Benthic Community Composition Results 



A detailed description of the benthos data is available in the contractor's report to 

 NO A A (Barnett et al., 1987). A summary of selected benthos parameters is listed in Table 

 20. No data are thus far available for the Oakland site. Among the four sites for which 

 data exist, total abundance was highest at SP and lowest at YB. Species richness was high 

 at TB and lower at all the San Francisco Bay sites: YB, SP, and VA. Dominance was 

 highest at SP, reflecting the abundance of the crustacean Ampelisca abdita. Dominance was 

 lowest at station 4 where the abundance of this amphipod was relatively low. Crustaceans 

 dominated the community in abundance at YB and SP, whereas molluscs were dominant at 

 VA and polychaetes were dominant at TB. Mean total biomass was distinctly high at VA 

 and TB, where molluscs and polychaetes were dominants, respectively, and was low at YB 

 and SP where crustaceans predominated. 



The amphipod A. abdita was the single most abundant organism found at the four sites, 

 occurring in a mean concentration of 3,061 individuals per O.lm^ at SP. At the SP site, the 

 dense population of tubes of A. abdita probably influenced the nature of the substrate. This 

 species was also the dominant at YB, but in concentrations roughly an order of magnitude 

 lower than those at SP. The tubes of the abundant polychaete Asychis elongatus likely 

 influenced the character of the sediments at YB. Among the four sites, variability in species 

 composition was highest at YB. Molluscs, especially Mya arenaria, were dominant in both 

 abundance and biomass at VA, where a significant amount of shell hash was encountered in 

 the sediments. Many polychaetes, especially Exogone lourei and the mud-dwelling mussel 

 Musculista senhousia were among the most abundant biota at TB where crustaceans were 

 relatively rare. The byssal threads of M. senhousia may have provided a haven for some 

 infaunal species at TB. 



The dendrogram in Figure 4 produced by Barnett et al., (1987) summarizes the results of a 

 community classification test based upon analyses of community similarity among stations 

 and sites. The community similarity analyses performed on the data for each station 

 produced both normal (by station) and inverse (by species) dendrograms, which were 

 arranged on a single plot on a two-way coincidence diagram. The normal (vertical) 

 dendrogram contains clusters of stations based upon similarity of faunal composition. The 

 (horizontal) inverse dendrogram contains clusters of species that had similar distribution 

 and abundance patterns among stations. The relative abundance of each species is identified 

 by four symbols. 



The normal classification analysis identified four major cluster groups of stations, 

 labelled as "site groups" 1, 2, 3, and 4. Site TB (site group 4) was determined to be the most 

 dissimilar among the sites. Next, the dendrogram separated site VA from sites SP and YB. 

 The latter two sites were most similar. Each site group consisted of stations from only one 

 site indicating that faunal similarity was very high at stations within a site compared to 

 similarity among sites. 



The inverse classification analysis identified five species groups with similar 

 distribution and abundance patterns among stations. The fauna at site TB was composed 

 almost exclusively of group A species, mainly polychaetes either unique to that site or 

 rarely found at the other sites. Site TB also had some species from groups B and D. The 

 fauna at YB included mainly species from species groups C and D along with species from 

 groups B and E. Species group D was dominant at site SP and group E species were most 

 abundant at site VA. The inverse analysis confirmed the pattern observed with the normal 

 analysis: sites TB and VA were the most dissimilar in faunal composition. 



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