Infaunal benthos of a natural petroleum seep: Study of community structure

At 8 wk intervals, from December 1975 through March 1978, we took a census of the infaunal benthic invertebrates at a natural petroleum seep near Santa Barbara, California, USA and at an area nearby where no fresh petroleum occurred in the sediments. Both sites had in common 72% of the populations representing over 90% of the individuals, strongly suggesting that the two sites are part of the same community. At the petroleum seep there were higher densities of individuals in many populations (60%), but no dramatic difference in diversity (Shannon-Wiener or dominance-diversity). The seep populations tended to be more variable from one sampling period to the next, possibly due, at least in part, to large larval settlements there. The most abundant populations at the seep site were dominated by deposit feeders (14 of the 15 species considered), especially oligochaetes which are extremely rare at the comparison site. The discussion includes a hypothesis of trophic enrichment of the seep populations by bacterial growth stimulated by fresh petroleum.