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



## Appendix A-5: FDEP’s Nutrient-L VI Stressor-Response AnalysesTSD 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 nut rients was examined using Florida’s Lake Vegetation Index (LVI). Initial an alysis 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 relations hips in clear lakes (Table A5-1), but only the relationship between LVI and tota l phosphorus (TP) was significan t in colored lakes. The weak initial relationships are not surprising because macrophytes integrate lake nutrient conditions over time and primarily obtain nu trients through uptake from sedi ments rather than the water column. Therefore, FDEP evaluated the LV I response to long-term nutrient conditions, specifically average conditions one year prior to the LVI sampling event. Lake Vegetation Index samples from 91 clear an d 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 dur ing 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 a nd clear lakes (Figures A5-1 and A5-2). The analyses indicate that lake vegetation exhibits a significant advers e response to nutrients. However, the adjusted R 2 values were still low, primarily because th e macrophyte community also responds to other environmental factors such as sediment conditio ns, physical disturbance, introduction of exotic taxa, and the presence or abse nce of herbivores. Since the bi ology responds to the combined effect of all these factors along with nutrients, the natu ral variation in thes e 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 insuffici ently 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 nutri ent criteria to protec t populations of flora in Florida’s clear lakes.

Table A5-1. Spearman Rank Correlation Co efficients between Instantaneous Nutrient Concentrations and Fl orida’s Lake Vegetation Index All Lakes Clear Lakes Colored Lakes TP -0.396 -0.397 -0.364 TN -0.324 -0.314 -0.211TSD 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 1LVI

0 10 20 30 40 50 60 70 80 90 100

One Year Geometric Mean TN (mg/L) 0.1 1 10LVI

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 R 2 = 0.287 p <0.001 A)

B)

Figure A5-1. Relationships between LVI and geomet ric mean (A) TP and (B) TN in clear (color ≤ 40 platinum-cobalt units (PCU)) Fl orida lakes. The nutrient concen trations were calculated as the geometric mean of samples co llected within the la kes during the 365 day period prior to LVI collection. Solid black line is the least squares re gression 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 1LVI

0 10 20 30 40 50 60 70 80 90 100

One Year Geometric Mean TN (mg/L) 0.1 1 10LVI

10 20 30 40 50 60 70 80 90 100 y = -7.819 Ln(x) + 35.02 R 2 = 0.198 p =0.001

y = -11.5 Ln(x) + 60.02 R 2 = 0.124 p = 0.011 A)

B)

Figure A5-2. Relationships between LVI and geom etric mean (A) TP and (B) TN in colored (color > 40 PCU) Florida lakes. The nutrient concentrations we re calculated as the geometric mean of samples collected with in the lakes during the 365 day period prior to LVI collection. Solid black line is the least s quares regression and the blue dash ed lines are the 95% confidence interval of the regression. Note that x-axis is expressed on a log-scale.