their relative abundance without being heavily biased by the large 

 numbers of serpulids which inhabited many of the stations. The 

 Margalef diversity index can be designated as an index of dominance 

 diversity (Whittaker, 1965) since it indicates the numerical percentage 

 composition of the species present in the sample (Sanders 1968). 

 The more species are represented by equal numbers of individuals 

 the more diverse the fauna. When the numbers of individuals in the 

 various species differ greatly (i.e., when some species greatly 

 dominate the sample) the sample is less diverse. 



Dominance diversity, therefore, is a measure of how equally or 

 unequally the species divide the sample. The formula for Margalef 's 

 diversity index (Margalef 1957) is: 



J = Z P.lnP. 

 ^ r 



Where I is the dominance diversity index, P. is the number of organisms 

 in species i divided by the total number of organisms in the sample, 

 and In P . is the natural logarithm of P.. 



The structure of the animal populations in the quadrats were compared 

 with each other and ranked by similarity using the Bray and Curtis 

 (1957) similarity coefficient modified of Pearson et at (1967). 



The prominence value (PF) of Pearson et at (1967) was changed slightly 

 using the formula PV = AF rather than PV = A(P)'^I 1 where A is the 

 average number of individuals of a particular species and F is the 

 frequency of occurrence. Thus, if an organism had an average 

 abundance of 500 specimen? per square meter but was present only 

 twenty percent of the time, its prominence value would be 100. Since 

 this figure represents the average abundance of the organism with its 

 absence during any given month included in the average as 0, it was 

 felt more realistic and meaningful than the arbitrary use of (F) '2. 



Once prominence values were calculated for each species present at 

 the station, these were summed. The stations were compared to each 

 other by determining the minimum percentage of individuals of each 

 species shared in common using the formula given by Pearson et at 

 (1967): 



S = 2W/(a + b) 



Where S is the similarity index, a and b are the sums of the promi- 

 nence values of the two stations being compared, and W is the sum of 

 the smallest prominence values for each species shared in common. 

 This index varies from when no two species are shared in common 

 to 1 where both stations have identical population structures. 



Quarterly samples of sediments were collected from all stations for 

 analysis of foraminifera (shelled, microscopic protozoans). Divers 



31 



