### 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 abundanceand 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, theLagoon 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 requirementsfor 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 isbased 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 amacrophyte-based system to a phytoplankton or algal

### based system. The indicator of this transformation is 

### the loss of seagrass near urbanized areas, wherewater 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, seagrassesare indicators of water quality and ecological integri

### ty. Seagrass habitats are the foundation for a billiondollar per year coastal fisheries industry in east central Florida (Steward et al., 1996).

### Reductions in nutrients, dissolved andlor suspend

### ed 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 WaterResources, 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 JAWRASigua 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 isstrongly affected by water clarity, which can be moni

### tored using indicators such as suspended matter, color, and phytoplankton density. Excessive nutrients

### in the water column can induce the growth of phyto### plankton 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 thenorthern end of the estuary as tidal influence is mini

### mal 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 diffuseoverland 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 hydro

### dynamically into six segments (Figure 1). The threenorthern segments are characterized by wide expans

### es of water, small watersheds, low tidal ranges, and 

### low tidal flushing or exchange (Segment 1A — MosquitoLagoon, ML; Segment lB —BananaRiver, 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 urbanizationand 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. Pollutanthfrom these sources have resulted in organic enrich

### ment 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, nearthe Kennedy Space Center, is good and seagrass coverage is excellent (Virnstein and Morris, 1996). How

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

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

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

JOURNALOF THE AMERICAN WATER RESOURCES ASSOCIATION 125 JAWRASigua 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 andproximity to inlets. In the NIRL, development is cur

### rently 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, contribut

### ed 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 dou

### bled 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 south### ern BRL southward to Sebastian, with a current den

### sity of roughly 450 people per km2. From Sebastiansouthward, the land use is predominantly agricultur

### al, 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 pro

### found 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 Okeechobeebasins (Steward and VanArman, 1987). These inter

### and intra-basin drainage improvements augment thepre-development rates and volumes of freshwater dis

### charges, and, consequently, increase the loadings of 

### nutrients and soils to the IRL. In the NIRL and MLsub-basins, drainage hydrology should mimic histori

### cal patterns because drainage features have not been altered, and land use intensification is comparatively
low. 

### In the SIRL, water quality and seagrass coverageshave improved, especially near Sebastian Inlet, prob

### ably because of strong oceanic flushing through the inlet. However, there is also an enormous amount ofnutrient 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 ASSOCIATIONEstablishing 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 + agricultur

### al) 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 WaterImprovement 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 Legisla

## ture, Florida Department of Environmental Protection (FDEP), and the SJRWMD under the State of Florida Water Policy (Chapter 62-40) acknowledgethat the setting of PLRTs should be done in a scientif

### ic manner to be reasonably assured that the targeted resources can be restored or protected. A key strategy 

### is to pursue reductions in nutrients and dissolvedand/or suspended matter loadings, that will allow suf

### ficient 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 goalis 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 JAWRASigua 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 themajor plant communities in the Lagoon. Because con

### ditions 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 concentra

### tion target then would be established for eachsegment of the Lagoon. In order to set PLRTs, quantification of important input parameters must be donefirst. The IRL is appropriately characterized as a com

### posite 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). Listedbelow are the essential input parameters for each seg

### ment:



### • spatial distribution and temporal variation of nutrient concentrations; 

### • nutrient fluxes (diffusionallresuspension) andexchanges 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 tar

### gets. 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 phyto

### plankton nutrient uptake/release and growth rates,and light attenuation. The other method is based sim

### ply 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 dataover a range of time scales (event, diurnal, or seasonal).

JAWRA 128 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATIONEstablishing 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 con

### straints, 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 graphicalrelationship 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 preliminaryfive-step process for the establishment and implemen### tation 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 (concen

### tration 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 trophicstate index values (FDEP, 1996) or maximum concen

### trations 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 Concentra

### tion or Loadings (CNCILs) Between ImpactedSegments and Pristine Reference Area. The evaluation of CNC/Ls is accomplished through a variety of 

### means including the collection of data during themonthly ambient water quality sampling from the dif

### ferent 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 isdefined as the greatest nutrient concentration or load

### ings that a system can receive without negatively impacting seagrass (Step 1).


### Step 4. Determine Pollutant Load ReductionTarget (PLRTs). The PLRTs for N, P, and TSS can

### not 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 AssumptionsSigua and Steward

Figure 4. Relationship of Impacted and Pristine-Healthy Seagrass Environment inTermsof Required Nutrient (e.g., P) Reduction.

298), local governments through stormwater manage

### ment 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 distribu

### tion (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 stationwas 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 (ID C

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 ASSOCIATIONEstablishing 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 phyto

### plankton 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 ini

### tial, 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 under

### way through a contract with the University of Flori- 

### da. The PLR model is an effective tool in the development of technically defensible and refinedPLRTs for the IRL (Steward et at., 1996). The IRL### PLR 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 mat

### ter, and level of salinity; (2) relative abundance ofphytoplankton and seagrass epiphytes; and (3) resul### tant 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 realis

### tic 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 JAWRASigua 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. Managementapproaches for the protection and restoration of sea

### grass 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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