# 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 8 th 

## 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 Environm ental, Inc. under Contract No. T-98-06 for the
Tampa Bay Estuary Program.

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## ACKNOWLEDGMENTS

This project was conducted under the guid ance 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 Modelin g 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 I N T R O D U C T I O N

.................................................................................................... 1-1

2 M E T H O D S

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

3 R E S U L T S

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

4 D I S C U S S I O N S A N D CONCLUSIONS

..................................................................... 4-1

5 R E F E R E N C ES

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

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## 1 INTRODUCTION

The Tampa Bay National Estuary Progra m (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 environmenta l issues affecting Tampa Bay. The CCMP specifically addressed several majo r issues, including water quality and habitat loss. The loss of seagrasses in Ta mpa Bay drew particular interest, given their important role in the ecological functioning of the bay.

Seagrasses provide critical habitat for recr eationally and commercially important fish and invertebrate species. Seagrasses in Florida provide juvenile nursery and adult feeding areas for red drum, spotted seatrou t, spot, silver perch, sheepshead, snook, shrimp, and the bay scallop (Zieman and Zieman, 1989). Seagrass meadows are also important feeding areas for the Florid a manatee. Seagrasses serve to improve water quality by reducing nutrients in th e 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 scarri ng and currents, seagrass disease, and location of seagrass beds in relation to offshore transverse sandbars. The TBNEP Technical Advisory Committee (TAC) recogn izes that these and other factors may affect progress towards reaching adopted seag rass restoration goals, and is currently initiating actions which will assist with addre ssing factors in addition to water clarity and quality (including the initiation of meas uring the depth of water at various seagrass beds around the bay, and the mon itoring of potential seagrass disease).

Actions needed to address additional factor s, such as the bay-wide effects of prop scarring or the location of various seagra ss 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 se agrass species. Turtle grass, the most common seagrass species in Tampa Bay, ha s an estimated light requirement of

20.5% of the incident subsurface light (Dixon and Leverone, 1995). For the purposes of seagrass restoration target settin g, this was assumed to be the minimum light requirement for seagrasses in Tampa Bay (Janicki and Wade, 1996).

To address the issue of se agrass habitat loss, recommended seagrass protection and restoration targets were developed (Janicki et al., 1995). Seagrass restoration targets were determined by comparing 1990 seag rass extent to that observed in 1950. Seagrass targets were defined as those porti ons 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.

1-1To strive toward the seagrass restoration goal, the TBNEP developed the Nitrogen Management Strategy. The Strategy seeks to prevent future imp acts due to excessive nitrogen loadings to Tampa Bay. To addre ss 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 a nd between chlorophyll a concentrations and light attenuation (Janicki and Wade, 1996).

The objective of this report is to re-evalu ate the empirical relationship between TN loads and chlorophyll a concentrations usin g the water quality data collected since 1994 by the Environmental Protecti on Commission of Hillsborough County (EPCHC) and the 1995-1998 loading estimates developed by the TBEP (Pribble et al., 2001).

1-2

## 2 METHODS

A principal working hypothesis was ear lier developed that observed chlorophyll a concentrations are directly related to tota l nitrogen loads, from external sources and from internal exchange of nitrogen be tween bay segments (Janicki and Wade, 1996). Regression results suggest that the va riation 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 manne r. First, a series of simple dilution equations were written for the segments of Tampa Bay a nd fit to the observed salinity data and freshwater inflow data. For example, th e equation for the Old Tampa Bay segment

was written as follows:

) (

, o b b o o o C C E C Q

− +

− = o

W 0

where: W o = the external load of a disso lved substance to Old Tampa Bay from its watersh (kg/month),

Q
o = the freshwater inflow rate to Old Tampa Bay from its watershed (m 3 /month),

C
o = the concentration of a disso lved substance in Old Tampa Bay (kg/m 3 ),

C b = the concentration of a dissolved substance in the adjacent subsegment of Middle Tampa Bay (kg/m 3 ), and

E o, b = a transfer coefficient from Old Tampa Bay to the adjacent subsegment of Middle Tampa Bay (m 3 /month).

A dilution equation was written in this manner for each of the five modeled segments (Old Tampa Bay, Hillsborough Ba y, and the three s ubsegments of Middle Tampa Bay). To estimate the average net tr ansfer 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 le ft only the transfer coefficient terms (E)

unknown in the series of e quations. The transfer coefficient terms were then estimated for the dry season (October-May) and the wet season (June-September).

After estimation of the exchange coefficient s between the segments, the series of equations were used to pred ict monthly nitrogen concentrations. To account for the nonconservative properties of nitrogen in the estuarine system, the predicted

2-1concentrations were related to the obser ved concentrations using least squares regression methods, as follow:

s t N

, ˆ

s t, s t, N β =

where: N t,s = the observed nitrogen conc entration in month t and bay

segment s,

N ˆ t,s = the nitrogen concentrati on 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 d egree of the adjustment for nonconservative processes varied monthly and typically fe ll within plus or minus 5% to 20%.

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

follows:

s t s s t L

, , β α + = s t, C

where: C t, s = average chlorophyll a conc entration at month t and in segment s

,

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

α t,s and β t,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 expla ined by the variation in total nitrogen load. Monthly specific regression intercept terms were used to avoid any potentially confounding effects of seasonality in the i ndependent and dependent variables.

Water quality data collected by the EP CHC for the period 1995 through 1998 were obtained and incorporated into the TBEP water quality database (Janicki Environmental, 2000). External TN loadin g 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.

2-2Initially, the TN load - chlorophyll a regr ession was used to estimate chlorophyll a given the estimated TN loads during 1995 through 1998. Comparisons of the predicted and observed chlorophyll a valu es, for both monthly and annual mean conditions, were then made. Secondly, th e residuals (i.e., the difference of the observed and predicted chlorophyll a values ) were examined to assess whether any important differences were observed be tween 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 comp arison 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 lo ad - 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 be tween predicted and observed values. Data points above the lin e represent months when the predicted chlorophyll a value was greater than that which was observed. Data points below the line represent months when the predic ted 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 point s were in close proxim ity to the line. If any bias in the predicted chlorophyll a va lues exists then a disproportionate number of points would be found either above (obs erved > 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 pred icted and observed chlorophyll a values

indicate that there is little difference be tween the regression results from the 1986- 1994 and 1995-1998 periods. Tabl e 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 gr eater 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

3-1Middle 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 predicte d and observed chlorophyll a values.

1 9 9 5 1 9 9 6 1 9 9 7 1 9 9 8 Bay Segment Observed P r e d i c t e d Observed P r e d i c t e d Observed P r e d i c t e d Observed P r e d i c t e d

O T B 8

. 3 7

. 7 6

. 9 8

. 6 8

. 5 8

. 9 1 1

. 6 9

. 9

H B 1 5

. 1 1 3

. 9 9

. 6 1 3

. 1 1 2

. 4 1 3

. 2 1 8

. 4 1 5

. 0

M T B

- 3 1 1 2

. 6 1 0

. 4 6

. 5 9

. 6 9

. 5 1 1

. 0 1 3

. 9 1 1

. 1

M T B

- 3 2 8

. 5 7

. 9 5

. 8 7

. 2 7

. 0 8

. 4 1 0

. 7 8

. 3

M T B

- 3 3 5

. 5 5

. 1 3

. 0 5

. 3 5

. 6 5

. 9 6

. 4 8

. 7

Differences Between Residuals from th e TN Load - Chlorophyll a Regressions During 1986-1994 and 1995-1998

To examine any differences in the rela tionship between predicted and observed chlorophyll a concentrations durin g the two periods, 1986-1994 and 1995-1998, the residuals (difference between predic ted and observed chlorophyll a) were plotted for each month and for each ba y segment for the two periods. Residual chlorophyll a values are show n as a function of month in Figure 3-7, and as a function of bay segment in Figure 3-8, fo r 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 re lationship between nitrogen loading and algal biomass has been relatively consistent dur ing the period of 1986 through 1998.

3-2Figure 3-1. Relationship betweenmean monthly observed and predicted chlorophyll a, all bay segments.3-3Figure 3-2. Relationship between mean monthly observed and predicted chlorophyll a, Old Tampa Bay.3-4Figure 3-3. Relationship between mean monthly observed and predicted chlorophyll a, Hillsborough Bay.3-5Figure 3-4. Relationship between mean monthly observed and predictedchlorophyll a, Middle Tampa Bay 31.3-6Figure 3-5. Relationship between mean monthly observed and predictedchlorophyll a, Middle Tampa Bay 32.3-7Figure 3-6. Relationship between mean monthly observed and predictedchlorophyll a, Middle Tampa Bay 33.3-8Figure 3-7. Residuals for the 1986-1994 and 1995-1998periods, by month, for all bay segments.3-9Figure 3-8. Residuals for the 1986-1994 and 1995-1998periods, by bay segment, for all months.3-10

## 4 DISCUSSION AND CONCLUSIONS

The re-evaluation of the TN load – chlorophyll a model is one component of the overall examination of the nitrogen mana gement 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 loadin g 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 fr om the watershed. Based on the outcome of these analyses, the relationship between nitrogen loading and algal biomass has been relatively consistent during th e period of 1986 through 1998. It is recommended that the empirical relationshi ps between TN loads and chlorophyll a concentrations developed fr om the 1986-1994 water quality data continue to be employed as part of the model suite re lating 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: Su rface Water Improvement and Management Program, Southwest Florida Water Mana gement District. Submitted by: Mote Marine Laboratory, Sarasota, Florida. Mote Marine Laborat ory Technical Report

Number 425.

Janicki, A. and D. Wade. 1996. Estimatin g critical external nitrogen loads for the Tampa Bay Estuary: An empirically based approach to setting management targets. Prepared for: Tampa Bay National Estuar y Program. Technical Publication #06-96.

Janicki et al., 1995. Habitat protection a nd restoration targets for Tampa Bay. Prepared for: Tampa Bay National Estuar y Program. Technical Publication #07-93.

Janicki Environmental, Inc. 2000. Ta mpa 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 co mmunity profile. U.S. Fish and Wildlife Service. Biol. Rep. 85(7.25).

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