Macroinfaunal communities of the Santa Maria Basin on the California outer continental shelf and slope
Marine Ecology Progress Series
We present results of a 2.5 yr survey of the rnacro~nfauna along the California coast north of Point Conception, between Purisima Point and Point San L u~s (USA), at outer-shelf and slope depths of 90 to 565 m. The study area, representing the southern offshore portion of the Santa Mdria Basin, IS a n oceanographically complex and productive region, which is also believed to contain major petroleum deposits. This area supports a highly diverse and abundant macroinfauna, represented mostly by
... sented mostly by crustaceans (34 %), polychaetes (31 "L), and molluscs (18 ' ' !L). Average numbers of species and densities (up to 151 0.lm-' and 28826 m-l, respectively. at water depths under 200 m) equal those reported for other productive regions, such a s Georges Bank and the North Sea. Spatial differences in the composition of these assemblages are related foremost to water depth in addition to variations in sedimentary and other depth-associated physical variables. Patterns of decreasing abundances and diversity with increasing depth appear to be due partly to a bottom dissolved-oxygen gradient (3.1 m1 1-' at 90 m to 0.6 m1 I-' at 565 m ) , which includes values at the low end that are below the oxygen tolerance of many b e n t h~c invertebrates. Variations in the percentage of sand explain further d~fferences that segregate some stations of comparable depth and oxygen Irvels. Macrofaunal variables also show significant temporal fluctuations, although distinct seasonal cycles are hard to detect and are not repeated throughout all sampling years and stations. The occurrence of density peaks in the sprlng durlng one or more sampling years at several of the stations, however, suggests a benthic response to upwelllng events. which are known to contribute to increases In new primary production and ultimately to increased energy supplies to the benthos. Results of thls study provide a basis for beg~nning to understand natural sources of variation in the benthos of the region, which should be considered in efforts to assess potential impacts of future oil development.