) contributions were unacceptably high (Figure 2) and that measures should be taken to reduce this

problem (Harper, 1995). Some possible measures are

listed below:

• reduce inter-basin freshwater discharges to

restore healthy salinity regimes and reduce external

loadings; • install best management practice facilities as a

retrofit strategy in existing developments (baffle box,

curb inlet traps, retention and detention ponds);

• encourage the use of swales and erosion control

methods that stabilize canal banks;

• encourage the establishment of local stormwater

utilities with runoff reduction incentives based on

percent of pervious area on their property or other

stormwater treatment methods employed; and

• fully implement Florida Law 90-262 which calls

for the elimination of existing and prohibition on new

domestic wastewater effluent discharge to the

Lagoon.

"Seagrass": Resource Target in the IRL. Seagrass is a valuable living resource in the IRL because

it provides habitat and nursery areas for estuarine

animals (Virnstein et al., 1987). It can enhance water

quality by nutrient uptake and sediment stabilization. Seagrass beds are one of the most productive

ecosystems on the planet (Zieman, 1982).

Seagrass serves as an indicator of overall Lagoon

condition because of its sensitivity to water quality

constituents, integrated over hours, days, months, or

years (Virnstein and Morris, 1996). Seagrass beds have been identified in the IRL Surface Water

Improvement and Management Plan as the most critical habitat in the Lagoon (Steward et al., 1994).

Principles and Processes: Establishing Pollutant Load Reduction Target. The Florida Legislature, Florida Department of Environmental

Protection (FDEP), and the SJRWMD under the State

of Florida Water Policy (Chapter 62-40) acknowledge

that the setting of PLRTs should be done in a scientific manner to be reasonably assured that the targeted

resources can be restored or protected. A key strategy

is to pursue reductions in nutrients and dissolved

and/or suspended matter loadings, that will allow sufficient penetration of sunlight for the expansion of

seagrass to pre-defined depths and aerial coverages. The preliminary target is to extend and to maintain

seagrass beds in water depths up to 1.7 m. This goal

is based on the maximum depth of seagrass in undisturbed "healthy" areas (Virnstein and Morris, 1996).

In addition, the relationship between seagrass growth

and nutrient levels will have to be established to predict seagrass response to changes in nutrient level.

Figure 2. Different Sources of Point and Nonpoint Sources of Nitrogen and

Phosphorus Loadings in the IRL (Source: Woodward -Clyde, 1994).

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 127 JAWRA
Sigua and Steward

The development of PLRTs requires information on

waterbody nutrient concentrations, external nutrient

loading rates, sediment nutrient flux estimates, and

nutrient turnover (uptake + release) rates of the

major plant communities in the Lagoon. Because conditions in the Lagoon vary spatially (Sigua et al., 1995; Sigua et al., 1998), it is impossible to set a

meaningful target for the entire Lagoon. The Lagoon

needs to be divided into several relatively homogeneous segments (Figure 1) and a nutrient concentration target then would be established for each

segment of the Lagoon. In order to set PLRTs, quantification of important input parameters must be done

first. The IRL is appropriately characterized as a composite of several segments of differing hydrodynamic

(Sheng, 1996), water quality (Sigua et a!., 1995; Sigua

et al., 1996; Sigua et al., 1998; Sigua et al., 1999), and

biological features (Virnstein et al., 1987). Listed

below are the essential input parameters for each segment:

• spatial distribution and temporal variation of

nutrient concentrations;

• nutrient fluxes (diffusionallresuspension) and

exchanges in sediments-water columns and benthicdetritus;

• rate and mass of nutrient decomposition/partitioning in water column and sediments;

• transformation and transport of nutrients to and

from water column and sediments;

• inflow of nutrients (mass and rate) from point

and nonpoint sources;

• flux of nutrients (mass and rate) through neighboring segments and inlets;

• optimum nutrient uptake and enrichment of seagrass for optimum productivity; and

• flow data measurements.

METHODS FOR ESTABLISHING POLLUTANT

LOAD REDUCTION TARGETS

The process of developing targets requires the

establishment of light requirements for seagrass and, consequently, the thresholds for nutrients and suspended sediment concentrations/loads to restore and

maintain those light requirements in the Lagoon.

Two methods are being employed to develop such targets. One method is the application of a predictive,

three-dimensional (3-D) model that accounts for the

Lagoon's essential interactive processes: hydrology,

hydrodynamics, water quality, macrophyte and phytoplankton nutrient uptake/release and growth rates, and light attenuation. The other method is based simply on the comparison of loading rates between a

developed sub-basin with associated seagrass impacts

and an undeveloped sub-basin with adjacent healthy

seagrass coverages. The latter method will provide

the initial, preliminary targets, which can be tested,

confirmed, or modified by application of the 3-D

model. These two methods are described below.

A. Indian River Lagoon-Pollutant Load Reduction

(IRL-PLR) Model

The SJRWMD entered into an agreement with the

University of Florida in 1994 for the development of

the IRL-PLR Model. The IRL- PLR model is a 3-D,

process based computer simulation model that uses

dynamic equations calibrated for a number of inter

acting proceses: hydrology, hydrodynamics, salinity,

water quality, and light attenuation (Figure 3). The

model will be used by the SJRWMD as a tool for the

setting and testing of PLRTs or numerical targets for

the reduction of N, P, and dissolved/suspended matter

loadings to the IRL. The development and use of the

IRL-PLR model is based on the premise thai,

improved water clarity will restore seagrass beds.

The model integrates various sub-models or mode]

components, and can run in sequence with an option

to omit those components not required for a specific

simulation (Steward et al., 1996). Integration of the

components will proceed in a step-wise fashion begin

ning with the coupling of the hydrologic, hydrodynam

ic, and salinity models. These models must be fully

linked, and calibrated before incorporating the water

quality model. Then the light attenuation model will

be incorporated to produce the final quantitative link

among sub-basin loadings (Figure 3). Once the model

is calibrated and verified for the full range of expected

meteorological and hydrological conditions (e.g., storms, dry periods, etc.), then it will be able to

describe the Lagoon's response to any realistic combi

nations like land use intensification or increased

nutrient loading.

It is expected that at least five years is required to

set up, calibrate, and verify the model before refined

PLRTs can be established. This length of time is need

ed because of several reasons:

• the large size of the Lagoon;

• the number of component processes (hydrology,

hydrodynamics, salinity regimes, water quality, and

light attenuation) that needs to be integrated; and

• the number of variables associated with each

component that require simultaneously collected data

over a range of time scales (event, diurnal, or seasonal).

JAWRA 128 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
Establishing Pollutant Load Reduction Targets for the Indian River Lagoon, Florida

Loadings

Figure 3. Seagrass as the "Target Resource" for the Establishment

of PLRTs in the IRL Basin (adapted from Steward et al., 1996).

Following its development, the IRL-PLR model will

be used in an optimization procedure, that incorporates economic, regulatory, and other technical constraints, to establish final PLRTs and allocate these

reduction targets among the watershed sources in an

equitable manner.

B. Impact Analyses Based on Pristine-Healthy

Seagrass Environment

Initial PLRTs will be set by comparison of loading

rates and amounts for basins that have impacted and

healthy seagrass beds. Figure 4 shows a graphical

relationship of impacted and pristine-healthy seagrass environment in terms of required nutrient (e.g.,

P) reduction. This method provides initial, prelimi- nary targets that can be tested, confirmed, or modified by application of the 3-D model.

The SJRWMD is proceeding with a preliminary

five-step process for the establishment and implementation of the PLRTs based on pristine-healthy seagrass beds for the Lagoon. These five-step process is

given below.

Step 1. Define Water Quality Targets (concentration and Loading) for Each Segment. For each

segment in the Lagoon, specific water quality targets will be established based on an understanding of the

needs of the seagrass resource target. Water quality

targets could be the average of inter-basin's trophic

state index values (FDEP, 1996) or maximum concentrations of N, P, or suspended matter to ensure that

sufficient sunlight is reaching target depths within

each segment to allow seagrass growth. The prelimi- nary target is to re-establish and maintain seagrass

beds in water depths up to 1.7 m. This goal is based

on the maximum depth of seagrass in undisturbed

"healthy" areas (Virnstein and Morris, 1996). It is

assumed that the biological productivity and diversity

in the Lagoon is dependent upon healthy seagrass, which is dependent upon good water quality and clarity.

Step 2. Quantify Current Nutrient Concentration or Loadings (CNCILs) Between Impacted

Segments and Pristine Reference Area. The evaluation of CNC/Ls is accomplished through a variety of

means including the collection of data during the

monthly ambient water quality sampling from the different impacted areas and pristine reference area

(e.g., site "IRLIO2").

Step 3. Establish Allowable Nutrient Concentration or Loadings (ANCILs). The ANC/Ls is the

maximum nutrient concentration or loading to the

receiving waters which will enable the achievement of

the IRL segment target loading capacity, which is

defined as the greatest nutrient concentration or loadings that a system can receive without negatively

impacting seagrass (Step 1).

Step 4. Determine Pollutant Load Reduction

Target (PLRTs). The PLRTs for N, P, and TSS cannot exceed the difference between the ANC/Ls and

CNC/Ls. Where data are estimated, the PLRTs are

considered provisional.

Step 5. Implementation and Assessment.

Implementation of the PLRTs will occur through

many means and at variety of governmental levels.

Examples include water control districts through

revised plans of surface water management (Chapter

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 129 JAWRA

PLRT

Model

•1

Hydrodynamic

Sub-Model

• Water Quality

Sub-Model

Water Quality I

• Light Attenuation

Sub-Model

Light Attenuation

I.

-. LightRequirements

• Literature Data

and Assumptions
Sigua and Steward

Figure 4. Relationship of Impacted and Pristine-Healthy Seagrass

Environment in Terms of Required Nutrient (e.g., P) Reduction.

298), local governments through stormwater management plans and utilities, and federal government

through point and nonpoint source (319) programs.

Example of Impact Analyses

As mentioned earlier, this method is based simply

on the comparison of loading rates between subbasins with seagrass impacts and an undeveloped

sub-basin with adjacent healthy seagrass coverages. This example of impact analysis shall be limited to P

reduction only (Figure 4), but the method is also

applicable to N and TSS reduction.

First, the long-term (1988-1994) P spatial distribution (north-south) from water quality station near or

at seagrass beds in the IRL was analyzed graphically

(Sigua et al, 1998; Sigua et al, 1999). Figure 4 shows

the north-south P distribution in the Lagoon. An

increasing north-to-south concentration gradient

(Sites IRLIO7 to IRLIRJ12) for P exists in the IRL.

The concentration of P from each impacted station

was compared with long-term mean of P concentration for the station having adjacent healthy seagrass

coverage. In general, all stations (IRLVO2 to

IRLIRJ12) were compared with station "IRLIO2"

(mean P concentration 0.056 mg L', and dense,

healthy, and continuous seagrass bed) to determine

the absolute difference of P concentration between the

sites (Figure 4).

Second, the required reduction in P concentration,

and the corresponding percent P reduction to attain

the P concentration at site "IRLIO2" were determined.

Results of these analyses are given in Table 2. Except

for station IRLIO7, all other stations would require

reduction in P concentration to attain the P level

exhibited by site "IRLIO2." Moreover, stations located

in the SCIRL and SIRL will require more P reduction

than those stations in the ML, BRL, and NIRL (Table

2). The required average percent reduction to attain

the "IRLIO2" concentration for the whole IRL are a

follows: NIRL (21.6) < ML (28.5)  BRL (28.8)

SCIRL (36.3) < SIRL (55.1). Results from this method

provide initial, preliminary targets that can be tested,

confirmed, or modified during evaluation with the 3-I)

model.

SUMMARY AND RECOMMENDATIONS

The Florida Legislature, FDEP, and the SJRWMT)

acknowledge that the setting of PLRTs should not be

done in an arbitrary manner. It should be done in a

-I-IMPACFEI) STATION —PRISTINE(BASELINE)

P TARGET COI.V. OF P (IJJqPACThD SUES) -COI. OFF

0.14 P TARGET @ IRLV11 = 0.082 0056 0 026 mgfL

0.12 P TARGET @ IRURJO5 01

0.10

0.08

(IDC

0.06

0.04

C

E 0.02

0.00 . . . , . . . , • 4,1D 4

STATION (NORTH - SOUTH)

IRUO2 STATION 147TH

DENSE, CONTIMJOUS

SEA GRASS, (PRISTINE

SITE asBASELINE)

JAWRA 130 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
Establishing Pollutant Load Reduction Targets for the Indian River Lagoon, Florida

scientifically defensible manner to be reasonably

assured that the living resources will be restored or

protected (Steward et al., 1996).

TABLE 2. Required Reduction of P Concentration and Estimated

Percent P Reduction for the Impacted Areas to Attain

Healthy Seagrass Conditions Described in Site

"IRLIO2" (pristine reference site).

Water

Quality Station

Near

Seagrass Beds

Latitude/

Longitude Coordinates

(percent)

Required Reduction

in P

Concentration

(mg L1)

Percent P

Reduction to

Attain the

Condition

at Site

"102"

IRLVO2 29.03N 80.91W 0.026 31.7

IRLVO5 29.O1N 80.91W 0.016 22.2

IRLV11 28.95N 80.84W 0.025 30.7

IRLV17 28.88N 80.84W 0.023 29.1

IRLBO2 28.43N 80.64W 0.021 27.3

IRLBO6 28.28N 80.63W 0.025 28.2

IRLBO9 28.20N 80.63W 0.033 30.7

1RL107 28.60N 80.80W (0.011) (24.4)

IRLI1O 28.50N 80.77W 0.002 3.5

IRLI12 28.41N 80.74W 0.020 26.3

IRLI16 28.28N 80.68W 0.030 34.9

IRLI18 28.19N 80.65W 0.028 33.3

IRLI21 28.13N 80.62W 0.031 35.6

1RL123 28.07N 80.59W 0.035 38.5

1RL127 27.95N 80.53W 0.034 37.8

IRLIRJO1 27.80N 80.45W 0.020 26.3

IRLIRJO4 27.69N 80.39W 0.056 50.0

IRLIRJO5 27.66N 80.38W 0.094 62.7

IRLIRJO7 27.62N 80.38W 0.062 52.5

IRLIRJ1O 27.70N 80.39W 0.062 52.5

IRLIRJ12 27.61N 80.37W 0.076 57.6

The process of developing targets requires the

establishment of light requirements for seagrass and, consequently, the minimum thresholds for nutrients

and suspended sediment concentrations/loads to

restore or maintain those light requirements in the

Lagoon. Two methods are being employed to develop

such targets. One method is the application of a predictive, 3-D model that will account for the essential

interactive processes of the Lagoon, i.e., hydrology,

hydrodynamics, water quality, macrophyte and phytoplankton nutrient uptake/release and growth rates, and light attenuation. The other method is based on

the comparison of loading rates between a developed

sub-basin with associated seagrass impacts and an

undeveloped sub-basin with adjacent healthy seagrass coverages. The latter method provides the initial, preliminary targets, which can be tested, confirmed, or modified by application of the 3-D

model.

Acceptance of the preliminary PLRTs for the

Lagoon could serve as numerical restoration targets

for seagrass or other biological resources in estuarine

system. The allowable N, P, and TSS concentration or

loadings expressed as maximum limits for seagrass

productivity can then be translated into numerical

IRL-PLRTs. Following its development, the IRL-PLR

model wi]l be used in an optimization procedure that

incorporates economic, regulatory, and other technical

constraints, to establish final PLRTs and allocate

reduction targets among the watershed sources in an

equitable manner.

Development of a lagoon-wide PLR model is underway through a contract with the University of Flori- da. The PLR model is an effective tool in the

development of technically defensible and refined

PLRTs for the IRL (Steward et at., 1996). The IRLPLR model is process-based (dynamical), threedimensional (3-D) and predictive. Given any set of

initial physical and boundary conditions, the model

will be capable of predicting the Lagoon's responses

(by segment) to given levels of fresh water, nutrient, and suspended matter loadings. The responses are

(1) concentration of nutrients, color, suspended matter, and level of salinity; (2) relative abundance of

phytoplankton and seagrass epiphytes; and (3) resultant level of light attenuation, which can help indicate whether seagrass coverage targets could be met.

It is expected that at least five years is required to

set up, calibrate, and verify the model before refined

PLRTs can be established. At least this length of time

is needed because of the large size of the Lagoon and

the number of component process that needs to be

integrated (e.g., hydrology, hydrodymanics, salinity

regimes, water quality, and light). It should be recognized that it would not be realistic to assign one set of PLRTs for the entire Lagoon.

The IRL is appropriately characterized as a composite

of several segments of differing hydrodynamic (Sheng,

1996), water quality (Sigua et al., 1995; Sigua et al.,

1996; Sigua et at., 1998), and biological features (Vim- stein et al., 1987). Pollutant load reduction targets

can be established for each Lagoon segment and then

apportioned among each of the segment's hydrologic

sub-basins from Ponce de Leon to Ft. Pierce Inlet in

some equitable fashion. It is more appropriate for the

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 131 JAWRA
Sigua and Steward

Lagoon to be divided into several relatively homogeneous segments, and then apply or recommend N or P

load reductions to that segment to achieve a specific,

maximum N and P concentration. Management

approaches for the protection and restoration of seagrass based on specific nutrient targets could also

vary among segments in the Lagoon.

ACKNOWLEDGMENTS

The authors wish to express their gratitude and appreciation to

Ms. Wendy Tweedale, Environmental Sciences Division, SJRWMD

for her technical assistance, especially in the review of the

manuscript and preparation of figures and other graphics. For their

help and assistance in the review of this manuscript, special thanks

are extended to the following individuals: Dr. Robert Virnstein,

Environmental Sciences Division, SJRWMD; Dr. Getachew

Belaineh, Engineering Division, SJRWMD; and Dr. Apurba Borah,

Engineering Division, SJRWMD.

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