Numeric Nutrient Criteria Recommendations 

for the Tampa Bay Estuary 

SUMMARY AND TECHNICAL SUPPORT 

DOCUMENTS 

Prepared for: 

Tampa Bay Estuary Program 

Prepared by: 

Janicki Environmental, Inc. 

22 February 2011
1 

Objective 

The objective of this document is to provide a summary of the recent work conducted by the 

Tampa Bay Estuary Program (TBEP) to provide the U.S. Environmental Protection Agency (EPA) and 

the Florida Department of Environmental Protection (FDEP) recommendations regarding the 

establishment of numeric nutrient criteria for the Tampa Bay Estuary. For reference, the following 

map of Tampa Bay depicts the four major bay segments referred to in this summary and the 

attached technical documents. 

Old Tampa

Bay

Hillsborough

Bay

Middle Tampa

Bay

Lower

Tampa

Bay

Gulf

of

Mexico
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The Need for Numeric Nutrient Criteria 

The FDEP began development of numeric nutrient standards in December 2001. The FDEP formed 

a technical advisory committee and an agency work group to assist in identifying appropriate 

nutrient standards. FDEP has conducted a number of workshops and meetings as well as several 

studies since 2002. 

In 2008, several environmental groups filed suit against EPA in Federal Court alleging that EPA had 

determined in 1998 that Florida’s current narrative nutrient standard did not comply with the Clean 

Water Act and that EPA had not established numeric nutrient standards pursuant to Section 

303(c)(4)(B) of the Clean Water Act. As a consequence of this lawsuit, EPA sent FDEP a letter on 

January 14, 2009 finding that FDEP’s narrative nutrient standard did not comply with the Clean 

Water Act and directing the State of Florida to develop numeric nutrient standards for rivers and 

lakes by January 2010 and estuarine and coastal waters by January 2011. EPA stated that it would 

adopt its own nutrient standards if FDEP could not meet these deadlines. In August 2009, the suit 

plaintiffs and EPA agreed to a Consent Decree formally establishing the deadlines and determined 

that EPA would be responsible for establishing numeric criteria for Florida waters. 

Management of the Tampa Bay Estuary 

The Tampa Bay Comprehensive Conservation and Management Plan (TBNEP, 1996) established the 

restoration of seagrass in the bay to levels estimated in the 1950s as a primary goal for overall bay 

restoration. In establishing and addressing this goal, a conceptual paradigm was developed to 

identify the primary, manageable factors thought to influence the recovery and sustainability of 

seagrass resources within the bay. Reduced water clarity as a result of excessive nitrogen loads to 

the bay and resulting light attenuation by phytoplankton responding to these loadings were the key 

water quality indicators by which seagrass recovery could be managed. A number of studies in the 

1990s clearly established that nitrogen was the limiting nutrient in the Tampa Bay estuary and that 

phosphorus loadings to the bay from the enriched Bone Valley region were not controlling 

estuarine production. 

In November 2002, the Florida Department of Environmental Protection (FDEP) concluded that the 

Tampa Bay Nitrogen Management Consortium’s (TBNMC) nitrogen management strategy provided 

reasonable assurance that the state water quality criteria for nutrients would be met in Tampa Bay. 

Prior to this state determination, the U.S. Environmental Protection Agency (EPA) recognized a 

1998 action by FDEP that proposed a total maximum load (“federally-recognized TMDL”) of 

nitrogen that could be discharged to the bay annually and still meet state water quality standards 

related to nutrients. Both FDEP’s reasonable assurance determination and the total maximum 

nitrogen loading recognized by EPA are based on statistical modeling and data analyses peerreviewed by the TBEP, its partners, and state and federal regulators. Thus, the TBNMC’s nitrogen 

loading targets developed for the major bay segments of Tampa Bay have been acknowledged by 

both FDEP and EPA as protective nutrient loads for this estuary. A five-year renewal of the Tampa 

Bay Reasonable Assurance (RA) was recently approved by order of the FDEP Secretary. 

The Southwest Florida Water Management District recently reported on the seagrass acreage in 

Tampa Bay from its survey conducted in 2010. The results from this survey show an increase of 

approximately 3,250 acres since the 2008 survey (Figure 1). Therefore, there is tangible evidence 
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that the TBNMC nitrogen loading strategy continues to support seagrass recovery in the Tampa Bay 

Estuary. 

Figure 1. Tampa Bay seagrass coverage. Data source: Southwest Florida Water Management District. 

Recent results from the Environmental Protection Commission of Hillsborough County ambient 

monitoring program indicate that the chlorophyll targets were achieved in all four bay segments of 

Tampa Bay in 2010 (Figure 2). Therefore, there is tangible evidence that the TBNMC nitrogen 

loading strategy continues to achieve water quality targets in the Tampa Bay Estuary. 

Figure 2. Tampa Bay mean annual chlorophyll a concentrations for each of the four major bay segments, 

1974-2010. Data source: Environmental Protection Commission of Hillsborough County. 
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TBNMC Recommendation for Tampa Bay Numeric Nutrient Criteria 

In March 2010, the TBNMC provided comments and requests regarding the development by EPA 

of protective loads for the Tampa Bay Estuary as it relates to establishing numeric nutrient criteria 

for inland waters and estuaries in Florida. As part of this effort, the TBNMC provided 

recommended protective nutrient loads for the Tampa Bay Estuary. The TBNMC proposed TN and 

TP loading criteria for the four mainstem segments of Tampa Bay as follows: 

Proposed TN and TP loading criteria for the segments of Tampa Bay.

Segment TN Load (tons/year) TP Load (tons/year) 

Old Tampa Bay 486 104 

Hillsborough Bay 1451 1093 

Middle Tampa Bay 799 140 

Lower Tampa Bay 349 52 

EPA expectations are for both total nitrogen (TN) and total phosphorus (TP) protective loads. The 

recommended protective TN loads for the segments of Tampa Bay are those from the 1992-1994 

period, as utilized in the Final 2009 Reasonable Assurance Addendum: Allocation & Assessment 

Report and in the federally-recognized Tampa Bay TMDL. The TBNMC recommended protective 

TP loads from the same time period in its comments and requests to the EPA in March 2010. 

The TBNMC approach that has established state and federally-approved nitrogen loading targets for 

the estuary follows EPA’s technical guidance that quantitative stressor-response relationships are the 

most preferred methodology in establishing numeric nutrient criteria (EPA, 2010). Multiple lines of 

empirical evidence justify maintaining existing TN and TP loads to the Tampa Bay Estuary. Water 

quality and clarity in the Bay has improved tremendously since significant management actions 

were initiated starting in the 1980s, seagrass acreage has increased to the highest levels observed 

since the 1950s and continues to increase, and economically important fish and wildlife 

populations have been maintained since routine monitoring programs began in the 1990s. 

Continuing TBEP Input to EPA Regarding Numeric Nutrient Criteria 

The TBEP, in cooperation with the Sarasota Bay Estuary Program and Charlotte Harbor National 

Estuary Program, supported the development of a document that identified the potential methods 

for the estimation of numeric nutrient criteria for southwest Florida estuaries (Janicki Environmental, 

2010). This document identified several methods currently being considered by both EPA (EPA, 

2010) and FDEP (2010) to establish numeric nutrient criteria for Florida estuarine waters. 

In addition to the methods document, the TBEP has addressed several other issues associated with 

the establishment of numeric nutrient criteria for the Tampa Bay Estuary. These include: 

• Expression of recommended TN and TP criteria as concentrations.

• Demonstration that the proposed criteria provide full aquatic life support, especially 

achievement of dissolved oxygen (DO) standards. 

• The need for establishment of downstream protective values (DPVs) for terminal reaches 

that drain directly into Tampa Bay. 

• Consideration of the influence of infrequent non-anthropogenic events, such as hurricanes 

and El Niño conditions, on implementation of the proposed criteria. 
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The following summarizes the TBEP recommendations regarding these issues. Detailed evaluations 

for each issue can be found in the attached documents. 

- Concentration-based Criteria 

Previous efforts by the TBEP have developed strong relationships between nutrient supply to Tampa 

Bay and resultant chlorophyll a concentrations in the bay, and between chlorophyll a

concentrations and light availability for seagrasses. Thus, management actions have focused on 

controlling nitrogen loads to Tampa Bay, with measureable success as expressed by increases in a 

biological endpoint, seagrass acreage. The relationships are between nitrogen loads and 

chlorophyll a, however, not nitrogen concentrations in the bay and chlorophyll a. 

TBEP recognizes that EPA intends to establish criteria for TN and TP and that these criteria may be 

expressed as ambient concentrations. Although the TBEP recommendations for TN and TP criteria 

remain the TN and TP loads reported above, recommendations for concentration-based numeric 

nutrient criteria consistent with the TN and TP loading recommendations have been developed and 

are being provided by the TBEP (Janicki Environmental, 2011a), in the event that EPA determines 

that loadings cannot be used as numeric nutrient criteria. 

The Reference Period approach was selected to establish the proposed concentration-based 

numeric criteria for TN and TP. Based on a 1992-1994 reference period, segment-specific 

chlorophyll a targets have been identified and implemented as part of the Tampa Bay Nitrogen 

Management Strategy since 2000 (Janicki and Wade, 1996; Janicki, Wade, and Pribble, 2000). 

Using this similar and consistent approach, segment-specific annual geometric mean TN and TP 

concentrations from the 1992-1994 period were derived for this current effort. TN and TP 

concentration thresholds, as were developed for established, regulatory-recognized chlorophyll a 

thresholds, account for the inter-annual variability in the TN and TP concentrations observed from 

1992-2009. 

Application of the Reference Period approach resulted in the following recommendations for 

concentration-based TN and TP criteria for Tampa Bay. These criteria are: 

 Old Tampa Bay TN=0.93 mg/L TP=0.31 mg/L 

 Hillsborough Bay TN=1.01 mg/L TP=0.45 mg/L 

 Middle Tampa Bay TN=0.87 mg/L TP=0.29 mg/L 

 Lower Tampa Bay TN=0.74 mg/L TP=0.10 mg/L. 

The criteria referenced above should be assessed as an annual geometric mean from long-term 

monthly water quality monitoring stations currently used in the state’s chlorophyll a threshold 

assessments under the Tampa Bay RA determination. The assessment of TN and TP concentrations 

attainment should only occur when chlorophyll a thresholds are exceeded within a bay segment, 

and should coordinate with current regulatory assessments under the FDEP RA determination and 

EPA TMDL for TN loads in Tampa Bay. Further, compliance assessments should be conducted 

over five-year time frames, with no more than two consecutive years being greater than these 

established criteria if chlorophyll a thresholds are also exceeded during the same time period. This 
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approach is analogous to the chlorophyll a threshold assessments currently being conducted under 

the regulatory requirements for the FDEP RA determination and EPA TMDL for Tampa Bay. 

 

- Aquatic Life Support - Dissolved Oxygen 

The numeric nutrient criteria to be promulgated will need to provide full aquatic life support in 

each estuary. The aquatic life forms specifically influenced by excessive nutrient loadings to 

estuaries include seagrasses (affected by reduced water clarity due to excessive chlorophyll a

concentrations) and fish and benthic communities (affected by reduced DO conditions). Seagrass 

support is provided by maintenance of appropriate nutrient conditions and the resulting chlorophyll 

a concentrations as discussed above. Support of fish and benthic communities is provided by 

maintenance of appropriate nutrient conditions and the resulting DO conditions. 

The spatial and temporal distributions of DO concentrations in Tampa Bay’s major bay segments 

have been characterized, the principal drivers of low DO conditions in Tampa Bay have been 

investigated, and the relevance of the empirical distribution of DO concentrations to the FDEP’s 

Impaired Water Rule standard for DO have been evaluated with respect to the proposed numeric 

nutrient criteria for the Tampa Bay Estuary (Janicki Environmental, 2011b). The following 

conclusions can be drawn from these efforts: 

• A descriptive characterization of the spatial and temporal attributes of observed DO 

concentrations used over 30 years of data, collected by 4 different sampling agencies. 

Examination of the spatial distribution of DO samples shows that DO exceedances < 4 

mg/L were always less than 10% of the samples in all segments except Hillsborough Bay, 

never exceeded 15% of the samples in Hillsborough Bay, and are most likely to occur in 

Hillsborough Bay near the mouths of the Hillsborough River and Alafia River and along the 

western half of Hillsborough Bay. These are deeper areas, more likely to be stratified due 

to freshwater inputs, and have high organic sediment content. 

• The principal factor affecting DO in Tampa Bay is temperature. That is evident in both the 

descriptive temporal plots and in the generalized linear model assessed in the quantitative 

assessment of those factors affecting the probability of DO being less than 4 mg/L. The 

model results indicate that stratification, bottom type, and sample depth were other factors 

that contributed to the probability of low DO conditions (i.e., < 4 mg/L). Furthermore, it 

was determined that chlorophyll a concentrations were not a significant factor contributing 

to the probability of low DO conditions in Tampa Bay. In other words, the occurrence of 

DO values below 4 mg/L were not significantly related to observed chlorophyll a

concentrations at the time of sampling. 

• Based on the weight-of-evidence presented here, it is reasonable to conclude that the 

proposed numeric nutrient criteria are protective of full aquatic life uses with respect to DO. 

- Downstream Protection Values 

Downstream Protection Values (DPVs) are defined by EPA as those water quality criteria in flowing 

waters that ensure protection of designated uses in the downstream estuarine waters as required by 

the Clean Water Act under 40 CFR 131.10(b). For freshwater lakes, EPA has determined that a 
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DPV for stream tributaries that flow into a downstream lake is either the allowable concentration or 

the allowable loading of TN and/or TP applied at the point of entry into the downstream waterbody 

(EPA Freshwater Rule, finalized December 2010). Either expression (concentration or loading) may 

be used for assessment and source control allocation purposes, such as TMDLs. 

Based on input garnered from a January 28th, 2011, joint TBEP Technical Advisory Committee and 

TBNMC meeting and the adopted logic approved by EPA for DPVs for freshwater lakes, the TBEP 

staff contends that the existing federally-approved TMDL TN loading limits and the recommended 

TP loading limits meet the intent of DPVs for Tampa Bay major bay segments. Continued 

attainment of chlorophyll a thresholds in the major bay segments of Tampa Bay should provide 

sufficient evidence that the TN and TP contributions of tributaries draining to Tampa Bay are 

protective of the estuary. Therefore, the protective TN and TP loads recommended by the TBNMC 

in March 2010 to the EPA are sufficiently protective to attain in-bay chlorophyll a thresholds for 

Tampa Bay. 

- Recommendation for Consideration of Tidal Creeks as Unique Entities 

Questions have been raised as to whether the numeric nutrient criteria proposed for the estuary 

proper should apply to tidal creeks that drain to the estuary. Tidal creeks play an integral role in 

the ecological function of coastal estuaries (summarized in Janicki Environmental, 2011d). The 

treatment of tidal creeks in the implementation of the estuarine numeric nutrient criteria is, 

therefore, a significant issue. A thorough understanding of the ecological elements (e.g., faunal and 

floral species and communities), processes (e.g., primary productivity, nutrient cycling, secondary 

production), dynamics of tidal creeks (e.g., temporal fluctuations in dissolved oxygen) and function 

in exporting energy to estuarine and coastal ecotones is paramount to the establishment of 

ecologically appropriate nutrient criteria. Numeric nutrient criteria established for tidal creeks must 

consider the different ecological processes and functions that distinguish them from both from the 

freshwater systems upstream and the open estuary downstream. Only with careful consideration of 

these attributes can criteria be developed that will maintain the function of tidal creeks in support 

of the greater estuarine ecosystem. Recently, EPA’s SAB (SAB Draft Panel Discussion, Jan. 25, 

2011) concurred that tidal creeks warrant development of distinct criteria relative to the estuary 

proper. 

Studies of Tampa Bay tidal creeks have revealed compelling evidence that these systems represent 

unique ecotones within the greater Tampa Bay estuary. Tidal creeks play an integral role in the 

ecological function of coastal estuaries as sites of high primary and secondary production, nursery 

and refuge habitat for several species of economically important fish and decapod crustaceans, and 

foraging areas for large-bodied fishes, wading birds, and other piscivorous species. Higher nutrient 

concentrations in tidal creeks relative to the greater estuary may be required to support the higher 

levels of primary and secondary production in these systems. 

Analysis of fish collections in tidal creeks suggests that fishes inhabiting tidal creeks appear to be 

very tolerant to the typical DO conditions found in these systems. Both fish abundance and species 

richness data indicate that fish communities are relatively invariant to DO levels between 2-10 

mg/L. There are indications that at DO concentrations below 2 mg/L, both fish abundance and 

species richness decline. Species richness of fish and decapod crustaceans may be a more sensitive 

indicator of the aquatic-life support function of tidal creeks; however, these need further 
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quantification to eliminate the possibility that seasonal recruitment patterns of estuarine-dependent 

fishes are not correlated with seasonal variation in DO concentrations due to temperature. 

The most desirable approach to establish numeric nutrient criteria for these systems would be to 

develop stressor-response models. Stressor-response models require the identification of an 

indicator variable that can be used to evaluate the condition of the tidal creek. Moreover, stressorresponse models require identification of a threshold value above (or below) which the system 

would no longer fully support its designated use. It is important that the established criteria for tidal 

creeks also account for the fact that these systems by nature are more variable than their upstream 

or downstream counterparts. This variability is in part what makes these systems so productive and 

also so difficult to generalize. The timing and volume of freshwater inflows are physical drivers that 

exert a great deal of control on tidal creeks. Inflows are deterministic of salinity regimes, nutrient 

delivery, water depths, temperatures and the potential for salinity stratification in these systems. 

Inflows also may control access to these systems for both small recruit species looking for refuge 

and for large-bodied predators. Therefore, the quantification of the effects on inflows on these 

systems will be necessary both to determine appropriate criteria and in the evaluation process. The 

extent of tidal creeks in the Tampa Bay watershed to which these recommendations pertain 

include, but may not be limited to, the systems identified in the maps provided in Figure 3a,b 

below. 

Based on the recognized need to define distinct biological endpoints for tidal tributaries and water 

quality criteria to support them, TBEP staff recommends the following: 

• Recognize tidal tributaries as a separate waterbody class; and 

• Consider setting a schedule (i.e., within 3 years) by which time endpoints and 

criteria will be proposed, but do not attempt to set interim or final criteria with 

insufficient data. 

TBEP has dedicated funds to continue work in tidal tributaries in Tampa Bay and will commit to 

work with EPA to develop recommendations by September 2014. 
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Figure 3. a) Named tidal creeks within the Tampa Bay watershed.
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Figure 3. b) Approximate location of the upstream limit of tidal creeks and rivers as defined by 

empirical salinity data analysis of data from tributaries within the Tampa Bay watershed. 

- Implementation Considerations 

TBEP has addressed two key issues identified by the EPA regarding successful implementation of 

the proposed numeric nutrient criteria in Tampa Bay, namely the method to account for nonanthropogenic events, such as El Niño and hurricanes, and the allowable exceedance criteria (how 

often criteria may be exceeded before non-compliance is observed). Analyses were performed to 

direct input on these subjects (Janicki Environmental, 2011c), with the following conclusions: 

• The annual response time to recover from the maximum monthly chlorophyll a

concentration during a year is relatively short. Median annual response times are two 
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months or less in all segments, and average annual response times are three months or less 

in all segments. This indicates that the bay recovers very quickly from normal loading 

events.

• The typical response times to unusual events, such as El Niño, are longer and, depending 

upon the timing of such events, can span over parts of two successive years.

• Comparison of the two temporal assessment schemes (1 in 3 years) vs (2 in 5 years) 

suggested that the 2 in 5 rule was less likely to result in a violation due solely to natural 

variability.

- Final Implementation and Assessment Recommendations 

It is recommended that the assessment of compliance with the proposed numeric nutrient criteria 

be performed in a manner similar to that which has been proposed by TBEP for compliance with 

both the Tampa Bay RA determination and EPA TMDL. The goal of the estuarine numeric nutrient 

criteria is to provide full aquatic-life support within the estuary. The TBEP has determined that 

seagrasses are important indicators of desirable conditions in the bay and has defined the waterquality conditions (i.e., chlorophyll a concentrations) that allow for the maintenance and growth of 

seagrass beds in Tampa Bay. Therefore, TBEP bases its compliance assessment on the comparison 

of both observed chlorophyll a concentrations and seagrass extent to the goals that have been 

established. To date, this has proven to be a successful adaptive management approach for abating 

nutrient eutrophication in the Tampa Bay Estuary. 

References 

U.S. Environmental Protection Agency (EPA). 2010. Methods and Approaches for Deriving 

Numeric Criteria for Nitrogen/Phosphorus Pollution in Florida’s Estuaries, Coastal Waters, and 

Southern Inland Flowing Waters. 

Florida Department of Environmental Protetion (FDEP). 2010. DRAFT Overview of Approaches for 

Numeric Nutrient Criteria Development in Marine Waters. Division of Environmental Assessment 

and Restoration, Standards and Assessment Section, Florida Department of Environmental 

Protection, Tallahassee, FL 

Hagy, J.D. 2010. Chapter 6: Numeric Nutrient Criteria Development for Protection of 

Downstream Estuaries. Presentation to EPA Science Advisory Board, Dec 13-14, 2010, 

Washington, D.C. 

Janicki, A., and D. Wade. 1996. Estimating critical external nitrogen loads for the Tampa Bay 

Estuary: An empirically based approach to setting management targets. Tampa Bay National 

Estuary Program Technical Publication #06-96. 

http://www.tbeptech.org/TBEP_TECH_PUBS/1996/TBEP_06_96ExternalTNLoads.pdf 
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Janicki, A., D. Wade, and J.R. Pribble. 2000. Developing and Establishing a Process to Track the 

Status of Chlorophyll-a Concentrations and Light Attenuation to Support Seagrass Restoration Goals 

in Tampa Bay. Tampa Bay Estuary Program Technical Report #04-00.

http://www.tbeptech.org/TBEP_TECH_PUBS/2000/TBEP_04_00Chlor-A.pdf 

Janicki Environmental, Inc. 2010. Empirical Approaches to Establishing Numeric Nutrient Criteria 

for Southwest Florida Estuaries. Report prepared for the Tampa Bay Estuary Program, Sarasota Bay 

Estuary Program, and Charlotte Harbor National Estuary Program by Janicki Environmental, Inc. 

http://www.tbeptech.org/TBEP_TECH_PUBS/2010/TBEP_05_10_SW_Florida_NEP_NNC_MethodsJuly_14_2010.pdf 

Janicki Environmental, Inc. 2011a. Tampa Bay Numeric Nutrient Criteria: Task 1 – TN and TP 

Concentrations. Letter Memorandum. Prepared for Tampa Bay Estuary Program. 

http://www.tbeptech.org/NitrogenMgmtConsort/NNC/TBEP_NNC_TN_TP_Letter_Memo_16Feb201

1.pdf 

Janicki Environmental, Inc. 2011b. Tampa Bay Numeric Nutrient Criteria: Task 3 – Dissolved 

Oxygen. Letter Memorandum. Prepared for Tampa Bay Estuary Program. 

http://www.tbeptech.org/NitrogenMgmtConsort/NNC/TBEP_NNC_DO_Letter_Memo_16Feb2011.p

df 

Janicki Environmental, Inc. 2011c. Tampa Bay Numeric Nutrient Criteria: Task 4 – 

Implementation Issues. Letter Memorandum. Prepared for Tampa Bay Estuary Program. 

http://www.tbeptech.org/NitrogenMgmtConsort/NNC/TBEP_NNC_Implementation_Letter_Memo_1

6Feb2011.pdf 

Janicki Environmental, Inc. 2011d. Tampa Bay Numeric Nutrient Criteria: Tidal Creeks. Letter 

Memorandum. Prepared for Tampa Bay Estuary Program. 

http://www.tbeptech.org/NitrogenMgmtConsort/NNC/TBEP_NNC_Tidal_Creeks_Letter_Memo_16F

eb2011.pdf 

Tampa Bay National Estuary Program (TBNEP). 1996. Charting the Course. The Comprehensive 

Conservation and Management Plan for Tampa Bay. Published by the Tampa Bay National Estuary 

Program in cooperation with the U.S. Environmental Protection Agency, Region IV. 

http://www.tbep.org/tbep/download_charting_the_course.html