TAMPA BAY 

Numeric Nutrient Criteria: 

Task 1 - TN and TP Concentrations 

Letter Memorandum 

Prepared for: 

Tampa Bay Estuary Program 

Prepared by: 

Janicki Environmental, Inc. 

16 February 2011 
16 February 2011 1 

FOREWORD 

This letter memo was produced in partial fulfillment of Purchase Order #6584, TBEP Contract T-07-

01 - Development of Numeric Nutrient Criteria for Tampa Bay, Task 1. 

 
16 February 2011 2

ACKNOWLEDGEMENTS 

We wish to thank the partners of the Tampa Bay Estuary Program and members of the Tampa Bay 

Nitrogen Management Consortium for the numerous conversations providing direction and insight 

into concerns regarding numeric nutrient criteria establishment and appropriate methodology for 

developing the proposed criteria. 

 
16 February 2011 3

EXECUTIVE SUMMARY 

The Tampa Bay Estuary Program (TBEP) and the Tampa Bay Nitrogen Management Consortium 

(TBNMC) have recommended to EPA numeric nutrient criteria for Tampa Bay as segment-specific 

annual total nitrogen and total phosphorus loads. EPA has noted its intention to express the 

numeric nutrient criteria for Tampa Bay as TN and TP concentrations. This document provides 

segment-specific TN and TP concentrations consistent with the TN and TP loads recommended as 

numeric nutrient criteria. 

Establishment of numeric nutrient criteria is dependent on an understanding of the limiting nutrient 

within the water body of concern. For Tampa Bay, extensive data exist for evaluation of which 

nutrient, nitrogen or phosphorus, is limiting. Ambient water quality data strongly indicate that 

Tampa Bay is nitrogen limited, and this is supported by the results of nutrient addition bioassays. 

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 

seagrass acreage. The relationships are between nitrogen loads and chlorophyll a, however, not 

nitrogen concentrations in the bay and chlorophyll a. 

Previous efforts to link in-bay nitrogen concentrations to chlorophyll a concentrations have not 

produced sufficient explanatory relationships for use in nitrogen concentration target setting. The 

current effort, driven by EPA’s intention to express numeric nutrient criteria as in-bay 

concentrations, examines several methods to link nutrient concentrations to chlorophyll a

concentrations and/or nutrient loads. 

The final method selected for establishing proposed concentration numeric nutrient criteria for 

nitrogen and phosphorus was the reference period approach, based on the following rationale. 

Segment-specific chlorophyll a targets (values at this level or below indicate desirable conditions) 

have been established previously (Janicki and Wade, 1996). These targets were based on a 1992-

1994 reference period. In 2001, a protocol for assessing whether the Tampa Bay segments were 

achieving these targets was developed (Janicki Environmental, 2001). 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. 

Using the same approach, the segment-specific annual geometric mean nitrogen and phosphorus 

concentrations of the 1992-1994 period were derived as commensurate with the chlorophyll a

targets. These concentrations were increased by one standard deviation (as derived from 1992-

2009 data) to develop the proposed concentration numeric nutrient criteria, which serve as the 

threshold concentrations in the bay. 

 
16 February 2011 4

The following conclusions are drawn from the analyses completed for this effort: 

• Tampa Bay is nitrogen-limited as indicated by both ambient TN:TP ratios and nutrient 

addition bioassays. 

• There is no discernable relationship between TN loadings and in-bay TN concentrations or 

between TP loadings and in-bay TP concentrations in any bay segment. This is not because 

loadings do not affect in-bay concentrations, but because various other confounding factors, 

for which sufficient data are not available, play a role in relationships between loadings and 

concentrations. 

• On a monthly time scale, the relationships between either TN concentrations or TP 

concentrations and chlorophyll a concentrations do not explain a significant proportion of 

the variability in the chlorophyll a concentrations to support development of concentrationbased numeric nutrient criteria in any bay segment. 

 

• There are differences in TN concentrations and TP concentrations, particularly in 

Hillsborough Bay and Middle Tampa Bay, in those years when the chlorophyll a

concentration thresholds are met when compared to those observed in years when the 

chlorophyll a concentration thresholds are not met. However, there is a great deal of 

variability within the data obtained in either group of years and these differences are not 

recommended as the basis for the establishment of concentration-based numeric nutrient 

criteria for Tampa Bay. 

• The reference period approach is recommended for the establishment of concentrationbased TN and TP criteria for Tampa Bay. The segment-specific annual geometric mean 

nitrogen and phosphorus concentrations of the 1992-1994 period were increased by one 

standard deviation (as derived from 1992-2009 data) to develop the proposed concentration 

numeric nutrient criteria. 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 Reasonable Assurance 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 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.
16 February 2011 5 

1.0 Introduction and Objective 

The Tampa Bay Estuary Program (TBEP) and the Tampa Bay Nitrogen Management Consortium 

(TBNMC) have recommended numeric nutrient criteria to U.S. Environmental Protection Agency 

(EPA) for Tampa Bay (TBNMC, 2010). The criteria, as proposed to EPA, are segment-specific 

(Figure 1) and are expressed as annual total nitrogen (TN) and total phosphorus (TP) loads. These 

TN and TP loads are those for the reference period of 1992-1994, as discussed in the March 8, 

2010 comments to EPA. However, EPA has informed the TBEP that it intends to express the 

numeric nutrient criteria for Tampa Bay as TN and TP concentrations. 

The objective of this task is to develop segment-specific TN and TP concentrations consistent with 

the TN and TP loads recommended as numeric nutrient criteria by the TBEP and TBNMC. This is 

in keeping with recognition of the importance of maintaining consistency with existing 

management goals and specifically with the recent load allocations to comply with the existing 

TMDL for Tampa Bay. 

The following provides a discussion of nutrient limitation, description of the analyses completed 

and the results of each analysis, and the recommended TN and TP numeric nutrient criteria, 

expressed as concentrations, for each of the four mainstem bay segments. 

2.0 Nutrient Limitation 

The establishment of numeric nutrient criteria depends upon knowledge of the nutrient most likely 

limiting in the waterbodies of concern. Three major factors control whether nitrogen or 

phosphorus is more likely to be limiting (NRC, 2000): 

• the N:P ratio in external nutrient inputs; 

• the preferential loss from the photic zone of nitrogen or phosphorus due to biogeochemical 

processes such as denitrification, sedimentation, or absorption of phosphorus; and 

• the amount of nitrogen fixation. 

Marine systems, including estuaries, are generally considered nitrogen limited (Thomas, 1970a,b; 

Ryther and Dunstan, 1971; Boynton et al., 1982; Smith, 1984; Howarth, 1988, 2008; Howarth et 

al., 1988a,b; Nixon et al., 1996; Howarth and Marino, 2006; Chapra, 1997; National Research 

Council, 2000;), although there may be times and locations when phosphorus limitation may occur 

(Conley, 2000; Conley et al., 2009; Malone et al., 1996).
16 February 2011 6

Figure 1. Tampa Bay and its four major bay segments. 

Old Tampa

Bay

Hillsborough

Bay

Middle Tampa

Bay

Lower

Tampa

Bay

Gulf

of

Mexico
16 February 2011 7

Since nitrogen is considered the most likely limiting nutrient in estuarine systems, it has been 

identified as the primary nutrient of concern in estuarine ecosystems nationwide (Smith, 1984; 

NRC, 1993). As noted in Correll (1999), however, since estuaries are part of the transition zone 

between the open ocean and the phosphorus supplied from the land, it is possible that both 

phosphorus and nitrogen may be limiting in estuaries, dependent upon the time of year, location in 

the estuary, and nutrient supplies. Ryther and Dunstan (1971) noted the change from phosphorus 

limitation in freshwaters to nitrogen limitation in near-shore marine waters, although Hecky and 

Kilham (1988) indicated that the extent and severity of marine nitrogen limitation has not been 

conclusively determined, with other studies reporting estuarine phosphorus limitation in the spring 

and nitrogen limitation in the summer and fall (Fisher et al., 1992; Lee et al., 1996). Depending 

upon the relative rates of nitrogen and phosphorus supply, the limitation has been found to shift 

between nitrogen and phosphorus in coastal lagoons in the northeastern US (Taylor et al., 1995). It 

has also been documented that residence times play a significant role in determining the estuarine 

responses to nutrient loads ((Monson et al., 2002; Hagy et al., 2000; Borsuck et al., 2004; Boynton 

and Kemp, 2008). 

2.1 Methods to Determine Limitation 

There are two general methods that have been used to define which nutrient is limiting in a water 

body. They include: 

• a method that depends upon ambient water quality data collected over a wide range of 

environmental conditions, and 

• a method that involves experimental manipulation of nutrient conditions, either in the 

laboratory or in situ. 

2.1.1 Ambient Water Quality Data Methods 

This method depends upon a metric typically used to evaluate nutrient limitation: the nitrogen to 

phosphorus ratio (N:P ratio). The N:P ratio indicative of balanced conditions is typically taken to 

be 16:1 (molar), based on the work of Redfield (1934, 1958). This N:P ratio was based on the 

elemental composition of algae, under both laboratory and natural conditions. When N:P ratios are 

greater than 16:1 in a system, this is indicative of phosphorus limitation. However, there may be 

considerable variation in this ratio within an algal culture, dependent upon cell division status, light 

conditions, and precedent conditions (Correll, 1999; Correll and Tolbert, 1962; Terry et al., 1985). 

Molar ratios of N:P are easily determined from water quality monitoring data. In freshwater 

systems, the N:P ratio is usually higher than 16:1, indicating that phosphorus is usually most 

limiting to primary production in these ecosystems (Schindler, 1977; Elser et al., 2007). This 

becomes evident by examining the concentrations of the forms of nitrogen and phosphorus that are 

available for algal uptake. Little if any dissolved inorganic phosphorus (DIP) is generally found in 

relatively productive freshwaters while measurable concentrations of dissolved inorganic nitrogen 

(DIN) remain. 

Most marine systems are nitrogen limited because there are relatively low concentrations of 

dissolved inorganic nitrogen compared to dissolved phosphorus. Since Redfield’s observations 

were published, research has shown that ratios from 10:1 to 20:1 for N:P are typically found in 
16 February 2011 8

estuaries (Parsons et al., 1984). Howarth (1988) observed that the correlation between nitrogen 

and the primary production was better for estuaries that received nutrient concentrations with 

smaller N:P ratios than the one studied by Redfield. Several studies have led to the conclusion that 

estuaries receiving nutrient concentrations with high N:P ratios were limited by phosphorus and 

only those with low ratios are limited by nitrogen (Boynton et al., 1982). Boynton et al. (1982) and 

Howarth (1988) compiled data on the ratio of inorganic nitrogen to phosphorus in a variety of 

estuaries. Of the 27 studied by Howarth, 22 had N:P ratios below the Redfield ratio and may have 

been nitrogen limited. Because phytoplankton can assimilate some organic nutrient forms and all 

forms are relatively labile, it is useful to examine the ratio of total nutrient concentrations (TN:TP). 

Reductions of nutrient levels in a water body will usually result in reduction in algal growth. 

Reducing phosphorus, however, will have no effect unless the reduction results in an N:P ratio 

greater than 16:1. Phosphorus would then become the limiting nutrient. In contrast, a reduction of 

nitrogen concentrations will result in a reduction of primary productivity when the ratio is less than 

16:1. There are exceptions to this general rule. Some coastal areas are phosphorus limited due to 

strict phosphorus control measures or natural conditions and some freshwaters are nitrogen limited 

due to natural sources of phosphorus. 

2.1.2 Experimental Methods 

Experimental manipulation of nutrient conditions, either in the lab or in situ, typically involves

nitrogen and phosphorus additions to either a test alga or a phytoplankton assemblage singularly 

and in combination. The responses to the additions determine the limiting nutrient. If growth is 

found only during nitrogen addition, nitrogen-limitation is indicated. Conversely, if growth is 

found only during phosphorus addition, phosphorus-limitation is indicated. 

In situ methods have included: 

• limnocorrals or bags in which nutrient additions are made and resultant growth responses 

are measured (Shapiro, 1980; Lynch and Shapiro, 1981; Havens and DeCosta, 1986; Perez 

et al., 1994); 

• mesocosm studies in which water is collected and placed in separate containers or 

enclosures for application of separate treatments over multiple day time scales (Oviatt et al., 

1986; Taylor et al., 1995); and 

• whole-lake studies performed on entire lakes or portions of lakes separated by curtains 

(Schindler, 1974, 1975). 

In Florida, as part of its TMDL process, the Florida Department of Environmental Protection (FDEP) 

attempts to identify the limiting nutrient(s) in impaired waterbodies. The TMDL for a specific 

waterbody specifies the maximum amount of the limiting nutrient that may enter the waterbody, 

with this limitation being defined with the aim of improving water quality. If the N:P ratio does not 

clearly suggest the limiting nutrient, TMDLs for both nitrogen and phosphorus are typically defined. 

The primary method for determining the limiting nutrient employed by the FDEP is use of existing 

water quality data to derive ambient N:P ratios, but more complicated methods, including field 

tests and laboratory algal growth potential bioassays, have been employed. Per FDEP guidelines, 

receiving waters with ratios less than 10:1 (molar) are considered nitrogen limited, ratios of greater 

than 30:1 (molar) indicate phosphorus limitation, and ratios of 10-30:1 (molar) indicate colimitation (FDEP, 2002). 
16 February 2011 9

2.2 Confounding Factors 

Determination of the limiting nutrient based solely on N:P ratios estimated from water quality data 

or from experimental uptake rates should be performed with consideration of potentially 

confounding effects. Algal cell interior N:P ratios and uptake rates may vary due to: 

• cell division status (Correll and Tolbert, 1962), 

• light intensity or light quality (Wynne and Rhee, 1986), 

• light and temperature (Jahnke et ., 1986), and 

• P deprivation and then subsequent availability (Sicko-Goad and Jensen, 1976). 

Nutrient limitation in freshwaters, which are typically considered to be phosphorus limited, can 

vary seasonally. Summer nitrogen limitation in lakes can occur when photic zone inorganic 

nutrients are low (Elser et al., 1990). It has also been demonstrated that some estuaries show 

seasonal shifts in limitation (D’Elia et al., 1986; McComb et al., 1981; Conley, 2000). The best 

available information should be used to determine the limiting nutrient of a system before 

management decisions are made with the objective of improved water quality via nutrient load 

control. 

2.3 Nutrient Limitation in Tampa Bay 

Nutrient limitation in Tampa Bay has been examined using both the N:P ratio method and nutrient 

addition bioassays. 

2.3.1 TN:TP Ratios in Tampa Bay 

The average TN:TP ratios for the segments of the TBEP, both by weight and molar, were 

determined based on ambient water quality data, and are presented in Table 1 for the period 1981 

to 2009. The ratios were calculated by first calculating the monthly ratio for each segment based 

on data collected by the Environmental Protection Commission of Hillsborough County (EPCHC). 

The mean value of these monthly values within a year was calculated and the mean of these annual 

values was calculated. 

Table 1. Annual mean TN and TP concentrations and TN:TP in TBEP segments (1981-2009). 

Bay Segment TN 

(mg/L) 

TP 

(mg/L) 

TN:TP 

(Weight) 

TN:TP 

(Molar) 

Old Tampa Bay 0.71 0.27 3.5 7.7 

Hillsborough Bay 0.79 0.45 2.1 4.6 

Middle Tampa Bay 0.63 0.27 2.9 6.3 

Lower Tampa Bay 0.48 0.13 4.8 10.5 

All segments except Lower Tampa Bay have molar N:P ratios less than 10:1, while that for Lower 

Tampa Bay is just slightly greater than 10:1. According to the FDEP guidelines (FDEP, 2002), all 

segments would therefore be considered nitrogen-limited. While nitrogen limitation is indicated in 

all segments, the nutrient that is most limiting can vary seasonally (Malone et al., 1996; Conley et 
16 February 2011 10

al., 2009), so that areas that are generally nitrogen limited may by phosphorus-limited at times. In 

addition to nutrient limitation, phytoplankton growth may also be light-limited during certain parts 

of the year (Pennock and Sharp, 1994). 

Seasonal variation in nutrient limitation has been observed in other waterbodies (Fisher et al., 

1992; Lee et al., 1996; Malone et al, 1996; Conley et al., 2009). Season-specific TN:TP ratios were 

also estimated based on the 1981-2009 data (Table 2). These estimates continue to support the 

conclusion that Tampa Bay is nitrogen-limited. 

Table 2. Seasonal mean TN:TP ratios in TBEP segments (1981- 2009). 

Bay Segment 

Dry Season Wet Season 

TN:TP 

(Weight) 

TN:TP 

(Molar) 

TN:TP 

(Weight) 

TN:TP 

(Molar) 

Old Tampa Bay 3.6 8.0 3.2 7.1 

Hillsborough Bay 2.2 4.9 1.9 4.2 

Middle Tampa Bay 3.0 6.6 2.6 5.7 

Lower Tampa Bay 5.1 11.3 4.2 9.2 

2.3.2 Nutrient Addition Bioassays in Tampa Bay 

The City of Tampa Bay Study Group routinely performed bioassays throughout Tampa Bay during 

1993-2009 as part of the evaluation of the effects of discharge from the H.F. Curren wastewater 

facility (Johansson, 2009 – Attachment 1). Bioassays in late winter and late summer have been 

conducted for most of this period. The results of these 152 bioassays have supported that nitrogen 

is the primary limiting nutrient in the bay, with no results showing phosphorus as the limiting 

nutrient in the bay, including within that portion of the bay which receives the wastewater effluent. 

Therefore, the discharge phosphorus limitation is not required for this facility. 

It is concluded that the four mainstem bay segments in Tampa Bay are nitrogen-limited. This 

conclusion contributed to the development of TN loading targets for Tampa Bay (Janicki and Wade, 

1996), without consideration of TP loading targets in the original target setting effort. Since EPA is 

considering developing both TN and TP numeric nutrient criteria for Tampa Bay, the proposed 

numeric nutrient criteria expressed as TP loads were based on the same time period used to 

develop the proposed TN loading numeric nutrient criteria (comments submitted to EPA (TBNMC, 

2010). Table 3 presents the recommended protective nutrient loads established by the TBNMC and 

proposed to EPA. 

 
16 February 2011 11

Table 3. Protective nutrient loads for the Tampa Bay estuary established by the Tampa 

Bay Nitrogen Management Consortium, and accepted through separate administrative 

action by FDEP (acceptance of the 2002 RA, 2007 RA Update & 2009 RA Addendum) 

and EPA (establishment of the 1998 federally-recognized TMDL for Tampa Bay). 

Bay Segment 

Tampa Bay NMC Proposed 

Alternative Total Nitrogen 

Load expressed as tons/year 

Tampa Bay NMC Proposed 

Total Phosphorus Load 

expressed as tons/year 

Old Tampa Bay 486 104 

Hillsborough Bay 1,451 1,093 

Middle Tampa Bay 799 140 

Lower Tampa Bay 349 52 

Remainder of Lower Tampa Bay 629 112 

3.0 Tampa Bay TN and TP Criteria: Analyses and Results 

Multiple analyses were completed in the evaluation of potential TN and TP criteria expressed as inbay concentrations. These include: 

• examination of the relationships between TN and TP loadings to in-bay TN and TP 

concentrations, respectively; 

• examination of relationships between monthly TN and TP concentrations with chlorophyll 

a concentrations; 

• examination of relationships between annual TN and TP concentrations with chlorophyll a

concentrations; and 

• application of a reference period approach to establishing TN and TP concentration-based 

criteria. 

The data used in these analyses are defined in Attachment 2. The following describes these 

analyses and the results obtained. 

3.1 Evaluation of Relationships Between In-bay TN and TP Concentrations and TN and TP 

Loads

Since the current proposed TN and TP 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 and TP concentrations and TN and TP loads delivered to each segment. All 

four mainstem segments have annual TMDL TN loads recognized by both EPA and DEP. If 

significant relationships are found between the nutrient loads and their respective in-bay 

concentrations, then the proposed numeric nutrient criteria loads could be expressed as 

concentrations. 

Monthly segment-specific TN and TP concentrations were merged with monthly segment-specific 

TN and TP loads resulting in a dataset of monthly values of TN and TP concentrations and loads. 

Plots of these data were inspected, with TN and TP concentrations as functions of TN and TP loads, 

respectively, including various lag and cumulative load effects. Graphical results for each segment 
16 February 2011 12

are provided in Attachment 3 (TN) and Attachment 4 (TP). The relationships examined are between 

the in-bay nutrient concentrations and the following: 

• Current months load, 

• Previous month load, 

• Cumulative loads over current and previous month, and 

• Cumulative loads over current and two previous months. 

The graphical representations of the relationships were then used to guide evaluation of 

relationships between TN and TP concentrations and loads that may explain large proportions of 

the monthly variation in concentrations. 

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 (Table 4). In 

Old Tampa Bay, the natural log-transformed three-month cumulative TN load explained only 7% of 

the variation in in-bay TN concentrations. In Hillsborough Bay, the natural log-transformed twomonth cumulative TN load explained 33% of the variation in in-bay TN concentrations. In Middle 

Tampa Bay, 10% of the variation in in-bay TN concentrations was explained by the natural logtransformed three-month cumulative TN load, and 20% of the variation in in-bay TN 

concentrations in Lower Tampa Bay was explained by the natural log-transformed cumulative threemonth TN load. 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 in Tampa Bay. 

The relationships between TP concentrations and potential TP loading explanatory variables that 

were found varied appreciably among bay segments (Table 3). In Old Tampa Bay, the natural logtransformed three-month cumulative TP load explained 44% of the variation in TP concentrations. 

In Hillsborough Bay, the natural log-transformed current month TP load explained 33% of the 

variation in in-bay TP concentrations. In Middle Tampa Bay, 47% of the variation in TP 

concentrations was explained by the natural log-transformed three-month cumulative TP load, and 

20% of the variation in TP concentrations in Lower Tampa Bay was explained by the natural logtransformed cumulative three-month TP load. 

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

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

and the nutrient loads to those segments. 

Table 4. Best-fit regressions of TN and TP concentrations on TN and TP loads, respectively.

Segment – Variable Regression p > F r

2

Old Tampa Bay – [TN]  = 0.19 + 0.112 ∗ln Cumulative 3-Month TN Load <0.0001 0.07

Old Tampa Bay – [TP]  = −0.32 + 0.15 ∗ln Cumulative 3-Month TP Load <0.0001 0.44

Hillsborough Bay – [TN]  = −0.25 + 0.189 ∗ln Cumulative 2-Month TN Load <0.0001 0.20

Hillsborough Bay – [TP]  = 0.32 + 0.0008 ∗ln Current Month TP Load <0.0001 0.33

Middle Tampa Bay – [TN]  = 0.5 + 0.0005 ∗ln Cumulative 3-Month TN Load <0.0001 0.10

Middle Tampa Bay – [TP]  = −0.08 + 0.089 ∗ln Cumulative 3-Month TP Load <0.0001 0.47

Lower Tampa Bay – [TN]  = 0.3 + 0.036 ∗ln Cumulative 3-Month TN Load <0.0141 0.02

Lower Tampa Bay – [TP]  = 0.1 + 0.0005 ∗ln Cumulative 3-Month TP Load <0.0001 0.20
16 February 2011 13

3.2 Evaluation of Relationships Between Chlorophyll a and TN and TP Concentrations

The second data analysis approach examined the potential relationships between chlorophyll a

concentrations and either TN or TP concentrations in each bay segment. Chlorophyll a thresholds 

have been established by the TBEP as part of the nitrogen management plan for the bay. If 

significant relationships are found, then the chlorophyll a thresholds could be used to determine 

the corresponding nutrient concentrations for use as numeric nutrient criteria. There are two 

temporal scales that can be examined – monthly and annual. 

3.2.1 Evaluation of Relationships Between Monthly Chlorophyll a and TN and TP 

Concentrations

Initially, monthly segment-specific chlorophyll a and nutrient (TN and TP) concentrations were 

plotted. A series of variables based on the ambient TN and TP concentrations, including various 

lag concentrations, was examined. Graphical results are provided in Attachments 5 (TN) and 6 (TP) 

for each segment as scatter plots of chlorophyll a and the following: 

• Current month TN and TP concentration, 

• Previous month TN and TP concentration, 

• Average of current and previous months TN and TP concentrations, and 

• Average of current and two previous months TN and TP concentrations, 

The graphical representations of the relationships were then used to guide evaluation of 

relationships between chlorophyll a and TN and TP concentrations that may explain the monthly 

variation in chlorophyll a concentrations. 

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

than 24% of the variation in chlorophyll a (maximum r2

 = 0.24) (Table 5). In Old Tampa Bay, the 

mean two-month TN concentration explained 12% of the variation in in-bay chlorophyll a

concentrations. In Hillsborough Bay, the same-month TN concentration explained 33% of the 

variation in in-bay chlorophyll a concentrations. In Middle Tampa Bay, 17% of the variation in inbay chlorophyll a concentrations was explained by the same-month TN concentration, and only 

6% of the variation in in-bay chlorophyll a concentrations in Lower Tampa Bay was explained by 

the same-month TN concentration. 

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

than 27% of the variation in chlorophyll a concentrations (Table 5). In Old Tampa Bay, the samemonth TP concentration explained 15% of the variation in in-bay chlorophyll a concentrations. In 

Hillsborough Bay, the mean two-month TP concentration explained 17% of the variation in in-bay 

chlorophyll a concentrations. In Middle Tampa Bay and Lower Tampa, the same-month TP 

concentration explained 27% and 12%,respectively, of the variation in in-bay chlorophyll a 

concentrations. 

 
16 February 2011 14

 

Table 5. Best-fit regressions of monthly chlorophyll a concentrations on TN and TP 

concentrations. 

Segment – Variable Regression p > F r2

Old Tampa Bay – TN chl  =3.78 + 7.34*Mean 2-month TN Concentration <0.0001 0.12

Old Tampa Bay – TP chl  =5.51 + 14.94*Mean TP Concentration <0.0001 0.15

Hillsborough Bay – TN chl  =1.22 + 15.7* Mean TN Concentration <0.0001 0.24

Hillsborough Bay – TP chl  =6.82+15.65*Mean 2-month TP Concentration <0.0001 0.17

Middle Tampa Bay – TN chl  = 2.75 + 7.45* Mean TN Concentration <0.0001 0.17

Middle Tampa Bay – TP chl  = 3.45 + 15.59*Mean TP Concentration <0.0001 0.27

Lower Tampa Bay – TN chl  =3.02 + 2.9* Mean TN Concentration <0.0003 0.06

Lower Tampa Bay – TP chl  = 2.97 + 10.22*Mean TP Concentration <0.0001 0.12

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

and TP concentration criteria based on the relationships between the in-bay chlorophyll a

concentrations and the in-bay TN and TP concentrations in these segments. 

3.2.2 Evaluation of Relationships Between Annual Chlorophyll a and TN and TP Concentrations

The relationships between annual mean chlorophyll a and annual mean TN and TP concentrations 

were examined as a potential means of developing nutrient criteria. The annual mean TN and TP 

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 and TP concentrations for those years 

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

years when the threshold was exceeded (Figures 2-9). 

The results of this analysis indicate that on an annual basis, there are 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. Similar 

differences were found between the TP concentrations in those years in which the threshold 

chlorophyll a targets are met and those in which they are not met. However, these differences are 

not as discrete as seen in the TN concentrations. 
16 February 2011 15

Figure 2. Comparison of TN concentrations in Old Tampa Bay in those years when the chlorophyll a

thresholds were exceeded and those years when the chlorophyll a thresholds were not exceeded. 

Figure 3. Comparison of TN concentrations in Hillsborough Bay in those years when the chlorophyll a

thresholds were exceeded and those years when the chlorophyll a thresholds were not exceeded. 
16 February 2011 16

Figure 4. Comparison of TN concentrations in Middle Tampa Bay in those years when the chlorophyll a

thresholds were exceeded and those years when the chlorophyll a thresholds were not exceeded. 

Figure 5. Comparison of TN concentrations in Lower Tampa Bay in those years when the chlorophyll a

thresholds were exceeded and those years when the chlorophyll a thresholds were not exceeded.
16 February 2011 17

Figure 6. Comparison of TP concentrations in Old Tampa Bay in those years when the chlorophyll a

thresholds were exceeded and those years when the chlorophyll a thresholds were not exceeded. 

Figure 7. Comparison of TP concentrations in Hillsborough Bay in those years when the chlorophyll a

thresholds were exceeded and those years when the chlorophyll a thresholds were not exceeded. 
16 February 2011 18

Figure 8. Comparison of TP concentrations in Middle Tampa Bay in those years when the chlorophyll a

thresholds were exceeded and those years when the chlorophyll a thresholds were not exceeded. 

Figure 9. Comparison of TP concentrations in Lower Tampa Bay in those years when the chlorophyll a

thresholds were exceeded and those years when the chlorophyll a thresholds were not exceeded. 
16 February 2011 19

3.3 Reference Period TN and TP Concentrations as Concentration Criteria 

The analyses described above indicated that while there were significant relationships between 

monthly nutrient concentrations and loads, and between monthly chlorophyll a and nutrient 

concentrations, these relationships explained very little of the observed variation in the dependent 

constituent. The evaluations of annual average chlorophyll a and annual TN and TP concentrations 

did provide evidence that for some segments, there were differences in nutrient concentrations 

between those periods in which chlorophyll a targets were met and those in which they were 

exceeded. Based on these findings, 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 is 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 a 1992-1994 reference period. In 2000, a protocol for assessing whether the Tampa 

Bay segments were achieving these targets was developed (Janicki and Pribble, 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. 

Following this approach, numeric nutrient criteria for the four bay segments can be estimated. The 

1992-1994 annual geometric mean concentrations are: 

• Old Tampa Bay TN=0.75 mg/L TP=0.25 mg/L 

• Hillsborough Bay TN=0.82 mg/L TP=0.37 mg/L 

• Middle Tampa Bay TN=0.69 mg/L TP=0.24 mg/L 

• Lower Tampa Bay TN=0.57 mg/L TP=0.08 mg/L 

The following are the standard deviations of the mean annual TN and TP concentrations from the 

1992-2009 period (which includes all years since the chlorophyll a concentration and TN loading 

targets were established): 

• Old Tampa Bay TN=0.18 mg/L TP=0.06 mg/L 

• Hillsborough Bay TN=0.19 mg/L TP=0.08 mg/L 

• Middle Tampa Bay TN=0.18 mg/L TP=0.05 mg/L 

• Lower Tampa Bay TN=0.17 mg/L TP=0.02 mg/L 

In the same manner as the chlorophyll a thresholds were developed for Tampa Bay as described 

above, the proposed concentration-based TN and TP criteria are defined as the sum of the 

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

concentrations. Table 6 presents these proposed criteria. 

 
16 February 2011 20

Table 6. Proposed numeric nutrient criteria based on geometric mean of annual 

concentrations for the period 1992-1994 and the standard deviation of the long-term TN and 

TP concentrations. 

Segment TN Concentration

(mg/L) 

TP Concentration

(mg/L) 

Old Tampa Bay 0.93 0.31

Hillsborough Bay 1.01 0.45

Middle Tampa Bay 0.87 0.29

Lower Tampa Bay 0.74 0.10

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

annual TN and TP concentrations in Figures 10 through 17. The horizontal lines represent the 

proposed criteria; the data to the right of the vertical lines depict the TN and TP concentrations 

since the establishment of the reference period (1992-1994). 

Figure 10. Comparison of proposed TN concentration criterion for Old Tampa Bay to the annual 

geometric mean TN concentrations from 1981 through 2009. 
16 February 2011 21

Figure 11. Comparison of proposed TN concentration criterion for Hillsborough Bay to the annual 

geometric mean TN concentrations from 1981 through 2009. 

Figure 12. Comparison of proposed TN concentration criterion for Middle Tampa Bay to the annual 

geometric mean TN concentrations from 1980 through 2009. 
16 February 2011 22

Figure 13. Comparison of proposed TN concentration criterion for Lower Tampa Bay to the annual 

geometric mean TN concentrations from 1980 through 2009. 

Figure 14. Comparison of proposed TP concentration criterion for Old Tampa Bay to the annual geometric

mean TP concentrations from 1980 through 2009. 
16 February 2011 23

Figure 15. Comparison of proposed TP concentration criterion for Hillsborough Bay to the annual 

geometric mean TP concentrations from 1980 through 2009. 

Figure 16. Comparison of proposed TP concentration criterion for Middle Tampa Bay to the annual 

geometric mean TP concentrations from 1980 through 2009. 
16 February 2011 24

Figure 17. Comparison of proposed TP concentration criterion for Lower Tampa Bay to the annual 

geometric mean TP concentrations from 1980 through 2009. 

4.0 Conclusions

The following conclusions can be drawn from the analyses and results discussed above: 

• Tampa Bay is nitrogen-limited as indicated by both ambient TN:TP ratios and nutrient 

addition bioassays. 

• There is no discernable relationship between TN loadings and in-bay TN concentrations or 

between TP loadings and in-bay TP concentrations in any bay segment. This is not because 

loadings do not affect in-bay concentrations, but because various other confounding factors, 

for which sufficient data are not available, play a role in relationships between loadings and 

concentrations 

• On a monthly time scale, the relationships between either TN concentrations or TP 

concentrations and chlorophyll a concentrations do not explain a significant proportion of 

the variability in the chlorophyll a concentrations to support development of concentrationbased numeric nutrient criteria in any bay segment. 

• There are differences in TN concentrations and TP concentrations, particularly in 

Hillsborough Bay and Middle Tampa Bay, in those years when the chlorophyll a

concentration thresholds are met when compared to those observed in years when the 

chlorophyll a concentration thresholds are not met. However, there is a great deal of 

variability within the data obtained in either group of years and these differences are not 
16 February 2011 25

recommended as the basis for the establishment of concentration-based numeric nutrient 

criteria for Tampa Bay. 

• The reference period approach is recommended for the establishment of concentrationbased TN and TP criteria for Tampa Bay. The segment-specific annual geometric mean 

nitrogen and phosphorus concentrations of the 1992-1994 period were increased by one 

standard deviation (as derived from 1992-2009 data) to develop the proposed concentration 

numeric nutrient criteria. 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. 

5.0 References 

Borsuck, M.E., C.A.Stow, and K.E. Reckhow. 2004. Confounding effect of flow on estuarine 

response to nitrogen loading. Jour. Environ. Eng. 130:605-614. 

Boynton, W.R. and W.M. Kemp. 2008. Estuaries. pp. 809-856. In: Nitrogen in the Marine 

Environment. 2nd Edition. Capone, D.G., Bronk, D.A., Mulholland, M.R., and Carpenter, E.J. 

(eds.), Elsevier Inc., Burlington, Massachusetts. 

Boynton, W.R., W.M. Kemp, and C.W. Keefe. 1982. A comparative analysis of nutrients and other 

factors influencing estuarine phytoplankton production. In: Estuarine Comparisons. Kennedy, V.S. 

(ed.). Academic Press, San Diego. 

Chapra, S.C. 1997. Surface Water-Quality Modeling, McGraw-Hill, New York, N.Y. 

Conley, D.J. 2000. Biogeochemical nutrient cycles and nutrient management strategies. 

Hydrobiologia. 410:87-96. 

Conley, D.J., H.W. Paerl, R.W. Howarth, D.F. Boesch, S.P. Seitzinger, K.E. Havens, C. Lancelot, 

and G.E. Likens. 2009. Controlling Eutrophication: Nitrogen and Phosphorus. Science. 323:1014-

1015. 

Correll, D.L. 1999. Phosphorus: A rate limiting nutrient in surface waters. Poultry Science. 

78:674-682. 

Correll, D.L., and N.E. Tolbert. 1962. Ribonucleic acid-polyphosphate from algae. I. Isolation and 

physiology. Plant Physiol. 37:627-636. 

D’Elia, C.F., J.G. Sanders, and W.R. Boynton. 1986. Nutrient enrichment studies in a coastal plain 

estuary: Phytoplankton growth in large-scale continuous cultures. Can. J. Fish. Aquat. Sci. 43:397-

406. 
16 February 2011 26

Elser, J.J., E.R. Marzolf, and C.R. Goldman. 1990. Phosphorus and nitrogen limitation of 

phytoplankton growth in the freshwaters of North America: a review and critique of experimental 

enrichments. Can. J. Fish. Aquat. Sci. 47:1468-1477. 

Elser, J.J., M.E.S. Bracken, L.L Cleland, D.S. Gruner, W.S. harpole, H. Hillebrand, J.T. Ngai, E.W. 

Seabloom, J.B. Shurin, and J.E. Smith. 2007. Global analysis of nitrogen and phosphorus limitation 

of primary producers in freshwater, marine and terrestrial ecosystems. Ecology Letters 10, doi: 

10.1111/j.1461-0248.2007.01113x. 

FDEP. 2002. Unpublished guidelines for determining nutrient limitations in waterbodies under the 

Impaired Waters Rule. Watershed Assessment Division. Tallahassee. 

Fisher, T.R., E.R. Peele, J.W. ammerman, and L.W. Harding, Jr. 1992. Nutrient limitation of 

phytoplankton in Chesapeake Bay. Mar. Ecol. Prog. Ser. 82:51-63. 

Hagy, J.D., L.P. Sanford, and W.R. Boynton. 2000. Estimation of net physical transport and 

hydraulic residence times for a coastal plain estuary using box models. Estuaries. 23:328-340. 

Havens, K.E., and J. DeCosta. 1986. A comparison of phytoplankton responses to nutrient 

additions in acidic and circumneutral pH lakewater. Hydrobiologia. 137:211-222. 

Hecky, R.E., and P. Kilham. 1988. Nutrient limitation of phytoplankton in freshwater and marine 

environment: A review of recent evidence on the effects of enrichment. Limnol. Oceanogr. 

33:796-822. 

Howarth, R.W. 1988. Nutrient limitation of net primary productivity in marine ecosystems. 

Annual Review of Ecology and Systematics. 19:89-110. 

Howarth, R. W. 2008. Coastal nitrogen pollution: A review of sources and trends globally and 

regionally. Harmful Algae. 8:14-20. 

Howarth, R.W., and R. Marino. 2006. Nitrogen as the limiting nutrient for eutrophication in 

coastal marine ecosystems: Evolving views over three decades. Limnol. Oceanogr. 51:364-376. 

Howarth, R.W., R. Marino, J. Land, and J.J. Cole. 1988a. Nitrogen fixation in freshwater, 

estuarine, and marine ecosystems. 1. Rates and importance. Limnol. Oceanogr. 33:669-687. 

Howarth, R.W., R. Marino, and J.J. Cole. 1988b. Nitrogen fixation in freshwater, estuarine, and 

marine ecosystems. 2. Biogeochemical controls. Limnol. Oceanogr. 33:688-701. 

Jahnke, J., H.J. Rick, and L. Aletsee. 1986. On the light and temperature dependence of the 

minimum and maximum phosphorus contents in cells of the marine plankton diatom Thalassiosira 

rotula Meunier. J. Plankton Res. 8:549-555. 

Janicki, A.J., and J.R. Pribble. 2000. Establishing a process for tracking chlorophyll-a 

concentrations and light attenuation in Tampa Bay. Tampa Bay National Estuary Program 

Technical Publication #04-00. 
16 February 2011 27

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. 

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. 

Lee, Y.S., T. Seiki, T. Mukai, K. Takimoto, and M. Okada. 1996. Limiting nutrients of 

phytoplankton community in Hiroshima Bay, Japan. Water Res. 30:1490-1494. 

Lynch, M., and J. Shapiro. 1981. Predation, enrichment, and phytoplankton community structure. 

Limnol. Oceanogr. 26:86-102. 

Malone, T.C., D.J. Conley, T.R. Fisher, P.M. Glibert, L.W. Harding, and K.G. Sellner. 1996. Scales 

of Nutrient-Limited Phytoplankton Productivity in Chesapeake Bay. Estuaries. Vol. 19, No. 2. 

Dedicated Issue: Nutrients in Coastal Waters. pp. 371-385. 

McComb, A.J., R.P. Atkins, P.B. Birch, and R.J. Lukatelich. 1981. Eutrophication in the PeelHarvey estuarine system, Western Australia. In: Estuaries and Nutrients. Nielsen, B.J., and L.E. 

Cronin (eds). Humana Press. Clifton, NJ, p 569-582. 

Monsen, N.E., J.E. Cloern, and L.V. Lucus. 2002. A comment on the use of flushing time, residence 

time, and age as transport time scales. Limnol. Oceanogr. 47:1545-1553. 

National Research Council (NRC), 2000. Clean Coastal Waters: Understanding and Reducing the 

Effects of Nutrient Pollution. National Academy Press. Washington, D.C. 

National Research Council (NRC), 1993. Managing wastewater in coastal urban areas. National 

Academy Press. Washington, D.C. 

Nixon, S.W., et al. (15 co-authors). 1996. The fate of nitrogen and phosphorus at the land-sea 

margin of the North Atlantic Ocean. Biogeochemistry. 35:141-180. 

Oviatt, C.A., A.A. Keller, P.A. Sampou, and L.L. Beatty. 1986. Patterns of productivity during 

eutrophication: a mesocosm experiment. Mar. Ecol. Prog. Ser. 28:69-80. 

Parsons, T.R., Y. Maita, and C.M. Lalli. 1984. A manual of chemical and biological methods for 

seawater analysis. Pergamon Press. 

Pennock, J.R., and J.H. Sharp. 1994. Temporal alternation between light- and nutrient-limitation of 

phytoplankton production in a coastal plain estuary. Mar. Ecol. Prog. Ser. 111:275-288. 

Perez, E.A.A., J. DeCosta, and K.E. Havens. 1994. The effects of nutrient addition and pH 

manipulation in bag experiments on the phytoplankton of a small acidic lake in West Virginia, 

USA. Hydrobiologia. 291:93-103. 
16 February 2011 28

Redfield, A.C. 1934. On the proportion of organic derivatives in sea water and their relation to the 

composition of plankton. James Johnstone Memorial Volume. Liverpool University Press. pp. 

176-192. 

Redfield, A.C. 1958. The biological control of chemical factors in the environment. Am. Sci. 

46:205-222. 

Ryther, J.H., and W.M. Dunstan. 1971. Nitrogen, phosphorus, and eutrophication in the coastal 

marine environment. Science. 171:1008–1013. 

Schindler, D.W. 1974. Eutrophication and recovery in experimental lakes: Implications for lake 

management. Science. 184:897-899. 

Schindler, D.W. 1975. Whole-lake eutrophication experiments with phosphorus, nitrogen and 

carbon. Verh. Int. Verein. Limnol. 19:3221-3231. 

Schindler, D.W. 1977. The evolution of phosphorus limitations in lakes. Science. 195:260-262. 

Shapiro, J. 1980. The importance of trophic-level interactions to the abundance and species 

composition of algae in lakes. In: Hypertrophic Ecosystems, Developments in Hydrobiology. 

Barica, J. and L.R. Mur (eds). Junk, The Netherlands Vol. 2. pp.105-116. 

Sicko-Goad, L., and T.E. Jensen. 1976. Phosphate metabolism in blue-green algae. II. Changes in 

phosphate distribution during starvation and the “polyphosphate overplus” phenomenon in 

Plectonema boryanum. Am. J. Bot. 63:183-188. 

Smith, S.V. 1984. Phosphorus versus nitrogen limitation in the marine environment. Limnol. 

Oceanogr. 29:1149-1160. 

Tampa Bay Nitrogen Management Consortium (TBNMC). 2010. Letter to Water Docket, U.S. 

Environmental Protection Agency, March 8, 2010. 

Taylor, D., S. Nixon, S. Granger, and B. Buckley. 1995. Nutrient limitation and the eutrophication 

of coastal lagoons. Mar. Ecol. Prog. Ser. 127:235-244. 

Terry, K.L., J. Hirate, and E.A. Laws. 1985. Light-, nitrogen-, and phosphorus-limited growth of 

Phaeodactylum tricornutum Bohlin strain TFX-1: chemical composition, carbon partitioning, and 

the diel periodicity of physiological processes. J. Exp. Mar. Biol. Ecol. 86:85-100. 

Thomas, W. H. 1970a. On nitrogen deficiency in tropical parameters in poor and rich water. 

Limnol. Oceanogr. 15:380-385. 

Thomas, W. H. 1970b. Effect of ammonium and nitrate concentration on chlorophyll increases in 

natural tropical Pacific phytoplankton populations. Limnol. Oceanogr. 15:386-394. 

Wynne, D., and G.Y. Rhee. 1986. Effects of light intensity and quality on the relative N and P 

requirement (the optimum N:P ratio) of marine planktonic algae. J. Plankton Res. 8:91-103. 
Attachment 1 

Results of City of Tampa Bay Study Group Bioassays (Johansson, 2009) 
 

 

NUTRIENT ENRICHMENT STUDIES OF NATURAL PHYTOPLANKTON 

POPULATIONS IN TAMPA BAY 

A SUMMARY OF RESULTS 

JUNE 1993 TO AUGUST 2009 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

J.O.R. JOHANSSON 

THE CITY OF TAMPA 

WASTEWATER DEPARTMENT 

BAY STUDY GROUP 

NOVEMBER 16, 2009 
NUTRIENT ENRICHMENT STUDIES OF NATURAL PHYTOPLANKTON 

POPULATIONS IN TAMPA BAY. A SUMMARY OF RESULTS 

 JUNE 1993 TO AUGUST 2009

 

INTRODUCTION 

The Bay Study Group (BSG) has been conducting nutrient enrichment studies (bioassays) on 

natural phytoplankton populations at four locations in Tampa Bay since 1993 (Figure 1). This 

report will summarize findings for all tests that have been performed to date, from June 1993 

through August 2009. 

Figure 1. Phytoplankton nutrient bioassay monitoring stations in Tampa Bay. 

METHODS 

 

The bioassays have been performed on the natural phytoplankton population collected from 

surface waters of the four stations, with one station located in each of the four major segment of 

Tampa Bay, as shown below: 

 Hillsborough Bay - COT4 

 Old Tampa Bay - COT40 

 Middle Tampa Bay - COT13 

 Lower Tampa Bay - COT95 

COT 4

COT 13

COT 95

COT 40
Bioassay measurements were performed on a quarterly schedule from June 1993 through August 

1995 for all stations except COT95. At this station monthly tests were conducted during the first 

year of the program. From February 1996 to the present, bioassays have been performed twice 

per year, during late winter and late summer. 

The bioassay method used is similar to a method used in Chesapeake Bay waters (see Fisher et 

al. 1992a and b). An outline of the specific method used by the BSG is provided here. 

A large volume of surface water is collected for the following nutrient treatments. Each treatment 

is conducted in duplicate on 3l samples: 

 Controls (no nutrient addition). 

 N-additions (NH3-N added to reach a final concentration in the sample of near 50uM). 

 P-additions (PO4-P added to reach a final concentration in the sample of near 5uM). 

 N+P-additions (combination of the N-additions and P-additions). 

The treatment samples are incubated under natural sunlight in ambient bay water temperatures 

for an appropriate incubated period (usually 48h for the late winter and 24h for the late summer 

tests). 

The growth response of the natural phytoplankton community to the different treatments is 

determined through measurements in changes of algal biomass, measured as chlorophyll-a. 

Paired t-test statistics (p<0.05) and non-statistical evaluations of treatment responses are used to 

interpret the bioassay results and to group the growth response to the nutrient additions into the 

following response categories: 

 Exclusive N limitation: (1) the addition of P induced no response relative the control, and (2) 

the addition of N alone had virtually the same effect as the addition of N+P. 

 Primary N limitation: (1) the addition of P alone induced little response relative the control, 

(2) the addition of N alone induced a response, and (3) the addition of N+P induced the 

largest response. 

 Balanced NP limitation: (1) the addition of N and P alone induced no response relative the 

control, (2) the addition of N+P induced a large response. 

 Exclusive P limitation: (1) the addition of N induced no response relative the control, and (2) 

the addition of P alone had virtually the same effect as the addition of N+P. 

 Primary P limitation: (1) the addition of N alone induced little response relative the control, 

(2) the addition of P alone induced a response, and (3) the addition of N+P induced the 

largest response. 

 No response to any nutrient addition, indicating nutrient saturation, light limitation, and/or 

insufficient incubation time. 
RESULTS 

A total of 152 bioassay experiments have been conducted since the start of the program in June 

1993. 

The growth response of the natural phytoplankton community to the different nutrient treatments 

were grouped into the response categories described above. The result of these analyses is 

summarized in Table 1. 

 Table 1. Results from natural phytoplankton nutrient bioassays in the four major subsections of Tampa Bay, 

 1993 – 2009.

Bioassay response Hillsborough Bay

COT4

Old Tampa Bay

COT40

Middle Tampa Bay

COT13

Lower Tampa Bay

COT95

Exclusive N limitation 29 25 28 39

Primary N limitation 2 2 2 1

Balanced 0 0 0 0

Exclusive P limitation 0 0 0 0

Primary P limitation 0 0 0 0

No response 0 3 0 0 

DISCUSSION AND CONCLUSION 

One-hundred-fifty-two natural phytoplankton community nutrient limitation experiments have 

been conducted in Tampa Bay by the BSG from 1993 to the present. The strong nitrogen 

dependence by the Tampa Bay phytoplankton community is obvious in all four bay segments; of 

the 152 bioassay tests conducted to date, 149 indicate that nitrogen was the stronger limiting 

nutrient. None of the 152 tests have indicated that phosphorous was the stronger limiting 

nutrient. However, three tests in OTB have shown a lack of phytoplankton growth response by 

either N or P additions. 

 

REFERENCES 

 

Fisher, T.R., A.B. Gustafson, K.G. Sellner, and R.B. Lacouture.1992a. Nutrient bioassays in 

Chesapeake Bay to assess nutrients limiting algal growth. Progress Rep. MD Dept. of the 

Environment, Jan. 1992, Baltimore, MD. 

 

Fisher, T.R., E.R. Peele, J.W. Ammerman, and L.W. Harding, Jr. 1992b. Nutrient limitation of 

phytoplankton in Chesapeake Bay. Mar. Ecol. Prog. Ser., 82:51-63. 

 
Attachment 2 

Data Sources 

 
The following data sources were used in the analyses performed for this task: 

 The water quality data were obtained from the Environmental Protection Commission of 

Hillsborough County (EPCHC). Their sampling program collects surface samples on a 

monthly basis from the following series of fixed stations. Among others, the primary 

analytes included: 

- Chlorophyll a 

- TN 

- TP 

- Salinity. 

Old Tampa Bay 

46 

60 

64 

47 

65 

63 

66 

40 

41 

67 

50 

38 

51 

68 

 36 

Hillsborough Bay 

 44 

 70 

 52 

 6 

 71 

 7 

 55 

 08 

 73 

 80 

Middle Tampa Bay 

 11 

 9 

 81 

 84 

 33 

 32 

 13 

 14 

 82 

 28 

 16 

 19 

Lower Tampa Bay 

 25 

 23 

 21 

 91 

 90 

 24 

 95 

 92 

 93 

 The TN and TP loading estimates were obtained from a series of reports produced for the 

TBEP (1985-2002) and FDEP (2003-2007). These are: 

Zarbock, H., A. Janicki, D. Wade, D. Heimbuch, and H. Wilson. 1994. Estimates of 

Total Nitrogen, Total Phosphorus, and Total Suspended Solids Loadings to Tampa 

Bay, Florida. Technical Publication #04-94 of the Tampa Bay National Estuary 

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

National Estuary Program. St. Petersburg, FL. 

Zarbock, H.W., A.J. Janicki, and S.S. Janicki. 1996. Estimates of Total Nitrogen, 

Total Phosphorus, and Total Suspended Solids Loadings to Tampa, Bay, Florida. 

Technical Appendix: 1992-94 Total Nitrogen Loadings to Tampa Bay. Technical 

Publication #19-96 of the Tampa Bay National Estuary Program. Prepared by 

Coastal Environmental, Inc. Prepared for Tampa Bay National Estuary Program. St. 

Petersburg, FL. 
Pribble, R., A. Janicki, H. Zarbock, S. Janicki, and M. Winowitch. 2001. Estimates of 

Total Nitrogen, Total Phosphorus, Total Suspended Solids, and Biochemical Oxygen 

Demand Loadings to Tampa, Bay, Florida: 1995-1998. Technical report #05-01 of 

the Tampa Bay Estuary Program. Prepared by Janicki Environmental, Inc. Prepared 

for Tampa Bay Estuary Program. St. Petersburg, FL. 

Poe, A., K. Hackett, S. Janicki, R. Pribble, and A. Janicki. 2005. Estimates of Total 

Nitrogen, Total Phosphorus, Total Suspended Solids, and Biochemical Oxygen 

Demand Loadings to Tampa Bay, Florida: 1999-2003. Technical Report #02-05 of 

the Tampa bay Estuary Program. Prepared by Janicki Environmental, Inc. Prepared 

for Tampa Bay National Estuary Program. St. Petersburg, FL. 

Janicki Environmental, Inc. 2008. Estimates of Total Nitrogen, Total Phosphorus, 

Total Suspended Solids, and Biochemical Oxygen Demand Loadings to Tampa Bay, 

Florida: 2004-2007. Prepared by Janicki Environmental, Inc. Prepared for Florida 

Department of Environmental Protections. Tallahassee, FL. 
Attachment 3 

Monthly TN Concentrations and Monthly TN Loads 
Old Tampa Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

Current Month TN Load (tons/month)

0 100 200 300

Hillsborough Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

Current Month TN Load (tons/month)

0 100 200 300 400 500 600 700 800 900 1000
Middle Tampa Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

Current Month TN Load (tons/month)

0 100 200 300 400 500 600

Lower Tampa Bay Monthly Mean TN Concentration (mg/L)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

Current Month TN Load (tons/month)

0 100 200 300 400 500 600 700 800 900
Old Tampa Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

Previous Month TN Load (tons/month)

0 100 200 300

Hillsborough Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

Previous Month TN Load (tons/month)

0 100 200 300 400 500 600 700 800 900 1000
Middle Tampa Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

Previous Month TN Load (tons/month)

0 100 200 300 400 500 600

Lower Tampa Bay Monthly Mean TN Concentration (mg/L)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

Previous Month TN Load (tons/month)

0 100 200 300 400 500 600 700 800 900
Old Tampa Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

Cumulative Two-Month TN Load (tons)

0 100 200 300 400 500

Hillsborough Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

Cumulative Two-Month TN Load (tons)

0 200 400 600 800 1000 1200 1400 1600 1800 2000
Middle Tampa Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

Cumulative Two-Month TN Load (tons)

0 100 200 300 400 500 600 700 800 900

Lower Tampa Bay Monthly Mean TN Concentration (mg/L)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

Cumulative Two-Month TN Load (tons)

0 100 200 300 400 500 600 700 800 900 1000 1100
Old Tampa Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

Cumulative Three-Month TN Load (tons)

0 100 200 300 400 500 600

Hillsborough Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

Cumulative Three-Month TN Load (tons)

0 1000 2000 3000
Middle Tampa Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

Cumulative Three-Month TN Load (tons)

0 100 200 300 400 500 600 700 800 900

Lower Tampa Bay Monthly Mean TN Concentration (mg/L)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

Cumulative Three-Month TN Load (tons)

0 200 400 600 800 1000 1200 1400
Old Tampa Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

ln Current Month TN Load (tons/month)

1234 5 6

Hillsborough Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

ln Current Month TN Load (tons/month)

3 4 5 6 7
Middle Tampa Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

ln Current Month TN Load (tons/month)

1234 5 6 7

Lower Tampa Bay Monthly Mean TN Concentration (mg/L)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

ln Current Month TN Load (tons/month)

1234 5 6 7
Old Tampa Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

ln Cumulative Two-Month TN Load (tons)

234 5 6 7

Hillsborough Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

ln Cumulative Two-Month TN Load (tons)

4 5 6 7 8
Middle Tampa Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

ln Cumulative Two-Month TN Load (tons)

234 5 6 7

Lower Tampa Bay Monthly Mean TN Concentration (mg/L)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

ln Cumulative Two-Month TN Load (tons)

234 5 6 7
Old Tampa Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

ln Cumulative Three-Month TN Load (tons)

3 4 5 6 7

Hillsborough Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

ln Cumulative Three-Month TN Load (tons)

4 5 6 7 8
Middle Tampa Bay Monthly Mean TN Concentration (mg/L)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

ln Cumulative Three-Month TN Load (tons)

3 4 5 6 7

Lower Tampa Bay Monthly Mean TN Concentration (mg/L)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

ln Cumulative Three-Month TN Load (tons)

3 4 5 6 7 8
Attachment 4 

Monthly TP Concentrations and Monthly TP Loads 
Old Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

Current Month TP Load (tons/month)

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Hillsborough Bay Monthly Mean TP Concentration (mg/L)

0.00

1.00

2.00

3.00

Current Month TP Load (tons/month)

0 1000 2000 3000
Middle Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

Current Month TP Load (tons/month)

0 100 200 300

Lower Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

Current Month TP Load (tons/month)

0 100 200 300
Old Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

Previous Month TP Load (tons/month)

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Hillsborough Bay Monthly Mean TP Concentration (mg/L)

0.00

1.00

2.00

3.00

Previous Month TP Load (tons/month)

0 1000 2000 3000
Middle Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

Previous Month TP Load (tons/month)

0 100 200 300

Lower Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

Previous Month TP Load (tons/month)

0 100 200 300
Old Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

Cumulative Two-Month TP Load (tons)

0 20 40 60 80 100 120 140 160 180 200

Hillsborough Bay Monthly Mean TP Concentration (mg/L)

0.00

1.00

2.00

3.00

Cumulative Two-Month TP Load (tons)

0 1000 2000 3000 4000
Middle Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

Cumulative Two-Month TP Load (tons)

0 100 200 300 400

Lower Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

Cumulative Two-Month TP Load (tons)

0 100 200 300 400 500
Old Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

Cumulative Three-Month TP Load (tons)

0 20 40 60 80 100 120 140 160 180 200 220

Hillsborough Bay Monthly Mean TP Concentration (mg/L)

0.00

1.00

2.00

3.00

Cumulative Three-Month TP Load (tons)

0 1000 2000 3000 4000
Middle Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

Cumulative Three-Month TP Load (tons)

0 100 200 300 400

Lower Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

Cumulative Three-Month TP Load (tons)

0 100 200 300 400 500
Old Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

ln Current Month TP Load (tons/month)

1234 5

Hillsborough Bay Monthly Mean TP Concentration (mg/L)

0.00

1.00

2.00

3.00

ln Current Month TP Load (tons/month)

3 4 5 6 7 8
Middle Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

ln Current Month TP Load (tons/month)

-1 0 1 2 3 4 5 6

Lower Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

ln Current Month TP Load (tons/month)

01234 5 6
Old Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

ln Cumulative Two-Month TP Load (tons)

234 5 6

Hillsborough Bay Monthly Mean TP Concentration (mg/L)

0.00

1.00

2.00

3.00

ln Cumulative Two-Month TP Load (tons)

4 5 6 7 8 9
Middle Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

ln Cumulative Two-Month TP Load (tons)

01234 5 6

Lower Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

ln Cumulative Two-Month TP Load (tons)

1234 5 6
Old Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

ln Cumulative Three-Month TP Load (tons)

234 5 6

Hillsborough Bay Monthly Mean TP Concentration (mg/L)

0.00

1.00

2.00

3.00

ln Cumulative Three-Month TP Load (tons)

4 5 6 7 8 9
Middle Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

ln Cumulative Three-Month TP Load (tons)

1234 5 6

Lower Tampa Bay Monthly Mean TP Concentration (mg/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

ln Cumulative Three-Month TP Load (tons)

1234 5 6 7
Attachment 5 

Monthly Chlorophyll a Concentrations and Monthly TN Concentrations 
Old Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

Monthly Mean TN (mg/L)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1

Hillsborough Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

 40.0

 50.0

 60.0

Monthly Mean TN (mg/L)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1
Middle Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

Monthly Mean TN (mg/L)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

Lower Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 1.0

 2.0

 3.0

 4.0

 5.0

 6.0

 7.0

 8.0

 9.0

 10.0

 11.0

 12.0

 13.0

 14.0

 15.0

Monthly Mean TN (mg/L)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3
Old Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

Previous Month TN Concentration (mg/L)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1

Hillsborough Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

 40.0

 50.0

 60.0

Previous Month TN Concentration (mg/L)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1
Middle Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

Previous Month TN Concentration (mg/L)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

Lower Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 1.0

 2.0

 3.0

 4.0

 5.0

 6.0

 7.0

 8.0

 9.0

 10.0

 11.0

 12.0

 13.0

 14.0

 15.0

Previous Month TN Concentration (mg/L)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3
Old Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

Average Two-Month TN Concentration (mg/L)

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7

Hillsborough Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

 40.0

 50.0

 60.0

Average Two-Month TN Concentration (mg/L)

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9
Middle Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

Average Two-Month TN Concentration (mg/L)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

Lower Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 1.0

 2.0

 3.0

 4.0

 5.0

 6.0

 7.0

 8.0

 9.0

 10.0

 11.0

 12.0

 13.0

 14.0

 15.0

Average Two-Month TN Concentration (mg/L)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Old Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

Average Three-Month TN Concentration (mg/L)

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

Hillsborough Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

 40.0

 50.0

 60.0

Average Three-Month TN Concentration (mg/L)

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7
Middle Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

Average Three-Month TN Concentration (mg/L)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3

Lower Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 1.0

 2.0

 3.0

 4.0

 5.0

 6.0

 7.0

 8.0

 9.0

 10.0

 11.0

 12.0

 13.0

 14.0

 15.0

Average Three-Month TN Concentration (mg/L)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Attachment 6 

Monthly Chlorophyll a Concentrations and Monthly TP Concentrations 
Old Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

 40.0

 50.0

Monthly Mean TP (mg/L)

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70

Hillsborough Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

 40.0

 50.0

 60.0

Monthly Mean TP (mg/L)

0.00 1.00 2.00 3.00
Middle Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

Monthly Mean TP (mg/L)

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80

Lower Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 1.0

 2.0

 3.0

 4.0

 5.0

 6.0

 7.0

 8.0

 9.0

 10.0

 11.0

 12.0

 13.0

 14.0

 15.0

Monthly Mean TP (mg/L)

0.00 0.10 0.20 0.30 0.40 0.50 0.60
Old Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

 40.0

 50.0

Previous Month TP Concentration (mg/L)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Hillsborough Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

 40.0

 50.0

 60.0

Previous Month TP Concentration (mg/L)

0123
Middle Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

Previous Month TP Concentration (mg/L)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Lower Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 1.0

 2.0

 3.0

 4.0

 5.0

 6.0

 7.0

 8.0

 9.0

 10.0

 11.0

 12.0

 13.0

 14.0

 15.0

Previous Month TP Concentration (mg/L)

0.0 0.1 0.2 0.3 0.4 0.5 0.6
Old Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

 40.0

 50.0

Average Two-Month TP Concentration (mg/L)

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Hillsborough Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

 40.0

 50.0

 60.0

Average Two-Month TP Concentration (mg/L)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7
Middle Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

Average Two-Month TP Concentration (mg/L)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Lower Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 1.0

 2.0

 3.0

 4.0

 5.0

 6.0

 7.0

 8.0

 9.0

 10.0

 11.0

 12.0

 13.0

 14.0

 15.0

Average Two-Month TP Concentration (mg/L)

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45
Old Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

 40.0

 50.0

Average Three-Month TP Concentration (mg/L)

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60

Hillsborough Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

 40.0

 50.0

 60.0

Average Three-Month TP Concentration (mg/L)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4
Middle Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 10.0

 20.0

 30.0

Average Three-Month TP Concentration (mg/L)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Lower Tampa Bay Monthly Mean Chlorophyll a (ug/L)

 0.0

 1.0

 2.0

 3.0

 4.0

 5.0

 6.0

 7.0

 8.0

 9.0

 10.0

 11.0

 12.0

 13.0

 14.0

 15.0

Average Three-Month TP Concentration (mg/L)

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40