Status Review

and Key Issues for the 

Nutrient Criteria to Protect 

Florida’s Lakes

Ken Weaver

Standards and Assessment Section

Florida Department of Environmental 

Protection

June 3, 2009

1
Lakes Nutrient Criteria 

Development Overview

 DEP has demonstrated significant and strong 

nutrient relationships with chlorophyll a

 DEP proposes using chlorophyll a relationships 

as primary line of evidence

 LVI response provides additional support for 

criteria development in clear lakes

2
Chlorophyll a Thresholds 

 To decide on appropriate chlorophyll a thresholds, first 

looked at Trophic State Index 

 Carlson (Kent State University) created an interpretation 

scheme for the TSI based on nutrient/chlorophyll a

responses in northern lakes

 TSI 30 – 40: Exhibit classical oligotrophy, but some shallower 

lakes will become anoxic in the hypolimnion during the summer

 TSI 40 – 50: Mesotrophic conditions, Water moderately clear, but 

increasing probability of anoxia in hypolimnion during summer. 

 TSI 50 – 60: Lower boundary of classical eutrophy

 Florida lakes do not have certain attributes of northern 

lakes, including the presence of cold-water or cool-water 

fisheries and fully oxygenated hypolimnetic (bottom) 

areas

3
Chlorophyll a Thresholds 

(continued)

 Salas and Martino (1991) proposed an alternate TSI 

categorization based on their work in phosphorus limited 

warm-water tropical lakes

 TSI in mesotropic warm-water lakes range from 50 to 60

 TSI of 60 is equivalent to a chlorophyll a of 20 μg/L

 Eutrophy occurs at a TSI ≥70

 Oligotrophy occurs at TSI ≤ 40

4

TSI Category TP (ug/L) Chlorophyll a

40 Oligotrophic 21.3 5

50 Mesotrophic 39.6 10

70 Eutrophic 118.7 40
Chlorophyll a Thresholds 

(continued)

 IWR TAC recommended TSI threshold of 60 for 

most Florida Lakes

 However, TAC realized that this may not be 

protective of oligotrophic lakes

 PCA analysis classified TSI and related 

parameters by color

 Lakes with Color ≤ 40 PCU had significantly lower 

TSI (oligotrophic lakes)

 DEP recommended and TAC agreed to a TSI 

threshold of 40 for clear lakes (≤ 40 PCU )

 These TSIs equate to chlorophyll a of 20 and 5 

μg/L for colored and clear lakes, respectively 5
Lines of Evidence Supporting 

Protective Chl-a Thresholds

6

Line of Evidence Chlorophyll a target State

Paleolimnological studies 14 to 20 μg/L and higher for some lakes

(Average condition) Florida

Expert opinion 20-33 μg/L

(Annual or summertime averages)

Virginia, Iowa, West 

Virginia, Maryland

Fisheries responses 

(warmwater)

35-60 μg/L

(Summer average)

Virginia

Lake user perceptions 20-25, up to 30 μg/L in colored lakes

(Instantaneous) Texas and Florida

Existing levels approach 5-27 μg/L

(Average condition) Alabama

Reference lake approach 9-18 μg/L in colored lakes

(Average condition) Florida, using 75th percentile
Relationships between Chlorophyll a

and Cyanobacteria

 Cyanobacteria blooms can be toxic and present a 

health risk to people recreating in and on the 

water

 World Health Organization

 A chlorophyll a level of 10 μg/L in which cyanobacteria 

are dominant presents a relatively low probability of 

mild effects

 50 μg chl-a/L in which cyanobacteria are dominant 

presents a moderate risk of adverse health effects

 Cyanobacteria are usually an unfavorable food 

source to zooplankton and many other aquatic 

animals 7
Relationship between chlorophyll a

percent Cyanobacteria 

8

 

April 2000 July 2006

5 50 500

-20

0

20

40

60

80

100

120

 Dominated by HAB 

 Not Dominated by HAB

Chlorophyll a (μg/L)

Cyanobacteria Percent

• 1,364 phytoplankton samples from small and large lakes sampled between 2000 and 

2006 in Florida’s probabilistic sampling network

• A small sub-set of individual lakes with repeat visits did show relationships
Chlorophyll a Thresholds 

 Based on multiple lines of evidence, an annual 

average chlorophyll a of 20 μg/L in colored lakes 

is protective of designated uses

 5-10 μg/L in clear lakes

 “Maintain existing condition approach”

 IWR TSI of 40 primarily targeted at a specific 

geographic region of Florida (the panhandle)

 Coldwater fishery: 3-5 μg/L

 TetraTech reference lake: 8 μg/L

 Salas and Martino TSI 50 (mesotrophic): 10 μg/L

 Seeking TAC feedback

9
Response Seasonality

Clear Lakes Colored Lakes

10

 

Jan.-April, Oct.-Dec. May-Sept.

Chlorophyll 

a (μg/L)

0.01

0.1

1

10

100

1000

Jan.-April, Oct.-Dec. May-Sept.

Chlorophyll 

a (μg/L)

0.1

1

10

100

1000

10000

• Similar chlorophyll a levels in summer as in spring, fall, 

and winter

• Chlorophyll a nutrient responses were not significantly 

different by season

• Recommend that the chlorophyll a response be 

evaluated as an annual average
Do we Need both N and P?

 Great deal of recent scientific debate 

(Schindler et al., 2008) 

 Emphasis on controlling eutrophication in 

freshwater lakes has been focused heavily on 

decreasing inputs of phosphorus

 Many studies in lakes and estuaries still 

conclude that nitrogen must be controlled as 

well as, or instead of, phosphorus to reduce 

eutrophication (Schindler et al., 2008; 

Howarth & Marino, 2006). 

11
Colored Lake Chl-a Response 

to Total Phosphorus

12

Annual Geometric Mean TP (mg/L)

0.01 0.1 1

Annual Geometric Mean Chl-a (μg/L)

1

10

100

Ln (y) = 1.128 Ln(x) + 5.729

R2 = 0.581

Regression Line

50% Prediction Interval

Chl-a typically < 20 Chl-a typically > 20

Range of Uncertainty

0.05 0.157
Colored Lake Chl-a Response 

to Total Nitrogen

13

0.05 0.157

Annual Geometric Mean TN (mg/L)

0.1 1 10

Annual Geometric Mean Chl-a (μg/L)

0.1

1

10

100

1000

Ln (y) = 1.965 Ln(x) + 1.995

R2 = 0.645

Regression Line

50% Prediction Interval

1.23 2.25
Colored Lake Chl-a Response to 

Nutrients

(based on lakes with color >40-140 PCU)

Total Phosphorus

Annual Geometric Mean TN (mg/L)

0.1 1 10

Annual Geometric Mean Chl-a (μg/L)

0.1

1

10

100

1000

Ln (y) = 1.965 Ln(x) + 1.995

R2 = 0.645

Regression Line

50% Prediction Interval

Total Nitrogen

Annual Geometric Mean TP (mg/L)

0.01 0.1 1

Annual Geometric Mean Chl-a (μg/L)

1

10

100

Ln (y) = 1.128 Ln(x) + 5.729

R2 = 0.581

Regression Line

50% Prediction Interval

14
Florida Colored Lake LVI Response to 

Nutrients 

LVI Paired to 1-year Annual Average 

N and P (n=52)

Phosphorus Response Nitrogen Response

15

One Year Geometric Mean TP (mg/L)

0.01 0.1 1

LVI

0

10

20

30

40

50

60

70

80

90

100

y = -7.819 Ln(x) + 35.02

R2 = 0.198

p=0.001

One Year Geometric Mean TN (mg/L)

0.1 1 10

LVI

10

20

30

40

50

60

70

80

90

100

y = -11.5 Ln(x) + 60.02

R2 = 0.124

p= 0.011
Draft Colored Lake TP and TN 

Criteria

Stressor Variables Response Variable

 Chlorophyll a ≤ 20 μg/L 

as annual geometric 

mean

 Potential acute threshold: 

>40 μg/L in less than 

10% of samples on 

annual basis

 Additional IWR threshold

 62-302?

16

50% Prediction Interval

Stressor Lower Threshold Upper Threshold

TP (mg/L) 0.05 0.157

TN (mg/L) 1.23 2.25
Clear Lake Chlorophyll a Response to 

Total Phosphorus

(n=195 lakes, 504 Lake-years)

Annual Geometric Mean TP (mg/L)

0.001 0.01 0.1 1

Annual Geometric Mean Chl-a (μg/L)

0.1

1

10

100

1000

Ln (y) = 1.136 Ln(x) + 6.370

R2 = 0.677

Regression Line

50% Prediction Interval

17
Clear Lake Chlorophyll a Response to 

Total Nitrogen

(n=195 lakes, 509 Lake-years)

Annual Geometric Mean TN (mg/L)

0.1 1 10

Annual Geometric MeanChl-a (μg/L)

0.1

1

10

100

1000

Ln (y) = 1.729 Ln (x) + 2.482

R2 = 0.769

Regression Line

50% Prediction Interval

18
Florida Clear Lake LVI Response to 

Nutrients 

LVI Paired to 1-year Annual Average 

N and P (n=91)

Annual Geometric Mean TP (mg/L)

0.001 0.01 0.1 1

LVI

0

10

20

30

40

50

60

70

80

90

100

y = -12.6 Ln(x) + 0.3

R2 = 0.441

Annual Geometric Mean TN (mg/L)

0.1 1 10

LVI

10

20

30

40

50

60

70

80

90

100

y = -15.74 Ln(x) + 43.16

R2

 = 0.287

Phosphorus Response Nitrogen Response

19
20

Alternative Analysis:

Algal Bloom Frequency
Conditional Probability Analysis

Probability that a TP Concentration will result in a 

10% Exceedance of 40 μg chl-a/L

21

Annual Geometric Mean Total Phosphorus (mg/L)

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Probability of >40 μg/ Chl-a in 

at least 10% of samples

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Fitted Logisitc Curve

Probability

95% C.I.

0.061

0.107
Conditional Probability Analysis

Probability that a TN Concentration will result in a 

10% Exceedance of 40 μg chl-a/L

22

 

 

Annual Geomteric Mean TN (mg/L)

0 2 4 6 8 10 12

Probability of >40 μg/ Chl-a in

at least 10% of Samples

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Fitted Logistic Curve

Probability

95% C.I.

1.23

2.09
Lines of Evidence Comparison

(Colored Lakes)

23

Annual Geometric Mean Total Phosphorus (mg/L)

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Probability of >40 μg/ Chl-a in 

at least 10% of samples

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Fitted Logisitc Curve

Probability

95% C.I.

0.061

0.107

 

 

Annual Geomteric Mean TN (mg/L)

0 2 4 6 8 10 12

Probability of >40 μg/ Chl-a in

at least 10% of Samples

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Fitted Logistic Curve

Probability

95% C.I.

1.23

2.09

50% Prediction Interval

Stressor Lower Threshold Upper Threshold

TP (mg/L) 0.05 0.157

TN (mg/L) 1.23 2.25

Bloom Frequency Conditional Probability

Annual Average Condition Regression