1

Using multiple lines of evidence for developing numeric nutrient criteria for

Halifax River Estuary, Florida

Joel S. Steward, Whitney C. Green, and Janice D. Miller

Department of Water Resources, St. Johns River Water Management District

June 17, 2010

Photos courtesy of Google Earth and its individual contributors: Dylan (Daytona Beach at top right), 

Sergeastrie (Tomoka R., top left), and JimGvo (barrier island near Ponce De Leon Inlet, bottom).
2

Introduction

The purpose of this report is to present a weight-of-evidence approach for developing

nutrient loading and concentration limit criteria for the Halifax River Estuary. The approach 

consists of multiple methods or lines of evidence whose collective results support a likely set of 

criteria. The format and content of this report will closely follow that of the POWERPOINT 

presentation prepared for the Northeast Florida Estuary and Coastal Numeric Nutrient Criteria 

Development Work shop in Jacksonville, Florida, March 3, 20101

. Please be aware that the 

March 31, 2010 POWERPOINT presentation for Mosquito Lagoon was preliminary; that this 

report contains revisions and supersedes the meeting presentation.

For the Halifax River Estuary (HRE), we used the weight-of-evidence approach because 

there is no known relationship between nutrients and a valued biological resource that can 

serve as a basis for nutrient criteria. Oysters are the primary, aquatic resource in the HRE, 

roughly analogous in form and function to seagrasses in the Indian River Lagoon system. 

Unfortunately, there is not a functional relationship between nutrient levels and oysters (as 

there is with seagrasses) that is appropriate for setting nutrient limits in the HRE. Therefore, 

we employed an approach that consists of three separate analyses (or lines of evidence) that 

generally assessed the trophic state-chlorophyll a-nutrient relationship in the HRE. Loading and 

concentration criteria for total nitrogen and total phosphorus were then determined based on

that assessment coupled with our interpretation of federal and state water policies (1972 

federal Clean Water Act; Chapter 62-40, Florida Administrative Code). We believe those 

policies are intended to maintain the current condition if it is either mesotrophic or 

oligotrophic; or are intended to attain mesotrophy, at minimum, if the current condition is 

worse due to cultural eutrophication. 

General Considerations and Methods

With respect to the Halifax River Estuary (HRE), as well as the other bar-built lagoonal 

estuaries oriented linearly along Florida’s east coast, development of nutrient criteria should 

consider at least 3 points:

(1) Spatial heterogeneity within the system, segmenting the system accordingly for the purpose 

of setting segment-specific criteria;

(2) Temporal shifts in water quality by which a certain period of record representing desirable 

trophic-water quality conditions (mesotrophy or oligotrophy) can be selected to develop 

protective criteria; and

 1 Presentation was accessed at http://www.dep.state.fl.us/water/wqssp/nutrients/estuarine.htm by the primary 

author on May 6, 2010.
3

(3) Multiple lines of evidence, helping to ensure a high level of certainty or confidence 

associated with the nutrient criteria derived from any one or all lines of evidence. 

The lines of evidence used for HRE are a reference period method, chlorophyll a –

nutrient regression analyses, and two empirical models that pertain to estuaries in general. The 

primary source for site-specific data required by these methods is Volusia County 

Environmental Management Department (VCEMD), and the full period of record for these data 

is 1992 - 2008.

Reference period. A specific period of time within the full period of record that provides

the lowest chlorophyll a (Chla) and nutrient levels along with a desirable (if not the lowest)

trophic state was selected to calculate annual and wet-season medians of [Chla], [TN], and [TP]. 

It is preferable that the reference period be 5 consecutive years or longer because Florida is 

subject to multi-year droughts or extreme wet conditions that can affect the magnitude of 

nutrient loading and attendant water quality in receiving waters. A reference period should 

sufficiently capture a number of variable rainfall years, but as a whole, not be biased by any 

extreme meteorological event or period. Because nutrient loadings and concentrations (and 

presumably [Chla] and TSI) in the HRE are largely a function of watershed rainfall-runoff, the 

selected reference period was not biased by prolonged drought or wet condition, but one that 

approximated the 30-year annual average rainfall for the region. A trophic condition was 

considered desirable for a given segment if the average of trophic state index (TSI) values, as 

applied to estuaries (FDEP 1996), was <50. 

Chla – [TN] or [TP] regression models. All data from the full period of record were 

reduced and organized into annual average, average monthly, and average monthly wet-season 

(peak production months of July – September only) data sets. Simple linear regression models 

were developed per segment from these reduced data sets: [Chla] regressed against [TN] and 

against [TP]. If a regression model had a moderate to strong correlation coefficient (r) (or 

coefficient of determination, r2

) and significance (p) statistic, it was used to predict median 

concentration limits based on the reference-period [Chla] medians (the de facto Chla target 

levels). We also performed multivariate regression analysis to identify major variables that may 

regulate algal production (i.e., Chla levels) in the HRE and, if statistically feasible, use as a 

complementary line of evidence to help set nutrient criteria.

General models. Two general nutrient models were applied to the HRE to predict limits 

or acceptable ranges of nutrient loadings and concentrations related to a desirable trophic

state (mesotrophy or oligotrophy). One model is an empirically defined relationship between 

water residence times and nutrient loading limits for mesotrophic Florida systems (Steward and 
4

Lowe 2010). Because the Florida TP loading-limit model is consistent with the Vollenweider 

(1976) TP critical loading model, the Vollenweider model equation was adjusted (specifically

the τw

upper mesotrophic [TP] = L

exponent) to fit the Florida models (Steward and Lowe 2010) to predict the upper 

orlower mesotrophic [TP], or an oligotrophic level of [TP]:

M(P) X τw

0.847/[z(1 + τw0.5

lower mesotrophic [TP] = L

)], which was empirically derived;

M(P) X τw

0.7

/[z(1 + τw

0.5

oligotrophic [TP] = L

)] as a theoretical derivation;

O(P) X τw

0.5/[z(1 + τw0.5)] as a theoretical derivation; whereby LM, LO = 

mesotrophic, oligotrophic loads (g/m2

/yr); τw = water residence time2; z= mean depth (m).

The N-analog of the Vollenweider (1976) model was not equivalent to or consistent with the 

Florida TN trophic loading model (Steward and Lowe 2010) and, thus, the Vollenweider model 

could not be used to predict [TN]. Therefore, the Dettmann (2001) model for estuaries was 

selected to predict [TN] based on a trophically desirable TN loading limit as determined by prior

application of the Florida model. The Dettmann (2001) model equation is:

[TN] = (LN X τw / V + [TN]s)(1/(1 + ατw)); whereby LN = TN loading (g/yr), τw = freshwater 

residence time (yr), V = volume of estuary (liters, L) segment, [TN]s = average [TN] at segment 

boundaries (g/L), α = 1st order rate coefficient for N loss (3.6 yr-1).

Among the three lines of evidence, the reference period provides the primary line of 

evidence because it constitutes the most direct analysis of site-specific data that represents a 

time when trophic state is desirable. The other two lines of evidence are model predictions

(incorporating site-specific data) and regarded as supporting evidence. 

Results and Discussion

Spatial considerations and segmentation. Some salient physical and hydrological 

characteristics of the HRE are listed below.

Length of system -- ~23 miles (37.5 km), wholly within Volusia County

Average width of estuary -- 0.56 miles (0.9 km)

Area of estuary -- ~13.8 mi2 (35.8 km2

Mean depth – 1.4 m from High Bridge near Bulow Cr. To S. Daytona

)

 1.7 m from S. Daytona to Ponce De Leon Inlet

Tidal amplitude – 0.7 m

Area of catchment -- ~340 mi2 (~880 km2

 2 Freshwater residence times (=R63 values) were scaled up for HRE to R96 values to better represent the longer time for 

nutrient detention, recycling, and trophic utilization of nutrients in lagoons as compared to freshwater systems (Steward and 

Lowe 2010). A tidal prism volume to freshwater inflow volume ratio was applied in the scaling of freshwater residence times to 

determine nutrient limit – trophic status relationship in estuaries. The R96 residence times are 0.0267 yr for S. HRE and 0.1248 

yr for N. HRE. The Dettmann (2001) equation uses freshwater residence times.

)
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Catchment area : Estuary area ratio -- ~24:1

Average annual rainfall -- ~50”/yr over last 30 yr

Land use -- 34% range/forest, 22% urban/residential, 5% agriculture, 39% water and other LU

The northern half of the HRE system (N. HRE), its estuary and catchment, is perceived as 

being distinct from the southern half (S. HRE) based on a number of characteristics. For 

example, even though the percent proportion of land uses by area are nearly equivalent 

between the S. HRE and N. HRE (Fig. 1, SJRWMD GIS file data on 2004 land use), their city 

population densities are quite different. The municipal population density in N. HRE, ~3,300 

people/mi2, is about 1.7 times greater than in the S. HRE with ~2000 people/mi2 (Volusia 

County Dept. of Economic Development 2009; www.volusia.org/economicdevelopment).

Hydrodynamic properties probably best describe the estuarine differences between N. 

and S. HRE, and are major factors mediating water quality and trophic response. John and 

Morris (1999) conducted a tracer study with a calibrated RMA2 model (U.S. Army Corps of 

Engineers) to determine two tidal null or node points within the HRE. A tidal node point is a

general location within the estuary where there is no net water flow in either direction (north 

or south). One point is located between South Daytona and Daytona Beach (Fig. 2a), and the 

other is located just south of High Bridge and east of the lower reach of Bulow Creek (Fig. 2b.).

Agriculture 

Other 

Range/Forest 

Residential 

Urban 

Water/Wetlands 

7%

2%

33%

14%

10%

35%

3% 3%

36%

11% 11%

37% N. HRE

(S. Daytona to Bulow)

S. HRE

(Ponce De Leon Inlet to S. Daytona)

Figure 1. Percent composition of major land uses in S. HRE and N. HRE (based on 2004 imagery).
6

 

 

 

The Daytona tidal node point marks the initial delineation between S. HRE and N. HRE

segments. The High Bridge node point generally marks the divide between HRE and Matanzas 

River estuary systems. The S. HRE and N. HRE share the same tidal height or amplitude of ~0.7 

m and are only slightly different in mean depth: 1.7 m and 1.4 m, respectively. However, the 

flushing rates or water residence times between the two segments are very different. The N. 

Tidal 

node

Tidal node

Bulow Cr.

Google Earth

High Bridge

N. HRE

S. HRE

N. HRE

Figure 2a. Location of tidal node at boundary between N. and S. HRE.

Figure 2b. Location of tidal node at boundary between HRE 

and Matanzas R. Estuary near High Bridge.
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HRE has an approximate freshwater residence time of 14 days3 and the S. HRE is 3 days (derived 

from Sheng et al. 2008), attributable to that segment’s close proximity to Ponce De Leon Inlet. 

As will be evident later in the presentation of results from the general models, water residence

time is an important factor that influences the critical nutrient loading limit required to achieve 

or maintain a desired trophic condition.

 

Spatial distributions of major habitat communities in the HRE estuary and catchment

appear to support the delineation between S. and N. HRE as described above. Two biological 

resource maps are shown (Fig. 3) as evidence of this support. One map shows the distribution 

of oyster reefs (SJRWMD 2006) and the other shows the distribution of aquatic and terrestrial 

habitat deemed regionally significant (Johnson et al. 1998). Oyster reef abundance in the S. 

HRE is clearly contrasted with the near absence of oyster reefs in the N. HRE (cause of loss is 

primarily due to dredge and fill development during the first 60 years of the 20th

 

century). The 

presence of regionally significant habitat is comparatively sparse in the N. HRE, especially along 

the estuary, as compared to more intact and contiguous habitats found in the S. HRE and in the 

southern portion of the Matanzas River estuary.

 3 Revised estimate that replaces the 20-day residence time that was presented at the March 3, 2010 workshop

SJRWMD Oyster Reef Imagery (2006) SJRWMD NCB Recon Report 

(Johnson et al. 1998)

Regionally Significant 

Habitat (includes salt 

marsh, oyster habitat as 

well as Fw and upland 

habitats)

Oyster reefs

Port 

Orange

Daytona 

Beach

S. HRE

N. HRE

N. HRE

S. HRE

Figure 3. Distribution of oyster and other significant habitat communities in the HRE
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Water quality conditions over the period of record also suggest a division of the HRE 

system into two, if not three segments. The network of VCEMD water quality sites, and their 

respective data, were spatially aggregated into three segments with the first two segments 

divided at the N. HRE and S. HRE boundary, as previously described; and a third segment of 

sites selected to represent the northernmost Tomoka area of N. HRE (Fig. 4). With an emphasis 

on those three segments, spatial trends for selected water quality parameters are shown in the 

following graphs.

 

Sites Segment

HL01-04 N. HRE/Tomoka

HL05-11 N. HRE/Daytona

HL12-20 S. HRE

Figure 4. Water quality sampling sites of Volusia County Environmental Management Department. 
9

Clear shifts in spatial gradients for salinity and turbidity exists (Fig. 5); one shift occurs 

from the Matanzas system (sites MAT and JXTR26) into the N. HRE and the other from N. HRE 

into S. HRE. The spatial breakpoints, particularly between N. HRE and S. HRE, occurs at tidal 

null point locations, which are indicative of the strong influence of hydrodynamics and full 

extent of the daily ocean/estuary exchange on the region’s salinity and turbidity. There is no 

obvious breakpoint between the Tomoka and Daytona areas of the N. HRE segment. 

0

5

10

15

20

25

Turbidity (ntu)

Site

Turbidity

Dry Season Wet Season Mean 1992-2008 +Std Dev

JXTR26 1997-2008

0

5

10

15

20

25

30

35

40

45

Salinity (ppt)

Site

Salinity

Dry Season Wet Season

Mean 1992-2008 +Std Dev

JXTR26 1997-2008

Tidal null point

N. HRE

Tomoka

N. HRE

Daytona S. HRE

Figure 5. Spatial distribution of salinity and turbidity ( mean and std deviation) by sampling site.
10

 

A similar spatial pattern exists for total suspended solids (TSS) as lower levels are 

consistent throughout the S. HRE as compared to N. HRE (Fig. 6). However, there is no spatial 

difference in TSS concentrations between the Tomoka and Daytona areas of N. HRE and 

between N. HRE and the S. Matanzas sites (MAT and JXTR26).

[Chla], [TN], and [TP] are clearly higher throughout most of the HRE in comparison to 

levels in the S. Matanzas (Fig. 7). Within the HRE system, spatial differences are not readily 

apparent (Fig. 7); however, non-parametric analyses indicate a significant difference between 

N. and S. HRE for [Chla] and [TN], not for [TP] (Kruskall-Wallis and Mann-Whitney; family α = 

0.15 for the 3 segments; individual α = 0.05 for pairs of segments).

Higher trophic state index (TSI) values4

The information above regarding spatial distribution of physical properties, habitat, and 

water quality support a recommendation for dividing the HRE into 2 segments, S. HRE and N. 

HRE (Tomoka and Daytona areas combined), for the development of segment-specific nutrient 

criteria. 

were calculated for N. HRE than for S. HRE (Fig. 

8); a spatial pattern of trophic state that, not surprisingly, reflects Chla and nutrient levels. Also 

in like fashion to nutrient levels and [Chla], the TSI values in the S. HRE continue to decline with 

decreasing distance to Ponce De Leon Inlet. 

 4 TSI values were calculated using the equation specified in FDEP (1996) and the value ranges for 

estuaries, as specified by FDEP (1996) are 0 – 49 good; 50 – 59 fair; and 60 – 100 poor.

0

10

20

30

40

50

60

70

TSS (mg/l)

Site

Total Suspended Solids

Dry Season Wet Season Mean 1992-2008 +Std Dev

JXTR26 1997-2008 N. HRE

Tomoka

N. HRE

Daytona S. HRE

Tidal null point

Figure 6. Spatial distribution of total suspended solids (mean and std deviation) by sampling site.
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0

5

10

15

20

25

Chl a Corr (μg/L)

Site

Chlorophyll a Corrected

Dry Season Wet Season Mean 1992-2008 +Std Dev

JXTR26 1997-2008

0

0.5

1

1.5

2

2.5

TN (mg/l)

Site

Total Nitrogen

Dry Season Wet Season

JXTR26 1997-2008

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

TP (mg/l)

Site

Total Phosphorus

Dry Season Wet Season Mean 1992-2008 +Std Dev

JXTR26 1997-2008

N. HRE

Tomoka

N. HRE

Daytona S. HRE

Tidal null point

---- N. HRE ---- ---- S. HRE ----

Figure 7. Spatial distribution of Chl a, TN, and TP (mean and std deviation) by sampling site.
12

 

Temporal considerations and the reference period method. TSI values throughout the HRE have 

decreased over time as shown in the comparison of mean TSI values between the 1992-2000 

and 2001-2008 periods (Fig. 8). The latter period’s TSI values strongly suggest upper 

mesotrophic conditions throughout the N. HRE, and lower mesotrophic to oligotrophic 

conditions in S. HRE. 

The decrease in trophic state from one decade to the next is a consequence of general 

reductions in [Chla] since 2000, a likely response to the precipitous decline in [TN] over the 

same timeframe (Fig. 9). The [TN] started to decline in the early 1990s, reaching much lower

and more stable levels from 1996 to 2008. The cause for the decline is unknown, but 

reductions in effluent loadings from domestic wastewater treatment facilities (WWTF) have 

occurred (Dennise Judy, FDEP Central District, pers. com., March 2010). 

0

10

20

30

40

50

60

TSI, mean 

± std dev

Trophic State Index 1992 - 2000

S. HRE

0

10

20

30

40

50

60

Site

2001 - 2008

N. HRE

Tomoka

N. HRE

Daytona

Figure 8. Spatial distribution of trophic state index values (means) 

for two time periods, 1992 – 2000 and 2001 – 2008.
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TP does not follow suit; the temporal trend in [TP] is flat over the full period of record.

It is likely that nonpoint sources and internal processes have controlled and are still controlling 

[TP] more so than WWTF loadings. 

Because TN:TP (and DIN:DIP) ratios are <5 (by weight) and [Chla] appears to be more 

responsive to [TN] than to [TP]; the HRE system overall may be N-limited. Data collected by 

University of Florida also support N limitation, but this characterization should be treated with 

caution for no bioassay analysis has been performed for verification (Edward Phlips, U.F., pers.

com., February 2010).

 

0

0.5

1

1.5

2

2.5

3

TN (mg/l)

Year

Total Nitrogen

Dry Season Wet Season

HL01-HL04

Low and stable in HRE 

overall since 1996

However, [TP ] trend was flat: 

0.14 – 0.17 mg/L long-term avg.

0

2

4

6

8

10

12

14

16

18

20

Chl a corr (μg/l)

Year

Chlorophyll a Corrected

Dry Season Wet Season

HL12-HL20

Possible response in Chla to TN 

Wet season spikes >11 μg/L reduction since wet season 2000

persist; concurrent with 

[TN] & [TP] increases, esp. 

in Aug. – Sept.

Figure 9. Temporal trends in [TN] , [TP], and [Chl a] for HRE overall from 1992 – 2008.
14

The 2000 -- 2008 period5

Reference-period results for Chla, TN, and TP are reported as annual median

concentrations and maximum wet-season median concentrations (based on the highest 

monthly values, typically observed in July, August, and September). Concentrations can be 

assessed as 10-year running medians (i.e., each successive monitoring year’s data replaces the 

earliest year’s data in the 10-year data set to develop a new 10-year median). The reference 

period results (Table 1) constitute the first and primary line of evidence in the development of 

nutrient criteria for N. and S. HRE.

is selected as a reference period for nutrient criteria 

development because (1) [TN] levels were about half the levels measured during the early 

1990s; (2) [Chla] levels dropped since the late 1990s, probably in response to the [TN] 

reduction, and are consistent with TMDL [Chla] targets established for other Florida estuaries

(typically ≤7 μg/L annual avg., Steward and Lowe 2010); and (3) trophic status was “good” -- 

upper mesotrophic in N. HRE, and mesotrophic tending toward oligotrophic in S. HRE. The [TP] 

trend was stable throughout the full period of record (1992 – 2008) and likely inconsequential 

with respect to the estuary’s trophic condition. 

Table 1. Halifax R. Estuary, Reference-Period Results – 1st line of evidence

Annual median, 2000 – 2008 (can be assessed against 10-yr running medians)

Segment Chla (μg/L) TN (mg/L) TP (mg/L)

North HRE 4.5 0.61 0.12

South HRE 3.5 0.39 0.12

Maximum wet-season median, 2000 – 2008 (Jul – Sep; can be assessed against 10-yr running medians)

Segment Chla (μg/L) TN (mg/L) TP (mg/L)

North HRE 7.9 0.90 0.18

South HRE 5.8 0.51 0.19

The maximum wet-season medians for Chla (7.9 and 5.8 μg/L) are well below the 

tentative indicator level of 12 μg/L that corresponds to dinoflagellate blooms or dominance 

(analysis of Edward Phlips, U.F., 2000 – 2004 data set for HRE). One of the dominant 

dinoflagellate species in the HRE is Prorocentrum minimum. In high densities, P. minimum have 

been associated with oyster mortality in Chesapeake Bay (NOAA 2007). It is not known

whether P. minimum densities have reached lethal levels in the HRE; archived samples remain 

to be analyzed (Edward Phlips, U.F., pers. com., February 2010).

 5 The reference period changed to 2000 – 2008 from 1996 – 2008. The latter period was presented as the 

reference period, along with related results, at the March 2010 FDEP workshop.
15

Chla – [TN], [TP] regression models. A covariant trend among average monthly [TN], 

[TP], and [Chla] was apparent (Fig. 10) and prompted development of simple regression models 

as described in General Considerations and Methods. 

If any of the regressions proved statistically significant, they were applied toward

setting median [TN] and [TP] based on [Chla] “targets” pre-determined by the reference-period

method. The annual average regressions were used to predict annual median concentrations. 

The average monthly regressions utilizing either the average July – September data sets or the 

all-months data sets, depending upon which data sets yielded the better statistical result (r and

p values), were used to predict maximum wet-season median limits. Results of the simple 

linear regressions are the second line of evidence (Table 2). 

Table 2. Halifax R. Estuary, Linear Regression Results – 2nd line of evidence

Annual median (1992 – 2008; annual avg data)

Segment Target Chla (μg/L) TN (mg/L) r p TP (mg/L) r p

North HRE 4.5 0.58 0.62 .008 not significant

South HRE 3.5 0.30 0.74 .0008 not significant

Maximum wet-season median (1992 – 2008; avg monthly; Jul – Sep only for TN, all months for TP)

Segment Target Chla (μg/L) TN (mg/L) r p TP (mg/L) r p

North HRE 7.9 0.90 0.69 .004 0.17 0.87 .0002

South HRE 5.8 0.41 0.76 .001 0.15 0.86 .0003

Multivariate analysis (step-wise regression) reveals that the major factor regulating 

[Chla] is water temperature, reflecting elevated [Chla] in summer (wet season) months. 

TP, mg/L x 10 

TN, mg/L x 10 

2

4

6

8

10

12

14

Chl a, μg/L

0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

2.5” 2.6” 4.3” 1.7” 1.8” 6.9” 6.2” 7.0” 7.7” 5.1” 2.9” 2.6”

rainfall

Figure 10. Average monthly [TN], [Chl a], and [TP] in the HRE overall. Monthly rainfall data are shown 

for comparison. The 1992 – 2008 period was used for both water quality and rainfall data.
16

Incorporating [TN] and [TP] together or separately did not strengthen the multivariate model. 

This lack of an enhanced relationship by nutrients does not necessarily reflect the system’s 

nutrient or trophic state (Borum 1996; Cloern 1991). Even if a system is nutrient rich, its 

trophic response could be limited by other factors. Given that the HRE system has relatively

low [Chla] and trophic state, other factors controlling [Chla] may be involved besides nutrients 

and water temperature (water residence times and turbidity are possible factors). Metrics 

related to water transparency or light availability -- Secchi depth and turbidity -- do not improve 

the multivariate relationship either; but that is more a consequence of low temporal variability, 

not magnitude. In fact, turbidities are comparatively and persistently high in the S. and N. HRE. 

HRE turbidities are typically over twice that observed in the IRL system (Fig. 11). 

High turbidities in the HRE may be a natural condition. The HRE is similar to many wellflushed, yet naturally turbid estuaries in the South Atlantic Bight region (NE Florida through 

South Carolina) that are dominated ecologically by oyster and marsh communities. A high level 

of turbidity or tripton is one major factor (short water residence time, appreciable tidal 

amplitude and strong tidal current being others) that constrains phytoplankton and submersed 

macrophyte production in favor of marsh production in the estuaries of northeast Florida, 

Georgia, and South Carolina (Lee and Jones 1981; Dame et al. 2000; Roman et al. 2000; Robert

0

2

4

6

8

10

12

14

16

18

Turbidity (ntu)

Year

Mean Turbidity in the HRE and IRL

Mean Turb. - N. HRE Mean Turb. - S.HRE Mean Turb. - IRL

N. HRE

S. HRE

IRL

Figure 11. Mean turbidity by year (1992 – 2008) for N. and S. HRE in comparison to the Indian River Lagoon (IRL).
17

Virnstein, Seagrass Ecosystems Analyst, pers. com., April 2010; Edward Phlips, U.F., pers. com., 

April 2010). 

General models. Both models (Steward and Lowe 2010; Dettmann 2001) require 

estimates of water residence time and external nutrient loadings to predict trophic state, 

loading limits and complementary concentrations of TN and TP. Estimates of external nutrient 

loadings for N. HRE and S. HRE are presented in Table 3.

Non-point surface water drainage conveys the largest percentage of the total external 

nutrient loading to HRE (78% - 89%), followed by the wastewater treatment facilities (8% -

21%). Atmospheric loading estimates, based on areal loading estimates for nearby North Indian 

River Lagoon (Gao 2009), constitute ~3% of the total TN loading and <1% of TP loading. 

Groundwater nutrient loading to the HRE was considered negligible by Scholl et al. 

(1980), possibly because the population in the HRE catchment is served by centralized 

wastewater treatment facilities, not septic tanks; and there are large withdrawals from both 

the surficial and Floridan aquifers for water supply (Rutledge 1985). 

Despite the conclusion reached by Scholl et al. (1980), the Floridan aquifer may be a 

significant, albeit un-quantified, source of TP to the HRE. A geologic confining layer, the 

Hawthorn Formation, ordinarily restricts water movement between the Floridan and surficial 

Table 3. Estimates of current (ca. 2004) external loadings of TN and TP to HRE

North HRE Sources TN (kg/yr) TP (kg/yr) 

Surface water drainage* 207,906 34,088 

Atmospheric 8,911 203 

WWTF** 41,015 9,203

Groundwater Un-quantified; assumed negligible by Scholl et al. 1980

 Total Loading: 257,832

(13.26 g/m2

 43,494

/yr) (2.24 g/m2/yr)

South HRE Sources TN (kg/yr) TP (kg/yr)

Surface water drainage* 196,284 32,927 

Atmospheric 7,516 171 

WWTF** 17,335 4,492

Groundwater Un-quantified; assumed negligible by Scholl et al. 1980

 Total Loading: 221,135

(13.04 g/m2

 37,590

/yr) (2.22 g/m2/yr)

*SJRWMD PLSM- estimated surface-water loadings based on 2004 land use and 2001-2008 avg. annual rainfall, which is 

nearly equivalent to 30-yr avg. annual rainfall. Major tributary streams are Tomoka R., Bulow Cr., 11th St. Canal, Reed 

Canal in N. HRE; Spruce Cr., Turnbull Cr., Halifax Canal in S. HRE. **Domestic WWTF: Daytona/Westside, Holly Hill, 

Ormond Bch in N. HRE; Port Orange, New Smyrna Beach (@27% of annual discharge) in S. HRE. WWTF loading estimates 

calculated from data obtained from FDEP, Tallahassee.
18

aquifers. That layer is nearly absent in the HRE catchment (Toth 1987) and what remains is a 

thin, discontinuous clay strata with slags of apatite (ferric6

The total external TN and TP loading estimates for the N. and S. HRE were normalized to 

estuarine surface water area (g/m

phosphate) (Jon Martin, U.F. 

Geology Dept., April 2010). Consequently, some degree of hydrologic connection exists 

between the aquifers, potentially allowing artesian upwelling of phosphorus-rich groundwater

into the HRE (~0.1 mg/L TP near HRE vs. 0.03 mg/L TP near Mosquito Lagoon; SJRWMD upper 

Floridan well data). Estimating annual TP loading from the upper Floridan is difficult at best. 

The inter-annual variability of the Floridan potentiometric surface elevation in coastal Volusia 

County is so large – roughly between +10 ft to -10 ft from mean sea level per year over the last 

10 years (David Toth, SJRWMD, pers. com., May 2010) – that any estimate of upwelled TP 

loading in one year can easily be offset by zero loading in another year. Nonetheless, over the 

long term, the contribution from the Floridan via upwelling or pumped withdrawal for surface

irrigation may be partly responsible for maintaining high [TP] in HRE waters.

2

The plotted points for TN and TP are located on or just below the lower mesotrophic 

line (LML) for S. HRE, indicating lower mesotrophy or oligo-mesotrophy; and just above the 

upper mesotrophic line (UML) for N. HRE, indicating eutrophy (Fig. 12). However, the TSI values 

and [Chla] in the N. HRE indicate (upper) mesotrophy. The inconsistency between these sets of 

results for N. HRE may be explained by high levels of turbidity (tripton) in N. HRE that could 

attenuate the trophic (Chla) response, as indicated by the low TSI, but cannot be accounted for 

in the Steward and Lowe (2010) model. The same phenomenon may exist in the S. HRE, but its 

current nutrient loading rates are sufficiently low to maintain oligo-mesotrophic conditions. 

However, for the sake of ensuring maintenance of mesotrophy in the N. HRE, some reduction of 

its current loading should be considered (especially if tripton levels happen to decline).

/yr) for input to the Steward and Lowe (2010) model. The N. 

and S. HRE areal loading estimates were then paired with their respective hydraulic loadings 

(mean depth/residence time) and plotted on a graph that describes TN or TP areal loading limit

(published TMDLs or PLRGs) as a function of estuarine hydraulic loading (Fig. 12). The line of 

best fit in the graph represents upper mesotrophy (UML) for Florida estuaries (based on 

Steward and Lowe 2010). The line in the plot representing lower mesotrophy (LML) was 

constructed using [TN] and [TP] limits for lower mesotrophy established by Steward and Lowe 

(2010), Smith (1998), and Baker et al. (1981) (further explanation provided in Steward and Lowe 

2010).

 6 FDEP determined that [Fe] in HRE exceeds the standard and commented that groundwater may be the major 

source (FDEP 2008).
19

The UML’s upper 95% confidence level was used to establish the N. HRE loading limit 

and, thus, the load reduction (Fig. 12). Accordingly, the TN and TP loading limits for N. HRE are

165,449 kg/yr TN and 25,700 kg/yr TP, requiring a 36% to 41% reduction of current TN and TP 

loadings, respectively. A significant percentage of that load reduction can be achieved by 

eliminating WWTF loadings (44% and 52% of the TN and TP load reduction, respectively), which 

contain a much higher fraction of labile N and P than non-point surface-water loadings (OECD 

N. HRE

S. HRE

-0.5

0.5

1

1.5

2

2.5

-0.5 0 0.5 1 1.5 2 2.5 3

log qs (z/τw; m/yr)

log TN load limit (g/m2/yr)

95% upper CI

LML

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

-0.5 0.0 0.5 1.0 2.0 2.5 3.0

log qs (z/τw; m/yr)

log TP load limit (g/m2/yr)

S. HRE

N. HRE

2.0

95% upper CI

LML

TN

TP

UML

UML

log TN Load Limit (≈UML) = 0.83(log qs) – 0.018; R2 = 0.96, p < 0.001

log TP Load Limit (≈UML) = 0.92(log qs) – 0.95; R2 = 0.97, p < 0.001

Figure 12. The log--linear plot of TN and TP load limits (g/m2/yr) vs. hydraulic loading (qs) adapted and 

improved from Steward and Lowe (2010) by the incorporation of additional data points from systems initially 

used to validate the models. A point lying above the 95% confidence interval for the line-of-best-fit (=upper 

mestrophy) is considered eutrophic; below the lower mesotrophic line (LML) is considered oligotrophic . 
20

1982). Therefore, it is possible that elimination of WWTF loadings alone, while maintaining 

current non-point loadings, would be sufficient to ensure mesotrophy in the N. HRE.

Finally, the Dettmann (2001) and Steward and Lowe (2010) models were used to predict

[TN] and [TP] in N. and S. HRE that correspond to their current loadings and loading limits. The 

TN and TP loading limits (current loadings or otherwise) and their predicted concentrations are 

presented as the 3rd line of evidence (Table 4). 

Summary and Conclusions

Results from the three lines of evidence are tabulated for ease of comparison (Table 5). 

Overall, the results from the HRE-specific regression models and the general models compare 

well with the reference-period results, increasing the level of confidence that numeric nutrient 

criteria for the N. HRE and S. HRE can be based on the 2000-2008 period of record. For the S. 

HRE, the results of the TN regression model do not compare well with those of the referenceperiod method. This inconsistency cannot be explained, but it is the exception in an otherwise 

solid, weight-of-evidence case for the HRE system.

For the N. HRE, the potential for current (ca. 2004) nutrient loadings to induce eutrophy 

may be mitigated by the estuary’s turbidity. Regardless of whether that turbidity could be

reduced or not, some level of nutrient load reduction is recommended as a margin of safety for 

maintaining the segment’s mesotrophic state. Toward that end, elimination or substantial 

reduction of WWTF loadings may be sufficient. We propose, as a starting point, that the TN 

and TP loading limits for the N. HRE be 216,817 kg/yr TN and 34,291 kg/yr TP, which are the 

current loadings less the WWTF loadings (Tables 4 and 5). The general models predict that the 

concentration limits associated with those loading limits are 0.57 mg/L TN and 0.16 mg/L TP as 

Table 4. Halifax R. Estuary current loading and possible loading limits for TN and TP 

with their model-predicted concentrations

North HRE TN: kg/yr mg/L* TP: kg/yr mg/L** 

Current loading (c. 2004) 257,832 0.62 43,494 0.20 

Loading limit, elimination of

treated domestic wastewater only

 216,817 0.57 34,291 0.16

Loading limit, full 36%, 41% 

reduction of current TN, TP

 165,449 0.50 25,700 0.12

South HRE TN: kg/yr mg/L* TP: kg/yr mg/L*** 

Current loading (c. 2004) 221,135 0.42 37,590 0.09, 0.18

Loading limit 222,000 0.42 38,000 0.09, 0.18

*Dettmann (2001) TN model equation was applied (General Considerations and Methods)

**Upper mesotrophic [TP] model equation was applied to N. HRE (see General Considerations and Methods)

***Two [TP] values reported because both the lower mesotrophic and oligotrophic [TP] model equations were applied to S. HRE

(see General Considerations and Methods)
21

annual medians. The three lines of evidence for N. HRE produced a tight range of annual and 

wet-season medians (Table 5). For example, the July-September medians are equivalent at 

0.90 mg/L TN and nearly so at 0.17 – 0.18 mg/L TP. The averages of all reported annual medians

for N. HRE (Table 5) are 0.60 mg/L TN and 0.16 mg/L TP, within 5% of the predicted 

concentrations based on the proposed loading limits (as reported in Table 5). 

For the S. HRE, the current (ca. 2004) loadings of TN and TP are proposed as loading 

limits (i.e., no reductions from current loadings appear necessary). Current TN and TP loadings

should maintain the oligo-mesotrophic condition in S. HRE. The predicted trophic state and 

concentration limits from the regression analysis and the general models support that 

conclusion and are generally consistent with the 2000-2008 reference-period results. 

Table 5. Summary of lines-of-evidence results: loading & concentration limits

for HRE intended to maintain trophic state and chlorophyll a (Chla) targets.

North HRE: mesotrophy (Chla targets: 4.5 μg/L annual median; 7.9 μg/L Jul – Sep median)

TN, kg/yr TP, kg/yr

A. Alternative loading limit

(current loading including WWTP discharge) 257,832 43,494

B. Alternative loading limit

(current loading excluding WWTP discharge) 216,817 34,291

Median [TN], mg/L

Annual 

 Median [TP], mg/L

Jul-Sep Annual Jul-Sep 

Ref. Period (’00-08 period) 0.61 0.90 0.12 0.18

Regressions (’92-08 data) 0.58 0.90 not sign. 0.17

General Models 0.62, 0.57a

 n.a. 0.2, 0.16a n.a.

South HRE: oligo-mesotrophy (Chla targets: 3.5 μg/L annual median; 5.8 μg/L Jul – Sep median)

TN, kg/yr TP, kg/yr

Proposed loading limit (current 

loading)

222,000 38,000

Median [TN], mg/L

Annual 

 Median [TP], mg/L

Jul-Sep Annual Jul-Sep 

Ref. Period (’00-08 period) 0.39 0.51 0.12 0.19

Regressions (’92-08 data) 0.30 0.41 not sign. 0.15

General Models 0.42 n.a. 0.135 (0.09, 0.18)

b

 n.a.

a 0.62 mg/L TN and 0.2 mg/L TP are predicted concentrations based on current loadings (ca. 2004); 0.57 mg/L TN and 0.16 mg/L TP are 

predicted concentrations if WWTF loadings are eliminated and the estimated loading limits are 216,817 kg/yr TN and 34,291 kg/yr TP.

b The midpoint between the two values in parentheses = 0.135 mg/L TP. The two [TP] values reported in parentheses are the results from

the lower mesotrophic and oligotrophic [TP] model equations applied to S. HRE (see General Considerations and Methods). 
22

Consequently, the S. HRE reference-period results are proposed as that segment’s nutrient 

criteria: 222,000 kg/yr TN and 38,000 kg/yr TP; 0.39 mg/L TN and 0.12 mg/L TP as annual 

medians; and 0.51 mg/L TN and 0.19 mg/L TP as Jul-Sep medians.

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