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

B3-1 

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

Stream Diatom Index Development In a process similar to that de scribed for the Stream Condition Index (SCI) and Lake Vegetation Index (LVI), FDEP has been developing a periphy ton assessment tool, the Stream Diatom Index (SDI) using a combination of the human distur bance gradient (HDG) and biological condition gradient (BCG) approaches. Unfortunately, the diat oms appear to be very strongly influenced by pH (as well as conductivity and color), whic h confounds the relationship between periphyton community response to human disturbance, includi ng nutrient enrichment effects. Figures B3-1 and B3-2 describe the relationship between th e Landscape Development Intensity Index (LDI) and pH. Note that in the minimally disturbed co ndition (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 wh en 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 Scal ing (NMS) analysis and the relationship between LDI and pH (Figure B3-1), Florida stre ams 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 hum an effects on stream systems. This fact is extremely significant when considering the resu lts from the BCG Workshop described below, and suggests that the expert group may actually be assessing the diatom re sponse to factors other than human disturbance, potentially pH/conductivity, likely related to thei r 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 correl ation 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 eval uated 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 follo wing 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 natura l 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 samp les 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

B3-2 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 SD I 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

B3-3 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 th e 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

B3-4 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.