1

Using multiple lines of evidence for developing numeric nutrient criteria for

Tolomato - Matanzas Estuary, Florida

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

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

 July 15, 2010

Palm Valley

St. Augustine

St. Augustine 

Inlet

Tolomato R. 

estuary

Guana R. 

estuary

N. Matanzas 

estuary

Matanzas 

Inlet

S. Matanzas 

estuary

Pellicer Cr.

High 

Bridge

Intracoastal Waterway at 

Palm Coast (K. Slater)

Intracoastal Waterway 

at Washington Oaks 

State Park (G. Dean)

Matanzas Inlet (Chads20)

St. Augustine bayfront looking 

across the Matanzas to Davis 

Shores (L. Mozolevskaya)

Looking north from Salt Run toward 

the inlet and Tolomato (G. Mann) 

The Tolomato 

(Sandbarhopper)

Tomoka R.

Halifax R. 

estuary

Photo contributions courtesy of GoogleEarth
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 Tolomato-Matanzas estuarine system in 

St. Johns and Flagler counties, Florida. The approach consists of multiple methods or lines of 

evidence whose collective results support a likely set of criteria.

For the Tolomato-Matanzas Estuary (TME), 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 definitive basis for nutrient criteria. Oysters are the primary, aquatic resource in 

the TME, 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 TME. 

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 

TME. Loading and concentration criteria for total nitrogen (TN) and total phosphorus (TP) 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 

culturally eutrophic. 

General Considerations and Methods

With respect to the TME, 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 representing desirable trophicwater quality conditions (mesotrophy or oligotrophy) can be selected to develop protective 

criteria; and

(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 TME are a reference period method, chlorophyll a –

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

sources for the site-specific data required by these methods are the St. Johns River Water 

Management District (SJRWMD), the Guana-Tolomato-Matanzas National Estuarine Research 

Reserve (GTMNERR), and the Shellfish Environmental Assessment Section of the Florida 
3

Department of Agriculture and Consumer Services (SEAS/FDACS). The full period of record for 

these aggregated water quality data is 1986 – 2009. The locations of TME sampling sites

monitored by the three agencies are shown in Fig. 1.

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 

Figure 1. Locations of water quality sampling sites monitored by the agencies that provided data 

used in this nutrient criteria analysis. The SJRWMD and GTMNERR provided the bulk of water 

quality data. Only salinity data were retrieved from the SEAS database for analysis.
4

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

extreme meteorological event of long duration. Because nutrient loadings and concentrations 

(and presumably [Chla] and trophic state) in the TME are partly a function of watershed rainfallrunoff, 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 wet-season monthly

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). 

General models. Two general nutrient models were applied to the TME 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 

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

(1976) TP critical loading model, the Vollenweider model equation was slightly modified (by 

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

upper or lower mesotrophic [TP]. The modified Vollenweider equations are as follows:

upper mesotrophic [TP] = LM(P) X τw

0.847/[z(1 + τw0.5)], which was empirically derived;

lower mesotrophic [TP] = LM(P) X τw

0.7

/[z(1 + τw

0.5)] as a theoretical derivation;

whereby LM = mesotrophic loads (g/m2/yr); τw = water residence time1; 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 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:

 

1

Freshwater residence times (=R63 values) were scaled up for TME to R99 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 R99 residence times are 0.27 yr for Tolomato, 0.0377 yr 

for N. Matanzas, and 0.063 yr for S. Matanzas.
5

[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 TME are listed below.

Length of system -- ~50 miles (~80.4 km), St. Johns County and Flagler County

Average width of estuary -- ~0.5 miles (0.8 km) in St. Johns County

 ~0.2 miles (0.32 km) in Flagler County

Area of estuary -- ~17 mi2(44 km2)

Mean depth – 3.88 m Tolomato R. Estuary, Palm Valley Bridge to St. Augustine Inlet

2.88 m North Matanzas R. Estuary, St. Augustine Inlet to Matanzas Inlet

 2.145 m South Matanzas R. Estuary, Matanzas Inlet to High Bridge

Tidal amplitude – 1.25 m

Area of catchment -- ~395 mi2(~1024 km2)

Catchment area : Estuary area ratio – 23

Average annual rainfall -- ~51” over last 30 yr; ~49” (2000 – 2009)

Land use – 38 - 43% range/forest, 15 - 28% urban/residential, ~1% agriculture (see Fig. 2)

The land use composition among the TME segments, from the Tolomato through S. 

Matanzas sub-basins, is similar (Fig. 2) (and similar to the adjacent Halifax R. Estuary catchment; 

see Steward et al. 2010a) with respect to the three major land uses: forest/rangeland, 

residential/urban, and agriculture. The percent composition of the 3 major land uses in the 

TME are ~38 – 43% forest/rangeland, ~15 -28% residential/urban, and ~1% agriculture

(SJRWMD 2004 land use, GIS spatial layer).
6

Like the Halifax R. Estuary, spatial differences within the TME may be best described by 

morphometric and hydrodynamic characteristics, which significantly mediate the system’s 

water quality and trophic response. The mean depths of the TME were significantly increased 

by the construction of the Intracoastal Waterway (ICW) in the early twentieth century. Among 

the three segments of the TME, the morphometry of the S. Matanzas estuary is defined largely 

by the ICW; indeed, much of its southern length and width is the ICW channel (see aerial photos 

on cover page). 

Water circulation in the TME is driven by strong tidal forces (~1.25 m tidal amplitude). 

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

Corps of Engineers) to determine 3 tidal null or node points within the TME. A tidal node point 

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

(north or south). The northernmost point is located near the Palm Valley Bridge (Fig. 3a), 

delineating the Tolomato’s northern terminus. A second tidal node point is located near the 

Crescent Beach Bridge (S.R. 206) where the tidal forces funneling through the St. Augustine

Inlet and Matanzas inlet counteract (Fig. 3b). The third point is located just south of High 

Bridge and marks the transition between Matanzas R. and Halifax R. estuaries (Fig. 3c). The 

tidal node point at Crescent Beach was evaluated in conjunction with water quality data to 

determine whether this is a reasonable location for a segment boundary. Tidal node points can 

coincide with points of transition for water quality gradients or biological resources, as 

observed in the Halifax R. Estuary and Mosquito Lagoon (Steward et al. 2010a, Steward et al. 

2010b).

Water residence time is the most important physical factor that determines the trophic 

response of an estuary to its external nutrient input. The freshwater residence time in the

Tolomato R. estuary is about 22 days, which is more than 4 to 5 times longer than in the 

Tolomato

(including Guana basin)

N. Matanzas 

S. Matanzas

0.5%

39.4%

2.0%

11.9%

3.2%

6.1%

36.9%

Agriculture 

Forest/Rangeland 

Other 

Residential 

Urban 

Water 

Wetlands 

1.3%

37.9%

1.4%

3.4% 7.7% 20.4%

27.9%

1.3%

42.9%

2.8%

15.2%

2.7%

3.9%

31.2%

Figure 2. Percent composition of major land uses in TME: Tolomato, N. Matanzas, and S. Matanzas 

(based on 2004 aerial imagery).
7

Matanzas (derived from Sheng et al. 2008). This indicates a nutrient residence time in 

Tolomato that is considerably longer than in Matanzas and a sensitivity to nutrient enrichment

(or a eutrophication susceptibility) that is higher than that for Matanzas with an equivalent 

nutrient input (g/m2estuary area/yr). 

 

Tidal node

Figure 3a. Location of tidal node at boundary between TME and 

the Lower St. Johns R. estuary

Guana

River
8

 

 

 

The spatial distributions of major aquatic habitat communities, oyster reef and salt

marsh (Spartina alterniflora, Spartina patens, and Juncus roemerianus are some of dominant 

marsh plant species) are fairly uniform throughout the TME (Johnson et al. 1998), and does not 

Tidal node

Tidal node

Bulow Cr.

Google Earth

Figure 3b. Location of tidal node at near Crescent Beach Bridge, midway 

between St. Augustine Inlet to the north and Matanzas Inlet to the south..

Figure 3c. Location of tidal node at boundary between the Matanzas and 

Halifax R. estuaries.
9

reveal or support any apparent segment delineation. Fortunately, the loss of oyster reef or 

marsh area throughout the TME system is very limited (unlike the N. Halifax R. Estuary; see 

Steward 2010a). Most oyster loss was due to dredge and fill development, mostly confined to 

the St. Augustine area and during the Spanish colonial period up to the 1960’s (prime examples 

are downtown St. Augustine and the Davis Shores residential development on the opposite 

shore from downtown). The TME basin contains vast stretches of nearly contiguous marsh,

oyster habitat, and minimally developed upland strand, much of it publicly owned and under 

conservation management, and is a major reason why NOAA and the State of Florida 

established the GTMNERR. Another major reason for establishing a NERR in this region is its

exceptionally good water quality.

Spatially, the quality of TME waters is good to excellent by nearly any metric used to 

gauge trophic water quality (nutrients, Chla, etc.). Two segmentation schemes for the TME 

were evaluated based on the longitudinal gradients of salinity, [TN], [TP], [Chla], and TSI values 

(Figs. 4 - 6). 

Mean salinity levels throughout the TME are very uniform, typically >30 ppt as longterm means except at the very southern and northern ends of the TME (most distant from 

inlets) where mean salinities are slightly lower, ranging between 27 to 30 ppt. Other than the 

clear inflection in the salinity gradient between the N. Halifax and S. Matanzas, there is no other 

apparent inflection point in salinity that would justify segmentation within the TME. 

Nonetheless, there is a small gradient shift in turbidity levels between N. Matanzas and 

Tolomato, as well as the stronger shift at the High Bridge tidal node point (Fig. 4). 

Unlike salinity and turbidity, the gradient shifts observed for TN and TP concentrations,

Chla levels, and TSI values are quite distinct (Figs. 5 and 6). With respect to those four

parameters, two types of gradients are evident and could support dividing the system into 2 or 

3 segments depending upon the management question or issue. One type of gradient shows a 

clear inflection point at the Crescent Beach tidal node point where concentrations of nutrient 

and Chla and TSI values are generally at their lowest point, and increasing in opposite directions 

toward the north and south ends of the TME. The other gradient type shows a low plateau of 

concentrations and TSI values throughout the N. Matanzas and higher, nearly equivalent 

plateaus in S. Matanzas and Tolomato. The latter gradient type reveals a three-segment 

scheme that is preferred for nutrient criteria development because there is tighter consistency 

in water quality within each of the three segments, potentially yielding more definitive criteria

(subject to less intra-segment variability) than the two-segment scheme.

Stringent non-parametric analyses indicate significant differences among the three 

segments for [TN] and TSI; and significant differences between N. Matanzas and the other two 

segments for turbidity, [Chla], and [TP] (Kruskall-Wallis; family α = 0.05 for the 3 segments).
10

 

0

5

1 0

1 5

2 0

2 5

3 0

3 5

4 0

Salinity (ppt)

Site

Salinity 

Dry Wet

High Bridge Matanzas Inlet St. Augustine InletTolomato

median: 31.9 ppt

1986 – 2009 period of record

N. Matanzas

median: 33.5 ppt

S. Matanzas

median: 31.5 ppt

0

2

4

6

8

1 0

1 2

1 4

1 6

1 8

Turbidity (ntu)

Site

Turbidity Dry Wet

High 

Bridge

Matanzas

Inlet

St. Augustine 

Inlet

Palm Valley 

Bridge

S. Matanzas

median: 7.8 ntu

N. Matanzas

median: 7.5 ntu

Tolomato

median: 9.4 ntu

Tidal 

node 

point

Figure 4. Spatial distribution of salinity and turbidity (mean and std deviation) by sampling site 

with median levels by segment included (1986 – 2009 period of record). The SEAS/FDACS sites 

are included only in the salinity graph; both graphs include the SJRWMD and GTMNERR sites.
11

 

 

0

2

4

6

8

1 0

1 2

1 4

1 6

CHLAC μg/l)

Site

Chlorophyll a (corrected) Dry Wet

0

0.05

0.1

0.15

0.2

0.25

0.3

TP (mg/l)

Site

Total Phosphorus Dry Wet

High Bridge

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2 TN (mg/l)

Site

Total Nitrogen DRY Wet

Palm Valley

Bridge

Matanzas Inlet St. Augustine 

Inlet

High Bridge

S. Matanzas

median: 0.49 mg/L

mean: 0.50 mg/L

N. Matanzas

median: 0.41 mg/L

mean: 0.43 mg/L

Tolomato

median: 0.53 mg/L

mean: 0.58 mg/L

S. Matanzas

median: 0.083 mg/L

mean: 0.10 mg/L

N. Matanzas

median: 0.072 mg/L

mean: 0.087 mg/L

Tolomato

median: 0.084 mg/L

mean: 0.098 mg/L

S. Matanzas

median: 3.9 mg/L

mean: 4.6 mg/L

N. Matanzas

median: 3.1 mg/L

mean: 3.4 mg/L

Tolomato

median: 4.3 mg/L

mean: 5.0 mg/L

Mean ± s.d.

Mean ± s.d.

Mean ± s.d.

Tidal 

node 

point

Figure 5. Spatial distribution of [TN], [TP], and [Chla] (mean ± std deviation) by SJRWMD or 

GTMNERR sampling site with segment mean and median included (1986 - 2009 period of record).
12

The lowest trophic state index (TSI) values2occur in the N. Matanzas segment, reflecting 

the segment’s low Chla and nutrient levels. The very short water residence time (oceanic 

flushing via St. Augustine and Matanzas inlets) is the likely reason for this spatial trend. 

The information above regarding the spatial distribution of physical properties and 

trophic water quality support a recommendation for dividing the TME into 3 segments –

Tolomato, N. Matanzas, and S. Matanzas -- for the development of segment-specific nutrient 

criteria. 

 Temporal considerations and the reference period method. Time series graphs for [TN], 

[TP], [Chla], and TSI values are shown below (Figs. 7 -10), comparing two long-term (quasi-

 

2

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

1 0

2 0

3 0

4 0

5 0

6 0

HL01 JXTR26 MAT MRT GTMFM JXTR21 MR312 JXTR17 GTMPI TOL TSI

Site

TSI

High Bridge Matanzas Inlet St. Augustine Inlet

Mean + STDEV

Palm Valley Bridge

S. Matanzas

median: 38.7

mean: 38.6

N. Matanzas

median: 36.3

mean: 35.2

Tolomato

median: 41.9

mean: 41.2

Trophic State Index

Figure 6. Spatial distribution of TSI values by SJRWMD or GTMNERR sampling site with segment 

mean and median included (1986 - 2009 period of record).
13

decadal) periods, 1986 – 1999 and 2000 – 2009. Differences between the periods are slight or 

moderate, albeit statistically significant (Kruskall-Wallis; α = 0.05; seasonal Kendall trend 

analysis, p ≤ 0.03). For the N. and S. Matanzas segments, significant differences exist between 

the two periods as [TP], [Chla], and TSI levels increased and [TN] decreased into the latter 

period. For the Tolomato segment, there are significant differences for three parameters: [TP] 

and [Chla] increased and [TN] decreased from the early to later period, while TSI exhibited no 

trend. 

It is important to note that a strong temporal bias exists in the 1986 – 1999 record in 

which 64 – 84% of the data were collected in 1997 – 1999 and during an extended drought 

(1998 – 2000). The sampling bias was due to an expansion of the TME monitoring program in

1997. Sampling sites were added (one SJRWMD site per segment was added in 1997 and two 

GTMNERR sites were added in 2001) and sampling frequency increased from quarterly to 

monthly. Consequently, there are substantially more data in the 2000 – 2009 record than in 

the 1986 – 1999 record. In addition, the 1986 – 1999 record is discontinuous; sampling was 

suspended at site MRT in S. Matanzas for 7 years (1990 through 1996); and throughout the 

TME, the 1986 – 1999 record is heavily weighted in its last 3 years (1997 – 1999) when sampling 

frequency tripled and the extended drought occurred. However, even with the removal of the 

drought data, [TP] trended upward throughout the TME slightly but significantly (seasonal 

Kendall, p ≤ 0.002) during the full, two-decade period of record. The rate of [TP] increase is 

approximately 0.001 mg/L per year based on the slope of the trend line (with or without 

drought data). S. Matanzas is the only segment in the TME where [Chla] and TSI may have 

responded to that [TP] incline (seasonal Kendall, p < 0.02 excluding drought data, p< 0.0001 all 

data) with annual rates of increase of ~0.06 μg/L and 0.16 units, respectively.

Compared to the 1986 – 1999 record, the 2000 - 2009 record is more representative of 

decadal trophic water quality conditions throughout the TME. The median or mean [Chla] and 

TSI values for the later period are indicative of good trophic quality. Annual TSI values were

usually in the mid-30’s to low 40’s (indicating mesotrophy, probably lower mesotrophy in N. 

Matanzas); and average annual Chla levels in all segments were below the target levels set for

Florida’s TMDL estuaries (typically ≤7 μg/L annual avg., Steward and Lowe 2010). Moreover, 

the average annual rainfall during 2000 – 2009 was close to normal (49”/yr as a basin-wide 

average) with no extended drought that would have a dampening effect on decadal medians 

and means. For those reasons, 2000 – 2009 was selected as the reference period for 

development of TME nutrient criteria. Even though S. Matanzas’ trophic state may have slightly 

deteriorated over the last 10 years, the current trophic state is good (TSI = 39), at least on par 

with the Tolomato segment (TSI = 42), and application of the 2000 - 2009 reference period to 

criteria development should promote maintenance of that trophic state. 
14

 

0

0.2

0.4

0.6

0.8

1

1.2

1.4 TN (mg/l)

Year

Total Nitrogen Dry Wet

Segment 1

0

0.2

0.4

0.6

0.8

1

1.2

1.4 TN (mg/l)

Year

Total Nitrogen Dry Wet

Segment 2

0

0.5

1

1.5

2

2.5 TN (mg/l)

Year

Total Nitrogen Dry Wet

Segment 3

S. Matanzas

N. Matanzas

Tolomato

Times series graphs of annual mean [TN] ± Std dev, dry 

and wet season, in the S. Matanzas, N. Matanzas and 

Tolomato segments. 

Long-term (decadal) mean and median concentration 

values for the 1986 – 99 and 2000 – 09 periods are 

included. The record for N. Matanzas only extends 

back to 1991. The bold horizontal lines represent the 

long-term medians.

0.53 mg/L mean

0.50 mg/L median

0.49 mg/L mean

0.45 mg/L median

0.50 mg/L mean

0.50 mg/L median

0.41 mg/L mean

0.37 mg/L median

0.62 mg/L mean

0.53 mg/L median

0.56 mg/L mean

0.52 mg/L median

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45 TP (mg/l)

Year

Total Phosphorus Dry Wet

Segment 1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7 TP (mg/l)

Year

Total Phosphorus Dry Wet

Segment 2

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9 TP (mg/l)

Year

Total Phosphorus Dry Wet

Segment 3

S. Matanzas

N. Matanzas

0.09 mg/L mean

0.069 mg/L median

0.10 mg/L mean

0.089 mg/L median

0.10 mg/L mean

0.068 mg/L median

0.083 mg/L mean

0.073 mg/L median

0.10 mg/L mean

0.084 mg/L median

0.10 mg/L mean

0.085 mg/L median

Tolomato

Times series graphs of annual mean [TP] ± Std dev, dry 

and wet season, in the S. Matanzas, N. Matanzas and 

Tolomato segments. 

Long-term (decadal) mean and median concentration 

values for the 1986 – 99 and 2000 – 09 periods are 

included. The record for N. Matanzas only extends 

back to 1991. The bold horizontal lines represent the 

long-term medians.

Figure 7.

Figure 8.
15

Reference-period results for Chla, TN, and TP (Table 1) are annual median

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

0

2

4

6

8

1 0

1 2

1 4

1 6

1 8

2 0 CHLAC (μg/l)

Year

Chla corrected Dry Wet

Segment 1

0

2

4

6

8

1 0

1 2 CHLAC (μg/l)

Year

Chla corrected Dry Wet

Segment 2

0

5

1 0

1 5

2 0

2 5

3 0 CHLAC (μg/l)

Year

Chla corrected Dry Wet

Segment 3

S. Matanzas

N. Matanzas

3.8 μg/L mean

2.9 μg/L median

5.0 μg/L mean

4.3 μg/L median

3.4 μg/L mean

2.7 μg/L median

3.5 μg/L mean 

3.1 μg/L median

4.4 μg/L mean 

3.2 μg/L median

5.3 μg/L mean

4.5 μg/L median

Tolomato

Times series graphs of annual mean [Chla] ± Std dev, 

dry and wet season, in the S. Matanzas, N. Matanzas 

and Tolomato segments. 

Long-term (decadal) mean and median concentration 

values for the 1986 – 99 and 2000 – 09 periods are 

included. The record for N. Matanzas only extends 

back to 1991. The bold horizontal lines represent the 

long-term medians.

0

1 0

2 0

3 0

4 0

5 0

6 0 TSI

Year

TSI Mean + STDEV

Segment 1

0

1 0

2 0

3 0

4 0

5 0

6 0 TSI

Year

TSI Mean + STDEV

Segment 2

0

1 0

2 0

3 0

4 0

5 0

6 0 TSI

Year

TSI Mean + STDEV

Segment 3

Times series graphs of annual mean Trophic State 

Index (TSI) values ± Std dev, dry and wet season, in the 

S. Matanzas, N. Matanzas and Tolomato segments. 

Long-term (decadal) mean and median concentration 

values for the 1986 – 99 and 2000 – 09 periods are 

included. The record for N. Matanzas only extends 

back to 1991. The bold horizontal lines represent the 

long-term medians.

36.4 mean

37.4 median

39.6 mean

39.3 median

34.5 mean

35.7 median

35.4 mean 

36.4 median

39.5 mean 

41.2 median

41.8 mean

42.2 median

Figure 9.

Figure 10.

S. Matanzas

Tolomato

N. Matanzas
16

values, which are typically measured in June – September (mean annual and wet-season values 

are also provided). 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 constitute the first and primary 

line of evidence in the development of nutrient criteria the TME. Selection of the 2000 – 2009 

as a reference period for TME is consistent with the reference periods selected for Halifax R. 

Estuary (2000 – 2008) and Mosquito Lagoon (2004 – 2008) (Steward et al. 2010a; 2010b). 

Table 1 . Tolomato-Matanzas Estuary, Reference-Period Results – 1st line of evidence

Annual median (mean), 2000 – 2009 (can be used to assess 10-yr running medians or means)

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

Tolomato 4.5 (5.3) 0.52 (0.56) 0.085 (0.096)

N. Matanzas 3.1 (3.5) 0.37 (0.41) 0.073 (0.083)

S. Matanzas 4.3 (5.0) 0.45 (0.49) 0.089 (0.103)

Maximum wet-season median (mean), (Jun – Sep; used to assess 10-yr running medians or means)

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

Tolomato 6.4 (7.4) 0.61 (0.65) 0.095 (0.105)

N. Matanzas 4.0 (4.4) 0.42 (0.46) 0.074 (0.085)

S. Matanzas 6.3 (7.2) 0.54 (0.62) 0.112 (0.127)

The maximum wet-season medians for Chla in the TME (4.0 – 6.4 μg/L; Table 1) are

relatively low in terms of the levels normally associated with harmful algal blooms, or HABs 

(e.g., ≥12 μg/L in the Halifax R. Estuary; analysis of Dr. Edward Phlips, U.F., 2000 – 2004 data set 

for S. Matanzas southward to northern Mosquito Lagoon). The HAB species encountered in the 

TME include the potentially toxic dinoflagellates Karlodinium veneficum, Cochlodinium 

polykrikoides, Prorocentrum minimum, and Takayama spp. the microflagellate 

Chrysochromulina sp. and the diatom Pseudo-nitzschia calliantha. Not one of these species was

observed in bloom proportions during the samplings (Edward Phlips, U.F., pers. com., July 

2010). The toxicity of the specific strains of these species encountered in TME is not known. All 

of the species have been linked to harmful effects in other ecosystems around the world. For 

example, high concentrations of P. minimum have been associated with oyster mortality in 

Chesapeake Bay (NOAA 2007). Compliance with the median or mean concentrations for Chla, 

TN, and TP, especially the wet season levels (Table 1) will not eliminate these HAB species, but 

should restrict their concentrations below bloom levels.
17

Chla – [TN], [TP] regression models. Covariance among average monthly [TN], [TP], and 

[Chla] was apparent (Fig. 11) 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 (see Table 1). Annual average and wet-season monthly average regressions are not 

significant and, thus, not used to predict annual and wet-season median concentrations. 

However, the monthly average regressions based on all months are significant with moderately 

strong correlation and determinant coefficients, and are used to predict maximum monthly, 

wet-season median concentrations (no annual medians or means were predicted). Results 

based on those linear regressions are the second line of evidence specific to wet-season criteria

(Table 2). 

TP *10

CHLAC

TN * 10

0

1

2

3

4

5

6

7

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

Average Monthly Nutrients, Chla, and Rainfall

TP *10 CHLAC TN * 10

3.1" 3.0" 3.9" 2.2" 2.3" 5.8" 4.8" 6.1" 7.5" 4.9" 2.4" 2.4"

avg. monthly rainfall based on average 

of measured rainfall at Jacksonville Beach, 

St. Augustine, Marineland, and Daytona

Month

Figure 11. Average monthly [TN], [Chl a], and [TP] in the TME overall. Monthly rainfall data are shown 

for comparison. The 1986 – 2009 period was used for the average monthly water quality data; and the 

1986 – 2008 period was used for average monthly rainfall.

Month

[TN] x 10, mg/L

[TP] x 10, mg/L

[Chla], μg/L
18

Table 2. Tolomato-Matanzas Estuary, Linear Regression Results – 2nd line of evidence

Maximum wet-season (Jun – Sep) median (1986 – 2009; all months used in the analysis)

Segment Target Wet Season Chla 

(μg/L)

TN (mg/L) r p TP (mg/L) r p

Tolomato 6.4 0.62 0.63 .028 0.110 0.68 .014

N. Matanzas 4.0 0.46 0.72 .008 0.095 0.81 .002

S. Matanzas 6.3 0.61 0.87 .0002 0.120 0.82 .001

The strongest [Chla] – nutrient regressions are strongest in the N. and S. Matanzas 

segments. TN:TP ratios are generally <7 (by mass) among the segments indicative of a system 

that is N-limited, but at times may tend toward a N+P, co-limited state3in the Tolomato 

segment. Dix and Phlips (2010) and Phlips et al. (2004) found phytoplankton production 

([Chla]) in the TME was subject to N-limitation primarily, followed by N+P as a secondary 

limitation. In contrast to our regression analysis (Table 2), Phlips et al. (2004) found that [Chla] 

was not correlated to either [TN] or [TP], but that a positive correlation between [Chla] and 

water temperature did exist, reflecting peak production during the warm months of late spring 

into early fall (similar to the trend observed in Fig. 11). They concluded that phytoplankton 

production is regulated more by physical factors, water residence times and temperature, than 

by nutrients. 

Whether or not a relationship between nutrients and Chla exists 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 TME system 

has relatively low [Chl a] and trophic state, it is likely that a combination of factors controlling

[Chl a] may be involved, including residence time, water temperature, nutrients, and possibly 

tripton (nonalgal turbidity). 

Turbidities are relatively high in the TME, particularly in comparison to the Indian River 

Lagoon (IRL). TME turbidities reached levels that were roughly twice that observed in the IRL

(Fig. 12), and were nearly consistent with levels in the South Halifax R. Estuary (S. HRE). The 

turbidity levels in the TME may be a natural condition, or nearly so. The TME 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. 

Turbidity or tripton is one factor (short water residence time, appreciable tidal amplitude and 

tidal current being other factors) 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 Virnstein, 

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

2010).

 

3

TN:TP ratio, by mass, <7 suggests N limitation; ~7.2 suggests a nutrient balance state (Redfield et al. 1963; Hecky 

et al. 1993).
19

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 Tolomato, N. Matanzas, and S. Matanzas are presented in Table 3.

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

nutrient loading to TME overall (68% for TN, 55% for TP), followed by the wastewater 

treatment facilities (WWTFs; 24.5% for TN, 37% for TP). The N. Matanzas segment is a notable 

exception to the basin-wide trend. N. Matanzas WWTF TN loading is approaching that of 

surface water drainage, and WWTF TP loading exceeds surface-water drainage TP loading 

(Table 3). 

Atmospheric loading estimates, based on areal loading estimates for Northeast Florida

(Pollman and Roy 2003), may constitute ~5% of the total TN loading and <1% of total TP 

loading. TME-specific measurements of groundwater loading are not available; instead,

estimated groundwater loading rates derived from a study conducted in the adjacent basin of 

the Lower St. Johns River basin (Ouyang 2005) were used to estimate TME groundwater 

loadings. The geohydrology (e.g., thicknesses of surficial aquifer and underlying confining layer)

of the St. Johns River study area is not appreciably different from that of the TME basin (Y. 

Ouyang and D. Toth, SJRWMD, pers. comm., July 2010).

0

2

4

6

8

1 0

1 2

1 4

1 6

1 8

Turbidity (ntu)

Year

Mean Turbidity in N. HRE, S. HRE, TME, and IRL

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

N. HRE

S. HRE

IRL

Figure 12. Mean turbidity by year (1992 – 2008) for TME in comparison to the North and South Halifax R. Estuary 

(N. HRE and S. HRE) and the Indian River Lagoon (IRL).

TME
20

The total external TN and TP loading estimates for the TME segments were normalized

to estuarine surface water area (g/m2/yr) for input to the Steward and Lowe (2010) model. 

These areal loading estimates for each segment 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. 

13). The line of best fit in the graph represents the upper mesotrophic boundary line (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).

Table 3. Estimates of current external loadings of TN and TP to TolomatoMatanzas Estuary ( 2004 land use, 2001 – 2008 avg. annual rainfall of 48”)

Tolomato Sources TN (kg/yr) TP (kg/yr) 

Surface water drainagea63,353 8,609 

Atmosphericb 8,350 161 

WWTFc 724 295

Groundwaterd 4,300 2,200

 Total Loading: 76,727

(5.96 g/m2/yr)

 11,264

 (0.82 g/m2/yr)

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

Surface water drainagea120,981 18,585 

Atmosphericb 10,607 204 

WWTFc101,186 22,803

Groundwaterd 4,300 2,200

 Total Loading: 237,074

(13.53 g/m2/yr)

 43,792

 (2.5 g/m2/yr)

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

Surface water drainagea170,557 23,474 

Atmosphericb 7,362 142 

WWTFc 25,806 11,326

Groundwaterd 4,300 2,200

 Total Loading: 208,025

(17.10 g/m2/yr)

 37,142

 (3.05 g/m2/yr)

a

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

close to the 30-yr avg. annual rainfall of 51”.

b

Atmospheric loading estimates were derived from a study applicable to the NE Florida coastal region (Duval and St. 

Johns counties) by Pollman and Roy (2003).

c

Domestic WWTFs: Marsh Landing in Tolomato; SR16 St. Johns County, Anastasia St. Johns County, and City of St. 

Augustine in N. Matanzas; Palm Coast, Beverly Beach, and Flagler Beach in S. Matanzas. WWTF annual loading 

estimates per facility were provided by FDEP, Northeast District, Jacksonville.

d

Groundwater estimates derived from a study in Duval County, Lower St. Johns R. basin, due west of the Tolomato, 

conducted by Ouyang (2005). Equivalency in loadings among segments is the result of same loading rate applied to the 

3 segments, which are nearly equivalent in shoreline length and surficial aquifer depth (~20 m). We are not aware of 

any groundwater nutrient loading study conducted in the TME basin.
21

The plotted points for TN and TP are located between the UML and LML for Tolomato 

and S. Matanzas segment and just below the LML for N. Matanzas, indicating lower mesotrophy 

or oligo-mesotrophy (Fig. 13). The TSI values for the TME also indicate mesotrophy with N. 

Matanzas exhibiting the lowest trophic state among the 3 segments. If the trophic positions of 

the points in the graphs are deemed suitable as a basis for nutrient criteria (according to state 

water policy as interpreted in the Introduction), then the current TN and TP loadings can be 

considered the loading limits.

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

[TN] and [TP] in the TME segments that correspond to their loading limits (ca. 2004 loadings). 

S. Matanzas

Tolomato

-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

N. Matanzas

-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)

N. Matanzas

S. Matanzas

Tolomato

2.0

95% upper CI

LML

TN

TP

UML

UML

log TN Load Limit (≈UML) = 0.83(log qs) – 0.018; R

2

= 0.96, p < 0.001

log TP Load Limit (≈UML) = 0.92(log qs) – 0.95; R

2

= 0.97, p < 0.001

Figure 13. 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 incorporation of additional data points from mesotrophic estuaries initially used to 

validate the models. The area above the 95% upper confidence interval associated with the line-of-best-fit (=upper 

mestrophic line or UML) is considered eutrophic; below the lower mesotrophic line (LML) is considered oligotrophic. 
22

The TN and TP loading limits 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 TME-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 TME can be based on the 2000-2009 period of record. For the Tolomato, the 

[TP] general model result (0.060 mg/L annual median or mean) does not compare well with the

reference-period results (0.086 mg/L median, 0.096 mg/L mean). This inconsistency cannot be 

explained, but it is the exception in an otherwise solid, weight-of-evidence case for the TME

system.

The [Chla] and trophic responses of the TME segments appear to be affected by water 

residence time, nutrient loading, and possibly tripton. Nutrient loading is the one factor that 

can be realistically controlled to manage the estuary’s trophic response. A very large 

proportion of the nutrient loading is treated wastewater effluent, especially in N. and S. 

Matanzas. Eventually, as development continues in the catchment, a limitation on WWTF

loadings may be required if maintenance of the current trophic status of mesotrophy to oligomesotrophy (N. Matanzas) is desired. In the meantime, ongoing efforts to implement extraregulatory stormwater treatment, promulgate basin-specific stormwater criteria (e.g.,

Outstanding Florida Waters designation by the state and/or Special Basin Designation by 

SJRWMD), and acquire natural lands for conservation (Haydt and Frazel 2003) will partially 

offset future nutrient loading increases. The addition of numeric nutrient criteria can help 

achieve a near-total offset and ensure protection of the current trophic state.

Table 4. Tolomato-Matanzas Estuary loading limits (ca. 2004 loadings) for TN and 

TP and their model-predicted concentrations

Tolomato segment TN: kg/yr mg/L TP: kg/yr mg/L 

Loading Limit & predicted 

concentration 76,727 0.48* 11,264 0.060**

N. Matanzas segment

Loading Limit & predicted 

concentration 237,074 0.38* 43,792 0.073***

S. Matanzas segment

Loading Limit & predicted 

concentration 208,025 0.45* 37,142 0.109€

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

** [TP] value is midpoint between values predicted by the upper mesotrophic model equation and the lower mesotrophic model 

equation (see General Considerations and Methods). *** [TP] value predicted by the lower mesotrophic model equation.

€

[TP] value predicted by the upper mesotrophic model equation (see General Considerations and Methods)
23

Toward that end, we propose that the nutrient criteria be the reference-period results and 

current external loading rates, which can be rounded up to the nearest 10,000 kg/yr for TN and 

to the nearest 1,000 for TP (e.g., Tolomato TN loading limit of 76,727 kg/yr rounded up to

80,000 kg/yr; see Table 5).

References

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

limits for TME segments intended to maintain trophic state and 

chlorophyll a (Chla) targets.

Tolomato: mesotrophy (Chla targets: 4.5 μg/L annual median; 6.4 μg/L Jun – Sep median)

 TN, kg/yr TP, kg/yr 

Proposed loading limit

(current loading)

 76,727 – 80,000 11,264 – 12,000

[TN] median (mean), mg/L

Annual Jun-Sep

 [TP] median (mean), mg/L

 Annual Jun-Sep 

Ref. Period (’00-09 period) 0.52 (0.56) 0.61 (0.65) 0.085 (0.096) 0.095 (0.105)

Regressions (’86-09 data) not sign. 0.62 not sign. 0.110

General Models 0.48 n.a. 0.060 n.a.

N. Matanzas: oligo-mesotrophy (Chla targets: 3.1 μg/L annual median; 4.0 μg/L Jun – Sep median)

 TN, kg/yr TP, kg/yr 

Proposed loading limit 

(current loading)

 237,074 – 240,000 43,792 – 44,000

[TN] median (mean), mg/L

Annual Jun-Sep

 [TP] median (mean), mg/L

 Annual Jun-Sep 

Ref. Period (’00-09 period) 0.37 (0.41) 0.42 (0.46) 0.073 (0.083) 0.074 (0.085)

Regressions (’86-09 data) not sign. 0.46 not sign. 0.095

General Models 0.38 n.a. 0.073 n.a.

S. Matanzas: mesotrophy (Chla targets: 4.3 μg/L annual median; 6.3 μg/L Jun – Sep median)

 TN, kg/yr TP, kg/yr 

Proposed loading limit 

(current loading)

 208,025 – 210,000 37,142 – 40,000

[TN] median (mean), mg/L

Annual Jun-Sep

 [TP] median (mean), mg/L

 Annual Jun-Sep 

Ref. Period (’00-09 period) 0.45 (0.49) 0.54 (0.62) 0.089 (0.103) 0.112 (0.127)

Regressions (’86-09 data) not sign. 0.61 not sign. 0.120

General Models 0.45 n.a. 0.109 n.a.
24

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