Tampa Bay Estuary Program 

Model Evaluation and Update: 

Nitrogen Load - Chlorophyll a Relationship

Prepared for: 

Tampa Bay Estuary Program 

Mail Station I-1/NEP 

100 8th Avenue SE 

St. Petersburg, FL 33701 

Prepared by: 

A. Janicki, R. Pribble, and S. Janicki. 

1155 Eden Isle Drive NE 

St. Petersburg, FL 33704 

July 2001
FOREWORD 

This report was prepared by Janicki Environmental, Inc. under Contract No. T-98-06 for the 

Tampa Bay Estuary Program. 

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ACKNOWLEDGMENTS

This project was conducted under the guidance of the TBEP and the Modeling 

Subcommittee of the TBEP TAC. Review and suggestions for the project were provided by 

the TBEP staff and members of the TAC Modeling Subcommittee. In particular, thanks are 

extended to Dick Eckenrod, Holly Greening, and the Modeling Subcommittee. 

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TABLE OF CONTENTS 

FOREWORD ........................................................................................................... i 

ACKNOWLEDGMENTS .............................................................................................ii 

1 INTRODUCTION ....................................................................................................1-1

2 METHODS...............................................................................................................2-1

3 RESULTS ..................................................................................................................3-1

4 DISCUSSIONS AND CONCLUSIONS .....................................................................4-1

5 REFERENCES............................................................................................................5-1 

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1 INTRODUCTION 

The Tampa Bay National Estuary Program (TBNEP) developed a Comprehensive 

Conservation & Management Plan (CCMP), entitled Charting the Course, which was 

the culmination of five years of effort aimed at bringing together the participants of 

the TBNEP to address the key environmental issues affecting Tampa Bay. The 

CCMP specifically addressed several major issues, including water quality and 

habitat loss. The loss of seagrasses in Tampa Bay drew particular interest, given 

their important role in the ecological functioning of the bay. 

Seagrasses provide critical habitat for recreationally and commercially important fish 

and invertebrate species. Seagrasses in Florida provide juvenile nursery and adult 

feeding areas for red drum, spotted seatrout, spot, silver perch, sheepshead, snook, 

shrimp, and the bay scallop (Zieman and Zieman, 1989). Seagrass meadows are 

also important feeding areas for the Florida manatee. Seagrasses serve to improve 

water quality by reducing nutrients in the water column and are an important 

component of the energy and nutrient cycles in coastal environments. 

Seagrass extent and condition can be impacted by many factors, including water 

quality, physical factors such as prop scarring and currents, seagrass disease, and 

location of seagrass beds in relation to offshore transverse sandbars. The TBNEP 

Technical Advisory Committee (TAC) recognizes that these and other factors may 

affect progress towards reaching adopted seagrass restoration goals, and is currently 

initiating actions which will assist with addressing factors in addition to water clarity 

and quality (including the initiation of measuring the depth of water at various 

seagrass beds around the bay, and the monitoring of potential seagrass disease). 

Actions needed to address additional factors, such as the bay-wide effects of prop 

scarring or the location of various seagrass beds, will be included in discussions of 

future monitoring needs.

In Tampa Bay, seagrasses typically grow at depths no deeper than six to eight feet. 

Light is one of the limiting factors on the depth at which seagrasses can be found. 

Light requirements for growth vary by seagrass species. Turtle grass, the most 

common seagrass species in Tampa Bay, has an estimated light requirement of 

20.5% of the incident subsurface light (Dixon and Leverone, 1995). For the 

purposes of seagrass restoration target setting, this was assumed to be the minimum 

light requirement for seagrasses in Tampa Bay (Janicki and Wade, 1996). 

To address the issue of seagrass habitat loss, recommended seagrass protection and 

restoration targets were developed (Janicki et al., 1995). Seagrass restoration targets 

were determined by comparing 1990 seagrass extent to that observed in 1950. 

Seagrass targets were defined as those portions of Tampa Bay that had seagrasses in 

1950, did not have seagrasses in 1990, and had not been permanently altered to 

preclude restoration of seagrasses. 

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To strive toward the seagrass restoration goal, the TBNEP developed the Nitrogen 

Management Strategy. The Strategy seeks to prevent future impacts due to excessive 

nitrogen loadings to Tampa Bay. To address these concerns, a paradigm that relates 

nitrogen loading to chlorophyll and seagrass was utilized, as shown in Figure 1-1. 

Figure 1-1. The TBNEP Nitrogen Management Strategy Paradigm. 

An empirical modeling approach was taken to define the relationships between TN 

loads and chlorophyll a concentrations and between chlorophyll a concentrations 

and light attenuation (Janicki and Wade, 1996). 

The objective of this report is to re-evaluate the empirical relationship between TN 

loads and chlorophyll a concentrations using the water quality data collected since 

1994 by the Environmental Protection Commission of Hillsborough County 

(EPCHC) and the 1995-1998 loading estimates developed by the TBEP (Pribble et 

al., 2001). 

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2 METHODS 

A principal working hypothesis was earlier developed that observed chlorophyll a 

concentrations are directly related to total nitrogen loads, from external sources and 

from internal exchange of nitrogen between bay segments (Janicki and Wade, 

1996). Regression results suggest that the variation in total nitrogen loads could 

explain the majority of the variation in chlorophyll a concentration. 

The estimates of nitrogen loading from internal exchange between segments were 

derived in the following manner. First, a series of simple dilution equations were 

written for the segments of Tampa Bay and fit to the observed salinity data and 

freshwater inflow data. For example, the equation for the Old Tampa Bay segment 

was written as follows: 

( ) 0 =Wo − QoCo + Eo,b Cb − Co

where: Wo= the external load of a dissolved substance to Old Tampa 

Bay from its watersh (kg/month), 

Qo= the freshwater inflow rate to Old Tampa Bay from its 

watershed (m3/month), 

Co= the concentration of a dissolved substance in Old Tampa 

Bay (kg/m3), 

Cb= the concentration of a dissolved substance in the 

adjacent subsegment of Middle Tampa Bay (kg/m3), and 

Eo,b= a transfer coefficient from Old Tampa Bay to the 

adjacent subsegment of Middle Tampa Bay (m3/month). 

A dilution equation was written in this manner for each of the five modeled 

segments (Old Tampa Bay, Hillsborough Bay, and the three subsegments of Middle 

Tampa Bay). To estimate the average net transfer of water and dissolved substance 

between the five bay segments, the system of equations was first solved using salt as 

a conservative dissolved substance. This left only the transfer coefficient terms (E) 

unknown in the series of equations. The transfer coefficient terms were then 

estimated for the dry season (October-May) and the wet season (June-September). 

After estimation of the exchange coefficients between the segments, the series of 

equations were used to predict monthly nitrogen concentrations. To account for the 

nonconservative properties of nitrogen in the estuarine system, the predicted 

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concentrations were related to the observed concentrations using least squares 

regression methods, as follow: 

Nt,s ˆ Nt,s = β t,s

where: Nt,s= the observed nitrogen concentration in month t and bay 

segment s, 

Nˆ t,s= the nitrogen concentration predicted by the simple 

dilution model in moth t and bay segment s, and 

 $t,s= a regression parameter. 

Results of the regression indicated that the variation in the observed nitrogen 

concentrations could be explained by the variation in the nitrogen concentrations 

predicted by the dilution model. The degree of the adjustment for nonconservative 

processes varied monthly and typically fell within plus or minus 5% to 20%. 

The relationship between total nitrogen load and chlorophyll a was then derived as 

follows: 

Ct,s = αt,s + β sLt,s

where: Ct,s = average chlorophyll a concentration at month t and 

in segment s,

 Lt,s = total nitrogen load at month t to segment s, and 

 αt,s and βt,s = regression parameters. 

Least squares regression methods were used to estimate the regression parameters 

(Janicki and Wade, 1996). The results of the regressions indicated that the variation 

in chlorophyll a concentrations could be explained by the variation in total nitrogen 

load. Monthly specific regression intercept terms were used to avoid any potentially 

confounding effects of seasonality in the independent and dependent variables. 

Water quality data collected by the EPCHC for the period 1995 through 1998 were 

obtained and incorporated into the TBEP water quality database (Janicki 

Environmental, 2000). External TN loading data were estimated for the 1995-1998 

period as described in Pribble et al. (2001). These data were used to evaluate the 

TN load - chlorophyll a model developed previously using data from the period 

1986 through 1994. 

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Initially, the TN load - chlorophyll a regression was used to estimate chlorophyll a 

given the estimated TN loads during 1995 through 1998. Comparisons of the 

predicted and observed chlorophyll a values, for both monthly and annual mean 

conditions, were then made. Secondly, the residuals (i.e., the difference of the 

observed and predicted chlorophyll a values) were examined to assess whether any 

important differences were observed between those from the 1986-1994 period and 

those of the 1995-1998 period. 

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3 RESULTS 

The results of the two methods used to evaluate the TN load - chlorophyll a 

regression for Tampa Bay are presented below. 

Comparison of Predicted and Observed Chlorophyll a Values Using the 1986-1994 

TN Load - Chlorophyll a Regressions 

The following figures present a comparison of the observed mean monthly 

chlorophyll a values for the 1986-1998 period to the chlorophyll a values predicted 

by the monthly TN loads and the TN load - chlorophyll a regressions developed 

from the 1986-1994 data (Janicki and Wade, 1996): 

• Figure 3-1 – All bay segments 

• Figure 3-2 – Old Tampa Bay 

• Figure 3-3 – Hillsborough Bay 

• Figure 3-4 – Middle Tampa Bay – 31 

• Figure 3-5 – Middle Tampa Bay – 32 

• Figure 3-6 – Middle Tampa Bay – 33. 

These plots include a line that represents a 1:1 relationship between predicted and 

observed values. Data points above the line represent months when the predicted 

chlorophyll a value was greater than that which was observed. Data points below 

the line represent months when the predicted chlorophyll a value was less than that 

which was observed. Close agreement in the predicted and observed chlorophyll a 

values would result in a plot where the points were in close proximity to the line. If 

any bias in the predicted chlorophyll a values exists then a disproportionate number 

of points would be found either above (observed > predicted) or below (observed 

< predicted) the line. In each plot, the 1995-1998 data are presented as circles 

while the 1986-1994 data are presented as plus signs (+). 

The comparisons of mean monthly predicted and observed chlorophyll a values 

indicate that there is little difference between the regression results from the 1986-

1994 and 1995-1998 periods. Table 3-1 presents a comparison of the mean annual 

predicted and observed chlorophyll a values by bay segment. The differences in the 

observed and predicted chlorophyll a values were typically greatest in 1996 and 

1998 in all bay segments. In 1995 and 1998, the observed mean annual 

chlorophyll a was typically greater than the predicted mean annual chlorophyll a in 

each segment. In 1996, the opposite was true, with the mean annual predicted 

chlorophyll a higher in each segment. In 1997 there was relatively good agreement 

between the predicted and observed mean annual chlorophyll a values. Overall, 

the difference in chlorophyll a values was approximately 1.1 :g/L in Old Tampa 

Bay, 2.0 :g/L in Hillsborough Bay, 2.4 :g/L in Middle Tampa Bay 31, 1.5 :g/L in 

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Middle Tampa Bay 32, and 1.3 :g/L in Middle Tampa Bay 33. These differences 

are relatively small compared to the observed mean annual values. 

Table 3-1. Mean annual predicted and observed chlorophyll a values. 

Bay 1995 1996 1997 1998

Segment Observed Predicted Observed Predicted Observed Predicted Observed Predicted

OTB 8.3 7.7 6.9 8.6 8.5 8.9 11.6 9.9

HB 15.1 13.9 9.6 13.1 12.4 13.2 18.4 15.0

MTB-31 12.6 10.4 6.5 9.6 9.5 11.0 13.9 11.1

MTB-32 8.5 7.9 5.8 7.2 7.0 8.4 10.7 8.3

MTB-33 5.5 5.1 3.0 5.3 5.6 5.9 6.4 8.7

Differences Between Residuals from the TN Load - Chlorophyll a Regressions 

During 1986-1994 and 1995-1998 

To examine any differences in the relationship between predicted and observed 

chlorophyll a concentrations during the two periods, 1986-1994 and 1995-1998, 

the residuals (difference between predicted and observed chlorophyll a) were 

plotted for each month and for each bay segment for the two periods. Residual 

chlorophyll a values are shown as a function of month in Figure 3-7, and as a 

function of bay segment in Figure 3-8, for each of the periods. Inspection of the 

residual plots suggests that the residuals are very similar for both periods, both for 

the same month and for the same bay segment. 

The results from this analysis reflect very good correspondence between predicted 

and observed chlorophyll a concentrations, and little change in residuals between 

the two periods. Thus, the relationship between nitrogen loading and algal biomass 

has been relatively consistent during the period of 1986 through 1998. 

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Figure 3-1. Relationship between mean monthly observed and predicted chlorophyll a, all bay segments
Figure 3-2. Relationship between mean monthly observed and predicted chlorophyll a, Old Tampa Bay
Figure 3-3. Relationship between mean monthly observed and predicted chlorophyll a, Hillsborough Bay
Figure 3-4. Relationship between mean monthly observed and predicted chlorophyll a, Middle Tampa B
Figure 3-5. Relationship between mean monthly observed and predicted chlorophyll a, Middle Tampa B
Figure 3-6. Relationship between mean monthly observed and predicted chlorophyll a, Middle Tampa B
Figure 3-7. Residuals for the 1986-1994 and 1995-1998 periods, by month, for all bay segments. 
Figure 3-8. Residuals for the 1986-1994 and 1995-1998 periods, by bay segment, for all months. 
4 DISCUSSION AND CONCLUSIONS 

The re-evaluation of the TN load – chlorophyll a model is one component of the 

overall examination of the nitrogen management strategy adopted by the Tampa Bay 

Estuary Program. The objective of this report is to re-evaluate the empirical 

relationship between TN loads and chlorophyll a concentrations using the water 

quality data collected since 1994 by the Environmental Protection Commission of 

Hillsborough County (EPCHC) and loading estimates for 1995-1998 developed by 

the TBEP. 

In general, the TN load - chlorophyll a model explains the observed variation in 

chlorophyll a as a function of TN loads from the watershed. Based on the outcome 

of these analyses, the relationship between nitrogen loading and algal biomass has 

been relatively consistent during the period of 1986 through 1998. It is 

recommended that the empirical relationships between TN loads and chlorophyll a 

concentrations developed from the 1986-1994 water quality data continue to be 

employed as part of the model suite relating external nitrogen loads to light 

attenuation. 

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5 REFERENCES 

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. Mote Marine Laboratory Technical Report 

Number 425. 

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

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

Prepared for: Tampa Bay National Estuary Program. Technical Publication #06-96. 

Janicki et al., 1995. Habitat protection and restoration targets for Tampa Bay. 

Prepared for: Tampa Bay National Estuary Program. Technical Publication #07-93. 

Janicki Environmental, Inc. 2000. Tampa Bay Estuary Program Water Quality 

Monitoring Program Database. Prepared for: Tampa Bay Estuary Program. 

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. Prepared for: Tampa Bay 

Estuary Program. Prepared by: Janicki Environmental, Inc. 

Zieman, R.C., R.T. Zieman. 1989. The Ecology of the seagrass meadows of the 

west coast of Florida: A community profile. U.S. Fish and Wildlife Service. Biol. 

Rep. 85(7.25). 

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