# Tampa Bay Estuary Program Model Evaluation and Update:

Chlorophyll a – Light Attenuation Relationship

Prepared for:



# Tampa Bay Estuary Program

Mail Station I-1/NEP

100 8 th 

## Avenue SE St. Petersburg, FL 33701

Prepared by:

1155 Eden Isle Drive NE St. Petersburg, FL 33704

March 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

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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-eva luate the empirical relationship between chlorophyll a concentrations and light attenuation using the water quality data collected since 1994 by the Environmen tal Protection Commission of Hillsborough County (EPCHC).

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## 2 METHODS

A principal working hypothesis was ea rlier developed that observed light attenuation is directly related to chlor ophyll a concentrations, turbidity, and color (Janicki and Wade, 1996). Regression re sults suggest that the variation in chlorophyll a concentrations could explain the majority of the variation in light attenuation as measured by the diffu se light attenuation coefficient, K d . Beer’s Law allows approximation of K d as follows:

z K d e I ) (

0 − × = z I

where: I
z = the incident light at depth z,

I 0 = the incident light just below the surface, and

K d = the diffuse light attenua tion coefficient (1/meters).

Giesen et al. (1990) reported that the diffuse attenuation coefficient K d can be approximated by a factor (that typically ra nges between 1.5 and 1.8) divided by the Secchi disc depth (expressed in me ters). Concurrently measured K d and Secchi disc depths reported by the City of Tampa Bay Study Group (Johansson, pers. comm.) were used to verify this relationship for Tampa Bay and to refine bay segmentspecific factors.

The depth to which a particular amount of light penetrates, expressed as the percentage of incident irradiance, can be estimated using the following rearrangement of Beer’s Law:

factor K Light

d × ×

− = ) ) 100 % ( ln 1 ( Z

where: Z =the depth to which the defined % light penetrates

% Light =the desired % light (d efined as 20.5% for this study)

K d factor =the bay segment-specific factor that relates K d and Secchi disc depth

The relationship between chlorophyll a c oncentrations and light attenuation, expressed as the depth to which 20.5% of the surface irradiance penetrates, was expressed by the following equation:

2-1) (Z ln â ) (C ln
t,s t,s t,s t,s × + = α

where: Z t, s = the depth to which 20.5% of surface irradiance penetrates in month t and bay segment s

C t, s = average chlorophyll a concentration in month t and bay segment s

α t,s and β
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 the observed depths to which 20.5% of surface irradiance penetrates could be explained by the variation in observed chlorophyll a concentrations. Monthly specific regression intercept terms were used to avoid any potentially confounding effects of seasonality in the independent and dependent variables. Turbidity data were also investigated as a possible exp lanation for a portion of the remaining unexplained variation in the light penetra tion data. No improvement in the model

fit was found.

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). These data were us ed to evaluate the chlorophyll a – light attenuation model developed previously us ing data from the period 1986 through

1994.

Initially, visual inspection of the relations hip between chlorophyll a concentrations

and Z 20. 5% was completed by plotting these da ta and identifying the period from which the data were obtained. Secondly, the chlorophyll a – Z regression was used to estimate Z given the observed chlor ophyll a concentrations during 1995 through 1998. Comparisons of the predicted and observed Z values, fo r both monthly and annual mean conditions, were also made. Thirdly, the residuals (i.e., the difference of the observed and predicted Z values) were related to observed turbidity and color

values to assess whether differences in turbidity or color could account for the unexplained variation in the Z values.

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## 3 RESULTS

The results of the three me thods used to evaluate the chlorophyll a – Z regression for Tampa Bay are presented below.

Visual Comparison of the Chlorophy ll a – Light Attenuation Relationship

The following figures present the chlor ophyll a – light attenuation relationship for all bay segments and for each of the bay segments:

• 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.

Specifically, the depth to which 20.5% of inci dent radiation penetrates (Z) is plotted as a function of chlorophyll a concentra tion. In each plot, the 1995-1998 data are presented as triangles while the 1986- 1994 data are presented as circles.

Clearly, there is considerable similarity in the chlorophyll a – Z relationships from the 1986-1994 and 1995-1998 periods . Close scrutiny of the data does suggest, however, that the 1995-1998 data tend toward shallower Z values for the same chlorophyll a concentrations. This seems to be particularly true when chlorophyll a

concentrations are less than 10 μ g/L. Therefore, there is some evidence that at lower chlorophyll a levels incident light pe netrated to somewhat shallower depths in the 1995-1998 period as compared to the 1986-1994 period.

Comparison of Predicted and Obse rved Z Values Using the 1986-1994 Chlorophyll a – Light Attenuation Regressions

The following figures present a comparison of the observed mean monthly Z values for the 1995-1998 period to the Z values pr edicted by the ambient mean monthly chlorophyll a concentrations and the chlo rophyll a – Z regressions developed from the 1986-1994 data (Janicki and Wade, 1996):

• Figure 3-7 – Old Tampa Bay

• Figure 3-8 – Hillsborough Bay

• Figure 3-9 – Middle Tampa Bay – 31

• Figure 3-10 – Middle Tampa Bay – 32

• Figure 3-11 – Middle Tampa Bay – 33.

3-1These 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 Z value was greater than that which wa s observed. Data points below the line represent months when the predicted Z value was less than that which was observed. Close agreement in the predicted and observed Z values would result in a plot where the points were in close prox imity to the line. If any bias in the predicted Z values exists then a dispr oportionate number of points would be found either above (observed > predicted) or below (observed < predicted) the line.

These comparisons of mean monthly predic ted and observed Z values also indicate

that there is a small but consistent bias in the observed Z values. In all bay segments, the observed Z valu es were generally less than those predicted by the regressions. This suggests that for the sa me chlorophyll a concentration the mean monthly Z values observed during the 1995-1998 period was generally less than that observed during the 1986-1994 period.

Table 3-1 presents a comparison of th e mean annual predicted and observed Z values by bay segment. The differences in the observed and predicted Z values were typically greatest in 1996 and 1997 in all bay segments. In both years, the predicted mean annual Z exceeded the obs erved mean annual Z. In 1995 there was relatively good agreement between th e predicted and observed mean annual Z values. In 1998, the observed mean a nnual Z exceeded the predicted value. Overall, the difference in Z values was a pproximately 0.3 meters in Old Tampa Bay and Middle Tampa Bay segments 32 and 33 and between 0.1 and 0.2 meters in Hillsborough Bay and Middle Tampa Bay segment 31.

Table 3-1. Mean annual pred icted and observed Z values.

1 9 9 5 1 9 9 6 1 9 9 7 1 9 9 8 Bay

S e g m e n t O b s e r v e d P r e d i c t e d Observed P r e d i c t e d O b s e r v e d P r e d i c t e d O b s e r v e d P r e d i c t e d

O T B 1

. 6 9 1

. 8 6 1

. 7 3 2

. 5 2 1

. 8 8 2

. 4 5 1

. 5 0 1

. 3 6

H B 1

. 1 1 1

. 1 2 1

. 2 6 1

. 5 8 1

. 1 6 1

. 4 0 0

. 9 9 0

. 9 5

M T B

- 1 1

. 2 4 1

. 2 4 1

. 3 5 1

. 9 3 1

. 0 7 1

. 3 1 0

. 9 8 0

. 8 7

M T B

- 2 1

. 7 9 1

. 9 5 1

. 8 8 2

. 4 8 1

. 8 4 2

. 5 2 1

. 4 2 1

. 2 6

M T B

- 3 1

. 8 1 2

. 0 5 2

. 3 3

. 3 2 1

. 9 1 2

. 2 5 1

. 4 7 1

. 3 8

Relationship Between Residuals from the Chlorophyll a – Light Attenuation Regressions and Other Variables

To examine the potential influence of ot her variables that can affect light attenuation, the residuals from the chlor ophyll a – light attenuation model estimated for the 1995-1998 period were plotted as a func tion of these variables. Specifically, turbidity and color can a ffect light attenuation.

3-2Residual Z values are plotted as a functi on of turbidity by bay segment in the following figures:

• Figure 3-12 – Old Tampa Bay

• Figure 3-13 – Hillsborough Bay

• Figure 3-14 – Middle Tampa Bay – 31

• Figure 3-15 – Middle Tampa Bay – 32

• Figure 3-16 – Middle Tampa Bay – 33.

Residual Z values are also plotted as a function of color by bay segment in the following figures:

• Figure 3-17 – Old Tampa Bay

• Figure 3-18 – Hillsborough Bay

• Figure 3-19 – Middle Tampa Bay – 31

• Figure 3-20 – Middle Tampa Bay – 32

• Figure 3-21 – Middle Tampa Bay – 33.

If either turbidity or color significan tly influenced the chlorophyll a - light attenuation relationship, then a discerna ble pattern in the residuals would be expected. Thus, the residual Z values would be greater at higher values of either turbidity or color. Inspecti on of the residual plots suggests that neither turbidity nor color is significantly relate d to the residual Z value.

3-3Figure 3-1. Relationship between chlorophyll a anddepth of 20.5% light attenuation, all bay segments.3-4Figure 3-2. Relationship between chlorophyll a anddepth of 20.5% light attenuation, Old Tampa Bay.3-5Figure 3-3. Relationship between chlorophyll a anddepth of 20.5% light attenuation, Hillsborough Bay.3-6Figure 3-4. Relationship between chlorophyll a and depth of 20.5% light attenuation, Middle Tampa Bay 31.3-7Figure 3-5. Relationship between chlorophyll a and depth of 20.5% light attenuation, Middle Tampa Bay 32.3-8Figure 3-6. Relationship between chlorophyll a and depth of 20.5% light attenuation, Middle Tampa Bay 33.3-9Figure 3-7. Relationship between observed and predicted depth of 20.5%light attenuation, Old Tampa Bay.3-10Figure 3-8. Relationship between observed and predicted depth of 20.5%light attenuation, Hillsborough Bay.3-11Figure 3-9. Relationship between observed and predicted depth of 20.5% light attenuation, Middle Tampa Bay 31.3-12Figure 3-10. Relationship between observed and predicted depth of 20.5% light attenuation, Middle Tampa Bay 32.3-13Figure 3-11. Relationship between observed and predicted depth of 20.5% light attenuation, Middle Tampa Bay 33.3-14Figure 3-12. Relationship between turbidity and residualsof the depth of 20.5% lightattenuation, Old Tampa Bay.3-15Figure 3-13. Relationship between turbidity and residualsof the depth of 20.5% lightattenuation, Hillsborough Bay.3-16Figure 3-14. Relationship between turbidity and residuals ofthe depth of 20.5% light attenuation, Middle Tampa Bay 31.3-17Figure 3-15. Relationship between turbidity and residuals ofthe depth of 20.5% light attenuation, Middle Tampa Bay 32.3-18Figure 3-16. Relationship between turbidity and residuals ofthe depth of 20.5% light attenuation, Middle Tampa Bay 33.3-19Figure 3-17. Relationship between color and residuals ofthe depth of 20.5% light attenuation, Old Tampa Bay.3-20Figure 3-18. Relationship between color and residuals ofthe depth of 20.5% lightattenuation, Hillsborough Bay.3-21Figure 3-19. Relationship between color and residuals ofthe depth of 20.5% light attenuation, Middle Tampa Bay 31.3-22Figure 3-20. Relationship between color and residuals ofthe depth of 20.5% light attenuation, Middle Tampa Bay 32.3-23Figure 3-21. Relationship between color and residuals ofthe depth of 20.5% light attenuation, Middle Tampa Bay 33.3-24

## 4 DISCUSSION AND CONCLUSIONS

The re-evaluation of the chlorophyll a – light attenuation model is one component

of the overall examination of the nitro gen management strategy adopted by the Tampa Bay Estuary Program. The objective of this report is to re-evaluate the empirical relationship between chlorophyll a concentrations and light attenuation using the water quality data collected si nce 1994 by the Environmental Protection Commission of Hillsborough County (EPCHC).

In general, the chlorophyll a – light model explains the observed variation in light attenuation as a function of chlorophyll a concentrations. However, there appears to be some bias in the predicted Z valu es at low chlorophyll a concentrations. Specifically, the depth to which 20.5% of the incident light penetrates was somewhat shallower during the 1995-1998 peri od than had been predicted by the

model based on the 1986-1994 data. The bi as is expressed at both monthly and

annual time scales.

To put the observed bias in th e Z values in perspective, the differences in the mean

annual Z values were converted to units of Secchi disc depth. Secchi disc depth is the specific indicator employed by the current monitoring program to evaluate spatial and temporal trends in light atte nuation. The EPCHC measures Secchi disc depth at 0.5 foot intervals. The mean differences by bay segment in the mean annual Z values for the 1995-1998 period were:

• Old Tampa Bay – 13 inches

• Hillsborough Bay – 5 inches

• Middle Tampa Bay 31 – 8 inches

• Middle Tampa Bay 32 – 15 inches

• Middle Tampa Bay 33 – 17 inches.

Clearly, the difference in predicted and observed mean annual Secchi disc depths

consistent with the Z values would be measurable given the existing monitoring program. Analysis of the Secchi disc depths observed in each bay segment show

that the mean annual Secchi disc de pths have decrease d during the 1995-1998 period compared to those of the 1986-1994 period. This decrease is on the same

order as those listed above.

Additional analyses were performed to evaluate possible reasons for the observed differences in the relationship be tween the 1986-1994 period and the 1995-1998 period. A regression model based on da ta from the extended time period of 1986 through 1998 was examined and the regressi on coefficients from this model were compared to those from the original model based on 1986-1994 data. Neither the monthly-specific intercept estimates nor th e slope estimates from these two models were significantly different.

4-1Another evaluation examined changes in th e methods employed to collect and/or analyze the chlorophyll samples between the two periods. Mr. Tom Cardinale, EPCHC, provided information relating to any changes in equipment and methods employed during the sampling record. No re levant differences in either field or laboratory methods were reported.

A hypothesis that might explain the obser ved bias is a chan ge in phytoplankton community structure that could result in changes in light attenuation per unit of algal biomass. Phytoplankton community composition data have been collected by the City of Tampa Bay Study Group since 1978, and can be used to test this hypothesis. Preliminary i ndications point toward a shift in community dominance in Hillsborough Bay from blue-green al gae prior to 1995 to diatoms in the succeeding period.

Based on the outcome of these analyses, no readily apparent cause for the small bias in the chlorophyll a – light attenuati on relationship could be found, but it may be that changes in phytoplankton commun ity composition have contributed to this bias. It is recommended that the empiri cal relationships between chlorophyll a concentrations and light attenuation 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. Continued m onitoring and analysis of the ambient chlorophyll a concentrations and water clarity is needed to ensure factors other than algal biomass are not contributing signifi cantly to water clarity in the bay.

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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.

Giesen, W.B.J.T, M.M. van Katijk, and C. den Hartog. 1990. Eelgrass condition and turbidity in the Dutch Wadden Sea. Aquatic Botany. Vol 37. pp.71-85.

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.

Johansson, R. 2001. Personal communi cation from Roger Johansson, February

2001.

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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