TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

Appendix A-5: FDEP’s Nutrient-LVI Stressor-Response Analyses 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

A5-1 

Appendix A-5. FDEP’s Nutrient-LVI Stressor-Response Analyses 

Macrophyte Analyses 

The relationship between lake macrophytes and nutrients was examined using Florida’s Lake 

Vegetation Index (LVI). Initial analysis evaluated the response of the LVI to nutrients using 

instantaneous nutrient measurements collected the same day as the LVI. These initial analyses 

showed weak, yet statistically significant relationships in clear lakes (Table A5-1), but only the 

relationship between LVI and total phosphorus (TP) was significant in colored lakes. The weak 

initial relationships are not surprising because macrophytes integrate lake nutrient conditions 

over time and primarily obtain nutrients through uptake from sediments rather than the water 

column. Therefore, FDEP evaluated the LVI response to long-term nutrient conditions, 

specifically average conditions one year prior to the LVI sampling event. 

Lake Vegetation Index samples from 91 clear and 53 colored lakes were paired with nutrient 

data from STORET, FDEP’s Ambient Program’s database (GWIS), and Florida’s biological 

database (SBIO). Nutrient data collected during the one year period prior to LVI sample 

collection were averaged using a geometric mean concentration. Only lakes with a minimum of 

three water chemistry samples during the period were analyzed further. Statistically significant 

relationships were found between LVI and 1-year geometric mean TP and total nitrogen (TN) 

concentrations in both colored and clear lakes (Figures A5-1 and A5-2). The analyses indicate 

that lake vegetation exhibits a significant adverse response to nutrients. However, the adjusted R2

values were still low, primarily because the macrophyte community also responds to other 

environmental factors such as sediment conditions, physical disturbance, introduction of exotic 

taxa, and the presence or absence of herbivores. Since the biology responds to the combined 

effect of all these factors along with nutrients, the natural variation in these other environmental 

factors blur the observed biological response to nutrients. With the exception of the clear lakes 

TP and LVI relationship, the LVI response to nutrients was insufficiently robust to be used as the 

basis to establish numeric nutrient criteria. The TP relationship could potentially be used as a line 

of evidence to support numeric nutrient criteria to protect populations of flora in Florida’s clear 

lakes. 

Table A5-1. Spearman Rank Correlation Coefficients between Instantaneous 

Nutrient Concentrations and Florida’s Lake Vegetation Index 

All Lakes Clear Lakes Colored Lakes 

TP -0.396 -0.397 -0.364 

TN -0.324 -0.314 -0.211 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

A5-2 

One Year Geometric Mean TP (mg/L)

0.001 0.01 0.1 1

LVI

0

10

20

30

40

50

60

70

80

90

100

One Year Geometric Mean TN (mg/L)

0.1 1 10

LVI

10

20

30

40

50

60

70

80

90

100

y = -12.6 Ln(x) + 0.3

R2 = 0.441

p<0.0001

y = -15.74 Ln(x) + 43.16

R2

 = 0.287

p<0.001

A)

B)

Figure A5-1. Relationships between LVI and geometric mean (A) TP and (B) TN in clear (color 

≤ 40 platinum-cobalt units (PCU)) Florida lakes. The nutrient concentrations were calculated as 

the geometric mean of samples collected within the lakes during the 365 day period prior to LVI 

collection. Solid black line is the least squares regression and the blue dashed lines are the 95% 

confidence interval of the regression. Note that x-axis is expressed on a log-scale. 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

A5-3 

One Year Geometric Mean TP (mg/L)

0.01 0.1 1

LVI

0

10

20

30

40

50

60

70

80

90

100

One Year Geometric Mean TN (mg/L)

0.1 1 10

LVI

10

20

30

40

50

60

70

80

90

100

y = -7.819 Ln(x) + 35.02

R2

 = 0.198

p=0.001

y = -11.5 Ln(x) + 60.02

R2

 = 0.124

p= 0.011

A)

B)

Figure A5-2. Relationships between LVI and geometric mean (A) TP and (B) TN in colored 

(color > 40 PCU) Florida lakes. The nutrient concentrations were calculated as the geometric 

mean of samples collected within the lakes during the 365 day period prior to LVI collection. 

Solid black line is the least squares regression and the blue dashed lines are the 95% confidence 

interval of the regression. Note that x-axis is expressed on a log-scale.