1 

## Using multiple lines of evidence for develop ing numeric nutrient criteria for Halifax River Estuary , Florida
Joel S. Steward, Whitney C. Green, and Jan ice D. Miller Department of Water Resources, St. Johns River Water Management District June 17, 2010

Photo s 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

T he purpose of this report is to present a weight

- of

- evidence approach for develop ing nutrient loading and concentration limit criteria for the Halifax River E stuary. The a pproach

cons ists of multiple methods or lines of evidence whose collective results support a l ikely 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, 2010 1

. 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 I ndian R iver L agoon 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 thr ee separate analyses ( or line s of evidence) that generally assess e d the trophic state

- c hl orophyll 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 polic ies ( 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 mesot roph ic or oligotroph ic ; or are intended to attain mesotrophy , at minimum, if th e current condition is worse due to cultural eutrophication.

General Considerations and Methods

With respect to the Halifax River E stuary (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) S patial heterogeneity within the system
, segmenting the system accordingly for the purpose
of sett ing 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 evide nce , helping to ensur e 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
, c hl orophyll a
– nutr ient regression analyses
, and two empirical models that pertain to estuaries in general. The primary source for site

- specific data required by these method s is Volusia County Environmental Management Department (VCEMD)

, and the full period of record for these data
is 199 2

- 2008

.

R eference period. A specific period of time within the full period of record that provid es the low est chlorophyll a ( Chl a ) and nutrient levels along with a desirable (if not the lowest ) trophic state was selected to calculate annual and wet

- season medians of [Chl a ], [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 load ing 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 [Chl a ] 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

.

Chl a
– [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 f rom these reduced data sets : [ Chl a ] regressed against [TN] and against [TP]

. If a regression model had a moderate to strong correlation coefficient ( r ) (or coefficient of determination, r 2 ) and significance ( p ) statistic, it was used to predict median
concentration limits based on the reference

- period [Chl a ] medians (the de facto Chl a target levels). We also performed m ultivariate regression analysis to identify major variables that may regulate algal production (i.e., Chl a 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 rela ted to a desirable trophic

state (mesotrophy or oligotrop hy). One model is an empirically defined relationship between
water residence times and nutrient loading limit s for mesotroph ic Florida systems (Steward and4 Lowe 2010). Because the Florida TP loadi ng

- limit model is consistent with the Vollenweider (1976) TP critical loading model

, the Vollenweider model equation was adjusted ( specifically
the τ w

upper mesotrophic [TP] = L exponent) to fit the Florida model s (Steward and Lowe 2010) to predict the upper

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

M (P) X τ w 0.847 /

[ z(1 + τ w 0.5 lower mesotrophic [TP] = L ) ] , which was empirically derived ;

M (P) X τ w 0. 7 /

[ z(1 + τ w 0.5 oligotrophic [TP] = L ) ] as a theoretical deriv ation;

O (P) X τ w 0.5 /

[ z(1 + τ w 0.5 ) ] as a theoretical derivation ; where by L M , L O = mesotrophic
, oligotrophic loads (g/m 2 /yr) ; τ w = water residence time 2 ; 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

app lication of the Florida model. The Dettmann (2001) model equation is : [TN] = (L N X τ w / V + [TN] s )(1/ ( 1 + ατ w ) ) ; whereby L N = TN loading (g/yr)

, τ w = freshwater
residence time (yr)

, V = volume of estuary (liters , L ) segment, [TN] s = average [ T N

] at segment boundar ies (g/L) , α = 1 st 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 . Th e other two lines of evidence are model predictions ( incorporating site

- specific data) and regarded as supporting evidence.

Results and Discussion

Spatial considerations and segmentation

. Som e s alient 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 mi 2 (35.8 km 2

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 mi 2 (~880 km 2

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 fr eshwater systems (Steward and Lowe 2010). A t idal 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 R 96 residence times are 0.0267 yr for S. HRE and 0.1248
yr for N. HRE. The Dettmann (2001) equation uses freshwater residence times. )5 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

p opulation de nsit i es are q uite different. T he municipal population density in N. HRE
, ~3,300 people/mi 2

, is about 1.7 times greater than in the S. HRE with ~2000 people/mi 2 (Volusia County Dept. of Economic Development 2009 ; www.volusia.org/economicdevelopment ).

H ydrodynamic properties probably best describe the estuarine difference s 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 c alibrated RMA2 model ( U.S. Army Corps of Engineers ) to determine two tidal null or node points with in the HRE. A t idal 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 n ode point marks the initial delineation between S. HRE and N. HRE segments . The High Bridge n ode 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. Th e 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.7 HRE has an approximate freshwater residence time of 14 days 3 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 fro m 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 developme nt during the first 60 years of the 20 th 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
southe rn 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 HRE8 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. T he spatial breakpoint s , particularly between N. HRE and S. HRE, occurs at tidal null point location s , 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 25Turbidity (ntu)

Site Turbidity

Dry Season Wet Season Mean 1992

- 2008 +Std Dev

JXTR26 1997

- 2008 0 5 10 15 20 25 30 35 40 45Salinity (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 concentration s between the Tomoka and Daytona areas of N. HRE and between N. HRE and the S. Matanzas sites (MAT and JXTR26).

[ Chl a ]

, [ 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) ; h owever, n on

- parametric analyses indicate a significant difference between
N. and S. HRE for [Chl a ] 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) values 4

The information above regarding spatial distribution of physical p roperties, 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 develop ment 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 Chl a and nutrient level s

. Also
in like fashion to nutrient level s and [ Chl a ]

, the TSI values in the S. HRE continue to decline with decreasing distance to Ponce De Leon I nlet.

4 TSI values were calculated using t he 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 70TSS (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. Spa tial distribution of total suspended solids (mean and std deviation) by sampling site.11 0 5 10 15 20 25Chl 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 2.5TN (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 0.7TP (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 200 1

- 2008 periods (Fig. 8) . The latter period’s TSI values strongly suggest upper mesotrophic c onditions 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
reduction s in [ Chl a ] 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 low er 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 60TSI,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

– 2008.13 TP does not follow suit; the temporal trend in [TP] is flat over the full perio d of record. It is likely that nonpoint sources and internal processes have c ontrolled a nd 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 Flo rida also support N limitation
, but this characterization s hould be treated with caution for no bioassay analysis has been performed for verification ( E dward Phlips, U.F., pers

.
com. , February 2010)

.

0 0.5

1 1.5

2 2.5

3TN (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 20Chl a corr (μg/l)

Year Chlorophyll a Corrected

Dry Season Wet Season

HL12

- HL20 Possible response in Chla to TN reduction since wet season 2000 Wet season spikes >11 μg/L 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 period 5

Reference

- period results for Chl a , 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). C oncentrations 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 devel opment 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 level s measured during the early
1990s ; (2) [Chl a ] levels dropped since the late 1990s
, probably in response to the [TN]
reduction , and are consistent with TMDL [ Chl a ] target s established for other Florida estuaries ( typically ≤7 μg/L annual avg.
, Steward and Lowe 2010) ; and (3) trophic stat us was “good”

-- upp er mesotroph ic 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**

**- P**  **eriod R**  **esults**

**– 1**  **st line of evidence**  Annual median, 2000

– 2008 (can be assess ed against 10

- yr running medians) Segment Chla (μg/L) TN (mg/L) TP (mg/L) North HRE 4. 5 0.6 1 0.12 South HRE 3.5 0. 39 0.1 2 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 0.9 0 0.1 8 South HRE 5.8 0.5 1 0.19

The maximum wet

- season medians for Chl a ( 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 E dward Phlips , U.F., 2000

– 200 4 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 know n whether P. minimum densities have reached lethal levels in the HRE ; archive d samples remain to be analyzed (E dward 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 Chl a
– [TN]

, [TP] regression models

. A covariant trend among average monthly [ TN ]

, [ TP ] , and [ Chl a ] 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 Jul y
– Sep tember data set s or the
all

- months data set s , depending upon which data sets yielded the be tter 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** 

**– 2**  **nd 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 .00 0 8 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. 6 9 .00 4 0.1 7 0. 87 .0 002 South HRE 5.8 0.41 0. 7 6 .00 1 0.1 5 0. 8 6 .0 003

Multivariate analysis (step

- wise regression) reveals that the major factor regulating [ Chl a ] is water temperature, reflecting elevated [Chl a ] 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 199 1). Even if a system is nutrient rich , its trophic response could be limited by other factors. Given that the HRE system has relatively low [Chl a ] and trophic state, other factors controlling [Chl a ] 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 ).

H igh turbid it ies in the HRE may be a natural condition . T he HRE is similar to many well

flushed

, 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 h igh level
of turbidity or tripton is one major factor (short water residence time , appreciable tidal ampli tude and strong tidal current being other s ) 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; R obert 0 2 4 6 8 10 12 14 16 18Turbidity (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 turbidit y 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; E dward Phlips, U.F., pers. com., April 2010).

General models

. Both models (Steward and Lowe 2010; Dettmann 2001) require
estimates of water residence time and exte rnal 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 convey s the largest percentage of the total external nutrient loading to HRE (7 8 %

- 8 9 %), followed by the wastewater treatment facilities (8 %

- 21%) . Atmospheric loading es timates, based on areal loading estimate s for nearby North I ndian
R iver L agoon (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 Florida n 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) e**  **xternal l**  **oadings 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; a ssumed negligible by Scholl et al. 1980
T

otal Loading : 257,832 (13. 26 g/m 2 43, 494 /yr) (2.2 4 g/m 2 /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; a ssumed negligible by Scholl et al. 1980
T

otal Loading : 221,135 (13. 04 g/m 2 37, 590 /yr) (2.2 2 g/m 2 /yr)

* SJRWMD PLSM

- estimated surface

- water loadings based on 2004 land use and 2001

- 200 8 avg. annual rainfall

, which is nearly equivalent to 30

- yr avg. annual rainfall . Major tributary streams are Tomoka R., Bulow Cr., 11 th 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 a nnua l 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 w hat remains is a thin, discontinuous clay strata with slags of apatite (ferric 6

The total external TN and TP loading estimates for the N. and S. HRE were normalized to estuarine surface water area (g/m phosphate) ( J on 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.
T he inter

- annual va riability of the Floridan potentiometric surface elevation in coastal Volusia County is so large

– r oughly between + 10 ft to

- 10 ft from mean sea level per year over the last
10 years (D avid Toth, SJRWMD , pers. com., May 2010 )

– that an y estimate of upwelled TP loading in one year can easily be offset by zero loading in another year

. Nonetheless, o ver 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 , t he T SI 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 level s of turbidity ( tripton ) in N. HRE that could

attenuate the trophic (Chl a ) response , as indicated by the low TSI , but cannot be account ed 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

- mesotrop hic 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 happe n to decline)

. /yr) for input to the Steward and Lowe (2010) model . The N. and S. HRE areal loading estimates were then p aired with their respective hydraulic loadings (mean depth/residence time) and plotted on a graph that describes TN or TP areal l oading limi t (published TMDLs or PLRGs) as a function of estuarine hydraulic loading (Fig. 12) . The line of best fit in the gr aph represent s 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), S mith ( 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 U ML’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 16 5
, 449 kg/yr TN and 2 5
, 700 k g/yr TP, requiring a 3 6 % to 4 1 % reduction of current TN and TP loadings , respectively. A significant percentage of that load reduction can be achieve d by eliminating WWTF loadings ( 44% and 52% of the TN and TP load reduction, respectively)

, which
contain a much high er 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 q s 

### (z/ τ w 

### ; 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 q s 

### (z/ τ w 

### ; 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 q s )

– 0.018 ; R 2 = 0.96, p < 0.001

log T P Load Limit ( ≈UML) = 0. 92 (log q s )

– 0. 95; R 2 = 0.97, p < 0.001

Figure 12. The log

-- linear plot of TN and TP load limits (g/m 2 /yr) vs. hydraulic loading (q s ) 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 (=uppe r mestrophy) is considered eutrophic; below the lower mesotrophic line (LML) is considered oligotrophic .20 1982)

. Therefore, i t is possible that e limination of WWTF loadings alone, while maintaining

current non

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

Finally, t he Dettmann (2001) and Steward and Lowe (2010) models were used to predict [TN] and [TP] in N. and S. HRE th at 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 3 rd line of evidence (Table 4)

.

Summary and Conclusions

R esults 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. HR E can be based on the 2000

- 2008 period of record. For the S. HRE, t he results of the TN regression model do not compare well with those of the reference

period method

. Th is inconsistency cannot be exp lained
, 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 (c a . 2004) nutrient loadings to induce eutrophy may be m itigated 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 mes o trophic state. Toward that end, elimination or substan tial 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 (Table s 4 and 5 ). The gene ral 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**  **concentration s**  

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

### TP : kg/yr mg/L * * Current loading ( c. 2004) 257,832 0.6 2 43, 494 0.20 Loading limit, elimination of treated domestic wastewater only 216,817 0. 57 34, 291 0.16

Loading limit, full 3 6 %, 4 1 % reduction of current TN, TP 165,449 0.5 0 2 5
, 700 0.12



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

### TP : kg/yr mg/L *** Current loading ( c. 2004) 221,135 0.4 2 37,590 0.09
, 0.1 8 Loading limit 222 ,000 0.4 2 38,000 0.09
, 0.1 8

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

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

** * Two [TP] values reported because both the lowe r 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 examp le, the July

- September medians are equivalent at 0.90 mg/L TN and nearly so at 0.17

– 0.18 mg/L TP. The average s of all reported annual medians
for N. HRE (Table 5 ) are 0.60 mg/L TN and 0.1 6 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 (c a . 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 **(Chl** a **) targets.**
North HRE : mesotrophy (Chl a targets : 4. 5 μg/L annual median; 7.9 μg/L Jul

– Sep median )



### TN
, kg/yr TP
, kg/yr

A. Alternative l oading limit (current loading including WWTP discharge ) 257,832 43,494



### B. Alternative l oading limit (current loading excluding WWTP discharge ) 216,817 34, 291



### M edian [TN], mg/L Annual M edian [TP], mg/L Jul

- Sep Annual Jul

- Sep Ref

. Period (’00

- 08 period ) 0.6 1 0.9 0 0.12 0.1 8 

### Regressions (’92

- 08 data ) 0. 58 0.90 not s ign

. 0.1 7 

### General Models 0.62, 0. 57 a n.a. 0.2, 0.1 6 a n.a.


### South HRE : oligo

- mesotrophy (Chl a targets: 3.5 μg/L annual median; 5.8 μg/L Jul

– Sep median) 

### TN, kg/yr T P, kg/yr Proposed l oading limit (current loading) 2 22 ,000 38
, 000



### M edian [TN], mg/L Annual M edian [TP], mg/L Jul

- Sep Annual Jul

- Sep Ref. Period (’00

- 08 period) 0. 39 0.5 1 0.1 2 0.19 

### Regressions (’92

- 08 data ) 0. 30 0. 41 not s ign

. 0. 1 5 

### General Models 0.4 2 n.a. 0.1 35 ( 0.09
, 0.1 8 ) b n.a.

a 0.62 mg/L TN and 0.2 mg/L TP are predicted concentrations based on current loadings (c a . 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 value s in parentheses = 0.135 mg/L TP. The two [TP] values reported in parentheses are the results from the lowe r 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 criteri a : 222,000 kg/yr TN and 38,000 kg/yr TP; 0. 39 mg/L TN and 0.1 2 mg/L TP as annual medians; and 0.5 1 mg/L TN and 0.19 mg/L TP as Jul

- Sep medians.

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