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

Appendix B-3: FDEP’s Stream Diatom Index (SDI) 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

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Appendix B-3. FDEP’s Stream Diatom Index (SDI) 

Stream Diatom Index Development 

In a process similar to that described for the Stream Condition Index (SCI) and Lake Vegetation 

Index (LVI), FDEP has been developing a periphyton assessment tool, the Stream Diatom Index 

(SDI) using a combination of the human disturbance gradient (HDG) and biological condition 

gradient (BCG) approaches. Unfortunately, the diatoms appear to be very strongly influenced by 

pH (as well as conductivity and color), which confounds the relationship between periphyton 

community response to human disturbance, including nutrient enrichment effects. Figures B3-1 

and B3-2 describe the relationship between the Landscape Development Intensity Index (LDI) 

and pH. Note that in the minimally disturbed condition (LDI <2), there is a wide range of pH, 

from about 4 to 8 standard units (SU) (Figure B3-2). However, as the systems experience more 

human disturbance, the pH tends to be above 6.5 SU, so that when the LDI value is higher than 

4, it is unlikely that a site will have a pH below 6.5 SU. 

Based upon a Non-metric Multidimensional Scaling (NMS) analysis and the relationship 

between LDI and pH (Figure B3-1), Florida streams were divided into two pH categories, < 6.5 

SU and > 6.5 SU, for development of the SDI. The SDI does not appear to clearly or predictably 

respond to objective measures of human disturbance (Figures B3-3 and B3-4), currently making 

it an unreliable tool for assessing adverse human effects on stream systems. This fact is 

extremely significant when considering the results from the BCG Workshop described below, 

and suggests that the expert group may actually be assessing the diatom response to factors other 

than human disturbance, potentially pH/conductivity, likely related to their lack of experience 

with Florida’s unique and variable background water quality conditions. 

Stream Diatom Index Calibration 

The BCG calibration process was also used for the SDI, with 15 nationally recognized 

periphyton experts involved in calibration workshop. The correlation between the expert’s 

average BCG ranking with low pH SDI scores is shown in Figure B3-5, and the correlation 

between the expert’s average BCG ranking with high pH SDI scores is shown in Figure B3-6. 

Site information and taxa lists for samples evaluated in the SDI BCG workshop are available in 

the United States Environmental Protection Agency’s (U.S. EPA) Proposed Methods and 

Approaches for Developing Numeric Nutrient Criteria for Florida’s Inland Waters (USEPA 

2009, Appendix 3K). 

The periphyton expert group was asked the following question: “In your opinion as an aquatic 

scientist, where specifically along the gradient would you see a point where critical changes lead 

to a loss of a balanced natural population of flora and fauna?” 

The average initial response to this question was Category 3.5. After discussion, the final 

average BCG score for this question was 3.1 (Table B3-1). However, establishing an impairment 

threshold near category 3 would result in 73% (51 out of 70) of FDEP’s previously described 

minimally disturbed nutrient benchmark site samples to be deemed impaired (an unacceptable 

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

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While the correlations between the BCG ranking and the SDI scores are statistically significant, 

the fact that there is no significant relationship between the BCG and human disturbance indicate 

the SDI should not be used as a bioassessment tool. The periphyton community appears to 

respond more strongly to pH and conductivity than to independent measures of human 

disturbance. 

Table B3-1. Nationally Recognized Periphyton Expert Responses 

BCG Category Initial Votes Final Votes 

Between 0/1 0 0 

Between 1/2 0 0 

Between 2/3 6 7 

Between 3/4 3 7 

Between 4/5 6 1 

Between 5/6 0 0 

Note: responses are to the question responses to the question, “In your opinion as 

an aquatic scientist, where specifically along the gradient would you see a point 

where critical changes lead to a loss of a balanced natural population of flora and 

fauna?” 

Figure B3-1. LDI and pH are strongly related for SDI development sites. The blue line indicated 

pH of 6.5, at which sites were divided for SDI development. 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

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Figure B3-2. There are strong relationships between the SDI and pH throughout various 

categories of human disturbance. 

Figure B3-3. Relationship between the SDI and the HDG in low pH sites (< 6.5 SU). Note lack 

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

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Figure B3-4. Relationship between the SDI and the HDG in high pH sites (> 6.5 SU). Note lack 

of strong association. 

Figure B3-5. Relationship between the SDI and the BCG in low pH sites (pH < 6.5 SU) as 

determined by an expert panel. 

Figure B3-6. Relationship between the SDI and the BCG in high pH sites (pH > 6.5 SU) as 

determined by an expert panel.