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Florida Numeric Nutrient Criteria

History and Status

Background

In recognition of the need to more proactively address impairment of State waters due to nutrients, 

the Department of Environmental Protection (DEP) implemented a detailed, EPA-approved plan 

for the development of numeric nutrient criteria and recently proposed revisions to Chapter 62-

302, FAC (Water Quality Standards) and Chapter 62-303, FAC (Impaired Waters Rule) to establish 

numeric nutrient criteria for lakes and streams. DEP selected the “dose-response” approach

(investigating the effects of nutrients on biological communities) as the primary method for the 

development of scientifically defensible numeric nutrient criteria, and has invested significant 

resources in:

• the development of biological assessment tools;

• the documentation of minimally disturbed reference conditions;

• the collection of large amounts of water quality and nutrient data; and 

• conducting a variety of studies to link nutrients to adverse effects on valued ecological 

attributes.

This process has required extensive methods development, staff training, and QA oversight to 

ensure the defensibility of the resulting products. The elements of this development and 

assessment process to date include such components as habitat assessment for streams and 

lakes, benthic invertebrate indices for streams and lakes, a vegetation index for lakes, and a 

periphyton index for streams. These activities represent significant investments in staff time and 

contractual services, with recent and planned funding associated with nutrient criteria 

development in Florida totaling nearly $20 million (Table 1). Extensive documentation of 

nutrient criteria study results, including statistical analyses and interpretation, are found at: 

http://www.dep.state.fl.us/water/wqssp/nutrients/ 

While the approved plan called for adoption of the criteria by the end of 2010, DEP accelerated its 

efforts to adopt numeric nutrient criteria in response to the Environmental Protection Agency’s 

(EPA) January 14, 2009 determination that numeric nutrient water quality criteria are necessary 

in Florida to implement the Clean Water Act. The determination letter established a schedule 

for criteria development, with criteria for lakes and streams due by January 14, 2010, and 

criteria for estuaries due by January 14, 2011. A part of a recent settlement agreement with 

Earth Justice, EPA is obligated to promulgate numeric nutrient criteria for Florida streams and 

lakes by October 15, 2010, unless EPA approves DEP’s proposed criteria prior to that date.
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Table 1. Funding associated with nutrient criteria development in Florida.

Project or Activity Cost to Date

Stream Condition Index/BioRecon Development $3,737,500

Lake Vegetation Index Development $358,750

Lake Condition Index Development $1,180,000

Stream Periphyton Index Development $1,020,000

Biocriteria Meetings and QA Activities $530,400

Stressor Identification Study $374,500

Nutrient Gradient Study $132,675

Stream Benchmark Site Determination $91,350

Longitudinal Study $38,400

Springs Nitrate Studies $384,000

Staff Technical Analysis and Coordination Time $292,000

Statistical Consultants $175,000

Nutrient Criteria Meeting Travel $33,200

Everglades TP Criterion Development $11,250,000

Total Numeric Nutrient Criteria Development $19,597,775

Implementation of the Narrative Nutrient Criteria

Florida currently uses a narrative nutrient standard to guide the management and protection of 

its waters. Rule 62-302.530, FAC, states that “in no case shall nutrient concentrations of body 

of water be altered so as to cause an imbalance in natural populations of flora or fauna.” The 

narrative criteria also states that (for all waters of the state) "the discharge of nutrients shall 

continue to be limited as needed to prevent violations of other standards contained in this 

chapter [Chapter 62-302, FAC]. Man-induced nutrient enrichment (total nitrogen or total 

phosphorus) shall be considered degradation in relation to the provisions of Sections 62-

302.300, 62-302.700, and 62-4.242, F.A.C.” 

DEP successfully established a numeric interpretation of this narrative criterion for Total 

Phosphorus (TP) in the Everglades through the use of the “dose-response” study approach. 

The criterion was ultimately based on maintaining TP concentrations at levels demonstrated to 

support healthy, well balanced populations of aquatic flora and fauna in minimally disturbed 

portions of the Everglades. However, there were extreme challenges associated with 

determining specific cause effect relationships in the Everglades and deriving an appropriately 

protective criterion. For example, biological response was not uniform across all microhabitats, 

and there were areas (e.g., bird rookeries) where naturally higher TP values led to small scale, 

non-anthropogenic changes in community structure. Further, stressors such as hydrologic 

modification and natural low DO regimes had to be sufficiently understood and accounted for in 

criterion development. Extensive dosing studies were conducted to better establish the type 

and magnitude of adverse biological changes associated with specific levels of TP. DEP’s 

experience in the Everglades highlighted the complexity of assessing biological response 

across the natural system and the difficulty in establishing a criterion that was neither under- nor

over-protective.
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While DEP has also successfully implemented the narrative criteria in both its NPDES 

Permitting Program and the TMDL Program, numeric nutrient criteria will improve the efficiency 

of these programs. For example, the numeric criteria will help expedite development of water 

quality-based effluent limits (WQBELs) for wastewater facilities that discharge nutrients to State 

surface waters. WQBEL development for nutrients has historically required extensive sitespecific evaluation to determine both the appropriate water quality target and the allowable 

loading to maintain that target, and the numeric nutrient criteria will expedite WQBEL 

development by providing clear targets for effluent limits. The criteria will also help DEP identify 

waters impaired by nutrients and provide water quality restoration targets for Total Maximum 

Daily Load (TMDL) development for these waters. The Impaired Waters Rule currently includes 

thresholds for nutrient impairment, but the thresholds are for response variables (chlorophyll a), 

and the numeric nutrient criteria will allow DEP to identify impaired waters based on nutrient 

concentrations alone. The nutrient criteria also provide clear water quality targets for nutrient 

TMDL development.

Public Involvement

DEP began the development of numeric nutrient criteria in 2002, when EPA first reached mutual 

agreement on DEP’s Numeric Nutrient Criteria Development Plan. From 2002 through 2009, DEP

conducted 22 meetings with the Nutrient Technical Advisory Committee (TAC), which consists of 

scientific experts from a variety of backgrounds including environmental groups, EPA, consultants, 

and state and local governments. 

DEP formally initiated rulemaking on numeric nutrient criteria in January, 2009, following receipt of 

EPA’s determination letter. Since then, DEP has held four publically noticed TAC meetings and 

three additional public rulemaking workshops. Issues and concerns rose by stakeholders, both 

verbally and in writing, were carefully considered, and modifications in the rules reflect the input. 

EPA Numeric Nutrient Criteria Guidance

EPA’s guidance for nutrient criteria development for streams (Nutrient Criteria Technical 

Guidance Manual: Rivers and Streams) describes three general approaches for the 

development of numeric nutrient criteria for streams. The most comprehensive and scientifically 

defensible approach (the state’s preferred approach) is to establish criteria to protect against 

dependably measured adverse biological responses. EPA suggests that an observed “doseresponse” relationship could be described by a model (e.g., bio-criteria), which in turn would link 

nutrient concentrations to the relative risk of environmental harm. DEP supports the “doseresponse approach”, since it establishes a cause/effect relationship between nutrients and 

valued ecological attributes, and is directly linked to maintaining designated uses. FDEP has

investigated the relationship between nutrients and biological measures such as the Stream 

Condition Index (SCI), Lake Vegetation Index (LVI), Stream Periphyton Index (SPI), and 

phytoplankton chlorophyll a.

In the absence of data quantitatively describing biological dose-response relationships (or the 

absence of statistically significant relationships), EPA recommends setting criteria based on an 

inclusive distribution of values obtained from minimally disturbed reference sites in a designated 

ecoregion. Using the “reference site approach”, EPA recommends setting criteria at an upper 

percentile value to represent a level of nutrient concentration that will inherently protect aquatic 

life. It should be noted that EPA’s guidance for streams and lakes suggests using the 75th

percentile when identifying reference sites via a “desktop, Best Professional Judgment” method. 
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EPA states that a higher percentile can be chosen when there is higher confidence in the 

reference site selection procedure (including such steps as biological confirmation, which DEP 

conducted) since the uncertainty regarding protection would be reduced. EPA’s more recent 

guidance for wetlands suggests that using the 75th to 95th percentile is appropriate. FDEP also 

supports the use of higher percentiles when there is high confidence that the reference sites are 

minimally disturbed, since it would ensure that nutrient concentrations associated with 

biologically healthy, well-balanced communities would be considered acceptable. 

However, the “reference site approach” is an inference model, and does not definitively 

demonstrate that exceeding the threshold established by the distribution of reference sites 

results in harm (impairment) to the aquatic life in a particular waterbody. Multiple factors can 

strongly influence the expression of biological responses to nutrients across waterbodies, such 

as water residence time, presence of grazers, availability of light (due to tree cover and/or 

water transparency), and availability of suitable habitat. The “reference site approach” identifies 

concentrations that are presumed to be inherently protective of the waterbody (including 

downstream waters) because these concentrations are associated with demonstrated minimally 

disturbed systems with healthy biological communities. Actual biological thresholds can be sitespecific and arguably more precise. 

The third and least preferred approach of those suggested by EPA is offered for use in 

situations where sufficient known reference sites are unavailable (either absent or not 

identifiable). This approach is often referred to as the “all sites approach”, and involves 

establishing criteria using a lower distribution (e.g., 5th to 25th percentile) of a pool of sites of 

undetermined ecological quality, as long as the pool is sufficiently large enough to represent all 

waters and can be presumed to reasonably reflect the full range of ambient conditions with a 

disturbance gradient from least to most impacted. However, after careful review, DEP has 

concluded that the “all sites approach” has limited defensibility, and therefore did not pursue it.

Criteria Development

Biological Health

DEP has invested significant resources into the development of biological assessment methods 

for streams and lakes because they provide the ability to directly measure whether a water 

body’s aquatic community meets the objective of the Clean Water Act (CWA). The primary 

methods are the Stream Condition Index (SCI), which consists of an examination of stream 

benthic macroinvertebrate communities, and the Lake Vegetation Index (LVI), which consists of 

sampling aquatic and wetland plants. For these indices, a human disturbance gradient (HDG) 

approach was utilized to identify community attributes that respond predictably to human stress 

(termed metrics). The HDG approach consists of landscape, habitat, hydrological, and water 

quality factors that are scaled to approximate the human disturbance present at series of sites. 

The SCI (with 10 metrics) and LVI (with 4 metrics) are scored from 0 to 100 points. 

DEP, in consultation with EPA, used two lines of evidence to set the thresholds for exceptional 

and impaired aquatic life conditions for the SCI and LVI. The primary method involved an 

examination of the lower distribution of minimally disturbed, rigorously verified reference site 

scores, with the intent of setting the impairment threshold at a level that would limit the number 

of reference sites that would be deemed to be impaired to 2.5%. The second approach 

included an examination of the results of expert opinion that was elicited through Biological 

Condition Gradient (BCG) workshops, where it was determined that a BCG level of 4 and above 

met the CWA goal for biological integrity. For the SCI, the exceptional threshold is a score of 64 
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and above, while scores below 40 are considered impaired (see Figure 1 for the relationship 

between the SCI and BCG). For the LVI, the exceptional threshold is a score of 78 and above, 

while scores below 46 are considered impaired. Another assessment tool under development, 

the Stream Periphyton Index, was also used to evaluate nutrient effects, although it has not yet 

been validated and calibrated. These indexes, as well as component metrics and thresholds, 

were used for examining dose-response relationships related to nutrients.

Figure 1. Relationship between the SCI and Biological Condition Gradient model.

Stream Criteria 

Clear Stream Nitrate-Nitrite Criterion

DEP examined data from several stream studies specifically designed to identify relationships 

between nutrients and biological response, and then analyzed the data using a wide variety of 

statistical techniques. Significant adverse biological responses to nitrate-nitrite were detected in 

clear streams (streams with a color of less than 40 platinum cobalt units). Results from 

experimental dosing studies suggested that nitrate concentrations ranging from 0.230 mg/L to 

0.261 mg/L stimulated the growth of nuisance algae under laboratory conditions. Although 

laboratory conditions do not accurately reflect ecological circumstances, where factors such as 

grazing, light limitation and other natural disturbances affect the growth of algae, they do 

demonstrate a cause-effect relationship nitrate and biological response. 

Because of the limitations of laboratory studies, DEP also examined field responses to nitrate 

enrichment to derive a protective criterion. Results of periphyton field surveys conducted at a 

large number of clear streams indicated that nitrate concentrations above 0.454 mg/L were 

associated with increased nuisance algal abundance. In addition, nearly two decades of 

scientific results from periphytometers deployed in the relatively low color (generally <40 PCU) 

Suwannee, Santa Fe, and Withlacoochee (north) Rivers indicated that significant increases in 

cell density and biomass, as well as adverse alterations in taxonomic community structure,

occurred at nitrate-nitrite concentrations above 0.441 mg/L (see results of change point analysis 

in Figure 2). 

Since the 0.441 mg/ L to 0.454 mg/L thresholds represent the lower range of nitrate 

concentrations for sites with excessive algal growth, an appropriate safety factor was needed to 

translate the thresholds into a protective criterion. A margin of safety was derived by averaging 
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the upper half-range and full range 95 percent confidence intervals, and then applied to the 0.44 

mg N/L change point to derive a nitrate-nitrite criterion of 0.35 mg/L. Given this approach, there 

is a high confidence (95% Confidence Interval) that adverse responses will not be observed at 

concentrations below 0.35 mg/L. 

Because the studies that were used to determine the criterion reflect a relatively long-term 

exposure, the criterion was expressed as a monthly median. The DEP decided not to establish 

a minimum sample size requirement for calculating a monthly median, and instead decided to 

express the criterion as not to be exceeded more than ten percent of the time, in 

acknowledgment that algal species are not expected to respond to short term increases in 

nutrient concentrations.

Mean NOx (Mg/L)

AFDW (g/Square-Meter)

0.0 0.2 0.4 0.6 0.8

0 2 4 6

Figure 2. Change point analysis showing a significant threshold between periphyton biomass 

and nitrate-nitrite.

Total Phosphorus (TP) and Total Nitrogen (TN) Criteria

DEP also conducted several studies specifically designed to identify relationships between TP 

and TN and biological response. Several hundred individual statistical analyses of the resulting 

data were performed using an exhaustive array of statistical techniques, including ordinary least 

squares regression, quantile regression, conditional probability analysis, and change point 

analysis. 
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Figure 3. Example of weak dose-response relationships quantified via statistical analyses. 

Quantile regression of the Stream Periphyton Index and TP and specific conductance.

Although statistically significant relationships were found between nutrients and some biological 

responses, the variability in biological response explained by nutrients alone was statistically 

weak (see Figure 3 as an example), and the Department concluded that the analyses were 

inadequate to establish nutrient thresholds. The results were not surprising because the 

expression of nutrients is very site-specific, depending on residence time, canopy cover and 

color (light limitation), and grazing, and because the biological responses can also be impacted 

by other factors including habitat and hydrology.

Because dose-response thresholds could not be identified, the DEP established numeric 

nutrient criteria for TP and TN in streams using the “reference site approach”, as recommended 

by EPA. As part of this approach, DEP divided the State into different regions based on their 

naturally occurring nutrient distributions and ecological attributes (Figure 4). 

Figure 4. Stream and river nutrient regions for Florida.

Reference sites were determined through a comprehensive, multi-step evaluation process in 

which potential benchmark sites were thoroughly verified to assure that they represented 

minimally disturbed conditions. Because there is high confidence that these reference sites are 

minimally disturbed and fully meet aquatic life use support, DEP recommends the criteria be 

established at the 90th percentile of the reference site distribution, except for the Bone Valley 

nutrient region where the 75th percentile was recommended because of data limitations. The 

resultant criteria are shown in Table 2 in bold font, along with some information about the 

available data. These criteria are expressed as annual geometric means that cannot be 

exceeded more than once every three years.
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Table 2. Stream reference site (benchmark site) distribution derived criteria.

Stream Benchmark Distribution Derived Criteria Limits

Parameter Region

WBID 

Years WBIDS

Mean 

LN

Std Dev 

LN

75th 

%tile

90th 

%tile

TP (mg/L) Panhandle 125 44 -3.662 0.768 0.043 0.069

Northeast 54 20 -3.205 0.714 0.066 0.101

North Central 180 18 -1.973 0.656 0.216 0.322

Peninsula 193 42 -2.746 0.464 0.088 0.116

Bone Valley 13 3 -1.211 0.491 0.415 0.559

TN (mg/L) Panhandle 123 42 -0.76 0.45 0.63 0.82

NE-NC-PenBV 400 85 -0.03 0.45 1.31 1.73

.

Figure 5. Note lack of relationship between TP and SCI throughout the range of reference site 

nutrient values, indicating full aquatic life use support, even at levels above the 90th percentile.

While criteria established using the benchmark approach are inherently protective of aquatic life, 

one disadvantage of using the benchmark approach is that it does not identify the specific 

nutrient levels at which biological impairment occurs (Figure 5). For this reason, it cannot 

be concluded a priori that adverse effects on aquatic life actually occur at concentrations above 

these values, and the Department recommends that a biological confirmation step is critical 

when determining impairment status. Although reference site derived values are inherently 

protective of downstream waters, quantitatively describing the protection of downstream waters 

is an extremely difficult challenge. Currently, there exists no adequate, statewide calibrated 
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model that could be used to numerically determine, without great uncertainty, protective nutrient 

loads for downstream lakes or estuaries. Until such models are developed, DEP has used a 

narrative statement to ensure downstream waters protection.

Lakes Criteria

After considering the stream nutrient regions and previously developed lake geographic regions, 

DEP determined regionalization was not needed or appropriate for lake nutrient criteria because 

color and alkalinity were the primary factors controlling lake response to nutrients. Color 

primarily affects lake response to nutrients by limiting light at very high color levels, but color is 

also an indirect indication of the source of the water reaching the lake. High water color (> 40 

PCU), which is imparted from breakdown of natural leaf litter, indicates that a lake is influenced 

by surface water runoff from forests and wetlands, and would contain higher natural nutrient 

levels than a rainfall driven system. Low color lakes (< 40 PCU) derive their water primarily

from rainfall, unless high alkalinity is also present, meaning higher phosphorus Floridan aquifer 

groundwater has influenced the system. 

After dividing lakes into categories of color and alkalinity, the Department determined

statistically strong, dose-response relationship between nutrients and chlorophyll a (an indicator 

of algal biomass or primary productivity). DEP then used multiple lines of evidence, including 

paleolimnology, fisheries success, expert opinion, lack of Harmful Algal Blooms, and user 

perception, to determine chlorophyll a levels that would be protective of designated aquatic life 

and recreational uses. DEP concluded that a chlorophyll a of 20 ug/L would protect human and 

aquatic life uses in both colored lakes and in clear, high alkalinity lakes. For clear, low alkalinity 

lakes, the protective chlorophyll a threshold was set at 9 ug/L. The 9 ug/L chlorophyll a

threshold in clear, low alkalinity lakes was further supported by Lake Condition Index (LCI) 

evidence. Lake Condition Index scores in lakes with chlorophyll < 5 ug/L were not statistically 

different from those in lakes with chlorophyll between 5 ug/L and 9 ug/L.

Statistical regressions were employed to determine the associated TN and TP criteria that 

would maintain the target chlorophyll a. While the regressions are statistically significant, there 

is sufficient uncertainty in the relationship that DEP used statistical prediction intervals to 

establish a “performance-based approach” in which the TN and TP criteria vary depending on 

measured chlorophyll a levels. 

Lake Type Response 

(Chl-a μg/L)

Stressor Lower Threshold Upper Threshold

Clear and 

Low 

Alkalinity 

(≤ 40 PCU 

and ≤ 50 

mg/L 

CaCO3)

9 TP (mg/L) 0.015 0.043

9 TN (mg/L) 0.85 1.14

Clear but 20 TP (mg/L) 0.030 0.087
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High 

Alkalinity 

(≤40 PCU 

but > 50 

mg/L 

CaCO3)

20 TN (mg/L) 1.0 1.81

Colored

20 TP (mg/L) 0.05 0.157

20 TN (mg/L) 1.23 2.25

In years when the chlorophyll a criterion is achieved, the draft rule automatically sets the 

nutrient criteria at the geometric mean of measured values. However, DEP added upper and 

lower boundaries to ensure that the criteria are not over- or under-protective. The chlorophyll a

criterion and the TN and TP criteria are expressed as annual geometric means that cannot be 

exceeded more than once every three years.

Remaining Issues

Although comprehensive and credible scientific information was used to inform the proposed 

numeric nutrient criteria, a number of issues remain that DEP would like to address, including:

• Establishment of discrete biological response thresholds in streams to TP and TN. DEP 

recently initiated a study that will examine biological responses to nutrients upstream 

and downstream of nutrient point source discharges to further evaluate a concentration 

versus response relationship;

• Determination of endpoints for the Rapid Periphyton Survey and the Linear Vegetation 

Survey that would be used to establish the algal abundance and thickness levels and 

aquatic plant changes that indicate interference with aquatic life use. DEP has initiated 

a study to accomplish these goals;

• Determination of the appropriate chlorophyll threshold in lakes that may have had 

historically higher chlorophyll levels, as determined by paleolimnological studies or other 

information. DEP is working with groups such as the Florida lakes Management Society 

to gather these types of data;

• Further evaluation of the applicability of other lake models, such as the Morphoedaphic 

Index, to refine the allowable nutrient levels in specific lake systems (analyses are 

currently underway); and 

• Quantitatively describing the maximum nutrient concentrations allowed for the protection 

of downstream waters. Currently, there exists no adequate, statewide calibrated model 

that could be used to numerically determine, without great uncertainty, protective 

nutrient loads for downstream lakes or estuaries. This is a daunting challenge with no 

current solution, prompting DEP to propose a narrative statement to ensure downstream 

waters protection.

Resolution of many of these above issues may lead to the establishment of Site Specific 

Alternative Criteria for particular waterbodies.

Type III Site Specific Nutrient Criteria
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Because nutrient dynamics are complex and the impacts are so site-specific, there will always 

be cases where statewide criteria are over-protective for specific waterbodies. To address this 

possibility, DEP developed rule language for a new process for developing Site Specific 

Alternative Criteria (SSAC) for nutrients. This new “Type III” SSAC process would require a 

demonstration that the SSAC is fully protective of designated uses based on the SCI and LVI, 

for streams and lakes, respectively. Under the draft rule, a Type III SSAC would be adopted if 

two spatially and temporally independent biological health assessments indicated that the 

existing nutrient regime supported healthy biota. To ensure that the SSAC is also protective of 

downstream waters, DEP also added a requirement that all downstream waters attain water 

quality standards related to nutrients. To protect high quality waters, a SSAC would not be 

allowed if a 20 point drop in the SCI or LVI occurred, compared with historic values.

TMDLs and SSACs

DEP also plans to adopt previously adopted nutrient Total Maximum Daily Loads (adopted in 

Chapter 62-304, FAC) as SSACs. DEP recommends this provision because the TMDLs:

• Establish site specific and sensitive responses to nutrient enrichment for a particular 

area;

• Are generated using data appropriate for a site specific assessment; 

• Establish a protective endpoint equivalent to numeric criteria; and

• Are more appropriate than a statewide criterion because they reflect geographically 

explicit protective conditions, and it would be counter-productive for statewide nutrient

criteria to supersede the TMDL.

Florida has currently adopted 135 nutrient TMDLs with an additional 39 pending adoption.

The recommended revisions to Chapter 62-303 (Impaired Waters Rule, IWR) are designed to 

update the rule to implement the proposed revisions to Chapter 62-302. The revisions will allow 

the Department to assess waters for nutrient impairment using the numeric nutrient criteria in 

addition to the current narrative nutrient impairment thresholds in the IWR, and to assess waters for 

biological impairment using the new SCI and LVI thresholds.