SARASOTA BAY TMDL SUPPORT 

Prepared for 

Sarasota Bay Estuary Program 

111 S. Orange Avenue, Suite 200W 

Sarasota, Florida 34236 

Prepared by 

5300 West Cypress Street 

Suite 200 

Tampa, FL 33607-1768 

DRAFT 

August 2007 
i Sarasota Bay TMDL Support 

DRAFT August 2007

Table of Contents 

1.0 Introduction .....................................................................................................................................1

1.1 Approach 1 – Acceptance of existing standards for chlorophyll-a.....................................3

1.2 Approach 2 – Establish site-specific and resource-linked targets for chlorophyll-a ..........3

2.0 Materials and Methods....................................................................................................................4

2.1 Site Identification................................................................................................................5

2.1.1 Segments 7, 8 10 and 11 (Sarasota Bay).............................................................5

2.1.2 Segments 1, 3, 13, 14 and 16...............................................................................6

2.2 Seagrass Coverage ...........................................................................................................7

2.3 Depth Measurements.........................................................................................................8

2.4 NOAA NOS Bathymetry...................................................................................................10

2.5 Depth Comparison ...........................................................................................................11

2.6 Chlorophyll-a and K Data.................................................................................................11

2.7 Chlorophyll-a Target.........................................................................................................11

3.0 Results ..........................................................................................................................................13

3.1 Water Depth Comparison ................................................................................................13

3.2 Chlorophyll-a Targets.......................................................................................................15

4.0 Discussion.....................................................................................................................................31

5.0 References....................................................................................................................................34

LIST OF TABLES 

Table 1. Verified Impaired Waterbodies in Sarasota Bay (from FDEP Group 3 Verified Impaired 

List (6/20/2005)).........................................................................................................................2

Table 2. Common names for portions of Sarasota Bay and corresponding SBEP bay 

segments. ..................................................................................................................................4

Table 3. Braun-Blanquet seagrass coverage method. ............................................................................7

Table 4. Segment and reference tidal station for field data.....................................................................9

Table 5. Results of Linear Model Regression between chlorophyll-a and K for each Bay 

Segment...................................................................................................................................12

Table 6. Average Corrected chlorophyll-a, Light Attenuation Coefficient, and PAR for each 

segment. ..................................................................................................................................17

Table 7. Segment 7 current and proposed water depth, K and target chlorophyll-a.............................17

Table 8. Segment 8 current and proposed water depth, K and target chlorophyll-a.............................20

Table 9. Segment 10 current and proposed water depth, K and target chlorophyll-a..........................21

Table 10. Segment 11 current and proposed water depth, K and target chlorophyll-a..........................23

Table 11. Segment 13 current and proposed water depth, K and target chlorophyll-a...........................25

Table 12. Segment 14 current and proposed water depth, K and target chlorophyll-a...........................27

Table 13. Segment 16 current and proposed water depth, K and target chlorophyll-a...........................29

Table 14. PBSJ Proposed chlorophyll-a targets for all segments and percent reduction required 

to reach the proposed 2050 target...........................................................................................32
Table of Contents 

ii Sarasota Bay TMDL Support 

DRAFT August 2007 

LIST OF FIGURES 

Figure 1. Site location in Segments 7,8,10 and 11. ..................................................................................5

Figure 2. Site location in Segments 13, 14 and 16. ..................................................................................6

Figure 3. Site Location in Segments 1 and 3............................................................................................6

Figure 4. Seagrass identification and coverage estimation ......................................................................7

Figure 5. Customized staff gauge used to measure water depth. ............................................................8

Figure 6. 1950’s NOS NOAA bathymetry data for Sarasota Bay. ..........................................................10

Figure 7. Average Water Depth at the 1950’s NOS and 2007 field sites. ..............................................13

Figure 8. Average water depth using both 1950’s bathymetry data and surveyed mapped and 

deep-edges..............................................................................................................................14

Figure 9. Distance from the mapped deep-edge to the field verified deep-edge....................................14

Figure 10. Correlation between Light attenuation and chlorophyll-a for Segment 1 (August 2001-

February 2006). .......................................................................................................................15

Figure 11. Correlation between Light attenuation and chlorophyll-a for Segment 3 (August 2001-

February 2006). .......................................................................................................................16

Figure 12. Correlation between Light attenuation and chlorophyll-a for Segment 7 (August 2001-

February 2006). .......................................................................................................................18

Figure 13. Current and future target chlorophyll-a concentrations for Segment 7....................................18

Figure 14. Correlation between Light attenuation and chlorophyll-a for Segment 8 (August 2001-

February 2006). .......................................................................................................................19

Figure 15. Current and future target chlorophyll-a concentrations for Segment 8....................................20

Figure 16. Correlation between Light attenuation and chlorophyll-a for Segment 10 (August 

2001-February 2006). ..............................................................................................................21

Figure 17. Current and Future target chlorophyll-a concentrations for Segment 10. ...............................22

Figure 18. Correlation between Light attenuation and chlorophyll-a for Segment 11 (August 

2001-February 2006). ..............................................................................................................23

Figure 19. Current and Future target chlorophyll-a concentrations for Segment 11. ...............................24

Figure 20. Correlation between Light attenuation and chlorophyll-a for Segment 13 (August 

2001-February 2006). ..............................................................................................................25

Figure 21. Current and Future target chlorophyll-a concentrations for Segment 13. ...............................26

Figure 22. Correlation between Light attenuation and chlorophyll-a for Segment 14 (August 

2001-February 2006). ..............................................................................................................27

Figure 23. Current and Future target chlorophyll-a concentrations for Segment 14. ...............................28

Figure 24. Correlation between Light attenuation and chlorophyll-a for Segment 14 (August 

2001-February 2006). ..............................................................................................................29

Figure 25. Current and Future target chlorophyll-a concentrations for Segment 16. ...............................30
1 Sarasota Bay TMDL Support 

DRAFT August 2007

1.0 Introduction 

In Southwest Florida, a substantial amount of research has focused on the relationships between 

population growth, changing land use patterns, pollutant loads, estuarine water quality, and 

seagrass health. In Tampa Bay, historical losses of seagrass coverage have been linked to both 

direct and indirect impacts (i.e., Lewis et al., 1985; Lewis, 1989; Haddad, 1989). In contrast, 

recent (1982–1996) increases in seagrass coverage in Tampa Bay have been linked to improved 

water quality (i.e., Johansson, 1991; Johansson and Ries, 1997; Lewis et al., 1998; Johansson 

and Greening, 1999). Improvements in water quality have in turn been attributed mostly to 

improvements in the treatment and disposal of wastewater discharges by the Cities of Tampa, St. 

Petersburg and Clearwater since the mid to late 1980s (Johansson and Greening, 1999). 

A similar situation exists in Sarasota Bay, where recent increases in seagrass coverage were 

related to reductions in anthropogenic nitrogen loads to the bay by the City of Sarasota and 

Manatee and Sarasota Counties (Kurz et al., 1999; Tomasko et al. 2005). 

Nitrogen loads into Tampa Bay and Sarasota Bay (generated primarily by domestic wastewater 

treatment facilities) were reduced by 57% and 46%, respectively, during the 1980s and 1990s 

(Tomasko et al. 2005). In response, both Tampa Bay and Sarasota Bay have lower phytoplankton 

concentrations, greater water clarity and more extensive seagrass coverage in recent years than in 

the early 1980s (Johansson and Greening, 1999; Kurz et al., 1999; Tomasko et al. 2005). As 

there is no evidence of a concurrent and explanatory trend in rainfall during this same time 

period, it was concluded that variation in rainfall could not account for the observed increase in 

seagrass coverage in these two bays (Tomasko et al. 2005). 

The following is the conceptual framework within which the topic of “impairment” should be 

considered regarding issues relating to the topic of Total Maximum Daily Loads (TMDLs) for 

Sarasota Bay. The bay itself is characterized as having stable or increasing seagrass coverage 

bay-wide, yet portions of the bay are characterized as “impaired” via the state of Florida’s 

Impaired Waters Rule (IWR). 

In Florida, the IWR (Chapter 62-303, Florida Administrative Code [FAC]) includes guidelines 

for chlorophyll-a in surface water bodies. Estuaries or estuary segments are included on the 

“planning list” for nutrients if their annual mean chlorophyll-a for any year is greater than 

11 μg / liter or if annual mean chlorophyll-a values have increased by more than 50% over 

historical values for at least two consecutive years. When comparing changes in chlorophyll-a to 

historical levels, historical levels are based on the lowest five-year average for the period of 

record. To calculate a five-year average, there must be annual means from at least three years of 

that five-year period. 

Waterbodies are placed on the verified list for impairment due to nutrients if there are sufficient 

data from the last five years preceding the planning list assessment, combined with historical 

data (if needed to establish historical chlorophyll-a levels or historical TSIs), to meet the data 

sufficiency requirements spelled out in FAC 62-303.350(2). 
Introduction 

Portions of Sarasota Bay have chlorophyll-A concentrations that are near the thresholds 

described in the IWR; and therefore, may have required pollutant load reductions assigned to 

them through the state’s Total Maximum Daily Load (TMDL) Program. 

Portions of Sarasota Bay that are currently listed as impaired are shown in Table 1. Also 

included is the bay segment that correlates with the Waterbody Identification number (WBID) of 

concern, as outlined in both the Sarasota Bay Estuary Program’s (SBEP) Framework for Action 

(1992) and also the SBEP’s Comprehensive Conservation and Management Plan (1995). 

Table 1. Verified Impaired Waterbodies in Sarasota Bay (from FDEP Group 3 

Verified Impaired List (6/20/2005)). 

Waterbody Name WBID SBNEP 

Segment Parameter of Concern 

Palma Sola Bay 1883 2 and 3 Nutrients (chlorophyll-a > 11 μg / liter) 

Palma Sola Bay North 1883 B 3 Bacteria (exceeds DOH thresholds) 

Palma Sola South 1883 C 3 Bacteria (exceeds DOH thresholds) 

Bowlees Creek 1896 NA Fecal Coliform Bacteria 

Bowlees Creek 1896 NA Total Coliform Bacteria 

Bowlees Creek 1896 NA Nutrients (chlorophyll-a) 

Philippi Creek 1937 NA Fecal Coliform Bacteria 

Philippi Creek 1937 NA Total Coliform Bacteria 

Roberts Bay 1968 D 13 Nutrients (Historic chlorophyll-a) 

Blackburn Bay 1968 F 16 Nutrients (Historic chlorophyll-a) 

Clark Lake 1971 NA Nutrients (TSI > 60) 

Clowers Creek Tidal 1975 AA NA Nutrients (chlorophyll-a) 

This report focuses on the review of current listing criteria for Sarasota Bay, and the 

development of potential alternative criteria for the water quality goal development for Sarasota 

Bay. 

While chlorophyll-a can be a useful indicator of impaired conditions in an estuary, interpreting 

the causes and effects of varying chlorophyll-a concentrations is challenging. Chlorophyll-a is 

known to be highly variable and responsive to a variety of factors, including nutrient loads, 

rainfall, salinity, residence time, season, and even the time of day. In addition, factors other than 

chlorophyll, such as turbidity and color, may also affect water transparency. If those factors 

affect water clarity to such a degree that they may also affect phytoplankton growth rates, than 

issues other than nutrient availability alone may need to be accounted for to develop potential 

nutrient limits for water quality protection and/or restoration. 

For Sarasota Bay, there appear to be two main courses of action that could be pursued, as relates 

to the issue of potentially biologically relevant values of chlorophyll-a, as summarized below. 

2 Sarasota Bay TMDL Support 

DRAFT August 2007 
Introduction 

3 Sarasota Bay TMDL Support 

DRAFT August 2007 

1.1 Approach 1 – Acceptance of existing standards for chlorophyll-a 

This approach would be to accept the state’s determination that a chlorophyll-a concentration of 

11 μg / liter, and the water clarity that accompanies such conditions, is an appropriate target. Or, 

that a 50 percent increase in levels of chlorophyll-a over “historical” levels is an indication of an 

impaired condition. If this approach is viewed as satisfactory, then Clowers Creek would be 

impaired for exceeding the 11 μg / liter standard, and Bowlees Creek, Roberts Bay and 

Blackburn Bay would be impaired for increases over historical levels of chlorophyll-a. 

1.2 Approach 2 – Establish site-specific and resource-linked targets for 

chlorophyll-a 

A second approach would be to develop site-specific and scientifically-defensible water clarity 

and chlorophyll-a goals that could be linked to a living resource. An approach such as this, using 

science-based management to protect the health of living resources, is similar to that currently 

being used by the Tampa Bay Estuary Program. 

This report summarizes efforts conducted in Sarasota Bay related to Approach 2, the 

development of site-specific and resource-linked targets for chlorophyll-a. 

In addition, this report also addresses the issue of whether or not “offshore” or “deep” edges 

from seagrass mapping efforts are the same as a “deep” edge that would be found from site visits 

and careful depth measurements. 

Finally, this report focuses on the potentially important issue of sea level rise, as it relates to the 

development of resource-based water clarity and chlorophyll-a targets for Sarasota Bay. 
4 Sarasota Bay TMDL Support 

DRAFT August 2007

2.0 Materials and Methods 

The techniques used in this report involved the following steps: 1) identification of “mapped” 

deep edges of seagrass meadows, 2) determining the “surveyed” deep edges of seagrass 

meadows, 3) assessing the percent coverage of seagrasses at these two depths, 4) determining the 

depths at these two potentially different locations, 5) determining the light levels at these depths, 

and 6) determining the levels of chlorophyll-a that would allow for adequate light penetration to 

target depths, and 7) determining the implications of sea level rise on potential chlorophyll-a 

targets. 

For this effort, the original Sarasota Bay segmentation scheme used in the SBEP’s Framework 

for Action (1992) and the Comprehensive Conservation and Management Plan (SBEP 1995) was 

utilized. Table 2 contains information on the relationship between portions of Sarasota Bay and 

the corresponding SBNEP bay segment numbers used in this report. 

Table 2. Common names for portions of Sarasota Bay and 

corresponding SBEP bay segments. 

Common Name for Waterbody SBEP Segment Numbers 

Anna Maria Sound 1 

Western Palma Sola Bay 2 

Eastern Palma Sola Bay 3 

Longboat Pass 4 

Western Upper Sarasota Bay – Manatee County 5 

Eastern Upper Sarasota Bay – 

Manatee County 6 

Western Upper Sarasota Bay – Sarasota County 7 

Eastern Upper Sarasota Bay – Sarasota County 8 

New Pass 9 

East of New Pass / Big Pass 10 

Offshore of Downtown Sarasota 11 

Big Pass 12 

Roberts Bay 13 

Little Sarasota Bay 14 

“Midnight Pass” 15 

Blackburn Bay 16 

Nearshore Gulf of Mexico 17 
Materials and Methods 

In greater detail, these steps are outlined below: 

2.1 Site Identification 

2.1.1 Segments 7, 8 10 and 11 (Sarasota Bay) 

PBS&J, working in concert with Janicki Environmental, verified seagrass coverage and water 

depth information along the mapped deep-edge of the seagrass beds within Sarasota Bay. As 

such, Janicki Environmental created a “Universe” of sampling stations along the deep-edge of 

the consistently mapped seagrass beds performed by the Southwest Florida Water Management 

District (SWFWMD). Twenty randomly selected stations were provided by Janicki 

Environmental for each of the segments in Sarasota Bay (7, 8, 10 and 11). PBS&J randomly 

selected eight sites for field verification of both seagrass coverage and water depth in each 

segment (Figure 1). The sites were visited on two calm weather days, February 20th and 21st

, 

2007. All sites were QA/QC for site accuracy; four sites were removed from analysis based on 

their proximity to the target site location. 

Figure 1. Site location in Segments 7,8,10 and 11. 

5 Sarasota Bay TMDL Support 

DRAFT August 2007 
Materials and Methods 

2.1.2 Segments 1, 3, 13, 14 and 16 

PBS&J identified 7-9 mapped deep-edge 

sampling points based upon the 2006 

SWFMWD seagrass coverage maps. Four 

sites were randomly selected to verify both 

seagrass coverage and water depth in 

segments 13, 14 and16 (Figure 2). This data 

was collected on May 4, 2007 during calm 

weather conditions. Five sites were 

randomly selected to verify both seagrass 

coverage and water depth in segments 1 and 

3 (Figure 3). These data were collected on 

May 14, 2007 during calm weather 

conditions. 

Figure 2. Site location in Segments 13, 14 and 16. 

Figure 3. Site Location in Segments 1 and 3. 

6 Sarasota Bay TMDL Support 

DRAFT August 2007 
Materials and Methods 

2.2 Seagrass Coverage 

At all mapped deep-edge sites, a modified Braun-Blanquet method was used to determine 

seagrass coverage (Figure 4; Table 3). Three assessments of seagrass coverage and species 

diversity were recorded at each site using a 1m2 quadrant. Each site was evaluated to determine 

if it satisfied the Virnstein et al. 2000 method for classification of the deep-edge of a seagrass 

bed, <10% seagrass coverage (Virnstein et al. 2000). If it was determined that the site had >10% 

seagrass coverage, PBS&J located the deep-edge. The location and water depth of the surveyed 

deep-edge were recorded. 

Figure 4. Seagrass identification and coverage estimation 

Table 3. Braun-Blanquet seagrass coverage method. 

Assessed 

Value Percent Seagrass Coverage 

0 No coverage 

0.1 Solitary short shoot 

0.5 Sparse of <1% coverage 

1 1-5% coverage 

2 6-25% coverage 

3 26-50% coverage 

4 51-75% coverage 

5 76-100% coverage 

6 Coverage outside of m2 placement 

7 Sarasota Bay TMDL Support 

DRAFT August 2007 
Materials and Methods 

2.3 Depth Measurements 

Water depth information was collected at a total of 89 sampling sites throughout all Segments. A 

total of 39 water depth measurements were recorded at the mapped deep-edge and an additional 

50 measurements were made at the surveyed deep-edge. Eleven of the 50 surveyed deep-edge 

sites were consistent with the mapped deep-edge. Three depths were recorded at each site using 

a customized staff gauge designed to reduce errors associated with the soft substrate (Figure 5). 

The weather condition was observed to reduce error associated with fetch and chop. All data 

were collected during calm bay conditions. The GPS coordinates were recorded for each station 

using a WAAS (Wide Area Augmentation System) enabled Garmin GPSmap 60CSx. 

Figure 5. Customized staff gauge used to measure water depth. 

All of the data were corrected for tide and referenced to Mean Low Water (MLW) (Table 4). The 

6-minute verified water level data from the St. Petersburg, Fl (8726520) tide station was used to 

model the tide for the Cortez, Sarasota and Venice tide stations (tidesandcurrents.noaa.gov). 

The tidal prediction for each station was used to modify the St. Petersburg data. The St. 

Petersburg tide was adjusted for high and low tide and a time shift was applied for each station. 

8 Sarasota Bay TMDL Support 

DRAFT August 2007 
Materials and Methods 

9 Sarasota Bay TMDL Support 

DRAFT August 2007 

Table 4. Segment and reference tidal station for field data. 

Segment Bay Tide Station

1 Anna Maria Sound Cortez 

3 Palma Sola Bay Cortez 

7 NW Sarasota Bay Cortez 

8 NE Sarasota Bay Cortez 

10 SW Sarasota Bay Sarasota 

11 SE Sarasota Bay Sarasota 

13 Roberts Bay Sarasota 

14 Little Sarasota Bay Sarasota 

16 Blackburn Bay Venice 
Materials and Methods 

2.4 NOAA NOS Bathymetry 

Bathymetry data was collected in Sarasota Bay by the U.S. Coast and Geodetic Survey between 

1953 and 1955 (Figure 6; www.estuarinebathymetry.noaa.gov). The majority of depth 

soundings were completed in 1954. Six surveys were completed collecting approximately 

52,299 depth values using a portable depth recorder. The average distance between readings was 

51 meters. The vertical datum was referenced to Mean Low Water. The horizontal datum was 

referenced to NAD83. The data provided bathymetry information for all Segments listed in 

Table 2. 

Figure 6. 1950’s NOS NOAA bathymetry data for Sarasota Bay. 

10 Sarasota Bay TMDL Support 

DRAFT August 2007 
Materials and Methods 

11 Sarasota Bay TMDL Support 

DRAFT August 2007 

2.5 Depth Comparison 

Using GIS, the closest NOS water depth point to each field site was selected and paired. The 

distance between the 1950’s NOS station and 2007 field site was determined. The 1950’s NOS 

and 2007 surveyed water depths of all 89 sites were statistically compared using a two-sample 

comparison with StatGraphics. Additionally, the water depth at the 2007 mapped deep-edge and 

the 2007 surveyed deep-edge sites were compared using a two-sample comparison. Again, the 

distance between the mapped deep-edge and surveyed deep-edge was determined. 

2.6 Chlorophyll-a and K Data 

Chlorophyll-a and the light attenuation coefficient (K) data for each segment were downloaded 

from the Florida STORET database –(www.dep.state.fl.us/water/storet). 

These data were available for all segments between August 2001 and February 2006. Light 

attenuation coefficients (K / meter) in Segment 1 and 3 were calculated from the raw underwater 

incident light readings where z=0.5m (personal communication, Greg Blanchard). Additionally, 

only uncorrected chlorophyll-a values were available for Segment 1 and 3. Corrected 

chlorophyll-a was available for all other segments. Statistical significance was determined based 

on the R2

 and sample size (Schefler 1979). 

The average K was determined for each segment based on the monthly averages from August 

2001 to February 2006. Percent Sub-surface Irradiance was calculated for each location at 4 

corresponding depths (Mapped Deep-Edge (1950’s), Mapped Deep-Edge (2007); Surveyed 

Deep-Edge (1950’s) and Surveyed Deep- Edge (2007); Formula 1), and then the average percent 

sub-surface irradiance (or PAR – photosynthetically active radiation) for each segment at each 

depth was determined. 

Formula 1: Percent PAR = +exp (-k*z)*100 

Where k=average light attenuation coefficient and z =water depth in meters at MLW 

2.7 Chlorophyll-a Target 

The chlorophyll-a target for segments 7, 8, 10, 11, 13, 14 and 16 were determined based on the 

average percent sub-surface irradiance at the Surveyed Deep-Edge (2007). The light attenuation 

coefficient was calculated based on PAR and the water depth at which light readings were made 

(Formula 2). 

Formula 2: K= -1/z * ln(Iz / Io) 

Where Iz = PAR at depth z (in meters) and Io = directly sub-surface PAR 
Materials and Methods 

12 Sarasota Bay TMDL Support 

DRAFT August 2007 

The current target chlorophyll-a concentration was calculated based upon the linear model 

relating K and chlorophyll-a for each segment (Table 5). A target chlorophyll-a concentration 

was also calculated based upon anticipated sea-level rise in Sarasota Bay for the years 2020 and 

2050 (after Clark 1992). Using the measured rate of sea-level rise between 1954 and 2007, sealevel rise was simulated for the next 13 and 43 years. The modified water levels were used to 

recalculate the new (lower) K value required to keep the same percent PAR at a location with 

additional water depths (due to sea level rise) and the revised chlorophyll-a target was calculated 

based on the new estimate of K. 

Table 5. Results of Linear Model Regression between chlorophyll-a and K for 

each Bay Segment. 

Segment 

No. Segment Name Chl- A significant? Linear Model 

1 Anna Maria Sound No 

3 Palma Sola Bay No 

7 Upper Big SB – West Side Yes – positive K = 0.5342 + 0.0241(chlorophyll-a) 

8 Upper Big SB – East Side Yes – positive K = 0.5408+ 0.0238 (chlorophyll-a) 

10 Lower Big SB – West Side Yes – positive K = 0.5628 + 0.0205 (chlorophyll-a) 

11 Lower Big SB – East Side Yes – positive K = 0.5025 + 0.0403(chlorophyll-a) 

13 Roberts Bay Yes – positive K = 0.7263 + 0.0364(chlorophyll-a) 

14 Little Sarasota Bay Yes – positive K = 0.7078 + 0.0404 (chlorophyll-a) 

16 Blackburn Bay Yes – positive K = 0.592 + 0.0564 (chlorophyll-a) 
3.0 Results 

3.1 Water Depth Comparison 

Tide-corrected water depth measurements from 89 sites were compared to the water depth 

estimates for these same locations using the bathymetry data compiled in 1953-1955 by the 

USGCS. The average (n = 89) water depth for sites visited in Sarasota Bay, based on the 1950’s 

bathymetry data, was 1.09 m at MLW compared to 1.26 m at MLW water depth for those same 

locations, but with directly measured water depths recorded in 2007 (Figure 7). The change in 

water depth was 0.17 m which is equivalent to 3.2 cm/decade. There was a highly significant 

difference between the 2007 water depths and the 1950’s data (N=89; p=0.01). This difference 

in water depths is within the range of documented rates of sea level rise in St. Petersburg during 

1950 to 1990 (2.4 cm/decade), and estimated rates of future sea level rise of approximately 5.2 

cm/decade, as outlined in Clark (1992) 

Additionally, the mapped deep-edge (2007) sites were compared to the surveyed deep-edge 

(2007) sites; the average water depth was 1.16 m at MLW and 1.34 m at MLW, respectively 

(Figure 8), but this difference was not statistically significant (N=39; p= 0.09). 

The average distance between the mapped deep-edge (2007) and the surveyed deep-edge (2007) 

was 42 m (Figure 9). 

Figure 7. Average Water Depth at the 1950’s NOS and 2007 field sites. 

13 Sarasota Bay TMDL Support 

DRAFT August 2007
Results 

Mapped DeepEdge (1950’s) Mapped DeepEdge (2007) Surveyed DeepEdge (1950’s) 

Surveyed 

Deep-Edge 

Figure 8. Average water depth using both 1950’s bathymetry data and surveyed 

mapped and deep-edges. 

0

50

100

150

200

250

PS-17PS-16PS-15PS-14PS-13

AM-9AM-8AM-7AM-6

BB-39BB-37BB-36BB-35

LSB-31LSB-30LSB-29LSB-28

RB-25RB-23RB-21

11-NE16-NE17-NE19-NE20-NE

03-NW04-NW05-NW20-NW

07-SE10-SE15-SE16-SE17-SE18-SE

08-SW17-SW18-SW20-SW

Distance (m)

Distance

Figure 9. Distance from the mapped deep-edge to the field verified deep-edge. 

14 Sarasota Bay TMDL Support 

DRAFT August 2007 
Results 

3.2 Chlorophyll-a Targets 

Segments 1 and 3

A significant correlation between chlorophyll-a and light attenuation was not found for Segments 

1 and 3 (Figures 10 and 11). In these parts of Sarasota Bay, prior studies concluded that nonchlorophyll suspended material (NCSM) was the dominant attenuator of light in the water 

column (Dixon and Kirkpatrick 1995). On average, NCSM explained 77% of the partitioned 

non-water attenuation for diffuse light in segment 1 and 58% in segment 3. Consequently, a 

chlorophyll-a target was not calculated for these segments. 

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 20 40 60 8

Uncorrected Chlorophyll a (ug/l)

Light attenuation coefficient (K per m)

0

Figure 10. Correlation between Light attenuation and chlorophyll-a for Segment 1 

(August 2001-February 2006). 

15 Sarasota Bay TMDL Support 

DRAFT August 2007 
Results 

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 5 10 15 20 25

Uncorrected Chlorophyll a (ug/l)

Light attenuation coefficient (K per m)

Figure 11. Correlation between Light attenuation and chlorophyll-a for Segment 3 

(August 2001-February 2006). 

Segment 7

Based on the data from August 2001 to February 2006, the average chlorophyll-a concentration 

and light attenuation coefficient were 5.4 μg/l and 0.6681 K per m, respectively (Table 6). 

Figure 12 shows the statistically significant relationship between chlorophyll-a and light 

attenuation for Segment 7. The linear model (Table 5) was used to produce target chlorophyll-a 

values for Segment 7 (Table 3). Based on the Surveyed Deep-Edge water depth and percent 

PAR, a chlorophyll-a target of 5.3 μg/l was calculated to maintain deep edge light requirements 

(Figure 14; Table 7). A potential IWR target was calculated based upon the chlorophyll a value 

which exceeds the historic value by 50% for WBID 1968B (Sarasota Bay; 5.5μg/l ). The 2020 

and 2050 target chlorophyll-a concentrations were calculated based on anticipated sea-level rise. 

A projected water depth of 1.63m in 2020 and 1.76m in 2050 was used. To maintain current 

water clarity conditions with increased water depth, a target chlorophyll-a concentration of 4.6 

μg/l is proposed for 2020. The proposed target chlorophyll-a concentration for 2050 is 2.6 μg/l. 

16 Sarasota Bay TMDL Support 

DRAFT August 2007 
Results 

17 Sarasota Bay TMDL Support 

DRAFT August 2007 

Table 6. Average Corrected chlorophyll-a, Light Attenuation Coefficient, and PAR 

for each segment. 

Segment 

Average 

Chl- a 

(μg / liter) 

Average Light Attenuation 

Coefficient (K per m) 

Percent PAR at the Surveyed 

Deep-Edge (2007) 

7 5.4 0.6681 35 

8 5.4 0.6681 36 

10 3.8 0.6372 34 

11 4.5 0.6837 47 

13 7.7 0.9993 31 

14 7.7 1.0180 37 

16 5.6 0.9105 46 

Table 7. Segment 7 current and proposed water depth, K and target chlorophyll-a. 

Segment 7 

PBSJ 

Proposed 

Target 

PBSJ 

Proposed 

2020 

PBSJ 

Proposed 

2050 

Depth (m at MLW) 1.58 1.63 1.76 

K (per m) 0.662 0.645 0.596 

Chl a (μg/l) 5.3 4.6 2.6 
Results 

y = 0.0241x + 0.5342

R2

 = 0.2191

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 10 20 30 4

Corrected Chlorophyll a (ug/l)

Light attenuation coefficient (K per 

m)

0

Figure 12. Correlation between Light attenuation and chlorophyll-a for Segment 7 

(August 2001-February 2006). 

5.5

4.6

2.6

5.3 5.4

0.0

2.0

4.0

6.0

8.0

10.0

12.0

Average 2001-

2006

 Potential IWR

Target

PBSJ Proposed

Target

2020 target 2050 target

Corrected Chloropyll a (ug

Figure 13. Current and future target chlorophyll-a concentrations for Segment 7 

18 Sarasota Bay TMDL Support 

DRAFT August 2007 
Results 

Segment 8

Based on the data from August 2001 to February 2006, the average chlorophyll-a concentration 

and light attenuation coefficient were 5.4 μg/l and 0.6681 K per m, respectively (Table 6). 

Figure 14 shows the statistically significant relationship between chlorophyll-a and light 

attenuation for Segment 8. The linear model (Table 5) was used to produce target chlorophyll-a 

values for Segment 8. Based on the Surveyed Deep-Edge water depth and K, a chlorophyll-a 

target of 5.0 μg/l was calculated to maintain current deep edge light requirements (Figure 15; 

Table 8). A potential IWR target was calculated based upon the chlorophyll a value which 

exceeds the historic value by 50% for WBID 1968B (Sarasota Bay; 5.5μg/l ). The 2020 and 2050 

target chlorophyll-a concentrations were calculated based on anticipated sea-level rise based on 

projected water depths of 1.61m and 1.74m, respectively. To maintain current water clarity 

conditions with increased water depth, a chlorophyll-a concentration of 4.3 μg/l is proposed for 

2020. The proposed target chlorophyll-a concentration for 2050 is 2.2 μg/l. 

y = 0.0238x + 0.5408

R2

 = 0.181

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 5 10 15 20 25 30

Corrected Chlorophyll a (ug/l)

Light attenuation coefficient (K per m)

Figure 14. Correlation between Light attenuation and chlorophyll-a for Segment 8 

(August 2001-February 2006). 

19 Sarasota Bay TMDL Support 

DRAFT August 2007 
Results 

Table 8. Segment 8 current and proposed water depth, K and target chlorophyll-a. 

Segment 8 

PBSJ 

Proposed 

Target 

PBSJ 

Proposed 

2020 

PBSJ 

Proposed 

2050 

Depth (m at MLW) 1.56 1.61 1.74 

K (per m) 0.660 0.643 0.594 

Chl a (μg/l) 5.0 4.3 2.2 

5.5

4.3

2.2

5.4 5.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

Average 2001-

2006

 Potential IWR

Target

PBSJ Proposed

Target

2020 target 2050 target

Corrected Chloropyll a (ug/l)

Figure 15. Current and future target chlorophyll-a concentrations for Segment 8. 

Segment 10

Based on the data from August 2001 to February 2006, the average chlorophyll-a concentration 

and light attenuation coefficient were 3.8 μg/l and 0.6372 K per m, respectively (Table 6). 

Figure 16 shows the statistically significant relationship between chlorophyll-a and light 

attenuation for Segment 10. The linear model (Table 5) was used to produce target chlorophylla values for Segment 10. Based on the Surveyed Deep-Edge water depth and K, a chlorophyll-a 

target of 1.9 μg/l was calculated to maintain current deep edge light requirements (Figure 17; 

Table 9). A potential IWR target was calculated based upon the chlorophyll a value which 

exceeds the historic value by 50% for WBID 1968B (Sarasota Bay; 5.5μg/l ). The 2020 and 2050 

target chlorophyll-a concentrations were calculated based on anticipated sea-level rise, therefore, 

projected water depths of 1.85m and 1.98m were used, respectively. To maintain current water 

clarity conditions with increased water depth, a target chlorophyll-a concentration of 1.2 μg/l is 

proposed for 2020. Due to further increases in water depth, a target chlorophyll-a concentration 

could not be derived for Segment 10 for 2050 conditions. 

20 Sarasota Bay TMDL Support 

DRAFT August 2007 
Results 

y = 0.0205x + 0.5628

R2

 = 0.0609

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25 30 35 40

Corrected Chlorophyll a (ug/l)

Light attenuation coefficient (K per m)

Figure 16. Correlation between Light attenuation and chlorophyll-a for Segment 

10 (August 2001-February 2006). 

Table 9. Segment 10 current and proposed water depth, K and target 

chlorophyll-a. 

Segment 10 

PBSJ 

Proposed 

Target 

PBSJ 

Proposed 

2020 

PBSJ 

Proposed 

2050 

Depth (m at MLW) 1.81 1.85 1.98 

K (per m) 0.602 0.588 0.549 

Chl a (μg/l) 1.9 1.2 N/A 

21 Sarasota Bay TMDL Support 

DRAFT August 2007 
Results 

1.9

5.5

N/A

3.8

1.2

0.0

2.0

4.0

6.0

8.0

Average 2001-

2006

 Potential IWR

Target

PBSJ Proposed

Target

2020 target 2050 target

Corrected Chloropyll a (ug/l)

Figure 17. Current and Future target chlorophyll-a concentrations for Segment 10. 

Segment 11

Based on the data from August 2001 to February 2006, the average chlorophyll-a concentration 

and light attenuation coefficient were 4.5 μg/l and 0.6837 K per m, respectively (Table 6). 

Figure 18 shows the statistically significant relationship between chlorophyll-a and light 

attenuation. The linear model (Table 5) was used to produce target chlorophyll-a values for 

Segment 11 (Table 10). Based on the Surveyed Deep-Edge water depth and K, a chlorophyll-a 

target of 6.4 μg/l was calculated to maintain current deep edge light requirements (Figure 19; 

Table 10). A potential IWR target was calculated based upon the chlorophyll a value which 

exceeds the historic value by 50% for WBID 1968B (Sarasota Bay; 5.5μg/l ). The 2020 and 

2050 target chlorophyll-a concentrations were calculated based on anticipated sea-level rise 

using projected water depths of 1.19m and 1.32m, respectively. To maintain current water 

clarity conditions with increase water depth, a target chlorophyll-a concentration of 3.3 μg/l is 

proposed for 2020. The proposed chlorophyll-a concentration for 2050 is 1.7 μg / liter. 

22 Sarasota Bay TMDL Support 

DRAFT August 2007 
Results 

y = 0.0403x + 0.5025

R2

 = 0.3239

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 10 20 30 40

Corrected Chlorophyll a (ug/l)

Light attenuation coefficient (K per m)

50

Figure 18. Correlation between Light attenuation and chlorophyll-a for Segment 

11 (August 2001-February 2006). 

Table 10. Segment 11 current and proposed water depth, K and target 

chlorophyll-a. 

Segment 11 

PBSJ 

Proposed 

Target

PBSJ 

Proposed 

2020

PBSJ 

Proposed 

2050

Depth (m at MLW) 1.14 1.19 1.32 

K (per m) 0.659 0.635 0.571 

Chl a (μg/l) 3.9 3.3 1.7 

23 Sarasota Bay TMDL Support 

DRAFT August 2007 
Results 

5.5

3.9

3.3

1.7

4.5

0.0

3.0

6.0

9.0

Average 2001-

2006

 Potential IWR

Target

PBSJ Proposed

Target

2020 target 2050 target

Chloropyll a (ug/l)

Figure 19. Current and Future target chlorophyll-a concentrations for Segment 11. 

Segment 13

Based on the data from August 2001 to February 2006, the average chlorophyll-a concentration 

and light attenuation coefficient were 7.7 μg/l and 0.9993 K per m, respectively (Table 6). 

Figure 20 shows the statistically significant relationship between chlorophyll-a and light 

attenuation. The linear model (Table 5) was used to produce target chlorophyll-a values for 

Segment 13. Based on the Surveyed Deep-Edge water depth and K, a chlorophyll-a target of 3.9 

μg/l was calculated to maintain current deep edge light requirements (Figure 21; Table 11). A 

TMDL target was developed based upon the chlorophyll a value which exceeds the historic value 

by 50% for WBID 1968D (Roberts Bay; 7.2 μg/l ). The 2020 and 2050 target chlorophyll-a 

concentrations were calculated based on anticipated sea-level rise and projected water depths of 

1.26m and 1.39m, respectively. To maintain current water clarity conditions with increased 

water depth, a target chlorophyll-a concentration of 5.5 μg/l is proposed for 2020. The proposed 

chlorophyll-a concentration for 2050 is 3.1 μg/l. 

24 Sarasota Bay TMDL Support 

DRAFT August 2007 
Results 

y = 0.0364x + 0.7263

R2

 = 0.4233

0.0

1.0

2.0

3.0

4.0

0 10 20 30 40 50 6

Corrected chlorophyll a (ug/l)

Light attenuation coefficient (K per 

0

m

Figure 20. Correlation between Light attenuation and chlorophyll-a for Segment 

13 (August 2001-February 2006). 

Table 11. Segment 13 current and proposed water depth, K and target 

chlorophyll-a. 

Segment 13 

PBSJ 

Proposed 

Target 

PBSJ 

Proposed 

2020 

PBSJ 

Proposed 

2050 

Depth (m at MLW) 1.21 1.26 1.39 

K (per m) 0.959 0.927 0.838 

Chl a (μg/l) 6.4 5.5 3.1 

25 Sarasota Bay TMDL Support 

DRAFT August 2007 
Results 

5.5

3.1

7.2 6.4

7.7

0.0

3.0

6.0

9.0

12.0

15.0

18.0

Average 2001-

2006

TMDL Target PBSJ Proposed

Target

2020 target 2050 target

Corrected Chloropyll a (ug/ Figure 21. Current and Fuure target chlorophyll-a concentrations for Segment 13. 

Segment 14

Based on the data from August 2001 to February 2006, the average chlorophyll-a concentration 

and light attenuation coefficient were 7.7 μg/l and 0.9105 K per m, respectively (Table 6). 

Figure 22 shows the statistically significant relationship between chlorophyll-a and light 

attenuation. The linear model (Table 5) was used to produce target chlorophyll-a values for 

Segment 14. Based on the Surveyed Deep-Edge water depth and K, a chlorophyll-a target of 7.7 

μg/l was calculated to maintain current deep edge light requirements (Figure 23; Table 12). A 

potential IWR target was calculated based upon the chlorophyll a value which exceeds the 

historic value by 50% for WBID 1968E (Little Sarasota Bay; 8.6 μg/l ). In comparison, the 

TMDL target derived by the Florida Department of Environmental Protection and adopted for 

the improvement of water quality in Segment 14 is 8.6 μg/l.The 2020 and 2050 target 

chlorophyll-a concentrations were calculated based on anticipated sea-level rise and projected 

water depths of 1.01m and 1.14m, respectively. To maintain current water clarity conditions 

with increased water depth, a target chlorophyll-a concentration of 6.6 μg/l is proposed for 2020. 

The proposed chlorophyll-a concentration for 2050 is 3.8 μg/l. 

26 Sarasota Bay TMDL Support 

DRAFT August 2007 
Results 

y = 0.0404x + 0.7078

R2

 = 0.4422

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 10 20 30 40 5

Corrected Chlorophyll a (ug/l)

Light Attenuation Coefficient (K per m)

0

Figure 22. Correlation between Light attenuation and chlorophyll-a for Segment 

14 (August 2001-February 2006). 

Table 12. Segment 14 current and proposed water depth, K and target 

chlorophyll-a. 

Segment 14 

PBSJ 

Proposed 

Target 

PBSJ 

Proposed 

2020 

PBSJ 

Proposed 

2050 

Depth (m at MLW) 0.97 1.01 1.14 

K (per m) 1.017 0.975 0.861 

Chl a (μg/l) 7.7 6.6 3.8 

27 Sarasota Bay TMDL Support 

DRAFT August 2007 
Results 

7.7

6.6

3.8

7.7

8.6

0.0

4.0

8.0

12.0

16.0

Average 2001-

2006

 Potential IWR

Target

PBSJ Proposed

Target

2020 target 2050 target

Corrected Chloropyll a (ug/l)

Figure 23. Current and Future target chlorophyll-a concentrations for Segment 14. 

Segment 16

Based on the data from August 2001 to February 2006, the average chlorophyll-a concentration 

and light attenuation coefficient were 5.6 μg/l and 0.9105 K per m, respectively (Table 6). 

Figure 24 shows the statistically significant relationship between chlorophyll-a and light 

attenuation for Segment 16. The linear model (Table 5) was used to produce target chlorophylla values for Segment 16. Based on the Surveyed Deep-Edge water depth and K, a chlorophyll-a 

target of 4.7 μg/l was calculated to maintain current deep edge light requirements (Figure 25; 

Table 13). An IWR target was calculated based upon the chlorophyll a value which exceeds the 

historic value by 50% for WBID 1968F (Blackburn Bay; 5.5μg/l ). The 2020 and 2050 target 

chlorophyll-a concentrations were calculated based on anticipated sea-level rise and projected 

water depths of 0.94 m and 1.08 m, respectively. To maintain current water clarity conditions 

with increased water depth, a target chlorophyll-a concentration of 4.0 μg/l is proposed for 2020. 

The proposed chlorophyll-a concentration for 2050 is 2.2 μg/l. 

28 Sarasota Bay TMDL Support 

DRAFT August 2007 
Results 

y = 0.0564x + 0.592

R2

 = 0.3536

0.0

1.0

2.0

3.0

4.0

5.0

0 10 20 30 4

Corrected Chlorophyll a (ug/l)

Light attenuation coefficient (K per m)

0

Figure 24. Correlation between Light attenuation and chlorophyll-a for Segment 

14 (August 2001-February 2006). 

Table 13. Segment 16 current and proposed water depth, K and target 

chlorophyll-a. 

Segment 16 

PBSJ 

Proposed 

Target 

PBSJ 

Proposed 

2020 

PBSJ 

Proposed 

2050 

Depth (m at MLW) 0.90 0.94 1.08 

K (per m) 0.858 0.820 0.718 

Chl a (μg/l) 4.7 4.0 2.2 

29 Sarasota Bay TMDL Support 

DRAFT August 2007 
Results 

4.7

4.0

2.2

5.5 5.6

0.0

3.0

6.0

9.0

12.0

Average 2001-

2006

IWR Target PBSJ Proposed

Target

2020 target 2050 target

Corrected Chloropyll a (ug/l)

Figure 25. Current and Future target chlorophyll-a concentrations for Segment 16. 

30 Sarasota Bay TMDL Support 

DRAFT August 2007 
31 Sarasota Bay TMDL Support 

DRAFT August 2007

4.0 Discussion 

The results of this project can be summarized as follows: 

 For much of the bay, correlations between chlorophyll-a and water clarity allow for the 

development of potential water clarity goals or “targets” 

 For much of the bay, Chlorophyll-a targets could be developed to protect seagrass resources 

as a “bio-indicator” of system health 

 However, Chlorophyll-a and water clarity does not appear to be correlated in either Palma 

Sola Bay or Anna Maria Sound 

 Detailed surveying of the “deep edge” of seagrass meadows results in a multi-year annual 

average light requirement of approximately 31 to 46 percent of sub-surface irradiance (at 

MLW) 

 Surveyed deep edges are typically 17 cm deeper than “deep edges” delimited by mapping 

techniques 

 Surveyed deep edges typically extend 30 to 40 meters farther offshore than edges delimited 

by mapping techniques 

 Based on a comparison between predicted depths from a 1950s-era bathymetric map and 

current depth measurements, it appears that the waters of Sarasota Bay are approximately 

17 cm deeper now (2007) than was the case in the 1950s 

 The rate of sea level rise (SLR) that appears to have been documented in Sarasota Bay is in 

line with estimates of the degree of SLR expected over a 50 year timeline 

 If future SLR occurs at a similar rate, water clarity would have to be improved, to allow the 

same light level to penetrate to the offshore edges of future seagrass meadows at their 

current locations 

 To maintain the same light penetration to future, deeper waters, chlorophyll-a levels would 

have to decrease by 51 to 56 percent by the year 2050, to allow seagrasses to maintain their 

present coverage (Table 14). 
Discussion 

32 Sarasota Bay TMDL Support 

DRAFT August 2007 

Table 14. PBSJ Proposed chlorophyll-a targets for all segments and percent 

reduction required to reach the proposed 2050 target. 

Segment 

PBSJ 

Proposed 

Target 

PBSJ 2020 

target 

PBSJ 

2050 

target 

% decrease 

(current to 

2050) 

7 5.3 4.6 2.6 52 

8 5.0 4.3 2.2 56 

10 1.9 1.2 N/A N/A 

11 3.9 3.3 1.7 56 

13 6.4 5.5 3.1 52 

14 7.7 6.6 3.8 51 

16 4.7 4.0 2.2 53 

It is not surprising that surveyed offshore edges of seagrass meadows, delimited via snorkeling, 

extend farther offshore and into deep waters than offshore edges delimited via aerial 

photography and photointerpretation. It would be difficult to believe that aerial imagery, 

regardless of the technique used, would be sufficient to pick up all the seagrass resources found 

in any particular area. However, the more relevant implication of this finding is that “edges” 

found by snorkeling would have deeper depths, and would have different light levels at those 

depths than locations depicted by a map as being a “deep edge.” For this reason, it is important 

that the type of “edge” is consistent with the literature, for developing minimum light 

requirement targets for seagrasses. For example, estimates of minimum light requirements for 

seagrasses determined in Tampa Bay (Dixon 1999), Sarasota Bay (Dixon and Kirkpatrick 1995), 

and Lemon Bay (Tomasko et al. 2001) range as high as 42 percent of immediately sub-surface 

irradiance. In all three of these locations, the “deep edge” used for estimating light requirements 

was defined by field surveying the site, and not by photointerpretation of aerial photography and 

mapping techniques. The choice of what “target” light level to use for maintaining seagrass 

coverage is an important one for setting water clarity targets, and thus for setting possible 

chlorophyll-a targets. 

An additional consideration for TMDL development in Sarasota Bay is whether or not there is 

evidence of system-wide improvements or degradation in water quality, based on ambient water 

quality monitoring programs or other objective assessments. That is, is “impairment” status 

derived from the rules and regulations outlined in the Impaired Waters Rule (IWR) consistent 

with more locally-collected data on trends in appropriate bio-indicators of system health? 

In the SBEP’s document “Framework for Action”, Lowrey (1992) concluded that of the 16 baywide segments (plus the nearshore waters of the Gulf of Mexico) in Sarasota Bay, roughly twothirds of those segments had trends in chlorophyll-a. Of those segments with trends, 93 percent 

of them had trends of decreasing values of chlorophyll-a, while only one segment had a trend of 

increasing values. Likewise, trends for TN were found in 43 percent of SBNEP segments, with 

twice as many having decreasing trends as increasing trends. For TP, 48 percent of bay 
Discussion 

33 Sarasota Bay TMDL Support 

DRAFT August 2007 

segments had significant trends, with only 10 percent of those segments showing signs of 

degradation over time (i.e., increasing values of TP). 

For water clarity (measured as Secchi disk depths) 29 percent of bay segments had trends over 

time – all of those trends were of improving water clarity (i.e., none showed evidence of 

degrading water clarity). 

In a later assessment of status and trends in water quality, Dixon and Heyl (1999) found that 

recent (1989 to 1998) trends of declining concentrations of total nitrogen and total phosphorus (9 

of 12 bay segments) were not accompanied by concurrent decreases in phytoplankton 

abundance, as no bay segments showed trends of decreasing chlorophyll-a concentrations during 

that same time period. The basis for this potential disconnect between declining nutrient 

concentrations and further decreases in phytoplankton abundance (between 1989 and 1998) 

merits further investigation, especially as it relates to the finding (discussed below) of continued 

increases in seagrass coverage in Sarasota Bay (i.e., Kurz et al. 1999, Tomasko et al. 2005). 

In the SBEP’s Framework for Action, the technical synthesis chapter (Tomasko et al. 1992) 

outlined the finding that seagrass biomass and productivity was a better indicator of segmentspecific nitrogen loads than either chlorophyll-a or Total Kjeldahl Nitrogen (TKN). A key 

finding of Tomasko et al. (1996) was that the status and trends of seagrass coverage in Sarasota 

Bay would be a better indicator of system “health” than traditional water quality monitoring 

programs. 

As seagrass coverage has been found to be increasing in Sarasota Bay in recent years (Kurz et al. 

1999, Tomasko et al., 2005), these results would indicate that findings of “impairment” for 

nutrients in various Sarasota Bay segments (e.g., Palma Sola Bay, Roberts Bay, Blackburn Bay) 

might be more a function of a potentially locally inadequate listing criteria via the IWR than a 

true indicator of a stressed ecosystem. 

However, results of the depth measurements, comparing 1950’s bathymetry estimates vs. field 

survey estimates from 2007 suggest that sea level rise could be an important factor to consider, 

when developing water quality goals for Sarasota Bay. The average increase in water depth of 

17 cm, comparing 1950’s depths to 2007 estimates, calculates out to a sea level rise of 

approximately 3.2 cm/decade. This difference in water depths is within the range of rates of 

measured sea level rise in St. Petersburg during 1950 to 1990 (ca. 2.4 cm/decade), and estimated 

rates of future sea level rise (ca. 5.2 cm/decade) as outlined in Clark (1992). Should these 

estimates of future sea level rise prove accurate, chlorophyll-a concentrations in Sarasota Bay 

would have to be reduced by 51 to 56 percent, to allow for the present-day light levels to 

penetrate to the same, but deeper, offshore edges of future seagrass meadows in the bay. 
34 Sarasota Bay TMDL Support 

DRAFT August 2007

5.0 References 

Clark P. 1992. Implications of a sea-level rise on the Sarasota Bay region. PP. 7.1-7.24. In: 

Sarasota Bay National Estuary Program. 1992 Framework for Action. 

Dixon, L.K. 1999. Establishing light requirements for the seagrass Thalassia testudinum: an 

example for Tampa Bay, Florida. Pp. 9-31, In: Bortone, S.A. (ed.). Seagrasses: Monitoring, 

Ecology, Physiology and Management. CRC Press, Boca Raton, Florida. 

Dixon, L. K. and M. Heyl (1999). Trend Analysis of Water Quality Data for the Sarasota Bay 

National Estuary Program. Sarasota, FL, Mote Marine Laboratory Technical Report No. 

645. 61 pp. plus Appendices. 

Dixon, L. K. and G. Kirkpatrick (1995). Light Attenuation with Respect to Seagrasses in 

Sarasota Bay, Florida. Sarasota, FL, Mote Marine Laboratory Technical Report No.407. 53 

pp. 

Haddad, K.D., 1989. Habitat trends and fisheries in Tampa and Sarasota Bays. In: Estevez, E. 

(Ed.), Tampa and Sarasota Bays: Issues, Resources, Status, and Management, NOAA 

Estuary-of the-month Seminar Series No. 11. National Oceanic and Atmospheric 

Administration, Washington, DC, pp. 113–128. 

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

indicators in Hillsborough Bay, Florida. In: Treat, S.F., Clark, P.A. (Eds.), Proceedings, 

Tampa Bay Area Scientific Information Symposium 2. Tampa Bay Regional Planning 

Council, St. Petersburg, Florida, pp. 157–176. 

Johansson, J., Ries, T., 1997. Seagrass in Tampa Bay: historic trends and future expectations. In: 

Treat, S. (Ed.), Proceedings, Tampa Bay Area Scientific Information Symposium 3. Tampa 

Bay Regional Planning Council, St. Petersburg, Florida, pp. 139–150. 

Johansson, J.O.R., Greening, H., 1999. Seagrass restoration in Tampa Bay: a resource-based 

approach to estuarine management. In: Bortone, S. (Ed.), Subtropical and Tropical Seagrass 

Management Ecology. CRC Press, Boca Raton, FL, pp. 279–293. 

Kurz, R. C., D. A. Tomasko, D. Burdick, T. F. Ries, K. Patterson, and R. Finck. 1999. Recent 

trends in seagrass distributions in southwest Florida coastal waters. In: Seagrasses: 

monitoring, ecology, physiology, and management. Edited by: S. A. Bortone. CRC Press 

LLC, Boca Raton, Florida. 

Lewis III, R.R., 1989. Biology and eutrophication of Tampa Bay. In: Estevez, E. (Ed.), Tampa 

and Sarasota Bays: Issues, Resources, Status, and Management, NOAA Estuary-of-themonth Seminar Series No. 11. National Oceanic and Atmospheric Administration, 

Washington, DC, pp. 89–112. 
References 

35 Sarasota Bay TMDL Support 

DRAFT August 2007 

Lewis, R.R., Durako, M.J., Moffler, M.D., Phillips, R.C., 1985.Seagrass meadows of Tampa 

Bay—a review. In: Treat, S.F., Simon, J.L., Lewis, R.R., Whitman, R.L. (Eds.), 

Proceedings, Tampa Bay Area Scientific Information Symposium. Florida Sea Grant 

Publication 65, Burgess, Minneapolis, pp. 210–246. 

Lewis, R.R., Clark, P.A., Fehring, W.K., Greening, H.S., Johansson, R.O., Paul, R.T. 1998. The 

rehabilitation of the Tampa Bay Estuary, Florida, USA, as an example of successful 

integrated coastal management. Marine Pollution Bulleting 37, 468-473. 

Lowrey, S. S., L. K. Dixon, et al. (1993). Water and Sediment Quality: Trends and Status for 

Sarasota Bay. Sarasota, Mote Marine Laboratory Technical Report No. 254. 108 pp. plus 

Appendices. 

SBEP 1992. Sarasota Bay Framework for Action. Sarasota Bay National Estuary Program, 

Sarasota, Fl. 

SBEP 1995. Sarasota Bay: The Voyage to Paradise Reclaimed, 1995. The Comprehensive 

Conservation and Management Plan for Sarasota Bay. Sarasota Bay National Estuary 

Program, Sarasota, Fl. 

Schefler, William C. 1979. Statistics for the Biological Sciences Second Edition, pp 25-A7. 

Tomasko, D., M. Alderson, P. Clark, J. Culter, K. Dixon, R. Edwards, E. Estevez, M. Heyl, S. 

Lowrey, Y. Sheng, and J. Stevely. 1992. Technical Synthesis of Sarasota Bay. Pp. 14.1-

14.16. In: Sarasota Bay National Estuary Program. 1992 Framework for Action. 

Tomasko, D.A., C.J. Dawes, and M.O. Hall. 1996. The effects of anthropogenic nutrient 

enrichment on turtle grass (Thalassia testudinum) in Sarasota Bay, Florida. Estuaries. 22: 

592-602. 

Tomasko, D.A. , D.L. Bristol, and J.A. Ott. 2001. Assessment of present and future nitrogen 

loads, water quality, and seagrass (Thalassia testudinum) depth distribution in Lemon Bay, 

Florida. Estuaries. 24: 926-938. 

Tomasko, D.A., Corbett, C.A., Greening, H.S., and G.E. Raulerson. 2005. Spatial and temporal 

variation in seagrass coverage in Southwest Florida: assessing the relative effects of 

anthropogenic nutrient load reductions and rainfall in four contiguous estuaries. Marine 

Pollution Bulletin. 50: 797-805. 

Virnstein, R. W., Avery, W. and Johansson, J.O.R. 2000. On Defining the “Edge” of a Seagrass 

Bed. Pp. 169-170. In Proceedings of a Symposium: Seagrass Management: It’s Not Just 

Nutrients! St. Petersburg, Florida.