PROPOSED 

NUMERIC NUTRIENT CRITERIA 

FOR TAMPA BAY 

Prepared for: 

Tampa Bay Estuary Program 

110 – 8th Ave. S.E. 

St. Petersburg, Florida 33701 

Prepared by: 

Janicki Environmental, Inc. 

1155 Eden Isle Drive NE 

St. Petersburg, Florida 33704 

September, 2011
1 

EXECUTIVE SUMMARY 

FDEP is developing a rule to establish numeric nutrient criteria for a number of water body types. 

The overarching FDEP goal is to “manage nutrients in surface (and groundwater) at loadings or 

concentrations that result in protection and maintenance of healthy, well-balanced aquatic 

communities”. Absent site-specific analyses, criteria should be based on a quantifiable linkage 

between anthropogenic nutrient enrichment and a biological response. FDEP recognizes the role 

of site-specific factors that affect numeric responses and proposes to base new standards on 

establishing a systematic numeric interpretation of the existing narrative criteria. Nutrient TMDLs, 

Site Specific Alternative Criteria (SSAC), and other site-specific actions written to achieve the 

narrative nutrient criteria should be given preference over more broadly applicable interpretations. 

The TBEP and the Tampa Bay Nitrogen Management Consortium (TBNMC) have developed a 

nitrogen management strategy linking seagrass growth to external nitrogen loadings. FDEP has 

accepted this strategy as providing reasonable assurance that the state water quality criteria would 

be met in Tampa Bay. In addition, the EPA has recognized the TN loading targets developed as 

part of the strategy as the TMDL for Tampa Bay. The TBEP and TBNMC have previously 

recommended to EPA numeric nutrient criteria as segment-specific annual TN loads. 

The recommended TN loads followed development of seagrass targets and corresponding water 

quality targets and thresholds for Tampa Bay. Extensive empirical evaluations found strong links 

between TN loads, resultant chlorophyll and light attenuation, and seagrass response. Water 

quality has been improving since the mid-80s, and corresponding responses in seagrass acreage 

have occurred. During this same period, however, TN loads often exceeded the TMDL targets. 

 The TBNMC further investigated relationships between TN loads and water quality, and found that 

residence time should be accounted for. As residence time shortens when freshwater inputs are 

greater, loadings move through the system more quickly and thus biological processes have less 

time to convert nutrients to chlorophyll. Given that both TN loads and hydrologic loads affect the 

chlorophyll within the bay, the annual hydrologic loads are considered along with the annual TN 

loads when assessing target compliance. 

A more appropriate predictor of the likelihood of good water quality, as defined by meeting the 

FDEP-approved chlorophyll a thresholds, is expected to be the amount of TN delivered per unit 

water. This is denoted as the Nitrogen Delivery Ratio, and is defined as the amount of TN 

delivered, in tons, per million cubic meters of freshwater delivered. The proposed criteria are 

expressed as the segment-specific Nitrogen Delivery Ratios observed during the 1992-1994 

reference period, provided in the table below. TBEP has also developed alternative criteria, 

specifically concentration-based TN criteria and both loading and concentration-based TP criteria 

for each bay segment. 

TBEP estuarine numeric nutrient criteria expressed as Nitrogen Delivery Ratio (tons TN per 

million m3 hydrologic load) based on 1992-1994 conditions. 

Segment Nitrogen Delivery Ratio Threshold 

(tons/million m3

) 

Old Tampa Bay 1.08 

Hillsborough Bay 1.62 

Middle Tampa Bay 1.24 

Lower Tampa Bay 0.97 
2 

1.0 Introduction 

The Florida Department of Environmental Protection (FDEP) is currently developing a rule to 

establish numeric nutrient criteria for a number of water body types. The following summarizes 

FDEP’s view on the development of numeric nutrient criteria in Florida (FDEP website, 2011). 

The overarching FDEP goal is to “manage nutrients in surface water (and groundwater) at loadings 

or concentrations that result in protection and maintenance of healthy, well-balanced aquatic 

communities”. Absent site-specific analyses, water quality criteria should be based on a 

quantifiable linkage between anthropogenic nutrient enrichment and a biological response (such as 

seagrass growth) that can be used to numerically interpret the narrative nutrient criteria. There is 

value in knowing whether nutrient concentrations are elevated to potentially environmentally 

harmful levels, but, conversely, it is equally important to identify adverse biological effects and 

determine if they are linked to nutrients before deciding that nutrient reduction strategies should be 

pursued. 

The Department recognizes the role of site-specific factors that affect nutrient responses and 

proposes to base new standards on establishing a systematic numeric interpretation of the existing 

narrative criteria. This concept is intended to implement Rule 62-302.530 (47)(b), FAC, which 

states, “in no case shall nutrient concentrations of a body of water be altered so as to cause an 

imbalance in natural populations of aquatic flora or fauna.” 

As envisioned by FDEP, the narrative nutrient criteria would continue to apply to all waterbodies, 

and numeric interpretations would be applied when adequate scientific information is available. 

The narrative criteria would be implemented using a hierarchical approach that numerically 

interprets the narrative nutrient criteria for each waterbody. 

Numeric interpretations are most accurate when determined as a site-specific function. Therefore, 

nutrient Total Maximum Daily Loads (TMDLs), Site Specific Alternative Criteria (SSAC), and other 

site-specific actions written to achieve the narrative nutrient criteria should be given preference 

over more broadly applicable interpretations. 

The first tier of the hierarchical approach entails an evaluation of previously established site-specific 

numeric interpretations of the narrative criteria (including TMDLs, SSACs, and other interpretations 

embodied in an official Department action) as the primary interpretation of the narrative nutrient 

criteria. Currently, Tampa Bay has an FDEP-approved Reasonable Assurance and Water Quality 

Based Effluent Limitation (WQBEL) in addition to an EPA-recognized TMDL. Therefore, Tampa Bay 

would be included in this first tier of the hierarchical approach. 

FDEP has also stated that if nutrient TMDLs are expressed as loads instead of concentrations, the 

loads do not have to be translated into concentrations to be deemed the numeric interpretation of 

the narrative nutrient criteria. This document presents loading-based numeric nutrient criteria for 

the four mainstem bay segments of Tampa Bay. 

Finally, FDEP recognizes that future TMDL rules may include a response target (chlorophyll a, for 

example) designed to implement the narrative nutrient criterion. Currently, the recommended 

assessment of the proposed numeric nutrient criteria for Tampa Bay is built on assessing the 

achievement of a series of segment-specific chlorophyll a targets. 
3 

In November 2002, FDEP concluded that the nitrogen management strategy developed by the 

Tampa Bay Estuary Program (TBEP) and Tampa Bay Nitrogen Management Consortium (TBNMC) 

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 

peer-reviewed 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 TBEP and TBNMC have previously recommended to EPA numeric nutrient criteria for the 

Tampa Bay estuary as segment-specific annual total nitrogen (TN) loads (TBNMC, 2010a). These 

recommendations were based on the previous work to develop a Reasonable Assurance (RA) plan 

and eventual nitrogen load allocations (TBNMC, 2010b). 

This document provides background information concerning the Tampa Bay estuarine system, 

including a brief physical characterization of the system and a summary of previous work 

completed with respect to Tampa Bay estuarine targets. In addition, this document provides the 

data and methodology utilized for developing estuarine numeric nutrient criteria for Tampa Bay, 

the targets resulting from application of the methodology, the proposed criteria, and the proposed 

methodology for implementation and compliance of the Tampa Bay estuarine numeric nutrient 

criteria. 

1.1 Tampa Bay and Its Watershed 

Tampa Bay is on the west central coast of Florida, and has been divided into seven bay segments 

for purposes of management activities. The bay covers approximately 1,030 km2

, extends 

approximately 56 km inland from the Gulf of Mexico, is 8-16 km wide along most of its length, and 

is the largest open water estuary in Florida (Figure 1-1). Tampa Bay is crossed by four major 

causeways, and has about 42 nautical miles of dredged channels with designed mean low water 

depths of 6-13 meters. The major shipping channel has been dredged from the mouth of the bay to 

the port facilities near Tampa. The average depth of the bay is approximately 4 meters. 

The four mainstem segments of the bay include Lower Tampa Bay, Middle Tampa Bay, 

Hillsborough Bay, and Old Tampa Bay (Figure 1-1). Lower Tampa Bay connects the mouth of the 

bay to the Gulf of Mexico, and is joined on the southeast by the Manatee River and Terra Ceia Bay 

segments and on the north by the Boca Ciega Bay segment. Middle Tampa Bay is northeast of 

Lower Tampa Bay, and connects to the Hillsborough Bay segment to the northeast and the Old 

Tampa Bay segment to the north. 
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The Tampa Bay watershed covers approximately 5,700 km2, and includes all or parts of Pasco, 

Pinellas, Hillsborough, Polk, Manatee, and Sarasota counties, with more than 2 million current 

residents and a projected population doubling by 2050. Land use is mixed, with about 40% of the 

watershed undeveloped, 35% agricultural, 16% residential, and the remaining commercial and 

mining. Major habitats in the Tampa Bay estuary include mangroves, salt marshes, and submerged 

aquatic vegetation. 

The watershed is divided into seven drainage basins, corresponding to the seven bay segments. 

The watershed contains various source types supplying freshwater runoff and associated nutrients 

and other pollutants to Tampa Bay (Greening and Janicki, 2006). These sources have been 

categorized as nonpoint sources (representative of runoff), atmospheric deposition (directly to the 

open water of the estuary), point sources (domestic and industrial wastewater facilities), springs, 

and groundwater. An additional source of nutrient loadings to the bay is loss from fertilizer 

handling operations at the two port facilities, in Hillsborough Bay and Lower Tampa Bay. 

The TBEP, in cooperation with local scientists and FDEP, has developed a tabular listing of 

symptoms of nutrient enrichment in Tampa Bay, along with potential explanations of these 

expressions, to aid in development of numeric nutrient criteria for the bay. This listing is provided 

in Tables 1-1 and 1-2 below. These tables provide a summary of potential responses expressed in 

the bay related to nutrient loads, and notes whether these responses are currently or historically 

found in the bay. 

 
Table 1-1. Checklist of nutrient enrichment symptoms in Tampa Bay.Response Variable Observed 

Historically 

or 

Currently? 

Explanation 

Low dissolved oxygen (DO) (hypoxia/anoxia)

Yes, limited 

in space and 

time 

Typically, the lowest dissolved oxygen conditions occur during the summer months in Hillsborough Bay; however, no apparent increasing trends have been detected for the areal extent of hypoxia during these months in this bay segment . Hypoxic events in this bay segmappear to be largely driven by physical processes rather than nutrient effects. 

Reduced clarity No. Long-term monitoring of bay water clarity via Secchi disk depths by the EPCHC has indicate

significant improvement. 

Increased chlorophyll aconcentrations 

No. Long-term monitoring of bay chlorophyll-a concentrations by the EPCHC has indicated a 

significant decline. 

Phytoplankton blooms (nuisance or toxic) 

Yes, limited 

in space and 

time 

Red tide blooms originating in the Gulf of Mexico have occasionally entered the lower bay. 

Most recently in Old Tampa Bay, nuisance blooms of Pyrodinium bahamense have occurred 

a result of anomalous spring/summer rainfall events in 2008 and 2009. Blooms did not reocin 2010, but did occur in late summer 2011. 

Problematic epiphyte growth 

No No significant trends in epiphyte growth have been reported as result of monitoring conduct

by the Tampa Bay Interagency Seagrass Monitoring Program. 

Problematic macroalgal growth 

Yes 

(historical), 

No (current) 

Macroalgal blooms have been reported and observed in the past; however, nuisance bloomhave not occurred over the recent period or have only been associated with distinct events arestricted to minor embayments (e.g. in 2003 in Bishop Harbor from FDEP-endorsed Piney Pcontrolled releases). 

Submerged aquatic vegetation (SAV) community changes or loss 

Yes, 

historically. 

Currently 

increasing. 

The Tampa Bay estuary is one of only a few estuaries worldwide that has shown recovery of seagrass resources despite continuing population growth and urbanization within the watersSeagrass coverage has increased by approximately 3,200 ha since 1982. 

Emergent or shoreline vegetation community changes or loss 

Yes, 

historically. 

Currently 

increasing. 

The 2010 Tampa Bay Habitat Master Plan Update has identified restoration and protection gfor critical coastal habitats in the Tampa Bay watershed. The TBEP and its partners have ado

a strategy to “restore the balance” of coastal habitats to levels commensurate to the 1950s. Significant progress has been made in restoring and protecting coastal habitats in Tampa Bay since 1996. 

Coral/hardbottom community changes or loss 

Unknown No information on change or loss has been developed. Current hardbottom communities w

Tampa Bay have been tentatively identified, but no historical reference exists for comparison

Impacts to benthic community 

No Benthic communities have been monitored since 1993 in Tampa Bay and have shown stable 

populations as evidenced by the Tampa Bay Benthic Index. 

Fish kills Yes, limited 

in space and 

time 

Periodic fish kills have occurred as a result of red tide blooms entering the lower bay and cowater induced shock; however, no significant kills have been reported as a direct result of nutrient inputs into the estuary. 
Table 1-2. Checklist of additional factors in Tampa Bay. Nutrient Factors Observed? Explanation Increased nutrient loading or concentrations 

No Phosphorus and nitrogen levels have been stable and/or declining in the major bay segment

of Tampa Bay over recent decades. Nitrogen delivery (i.e. N load per unit hydrologic input) from the watershed to the major bay segments of the estuary has been declining since 1985 

Mitigating factors that could prevent nutrient expression 

Yes Bay hydrologic residence times and circulation; highly-colored seasonal, freshwater inflows. 

Other significant stressors to biological communities 

No - 

Natural highnutrient episodes (e.g., seasonal or event driven) 

Yes During periods of extremely high rainfall (e.g., the El Niño event in 1998), other storm event

(hurricanes), and/or domestic/industrial spills elevated nutrient and/or chlorophyll-a concentrations have been observed. 
8 

2.0 Objective 

The objective of this document is to propose estuarine numeric nutrient criteria specific to the four 

mainstem segments of Tampa Bay: Old Tampa Bay, Hillsborough Bay, Middle Tampa Bay, and 

Lower Tampa Bay. The remaining three segments (Boca Ciega Bay, Terra Ceia Bay, and Manatee 

River) will be addressed in a separate document. The criteria are expressed as TN and TP loadings 

to the bay. The proposed TN loading criteria for the four mainstem segments were based on the 

nitrogen management strategy developed by the TBNMC. 

3.0 Development of Seagrass, Light Attenuation, Chlorophyll a, 

and Nitrogen Loading Targets 

This section provides detailed summary of work previously completed by the TBEP and TBNMC in 

establishment of the seagrass target, the associated water quality (light attenuation and chlorophyll) 

targets, and TN loading targets to each segment. An assessment of the seagrass, water quality, and 

TN loading data utilized in this effort is provided in Appendix A. Appendix B contains a 

description of the derivation of the recently proposed TP loading targets and TN and TP 

concentration thresholds which may be utilized as criteria if necessary. 

3.1 Background and Rationale

Between 1950 and 1990, an estimated 40-50% of the seagrass acreage in Tampa Bay was lost due 

to excess nitrogen loading and related increases in algae concentration which caused light 

limitation detrimental to seagrass survival and growth, in addition to shoreline modifications. In 

1980, all municipal wastewater treatment plants were required to provide Advanced Wastewater 

Treatment (AWT) for discharges directly to the bay and its tributaries. In addition to the significant 

reductions in nitrogen loadings from municipal wastewater treatment plants, stormwater regulations 

enacted in the 1980s also resulted in reduced nitrogen loads to the bay. Estimated TN loads in 

1976 are more than twice recent (2003-2007) estimates. 

The Comprehensive Conservation and Management Plan (CCMP) of the TBEP (TBEP, 1996a) 

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 (Figure 3-1). As depicted in the TBEP’s nitrogen 

management paradigm, 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 since the 1990s clearly established that nitrogen loads were 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 (Johansson, 1991, 2009; Janicki 

Environmental, 2011a). 
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10 

The segment-specific seagrass targets for the four mainstem bay segments were established as: 

 Old Tampa Bay 10,545 acres, 

 Hillsborough Bay 1,663 acres, 

 Middle Tampa Bay 8,930 acres, and 

 Lower Tampa Bay 7,030 acres. 

3.3 Development of Chlorophyll a, Light Attenuation, and Nitrogen Loading 

Targets 

The TBEP developed targets for light and chlorophyll a in support of the seagrass target for the bay, 

based on a specified percentage of incident light reaching segment-specific depths as determined 

from historical (1950s) seagrass distribution. Target depths were chosen based on conservative 

estimates of the deepest edges of seagrass meadows in the 1950s period (Janicki et al., 1995). The 

minimum light requirement at these targets depths was established as 22.5% of incident light, 

based on data obtained during a SWFWMD study conducted by Mote Marine Laboratory (Dixon 

and Leverone, 1995). Further evaluation included accounting for bottom reflectance, resulting in 

the light requirement target being adjusted to 20.5% of incident light (Janicki and Wade, 1996). 

Following establishment of depth and light targets, relationships were developed between water 

quality and light attenuation. Least squares regression methods were used to estimate parameters 

relating average monthly chlorophyll a concentrations to light availability (Janicki and Wade, 

1996). Chlorophyll a levels were related to light attenuation (via estimates derived from Secchi 

disk depths) using a functional form of Beers’ law in a linear regression and served as a proxy for 

light availability to seagrass in the bay (Figure 3-3). Given the minimum light requirement of 20.5% 

of incident light, predictions of chlorophyll a levels and Secchi disk depths (light penetration) 

necessary to restore seagrass to average depths observed in each of the major bay segments during 

the 1950s were used to assess the development of annual targets for these parameters (Janicki and 

Wade, 1996; Greening and Janicki, 2006). 

Figure 3-3. Relationship between chlorophyll a and depth of 20.5% light attenuation.
11 

The next step in the TBEP paradigm (Figure 3-1) was linking chlorophyll a concentrations to 

loadings. As described in Janicki and Wade (1996), the net flux of water and nitrogen between 

segments must be considered. First, a series of dilution equations were written for the four 

mainstem segments of the bay, and fit to observed salinity and freshwater inflow data. Next, the 

dilution equations were used to predict nitrogen concentrations, and a regression parameter was 

used to relate the observed and predicted nitrogen concentrations to account for the net of all nonconservative processes affecting nitrogen (such as sediment uptake or release) (Figure 3-4). 

The chlorophyll a targets developed from this effort were segment-specific. Potential chlorophyll a

targets were developed utilizing two methods: the TN loading-chlorophyll a model described 

above, and annual average values from the 1992-1994 period. The TN loading-chlorophyll a

model was used to identify the chlorophyll a needed for sufficient light to reach depths in each 

segment corresponding to recovery to seagrass target. Recall from above that the seagrass target 

was set as 95% of the sum of the protection and restoration areas. Based on improving seagrass 

coverage and water quality seen over the 1990–1996 period, secondary targets were developed 

from the average annual chlorophyll a levels seen during 1992-1994 (a period of time with high 

and low rainfall during which seagrass was expanding). The ultimate selection of bay segmentspecific chlorophyll a targets was determined as the average annual levels developed from the 

empirical model predictions (Janicki and Wade, 1996) or the 1992-1994 average annual levels – 

which ever were lower (TBEP, 1996b and 2001). 

The TBEP TAC further recognized that there may be years in which these targets may be exceeded 

without causing significant reductions in seagrass cover. This means that there is some allowable 

amount of variation that should not elicit a significant degradation in water quality and therefore 

seagrass coverage. This level of variation was defined as two standard errors around the period of 

record mean annual chlorophyll a concentrations in each segment. Therefore, a distinction is made 

between a target, i.e., a desired chlorophyll a concentration and a threshold, i.e., a chlorophyll a

concentration above which undesirable chlorophyll a concentrations exist. The chlorophyll a

threshold for each segment is the sum of the target and two standard errors around the mean annual 

chlorophyll a concentrations in each segment. These thresholds have been recognized both in the 

recent WQBEL and RA documents (FDEP, 2010; TBNMC, 2010b). The chlorophyll a targets and 

thresholds are shown in Table 3-1. 

Table 3 1. Tampa Bay chlorophyll a targets and thresholds. 

Segment Chlorophyll a Target 

(μg/L) 

Chlorophyll a Threshold 

(μg/L) 

Old Tampa Bay 8.5 9.3 

Hillsborough Bay 13.2 15.0 

Middle Tampa Bay 7.4 8.5 

Lower Tampa Bay 4.6 5.1 

Finally, TN loads from both external sources and intersegment transport were related to the 

observed chlorophyll a using a regression model (Figure 3-4) (Janicki and Wade, 1996). During this 

same time, the 1992-1994 average annual total nitrogen loads were established as the appropriate 

nitrogen load management targets by TBEP partners (Table 3-2) in order to support the maintenance 

of the chlorophyll a and light attenuation targets developed for each of Tampa Bay’s major bay 

segments. These targets have subsequently been adopted by EPA as the federally-recognized 

nitrogen TMDL for Tampa Bay. 
12 

Figure 3-4. Comparison of observed chlorophyll a and that predicted from the TN load – chlorophyll a

relationships, all four mainstem bay segments, for 1986-1998 and 1999-2007. 

 Table 3 2. Tampa Bay TN loading targets. 

Segment TN Load Target 

(tons/year) 

Old Tampa Bay 486 

Hillsborough Bay 1,451 

Middle Tampa Bay 799 

Lower Tampa Bay 349 

4.0 Proposed Criteria 

The TBNMC annual assessment of the water quality conditions in the four mainstem segments of 

the bay is accomplished by completion of the previously developed Decision Matrix, which 

compares annual average chlorophyll a and light conditions to targets (Janicki et al., 2000). The 

Decision Matrix results through 2010 are provided in Figure 4-1. Chlorophyll a values were less 

than the thresholds in most segments for most years following the 1992-1994 period (see Appendix 

A). Exceptions were linked either to excessive rainfall conditions or to an emergency action taken 

associated with the closure of an industrial point source (TBNMC, 2010b). 
13 

Figure 4-1. Decision Matrix results through 2010 showing assessment 

of chlorophyll a threshold values.

Year OTB HB MTB LTB

1975 No No No Yes

1976 No No No Yes

1977 No No No No

1978 No No No Yes

1979 No No No No

1980 No No No No

1981 No No No No

1982 No No No No

1983 No No No No

1984 Yes Yes No Yes

1985 No No No Yes

1986 No No Yes Yes

1987 No Yes No Yes

1988 Yes Yes Yes Yes

1989 No Yes Yes Yes

1990 No Yes Yes Yes

1991 Yes Yes Yes Yes

1992 Yes Yes Yes Yes

1993 Yes Yes Yes Yes

1994 No No No No

1995 No No No Yes

1996 Yes Yes Yes Yes

1997 Yes Yes Yes Yes

1998 No No No No

1999 Yes Yes Yes Yes

2000 Yes Yes Yes Yes

2001 Yes Yes Yes Yes

2002 Yes Yes Yes Yes

2003 No Yes Yes Yes

2004 No Yes Yes Yes

2005 Yes Yes Yes No

2006 Yes Yes Yes Yes

2007 Yes Yes Yes Yes

2008 Yes Yes Yes Yes

2009 No Yes Yes Yes

2010 Yes Yes Yes Yes
14 

During the development of the 2009 Reasonable Assurance (TBNMC, 2010b), the TBNMC 

investigated the temporal trends in TN loading. While the chlorophyll a thresholds were being 

met, the TN loading targets were exceeded in some years (Figure 4-2). The TBNMC investigated 

the relationships between TN loads and chlorophyll a to better understand how the bay responds to 

varying nitrogen loads. Non-anthropogenic factors can significantly influence the relationship 

between chlorophyll a and TN loadings. One such factor is rainfall and its role in determining 

estuarine residence time, which in turn has been shown to influence this relationship in many lakes 

and other estuaries. As residence time shortens, and loadings move more quickly out of the 

estuary, biological processes have less time to convert nutrients to chlorophyll. As residence time 

lengthens, loadings remain within the system longer, and thus more nutrients can be converted to 

chlorophyll. Given the same nutrient loads, different residence times within the system can result 

in very different expressions in water quality constituents. Given this paradigm, that both TN loads 

and hydrologic loads affect the chlorophyll a within the systems, the annual hydrologic loads to 

each of the segments must be considered along with the annual TN loads to establish loading 

thresholds for each segment. Figure 4-3 provides a schematic of these influences. 

Figure 4-2. Comparison of annual loading to the four mainstem segments to the segment-specific targets. 
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16 

Given the critical contributions of both TN load and hydrologic load to chlorophyll a

concentrations, the proposed criteria are expressed as the segment-specific Nitrogen Delivery 

Ratios observed during the 1992-1994 reference period. The proposed criteria for the four 

mainstem segments are provided in Table 4-1 below. 

Table 4-1. TBEP estuarine numeric nutrient criteria expressed as Nitrogen Delivery Ratio (tons 

TN per million m3 hydrologic load) based on 1992-1994 conditions. 

Segment Nitrogen Delivery Ratio Threshold 

(tons/million m3

) 

Old Tampa Bay 1.08 

Hillsborough Bay 1.62 

Middle Tampa Bay 1.24 

Lower Tampa Bay 0.97 

5.0 Implementation 

The TBNMC and TBEP have recommended that FDEP and EPA compliance assessments should first 

consider whether the mean annual bay segment chlorophyll a concentration is above the FDEPapproved threshold, and as a result, whether seagrass resources within the bay segment have been 

or are likely to be impacted (TBNMC, 2010b). The RA defines a series of management actions to 

be taken in response to various threshold exceedances. The following assessment strategy builds 

on that approved by FDEP for the Tampa Bay RA. 

The proposed compliance assessment strategy involves two steps. The initial step is the 

comparison of the mean annual chlorophyll a concentrations in each bay segment to the 

established chlorophyll a thresholds. Compliance is achieved if the threshold is met in that year. If 

the chlorophyll a threshold is exceeded in more than two years during any five-year period, an 

assessment of nitrogen loading (relative to the hydrologic loads, i.e., the Nitrogen Delivery Ratios) 

during that period will be completed. The justification for this “2-in-5 year” compliance assessment 

has been developed (Janicki Environmental, 2010b) and summarized in Appendix D. 

The recommendation for compliance assessment also hinges on the availability of the appropriate 

data for a defensible assessment. Thus, how the specific spatial and temporal averaging has been 

conducted in the development of the targets and thresholds as well as the compliance assessment is 

critical. The following discusses this aspect of the implementation of the proposed numeric 

nutrient criteria for Tampa Bay. 

From the initial efforts to establish defensible water quality targets through the development of the 

RA, the primary data source has been the Environmental Protection Commission of Hillsborough 

County (EPC). The EPC has monitored water quality at more than 50 sites on a monthly basis since 

1974. Investigation of the available data resulted in the identification of 46 sites for use in the 

establishment of water quality targets in Tampa Bay. The suite of segment-specific EPCHC sites 

utilized for establishment of segment-specific chlorophyll a targets and thresholds, and for the 

annual assessment, and location maps for each segment are provided in Appendix E. Defensible 

assessment will depend upon continued monitoring at these sites to ensure that the annual 

assessment for attainment of the FDEP-adopted chlorophyll a thresholds, implementation of the 

FDEP-approved Reasonable Assurance Plan, and implementation of the proposed TN loading 

criteria presented here. 
17 

6.0 References 

Avery, W. and J.O.R. Johansson. 2010. Data Summary from the Tampa Bay Interagency Seagrass 

Monitoring Program through year 2008. Technical Report #01-10 of the Tampa Bay Estuary 

Program. Prepared by the City of Tampa Bay Study Group. 

Badylak, S., E.J. Phlips, P. Baker, J. Fajans, and R. Boler. 2007. Distributions of Phytoplankton in 

Tampa Bay Estuary, U.S.A. 2002–2003. Bulletin of Marine Science, 80(2): 295–317. 

Bricker, S.B. & 6 others. 2008. Effects of nutrient enrichment in the nation’s estuaries: A decade of 

change. Harmful Algae. 8:21-32. 

Dixon, L.K., and J.R. Leverone. 1995. Light requirements of Thalassia testudinum in Tampa Bay 

Florida. Submitted to: Surface Water Improvement and Management Program, Southwest Florida 

Water Management District. Submitted by: Mote Marine Laboratory, Sarasota, Florida. 

Florida Department of Environmental Protection (FDEP) website. 2011. 

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

Florida Department of Environmental Protection (FDEP). 2010. Final Order Adopting Water 

Quality Based Effluent Limits for Point Source Discharges to the Tampa Bay Watershed. 

Greening, H. and A. Janicki. 2006. Toward reversal of eutrophic conditions in a subtropical 

estuary: water quality and seagrass response to nitrogen loading reductions in Tampa Bay, Florida, 

USA. Environmental Management Vol. 38, No. 2, pp. 163-178. 

Heil, C.H., M. Revilla, P.M. Gilbert and S. Murasko. 2007. Nutrient quality drives differential 

phytoplankton community composition on the southwest Florida shelf. Limnol. Oceanogr., 

52(3):1067–1078. 

Janicki, A.J., D.L. Wade, and D.E. Robison. 1995. Habitat protection and restoration targets for 

Tampa Bay. Tech. Pub #07-93, Tampa Bay National Estuary Program. Prepared by Coastal 

Environmental, Inc. 

Janicki, A.J. and D.L. Wade. 1996. Estimating Critical Nitrogen Loads for the Tampa Bay Estuary: 

An Empirically Based Approach to Setting Management Targets. Technical Publication #06-96 of 

the Tampa Bay National Estuary Program. Prepared by Coastal Environmental, Inc. 

Janicki, A., D. Wade, and 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. Technical Report #04-00 of the Tampa Bay Estuary Program. Prepared by Janicki 

Environmental, Inc. 

Janicki, A.J., R. Pribble and M. Winowitch. 2001. Estimation of the Spatial and Temporal Nature of 

Hypoxia in Tampa Bay, Florida. 2001.Technical Report #09-01 of the Tampa Bay Estuary Program. 

Prepared by Janicki Environmental, Inc. 
18 

Janicki Environmental, Inc. 2010a. Sarasota Bay Numeric Nutrient Criteria: Implementation 

Issues. Letter Memorandum Prepared for Sarasota Bay Estuary Program. Sarasota, Florida 

Janicki Environmental, Inc. 2010b. Charlotte Harbor Numeric Nutrient Criteria: Implementation 

Issues. Letter Memorandum Prepared for Charlotte Harbor Estuary Program. Ft. Myers, Florida 

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

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

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

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

Johansson, J.O.R. 1991. Long-term trends of nitrogen loading, water quality and biological 

indicators in Hillsborough Bay, Florida in Treat, S.F. and P. Clark, eds. Proceedings, Tampa Bay 

Area Scientific Information Symposium 2. Feb. 27 – Mar. 1, 1992, Tampa, FL. 528pp. 

Johansson, J.O.R. 2009. Nutrient enrichment studies of natural phytoplankton populations in 

Tampa Bay: A summary of results June 1993 to August 2009. City of Tampa Wastewater 

Department Bay Study Group. 

Johansson, J.O.R. and R.R. Lewis. 1992. Recent improvements of water quality and biological 

indicators in Hillsborough Bay, a highly impacted subdivision of Tampa Bay, Florida, USA 

pp.1199-1215 in R.A. Vollenwedier et al., eds. Marine Coastal Eutrophication: The Response of 

Marine Transitional Systems to Human Impact: Problems and Perspectives for Restoration. 

Proceedings of an International Conference, Bologna, Italy, 21-24 March 1990. 

Karlen, D., T. Dix, B. Goetting, S. Markham, C. Meyer, M. Flock and G. Blanchard. 2008. Tampa 

Bay Benthic Interpretive Report: 1993-2004 Bay Wide Report. 2008. Technical Report #05-08 of 

the Tampa Bay Estuary Program. Prepared by the Environmental Protection Commission of 

Hillsborough County. 

Lewis, R.R. III, R.G. Gilmore, Jr., D.W. Crewz, and W.E. Odum. 1985a. Mangrove habitat and 

fishery resources of Florida. In: W. Seaman, Jr. (Ed.). Florida Aquatic Habitat and Fishery 

Resources. Florida Chapter of the American Fisheries Society. 

Morrison, G., E.T. Sherwood, R. Boler, and J. Barron. 2006. Variations in water clarity and 

chlorophyll a in Tampa Bay, Florida, in response to annual rainfall, 1985-2004. Estuaries and 

Coasts. 29:926-931. 

Phlips, E.J., S. Badylak, E. Bledsoe and M. Cichra. 2006. Factors affecting the distribution of 

Pyrodinium bahamense var. bahamense in coastal waters of Florida. Marine Ecology Progress 

Series, 322:99-115. 

Poe, A., ed. 2006. Baywide Environmental Monitoring Report, 2002-2005. TBEP Technical 

Publication #06-06. 202pp. 

Pribble, R. 2006. Bay Characteristics. In “Baywide Environmental Monitoring Report, 2002-2005. Tampa 

Bay, Florida.” Eds. A. Poe, A.J. Janicki, and H. Greening. Tampa Bay Estuary Program Technical Publication 

#06-06. 
19 

Robison, D. 2010. Tampa Bay Estuary Program Habitat Master Plan Update. Technical Report #06-

09 of the Tampa Bay Estuary Program. Prepared by PBS&J. 

Sherwood, E.T. 2010. 2009 Tampa Bay Water Quality Assessment. 2010. Technical Report #02-10 

of the Tampa Bay Estuary Program. Prepared by the Tampa Bay Estuary Program. 

Tampa Bay Estuary Program (TBEP). 1994. Physical impacts to bottom habitats in Tampa Bay. 

Prepared for the Tampa Bay Estuary Program. Prepared by Coastal Environmental, Inc. 

Tampa Bay Estuary Program (TBEP). 1996a. 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. 

Tampa Bay Estuary Program (TBEP). 1996b. Final Action taken by TBNEP Management and Policy 

Committees, adopting goals for seagrass acreage, targets for segment specific chlorophyll-a 

concentrations, and a five-year nitrogen management strategy to “hold the line” on nitrogen 

loadings for each bay segment. June 14, 1996. 

Tampa Bay Estuary Program (TBEP). 2001. Final Action taken by TBEP Management and Policy 

Boards, extending through 2005 the previously adopted chlorophyll a concentrations for each bay 

segment, and the nitrogen management strategy to “hold the line.” May 11, 2001. 

Tampa Bay Nitrogen Management Consortium (TBNMC). 2009. Decision Memo #11. 

Incorporation of hydrological normalization into Tampa Bay Nitrogen TMDLs. Appendix A in: 

2009 Reasonable Assurance Addendum: Allocation and Assessment Report. 

Tampa Bay Nitrogen Management Consortium (TBNMC). 2010a. Tampa Bay Nitrogen 

Management Consortium Comments and Request Regarding the Development of Protective Loads 

for the Tampa Bay Estuary as It Relates to Establishing Numeric Nutrient Criteria for Lakes, Flowing 

Waters and Estuaries in Florida. Attachment to Letter from TBNMC to EPA, March 8, 2010. 

Tampa Bay Nitrogen Management Consortium (TBNMC). 2010b. 2009 Reasonable Assurance 

Addendum: Allocation and Assessment Report. TBEP Technical Publication #03-10. 

Tampa Bay Regional Planning Council (TBRPC). 1986. Habitat restoration study for the Tampa 

Bay Region. Prepared by TBRPC, St. Petersburg, FL. 

Waycott, M. & 13 others. 2009. Accelerating loss of seagrass across the globe threatens coastal 

ecosystems. Proceedings of the National Academy of Science. 106(30):12377-12381 

Wolter, K., and M.S. Timlin, 1993: Monitoring ENSO in COADS with a seasonally adjusted 

principal component index. Proc. of the 17th Climate Diagnostics Workshop, Norman, OK, 

NOAA/NMC/CAC, NSSL, Oklahoma Clim. Survey, CIMMS and the School of Meteor., Univ. 

of Oklahoma, 52-57. 

Download PDF.http://www.esrl.noaa.gov/psd/people/klaus.wolter/MEI/#ref_wt1
APPENDIX A.

Data Description and 

Assessment of Seagrass, Water Quality, and TN Loading

1.0 Data Description

This section provides identification and description of the data utilized for developing relationships 

between seagrass extent, water quality, and loadings. 

1.1 Seagrasses

An estimate of seagrass extent is available for 1950 (Figure 1-1). The areal coverage of the 1950 

seagrass beds was developed using data collected by the Florida Department of Natural Resources 

(FDNR) and the U.S. Fish and Wildlife Service (USFWS). Seagrass meadows were mapped from 

1:24,000-scale color photography, and obtained in digital form from the FDNR. A GIS coverage of 

the data was created, and horizontally rectified to geographically coincide with mapped data for 

the time period (Janicki et al., 1995). 

Aerial photography of seagrasses in the bay has been gathered approximately every two years since 

1988 by the Southwest Florida Water Management District (SWFWMD), resulting in seagrass 

coverages for 1988, 1990, 1992, 1994, 1996, 1999, 2001, 2004, 2006, 2008, and 2010. Some of 

the 1988 areas mapped as seagrasses were later found to actually represent attached macro-algae. 

From 1990 on, the aerial photography was field checked to verify presence of seagrasses (Figures 1-

1, 1-2, and 1-3). 

Additional aerial photography exists for 1982, as described in Lewis et al. (1985a). The 

methodology used in developing the 1982 seagrass coverage was different than that employed by 

the SWFWMD, so that it should be understood that direct comparisons of specific seagrass areas 

between the 1982 coverage and later coverages are not appropriate. However, the 1982 coverage 

does provide some measure of the baywide extent of seagrasses, and is useful for tracking seagrass 

changes on a baywide basis.
Figure 1-1. Tampa Bay seagrass, 1950 (FDNR and USFWS); 1990-1994 (SWFWMD), with four mainstem 

segments identified.
Figure 1-2. Tampa Bay seagrass, 1996-2004 (SWFWMD), with four mainstem segments identified.
Figure 1-3. Tampa Bay seagrass, 2006-2010 (SWFWMD), with four mainstem segments identified.
1.2 Water Quality and Loadings

Water quality data utilized previously for establishment of targets in Tampa Bay, and here for 

developing numeric nutrient criteria in the bay, are those collected by the Environmental Protection 

Commission of Hillsborough County (EPCHC). The EPCHC has conducted monthly monitoring of 

ambient water quality in Tampa Bay since 1972, with complete records for most stations since 

1974. Monitoring is performed at 52 fixed stations in Old Tampa Bay, Hillsborough Bay, Middle 

Tampa Bay, and Lower Tampa Bay. Monitoring is also performed in several bay tributaries. Water 

quality samples are collected at mid-depth, and are analyzed for nutrients, chlorophyll, and 

biochemical oxygen demand (BOD). Hydrolab measurements are taken of dissolved oxygen (DO), 

specific conductivity, temperature, and pH at the surface, mid-depth, and bottom. Water clarity is 

measured using a Secchi disc. Full descriptions of the EPCHC water quality monitoring program 

and data summaries are found in Boler (1998), including the methods employed for sampling and 

analysis. The selected sites displayed in Figure 1-4 are those utilized for target setting efforts in the 

past, and used for developing estuarine numeric nutrient criteria for the four mainstem segments.

 Figure 1-4. Locations of EPCHC water quality monitoring sites used in analyses. 
2.0 Assessment of Conditions

Evaluation of the success of the TBNMC nitrogen management strategy is accomplished by an 

annual review of water quality data and biannual review of baywide seagrass extent. The following 

sections provide segment-specific comparisons of observed conditions to the seagrass targets, 

chlorophyll a targets and regulatory thresholds, and TN loading targets. For reference, segmentspecific tables at the end of this appendix contains the annual mean chlorophyll a concentrations 

utilized in the Decision Matrix tool, the geometric mean annual TN and TP concentrations, and the 

annual hydrologic, TN, and TP loads for each of the four mainstem segments.

2.1 Old Tampa Bay

Time series plots of seagrass acreages, mean annual chlorophyll a concentrations, mean annual 

light attenuation and nutrient concentrations, and annual TN loadings are provided in Figures 2-1

through 2-6, respectively. Seagrasses in Old Tampa Bay have increased since 1990, with the 

exception of the period following the 1997-1998 El Niño event and during 2004 (Figure 2-1). The 

TBEP has funded several studies in Old Tampa Bay to evaluate the potential cause of the less rapid 

increase in seagrass acreage here as compared to the bay as a whole, and is initiating additional 

studies in the near future. As seen from the time series of annual chlorophyll a concentrations

(Figure 2-2), the regulatory threshold has been met in most years, indicating that light should be 

sufficient for seagrass recovery, with observed improvements in the available light (Figure 2-3). 

Improvements in TN and TP concentrations have also occurred (Figures 2-4 and 2-5, respectively). 

Evaluation of the external TN load to the segment, which has been related to chlorophyll a

concentrations via the empirical modeling effort (Janicki and Wade, 1996), shows that the target TN 

loads have been exceeded in some years since 1994, even in years when chlorophyll a targets and 

thresholds were met (Figure 2-6). The apparent contradiction in meeting chlorophyll a targets and 

thresholds while not meeting external TN loading targets has been related to the effects of varying 

hydrologic loading rates and residence time. 
Figure 2-1. Comparison of observed Old Tampa Bay seagrass acreage to target.

Figure 2-2. Comparison of observed Old Tampa Bay chlorophyll a concentrations

to target and regulatory threshold.
Figure 2-3. Time series of Old Tampa Bay annual mean light attenuation, 1980-2010.

Figure 2-4. Time series of Old Tampa Bay annual geometric mean TN concentrations, 1981-2009.
Figure 2-5. Time series of Old Tampa Bay annual geometric mean TP concentrations, 1980-2009.

Figure 2-6. Comparison of observed Old Tampa Bay TN loadings to target.
2.2 Hillsborough Bay

Time series plots of seagrass acreages, mean annual chlorophyll a concentrations, mean annual 

light attenuation and nutrient concentrations, and annual TN loadings are provided in Figures 2-7

through 2-12, respectively, for Hillsborough Bay. Seagrasses in this segment have increased 

considerably since 1990 (Figure 2-7). As seen from the time series of annual chlorophyll a

concentrations, the regulatory threshold has been met in most years since 1994, and continuously 

for more than a decade, so that water quality conditions are appropriate for the observed increase 

in seagrasses (Figure 2-8). This is reinforced by the time series of light attenuation (Figure 2-9), with 

improved light availability. Nutrient concentrations have declined as well (Figures 2-10 and 2-11), 

especially TP. As in Old Tampa Bay, evaluation of the external TN load to the segment shows that 

the target TN loads have been exceeded several times since 1994, during the same period in which 

the segment was meeting chlorophyll a targets and thresholds. This has been related to the effects 

of varying hydrologic loading rates and residence time. 

Figure 2-7. Comparison of observed Hillsborough Bay seagrass acreage to target.
Figure 2-8. Comparison of observed Hillsborough Bay chlorophyll a concentrations

to target and regulatory threshold.

Figure 2-9. Time series of Hillsborough Bay annual mean light attenuation, 1980-2010.
Figure 2-10. Time series of Hillsborough Bay annual geometric mean TN concentrations, 1981-2009.

Figure 2-11. Time series of Hillsborough Bay annual geometric mean TP concentrations, 1980-2009.
Figure 2-12. Comparison of observed Hillsborough Bay TN loadings to target.

2.3 Middle Tampa Bay

Time series plots of seagrass acreages, mean annual chlorophyll a concentrations, mean annual 

light attenuation and nutrient concentrations, and annual TN loadings are provided in Figures 2-13

through 2-18, respectively, for Middle Tampa Bay. Seagrasses in this segment have increased by 

almost 3,000 acres since 1990 (Figure 2-13). As seen from the time series of annual chlorophyll a

concentrations, the regulatory threshold has been met in most years since 1994, and continuously 

for more than a decade, so that water quality conditions are appropriate for the observed increase 

in seagrasses (Figure 2-14). This is supported by the time series of light attenuation (Figure 2-15), 

with improved light availability. Nutrient concentrations have declined as well (Figures 2-16 and 2-

17), especially TP. As in Old Tampa Bay and Hillsborough Bay, evaluation of the external TN load 

to the segment shows that the target TN loads have been exceeded many times 1994, during the 

same period in which the segment was meeting chlorophyll a targets and thresholds (Figure 2-18). 

This has been related to the effects of varying hydrologic loading rates and residence time. 
Figure 2-13. Comparison of observed Middle Tampa Bay seagrass acreage to target.

Figure 2-14. Comparison of observed Middle Tampa Bay chlorophyll a concentrations

to target and regulatory threshold.
Figure 2-15. Time series of Middle Tampa Bay annual mean light attenuation, 1980-2010.

Figure 2-16. Time series of Middle Tampa Bay annual geometric mean TN concentrations, 1981-2009.
Figure 2-17. Time series of Middle Tampa Bay annual geometric mean TP concentrations, 1980-2009.

Figure 2-18. Comparison of observed Middle Tampa Bay TN loadings to target.
2.4 Lower Tampa Bay

Time series plots of seagrass acreages, mean annual chlorophyll a concentrations, mean annual 

light attenuation and nutrient concentrations, and annual TN loadings are provided in Figures 2-19

through 2-24, respectively, for Lower Tampa Bay. Seagrass acreages in this segment have remained 

more or less steady since 1990, with an increase during the last decade to nearly the segment target

(Figure 2-19). As seen from the time series of annual chlorophyll a concentrations, the regulatory 

threshold has been exceeded only twice since 1994, so that water quality conditions are 

appropriate for the observed increase in seagrasses (Figure 2-20). This is supported by the time 

series of light attenuation (Figure 2-21), with improved light availability. Nutrient concentrations 

have declined as well (Figures 2-22 and 2-23). Evaluation of the external TN load to the segment 

shows that the target TN loads have been exceeded only three times since 1994 (Figure 2-24). As 

for the other segments, exceedance of target load while attaining chlorophyll a targets and 

thresholds has been related to the effects of varying hydrologic loading rates and residence time. 

Figure 2-19. Comparison of observed Lower Tampa Bay seagrass acreage to target.
Figure 2-20. Comparison of observed Lower Tampa Bay chlorophyll a concentrations

to target and regulatory threshold.

Figure 2-21. Time series of Lower Tampa Bay annual mean light attenuation, 1980-2010.
Figure 2-22. Time series of Lower Tampa Bay annual geometric mean TN concentrations, 1981-2009.

Figure 2-23. Time series of Lower Tampa Bay annual geometric mean TP concentrations, 1980-2009.
Figure 2-24. Comparison of observed Lower Tampa Bay TN loadings to target.
Old Tampa Bay mean annual chlorophyll a, geometric mean annual TN and TP concentrations, 

and annual hydrologic, TN, and TP loads.

Year Chl a

(μg/L)

TN Conc

(mg/L)

TP Conc

(mg/L)

TN Load 

(tons/yr)

TP Load 

(tons/yr)

Hydrologic Load

(106 m3/yr) 

1980 13.87 0.64

1981 12.03 0.56 0.43

1982 12.99 0.59 0.30

1983 17.54 0.86 0.41

1984 9.16 1.05 0.41

1985 11.16 0.70 0.39 419 114 513

1986 10.59 0.58 0.43 478 111 515

1987 9.30 0.89 0.40 607 131 594

1988 8.16 0.58 0.37 685 187 695

1989 9.35 0.73 0.39 493 96 459

1990 9.34 0.52 0.40 386 88 395

1991 8.29 0.52 0.30 519 109 518

1992 7.88 0.62 0.27 444 105 441

1993 7.59 0.73 0.25 453 97 420

1994 9.43 0.92 0.25 562 109 488

1995 11.52 0.81 0.26 653 276 687

1996 6.85 0.83 0.19 475 191 514

1997 8.36 0.70 0.18 758 158 780

1998 11.23 0.78 0.24 811 179 789

1999 8.13 0.62 0.12 394 146 446

2000 7.17 0.92 0.09 396 106 413

2001 8.60 0.89 0.13 444 107 507

2002 7.83 0.82 0.14 615 134 688

2003 11.72 0.52 0.16 841 185 784

2004 11.81 0.56 0.12 855 190 910

2005 8.40 0.47 0.13 470 119 544

2006 6.28 0.45 0.14 515 116 604

2007 8.64 0.41 0.14 390 91 484

2008 9.27 0.44 0.13

2009 12.12 0.49 0.14
Hillsborough Bay mean annual chlorophyll a, geometric mean annual TN and TP concentrations, 

and annual hydrologic, TN, and TP loads.

Year Chl a

(μg/L)

TN Conc

(mg/L)

TP Conc

(mg/L)

TN Load 

(tons/yr)

TP Load 

(tons/yr)

Hydrologic Load

(106 m3/yr) 

1980 22.64 0.94

1981 30.98 1.09 0.74

1982 35.95 1.02 0.63

1983 25.86 1.10 0.67

1984 13.05 1.04 0.68

1985 19.27 0.85 0.63 1321 1474 707

1986 16.78 0.64 0.60 1516 1384 880

1987 14.23 1.03 0.66 2454 1862 1087

1988 13.04 0.70 0.65 2101 2006 1177

1989 14.18 0.75 0.56 1236 1352 642

1990 12.78 0.62 0.47 1010 1149 499

1991 14.10 0.59 0.45 1731 1603 921

1992 11.09 0.73 0.38 1240 1102 717

1993 9.84 0.71 0.34 1206 1166 804

1994 18.65 1.05 0.38 1911 1019 1192

1995 18.70 0.90 0.40 2163 1782 1447

1996 9.63 0.75 0.30 1459 991 1000

1997 11.99 0.73 0.30 2012 2507 1270

1998 20.74 0.85 0.43 3023 1910 1933

1999 10.29 0.64 0.24 1204 838 743

2000 8.83 0.88 0.20 799 584 490

2001 10.10 0.86 0.23 1499 1071 941

2002 13.69 0.77 0.23 1723 1193 1159

2003 12.40 0.56 0.26 2922 2055 1873

2004 11.93 0.57 0.28 3359 3929 1774

2005 13.12 0.51 0.26 1540 1128 1035

2006 10.95 0.46 0.24 1337 757 697

2007 8.34 0.42 0.20 741 550 565

2008 11.58 0.47 0.23

2009 14.63 0.46 0.23
Middle Tampa Bay mean annual chlorophyll a, geometric mean annual TN and TP concentrations, 

and annual hydrologic, TN, and TP loads.

Year Chl a

(μg/L)

TN Conc

(mg/L)

TP Conc

(mg/L)

TN Load 

(tons/yr)

TP Load 

(tons/yr)

Hydrologic Load

(106 m3/yr) 

1980 14.67 0.61

1981 14.21 0.58 0.37

1982 18.78 0.63 0.30

1983 19.33 0.81 0.43

1984 8.80 0.82 0.39

1985 13.00 0.65 0.37 461 92 565

1986 8.18 0.49 0.37 679 117 671

1987 8.56 0.75 0.42 859 140 690

1988 7.65 0.53 0.38 1215 249 1048

1989 8.38 0.59 0.36 694 109 572

1990 8.35 0.49 0.34 546 77 458

1991 7.88 0.45 0.28 674 130 610

1992 6.47 0.57 0.25 808 164 663

1993 6.57 0.61 0.23 560 87 524

1994 11.03 0.94 0.25 1036 221 751

1995 10.28 0.73 0.25 985 378 912

1996 5.35 0.63 0.16 484 177 509

1997 7.96 0.66 0.18 936 157 875

1998 12.24 0.75 0.27 944 201 869

1999 7.23 0.56 0.14 498 75 521

2000 6.07 0.81 0.11 493 66 475

2001 7.73 0.77 0.13 877 203 823

2002 6.14 0.68 0.14 649 103 673

2003 7.76 0.46 0.16 1230 246 1037

2004 6.58 0.42 0.14 854 213 842

2005 7.66 0.39 0.15 519 121 574

2006 5.13 0.34 0.14 468 89 502

2007 5.54 0.35 0.13 427 58 466

2008 5.75 0.38 0.14

2009 6.47 0.40 0.14
Lower Tampa Bay mean annual chlorophyll a, geometric mean annual TN and TP concentrations, 

and annual hydrologic, TN, and TP loads.

Year Chl a

(μg/L)

TN Conc

(mg/L)

TP Conc

(mg/L)

TN Load 

(tons/yr)

TP Load 

(tons/yr)

Hydrologic Load

(106 m3/yr) 

1980 6.13 0.27

1981 6.15 0.34 0.12

1982 6.14 0.38 0.12

1983 7.48 0.54 0.19

1984 3.94 0.57 0.13

1985 4.91 0.47 0.12 293 141 359

1986 3.56 0.37 0.15 379 128 399

1987 3.51 0.54 0.17 379 76 403

1988 3.39 0.42 0.15 369 119 499

1989 5.09 0.48 0.14 444 81 362

1990 4.06 0.34 0.12 318 53 293

1991 3.61 0.31 0.09 331 59 343

1992 4.04 0.46 0.08 341 63 393

1993 3.56 0.50 0.07 323 39 324

1994 5.49 0.80 0.08 386 55 367

1995 4.52 0.54 0.09 314 180 469

1996 2.21 0.44 0.06 211 95 321

1997 4.59 0.46 0.10 458 41 531

1998 5.81 0.54 0.14 457 57 437

1999 4.23 0.46 0.09 222 13 317

2000 4.49 0.80 0.06 217 13 286

2001 4.76 0.73 0.07 270 30 428

2002 3.82 0.59 0.11 247 16 406

2003 3.41 0.37 0.11 490 41 547

2004 4.89 0.32 0.07 285 35 438

2005 6.15 0.34 0.07 213 22 343

2006 3.64 0.29 0.08 195 18 330

2007 3.11 0.25 0.08 179 12 288

2008 2.85 0.27 0.08

2009 4.42 0.31 0.08
APPENDIX B.

Alternative Criteria Expressed as TN Concentrations, and as

TP Loads and Concentrations

1.0 TN Concentrations

The TBEP has developed recommended segment-specific numeric nutrient criteria in terms of TN 

concentrations should these be necessary for Tampa Bay (Janicki Environmental, 2011a). The 

development of these criteria is described below.

Multiple analyses were completed in the evaluation of potential TN criteria expressed as in-bay 

concentrations, as described in Janicki Environmental (2011a). These included:

 examination of the relationships between TN loadings and in-bay TN concentrations;

 examination of relationships between monthly TN concentrations and chlorophyll a

concentrations; 

 examination of relationships between annual TN concentrations and chlorophyll a

concentrations; and

 application of a reference period approach to establishing TN concentration-based criteria.

Since the previously proposed TN criteria are expressed as loads, the simplest method to propose 

criteria expressed as in-bay concentrations would be based on the potential relationships between 

in-bay TN concentrations and TN loads delivered to each segment. All four mainstem segments 

have annual TMDL TN target loads recognized by both EPA and DEP. If significant relationships 

were found between the nutrient loads and their respective in-bay concentrations, then the 

proposed numeric nutrient criteria could be expressed as concentrations. 

Monthly segment-specific TN concentrations were merged with monthly segment-specific TN loads

resulting in a dataset of monthly values of TN concentrations and loads. Plots of these data were 

inspected, with TN concentrations as functions of TN loads, including various lag and cumulative 

load effects. 

 

No relationships were found between TN concentrations and potential TN load explanatory 

variables that explained more than 20% of the variation in TN concentrations in Hillsborough Bay 

or more than 10% of the variation in concentrations in the other three bay segments. This suggests 

that there are some in-bay processes (e.g., sedimentation, denitrification, and transport within the 

bay and exchange with the Gulf of Mexico) that affect the relationship between the TN loads and 

the resultant in-bay TN concentrations. 

The results of these analyses did not provide adequate evidence to support recommendations for 

TN concentration criteria based on the relationships between the in-bay nutrient concentrations and 

the nutrient loads to those segments. 

The next method used in evaluation of potential TN concentration criteria was to examine potential 

relationships between monthly TN concentrations and monthly chlorophyll a concentrations in 

each segment. Given the existing chlorophyll a targets and associated FDEP chlorophyll a
thresholds in each segment, sufficiently predictive relationships between nutrient and chlorophyll a 

concentrations would allow derivation of TN concentration criteria commensurate with the 

threshold values.

No relationships were found between chlorophyll a and TN concentrations that explained more 

than 24% of the variation in chlorophyll a concentrations.

The third method evaluated was to examine relationships between annual mean chlorophyll a and 

annual mean TN concentrations as a potential means of developing nutrient criteria. The annual 

mean TN concentrations were classified according to whether the annual chlorophyll a threshold in 

a given segment was met or not. Based on this classification, the TN concentrations for those years 

in which the threshold was met could be compared to those TN concentrations for those years 

when the threshold was exceeded.

The results of this analysis indicated that on an annual basis, there are apparent differences in the 

in-bay TN concentrations during years in which the threshold chlorophyll a targets are met and 

those in which they are not met. This is particularly the case in Hillsborough Bay and Middle 

Tampa Bay. These findings indicate that establishment of nutrient criteria should be linked to the 

annual chlorophyll a and loading targets already established for Tampa Bay.

The fourth approach to developing concentration-based numeric nutrient criteria was the reference 

period approach. Segment-specific chlorophyll a targets (values at this level or below indicate 

desirable conditions) have been previously established (Janicki and Wade, 1996). These targets 

were based on the 1992-1994 reference period. In 2000, a protocol for assessing whether the 

Tampa Bay segments were achieving these targets was developed (Janicki et al., 2000). This 

protocol, referred to as the Decision Matrix approach, considered the year-to-year variability in 

chlorophyll a concentrations and arrived at segment-specific chlorophyll a thresholds (values above 

this level indicate undesirable conditions). The threshold was the sum of the chlorophyll a target 

and 2X the standard error of the long-term chlorophyll a concentrations. FDEP has adopted these 

thresholds to assess compliance with the Tampa Bay Reasonable Assurance.

Applying the reference period approach to deriving concentration-based numeric nutrient criteria 

also considered the effects of year-to-year variability, similar to that included in the development of 

the segment-specific chlorophyll a targets and thresholds discussed above.

The 1992-1994 segment-specific annual geometric mean TN concentrations are provided below, 

with the standard deviation based on the 1992-2009 period:

Segment Geometric Mean Standard Deviation

 Old Tampa Bay 0.75 mg/L 0.18 mg/L

 Hillsborough Bay 0.82 mg/L 0.19 mg/L

 Middle Tampa Bay 0.69 mg/L 0.18 mg/L

 Lower Tampa Bay 0.57 mg/L 0.17 mg/L

The proposed alternative concentration-based TN criteria (Table 1-1) are defined as the sum of the 

annual geometric mean concentration and the standard deviation of the long-term mean annual TN 

concentrations. 
Table 1-1. TBEP estuarine numeric nutrient criteria for TN 

expressed as concentrations.

Segment TN Concentration (mg/L)

Old Tampa Bay 0.93

Hillsborough Bay 1.01

Middle Tampa Bay 0.87

Lower Tampa Bay 0.74

The proposed TN concentration criteria are compared to the observed geometric mean annual TN 

concentrations in Figures 1-1 through 1-4. The green horizontal lines represent the proposed 

alternative criteria; the data to the right of the vertical lines depict the TN concentrations since the 

reference period (1992-1994). 

Figure 1-1. Comparison of proposed alternative TN concentration criterion for Old Tampa Bay to the 

annual geometric mean TN concentrations from 1981 through 2009.
Figure 1-2. Comparison of proposed alternative TN concentration criterion for Hillsborough Bay to the 

annual geometric mean TN concentrations from 1981 through 2009.

Figure 1-3. Comparison of proposed alternative TN concentration criterion for Middle Tampa Bay to the 

annual geometric mean TN concentrations from 1980 through 2009.
Figure 1-4. Comparison of proposed alternative TN concentration criterion for Lower Tampa Bay to the 

annual geometric mean TN concentrations from 1980 through 2009.

2.0 TP Load and Concentration Alternative/Supplementary 

Criteria

The 1992-1994 average annual total nitrogen loads were established as the appropriate nitrogen 

load management targets by TBEP partners in order to support the maintenance of the chlorophyll a

and light attenuation targets developed for each of Tampa Bay’s major bay segments. The TN 

loading threshold criteria were defined as the segment-specific Nitrogen Delivery Ratio based on 

the mean 1992-1994 conditions. In keeping with this approach, the TP loading target and TP 

loading threshold criteria are defined as the segment-specific Phosphorus Delivery Ratio based on 

the mean 1992-1994 conditions. The TP loading targets and loading threshold criteria expressed as 

the Phosphorus Delivery Ratio are provided in Table 2-1. Time series plots of annual Phosphorus

Deliver Ratio to the threshold are provided in Figures 1-5 through 1-8, indicating that the threshold 

value has been exceeded infrequently during the recent (2000 on) period.

Table 2-1. TBEP estuarine numeric nutrient criteria for TP target loads and threshold 

criteria expressed as the Phosphorus Delivery Ratio.

Segment TP Load Target 

(tons/yr)

TP Load Threshold Expressed as Phosphorus 

Delivery Ratio (tons/million m3)

Old Tampa Bay 104 0.23

Hillsborough Bay 1093 1.28

Middle Tampa Bay 140 0.24

Lower Tampa Bay 112 0.14
Figure 2-1. Comparison of annual Phosphorus Delivery Ratio to proposed TP loading threshold criteria for 

Old Tampa Bay.

Figure 2-2. Comparison of annual Phosphorus Delivery Ratio to proposed TP loading threshold criteria for 

Hillsborough Bay.
Figure 2-3. Comparison of annual Phosphorus Delivery Ratio to proposed TP loading threshold criteria for 

Middle Tampa Bay.

Figure 2-4. Comparison of annual Phosphorus Delivery Ratio to proposed TP loading threshold criteria for 

Lower Tampa Bay.
In the same manner as described above for development of the TN criteria in terms of 

concentrations, evaluation of TP concentration criteria was completed. As for the TN concentration 

methodology, the most appropriate method was determined to be the Reference Period approach, 

in keeping with the existing TN loading targets and proposed threshold criteria, the proposed TN 

concentration criteria, and the proposed TP loading targets and threshold criteria. 

The 1992-1994 segment-specific annual geometric mean TP concentrations are provided below, 

with the standard deviation based on the 1992-2009 period:

Segment Geometric Mean Standard Deviation

 Old Tampa Bay 0.25 mg/L 0.06 mg/L

 Hillsborough Bay 0.37 mg/L 0.08 mg/L

 Middle Tampa Bay 0.24 mg/L 0.05 mg/L

 Lower Tampa Bay 0.08 mg/L 0.02 mg/L

The proposed alternative concentration-based TP criteria (Table 2-2) are defined as the sum of the 

annual geometric mean concentration and the standard deviation of the long-term mean annual TP

concentrations. 

Table 2-2. TBEP estuarine numeric nutrient criteria for TP

expressed as concentrations.

Segment TP Concentration (mg/L)

Old Tampa Bay 0.31

Hillsborough Bay 0.45

Middle Tampa Bay 0.29

Lower Tampa Bay 0.10

The proposed TP concentration criteria are compared to the observed geometric mean annual TP

concentrations in Figures 2-5 through 2-8. The green horizontal lines represent the proposed 

criteria. As seen in these graphics, the proposed TP concentration criteria have been met in most 

years in all segments since the early 1990s. 
Figure 2-5. Comparison of proposed TP concentration criterion for Old Tampa Bay to the annual 

geometric mean TP concentrations from 1980 through 2009.

Figure 2-6. Comparison of proposed TP concentration criterion for Hillsborough Bay to the annual 

geometric mean TP concentrations from 1980 through 2009.
Figure 2-7. Comparison of proposed TP concentration criterion for Middle Tampa Bay to the annual 

geometric mean TP concentrations from 1980 through 2009.

Figure 2-8. Comparison of proposed TP concentration criterion for Lower Tampa Bay to the annual 

geometric mean TP concentrations from 1980 through 2009.
APPENDIX C.

Comparison of Annual TN Loads to Annual Hydrologic Loads:

Nitrogen Delivery Ratio

A comparison of the annual TN loads to annual hydrologic loads for each bay segment between 

1995 and 2007 is shown graphically in Figures 1-4 below. The vertical axis is the annual TN load, 

and the horizontal axis is the annual hydrologic load. The slanted blue line represents the loads 

corresponding to those expected using the 1994-1994 Nitrogen Delivery Ratio, in terms of tons 

TN/year per million m3 hydrologic load/year. Points falling below the line represent those years 

when the Nitrogen Delivery Ratio was less than that observed during the 1992-1994 period. Points 

above the blue line in each graphic denote years when the loads would be in excess of the 

thresholds. The Hillsborough Bay threshold as denoted by the Nitrogen Delivery Ratio was 

exceeded in 2004 and 2006, and the Lower Tampa Bay threshold was exceeded in 1998. In all the 

remaining years for those bay segments, and in all years during the 1995-2007 period for Middle 

Tampa Bay and Old Tampa Bay, the Nitrogen Delivery Ratios were below the threshold.

Figure 1. Relationship between annual TN load and annual hydrologic load in Old Tampa Bay for 1995-

2007. The slanted blue line is what the expected relationship would be given the Nitrogen Delivery Ratio 

of 1.08 tons TN load/million m3 hydrologic load from the 1992-1994 period.
Figure 2. Relationship between annual TN load and annual hydrologic load in Hillsborough Bay for 1995-

2007. The slanted blue line is what the expected relationship would be given the Nitrogen Delivery Ratio 

of 1.62 tons TN load/million m3 hydrologic load from the 1992-1994 period.

Figure 3. Relationship between annual TN load and annual hydrologic load in Middle Tampa Bay for 1995-

2007. The slanted blue line is what the expected relationship would be given the Nitrogen Delivery Ratio 

of 1.24 tons TN load/million m3 hydrologic load from the 1992-1994 period.
Figure 4. Relationship between annual TN load and annual hydrologic load in Lower Tampa Bay for 1995-

2007. The slanted blue line is what the expected relationship would be given the Nitrogen Delivery Ratio 

of 0.97 tons TN load/million m3 hydrologic load from the 1992-1994 period.
APPENDIX D.

Implementation Issues and Compliance Assessment

Background

As part of this document, site specific numeric nutrient criteria are being proposed as alternatives to 

the application of regional or state wide model based standards. These site specific criteria rely on 

the identification of a response endpoint which has been defined for Tampa Bay as chlorophyll a

concentrations and their effect on light attenuation to seagrasses. The temporal scale has been 

defined as annually. The implementation of the Florida numeric nutrient criteria proposed by EPA 

will require the definition of an implementation and assessment cycle. Consideration of the 

potential ramifications of an assessment that is either too lenient or too strident is a critical element 

of the evaluation of alternative assessment cycles. Both EPA and FDEP are considering allowances 

of criteria exceedance due to natural variability. The draft freshwater numeric nutrient criteria 

proposed by EPA identified a 3-year assessment cycle which incorporates a “1 in 3” rule to allow 

one exceedance in a three-year assessment cycle to account for natural variability. The Florida 

Department of Environmental Protection (FDEP) currently uses a 5-year assessment cycle for 

evaluation of impairment in waterbodies, as well as NPDES, MS4, and other regulatory permitting 

cycles. FDEP was initially considering a “2 in 5” rule for compliance but has since proposed “1 in 

5” as the allowable exceedance frequency.

Climate anomalies and criteria exceedances

Janicki Environmental (2011a,b,c) examined the effects of these different temporal assessment 

schemes on the likelihood of concluding that a waterbody was in exceedance based solely on 

natural meteorological variability and concluded that the 2 in 5 rule would be most appropriate to 

allow for natural variability due to climatological anomalies and yet remain sensitive to detecting a 

deviation in water quality due to anthropogenic activities. The investigation was based on a 

conceptual model expressed by the EPA that the regulatory compliance assessment cycle should 

allow for natural disturbance patterns resulting from episodic events in Florida. These events could 

include hurricanes and ENSO-related droughts and floods that influence water quality independent 

of anthropogenic effects. The relationship between these events and water quality has been 

substantiated in several southwest Florida estuaries. (e.g. Morrison et al., 2006; Sherwood, 2010). 

Janicki Environmental (2011c) used long-term stream flow records to characterize natural variability 

in the hydrologic conditions in Tampa Bay since the 1930s. This estimate of natural variability was 

then used to test two potential temporal assessment schemes with respect to their ability to account 

for natural variability and identify exceedances related to anthropogenic activities. Since the 

objective of the rule is to account for exceedances due to natural environmental variability, the

analysis suggested that a “2 in 5” rule is more likely to absorb natural variability than the “1 in 3”

rule. Based on these analyses, the “1 in 3” rule would result in more exceedances due to natural 

variability alone and, therefore, be overly sensitive to non-anthropogenic variability compared to 

the “2 in 5” rule. For example, it is easy to see in Figure 1 that when El Niño or La Niña events 

occur they tend to last more than a single year. In fact, it’s quite possible for many of the single 

events to cross a calendar year. Further information on the frequency and duration of ENSO events 

can be found at: http://www.esrl.noaa.gov/psd/enso/mei/
Figure 1. Classification of El Niño and La Niña events as described in Janicki Environmental 2011a.

FDEP is also considering allowances for the misclassification of an impairment due to sampling 

variability. This type of misclassification is called the “Type 1 error” of “False Positive Rate”. The 

type 1 error rate is defined when establishing targets and threshold values using any estimate of a 

population of water quality data with variability. Such is the case with many of the estuaries that are 

developing criteria based on the reference period approach. An example of that discussion follows.

Type 1 Error and the Reference Period Approach

To begin this example, a hypothetical reference period distribution of chlorophyll a concentrations 

was generated. Chlorophyll a concentrations in estuaries typically exhibit “lognormal” distribution 

characteristics. This implies that when a natural log transformation is applied to the distribution, it 

becomes normally distributed which conforms to most parametric statistical tests. Therefore, a 

lognormal distribution of chlorophyll a concentrations was simulated using a mean of 6.75 μg/L 

and a standard deviation of 4.49 μg/L. This distribution composed of 10000 observations is shown 

in Figure 2. Once the data are natural log transformed, the distribution becomes bell shaped 

corresponding to the normal distribution (Figure 2). Since FDEP and EPA have proposed using 

annual geometric means as regulatory statistics for parameters with lognormal distributions such as 

chlorophyll a, we now consider the log transformed values to represent the data underlying this 

hypothetical evaluation and back transform the averages to obtain the geometric means.
Given the theoretical distribution of Figure 3, we can generate annual averages to simulate the 

product of a typical monitoring program. For example, we can randomly select 12 observations 

from the distribution of log values, and calculate an average and back transform that average to 

obtain the geometric mean. Further, we can repeat this experiment many times, each time 

selecting 12 samples and calculating the average of the log values. We expect each average to be 

close to the overall average of the reference period since it is a random sub-sample, however, we 

know that variability will exist due to sampling. For example, in Figure 4, we have generated 100 

annual averages from the reference period distribution. As can be seen in Figure 4, the distribution 

of annual averages is centered on a value very close to the overall average in Figure 3 which is 

expected. 

Figure 2. Simulated distribution of chlorophyll a concentrations.
Figure 3. Distribution of the natural log transformed values of the simulated distribution above.

Figure 4. Distribution of 100 annual averages of the log transformed chlorophyll

values using 12 samples per year.
To derive the geometric annual averages, we simply back transform the annual averages of Figure 

3. This distribution is shown in Figure 5. 

Figure 5. Geometric means taken from the simulated chlorophyll concentrations above.

Targets and Thresholds

Once the geometric means have been calculated we have all the information necessary to develop 

targets and thresholds for this theoretical reference period distribution.

The reference period target for this distribution can be defined as the overall average of all the 

geometric means. That is, 5.947 μg/L is our goal for the water body to ensure that it remains similar 

to the reference period condition we identified as being indicative of full aquatic life support. 

However, it can be seen from Figure 4 that the distribution of annual geometric means for this 

population ranges between 3.9 μg/L and 8.3 μg/L. Therefore, the distribution of geometric means 

can be considered a population of values that represent the expected distribution of annual 

averages of the reference period. Based on normal distribution theory, we can assign a threshold 

value to represent an annual average above which we would reject the hypothesis that the value 

was from the same population as the reference period distribution of annual geometric means. 

Using the z table of standard statistical tests, only 5% of a normal distribution with a mean and 

standard deviation of the distribution in Figure 4 would lie above the mean+1.65*the standard 

deviation of the population of geometric means. Therefore, a threshold can be constructed for this 

distribution that defines with 95% confidence what value a given geometric mean would need to 

exceed to reject the hypothesis that it belonged to the reference period distribution. For this 

distribution that threshold value equals 7.59 μg/L. 
The threshold value provides a level of certainty that a future single geometric mean value (i.e. a 

sample) is not declared to exceed the reference distribution simply due to sampling. As was 

demonstrated above, we know that some portion of the reference distribution values will be above 

the target value. In fact, nearly 50% of the annual geometric means will be higher than the target. 

However, only 5% of the values will be higher than the threshold value of 7.59 μg/L due to 

chance. This construct defines what is known as the Type I error rate or False Positive rate. That 

is, we are willing to accept that 1 in 20 times we may get a value that is higher than 7.59 μg/L that 

still belongs to the reference period distribution. As was seen in Figure 5, 5% of the values did 

exceed 7.59 μg/L even though we know the entire population of values. 

Therefore, the type I error rate is a value that is selected based on judgment and while 5% is 

typically used, there is no definitive rationale for 5% versus 1% or 10%. If we wish to be more 

conservative, we would select a lower type I error rate by choosing a larger value (e.g., 1.96) to 

multiple against the standard deviation to minimize the potential for falsely declaring that a value 

was different from the reference distribution. If we have reason to believe that the water body may 

not fully meet its designated use at values at the higher end of the reference distribution, one could 

select a higher type I error rate which in turn would equate to a smaller z score to multiply by the 

standard deviation (e.g., 1). For example, selecting 1 standard deviation equates to a type 1 error 

rate of 16%. Therefore, the type 1 error rate does obviously have implications on the compliance 

assessment that also need to be accounted for. 

Recommendations

Based on this analysis, it makes sense to allow for the potential that an anomalous event could 

result in higher annual geometric average than the reference period threshold value in more than 

one year in a 5 year cycle as is currently proposed by FDEP. 

Therefore, the recommended allowance as an exceedance frequency in a five year assessment 

cycle is to allow for no more than 2 exceedances in a 5 year assessment cycle.

It is possible that the type I error rate and the probability of an anomalous event could be estimated

such that both the probability of an exceedance occurring by chance and the probability that an 

exceedance occurs due to anomalous conditions could be integrated. That is, over a 5 year cycle 

one might calculate the probability of either an exceedance due to chance or an exceedance due to 

a natural anomaly. This was beyond the scope of this memorandum but one that might be pursued 

as the implementation process is refined. Because the proposed criteria are based on threshold 

values with known type 1 error rates, the criteria and the 2 in 5 year implementation criteria are 

inextricably linked and should not be separated. Further information supporting the use of the 2 in 

5 implementation rule is provided in Janicki Environmental (2011a,b,c). 
References

Janicki Environmental, Inc. 2011a. Sarasota Bay Numeric Nutrient Criteria: T

ask 4 - Implementation Issues. Letter Memorandum Prepared for Sarasota Bay Estuary Program. 

Sarasota, Florida

Janicki Environmental, Inc. 2011b. Charlotte Harbor Numeric Nutrient Criteria: Implementation 

Issues. Letter Memorandum Prepared for Charlotte Harbor Estuary Program. Ft. Myers, Florida

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

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

Morrison, G., E.T. Sherwood, R. Boler, and J. Barron. 2006. Variations in water clarity and 

chlorophyll a in Tampa Bay, Florida, in response to annual rainfall, 1985-2004. Estuaries and 

Coasts. 29:926-931.

Sherwood, E.T. 2010. 2009 Tampa Bay Water Quality Assessment. 2010. Technical Report #02-10 

of the Tampa Bay Estuary Program. Prepared by the Tampa Bay Estuary Program.

Wolter, K., and M.S. Timlin, 1993: Monitoring ENSO in COADS with a seasonally adjusted

principal component index. Proc. of the 17th Climate Diagnostics Workshop, Norman, OK, 

NOAA/NMC/CAC, NSSL, Oklahoma Clim. Survey, CIMMS and the School of Meteor., Univ. 

of Oklahoma, 52-57.

Download PDF.http://www.esrl.noaa.gov/psd/people/klaus.wolter/MEI/#ref_wt1
APPENDIX E.

Maps of EPCHC Monitoring Sites

Utilized for Water Quality Assessment in 

Each Segment
Figure 1. EPCHC sites used for assessment of water quality conditions in Old Tampa Bay.
Figure 2. EPCHC sites used for assessment of water quality conditions in Hillsborough Bay.
Figure 3. EPCHC sites used for assessment of water quality conditions in Middle Tampa Bay.
Figure 4. EPCHC sites used for assessment of water quality conditions in Lower Tampa Bay.