Table 3-1. Methods used to calculate components of the benthic index from raw benthic data. 

 Component Calculation Method 



Proportion of Expected Shannon-Wiener Diversity Index: H' = -£ p, log^ p, 



Shannon-Wiener Diversity Index where p. = proportion of total abundance represented by species /. 



Proportion of Expected Shannon-Wiener Diversity Index = 

 H's[ 2.618426 - 0.044795X + 0.007278X2 - 0.0001 IQX^ ] 

 where X = bottom salinity (ppt) 



Mean Abundance of Tubificids Calculate sum of total abundance of members of Family:Tubificidae and 



divide by number of grabs (3) 

 Percent Capitellids Calculate sum of abundance of members of Family:Capitellidae in each grab 



and divide by the total abundance of all fauna in that grab. Calculate average 



proportion in 3 grabs and multiply by 100. 

 Percent Bivalves Calculate sum of abundance of members of Class:Bivalvia in each grab and 



divide by the total abundance of all fauna in that grab. Calculate average 



proportion in 3 grabs and multiply by 100. 

 Percent Amphipods Calculate sum of abundance of members of Order:Amphipoda in each grab 



and divide by the total abundance of all fauna in that grab. Calculate average 



proportion in 3 grabs and multiply by 100. 



Table 3-2. Methods used to calculate the benthic index from the components listed in Table 1 . 



• Step 1 : Log transform abundances and arc-sine transform proportions. 



• Step 2: Standardize variables to mean = and standard deviation = 1 by applying the 



following formula to all of the data: 



X ' = S * (x - 0) + M 



where 



X.' = new standardized value 



S = desired standard deviation (1) 



M = desired mean (0) 



x^ = observation's original value 



= variable's mean 



s = variable's standard deviation 



X 



step 3: Calculate the discriminant score of the benthic index as follows using the 

 standardized variables from Step 2. 



Discriminant Score = ( 1 .5710 x Proportion of expected diversity) + 

 (-1 .0335 x Mean Abundance of tubificids) + 

 (-0.5607 X Percent capitellids) + 

 (-0.4470 X Percent bivalves) + 

 ( 0.5023 X Percent amphipods). 



Step 4: Calculate the benthic index by normalizing the discriminant scores from Step 3 

 using the following formula which includes the minimum (-3.21) and range (7.50) 

 of discriminant scores from the original test data used to develop the benthic index: 

 Benthic Index = ((Discriminant Score - (-3.21))/7.50) * 10 



3-9 



