Regional patterns in shallow water invertebrate assemblages on offshore oil and gas platforms along the Pacific continental shelf

Abstract

We explored biogeographical and local patterns in the composition of shallow water (≤18 m depth) invertebrate assemblages inhabiting California offshore oil and gas platforms using multivariate analysis of diver- conducted photographic data collected from 23 platforms in 2013–2014. We evaluated the potential importance of sea surface temperature (SST) and other physical and biological factors in driving observed patterns in these assemblages. Prior to this analysis, platforms were grouped into four regions based on local differences in annual mean SST. The composition of invertebrate assemblages varied significantly among the four regions, reflecting differences in the relative abundances of certain anemone, bryozoan, sponge, and bivalve taxa. However, invertebrate assemblages varied idiosyncratically among platforms within a region. Variation in platform assemblages was associated with SST across regions; however, assemblages of platforms in the southeast Santa Barbara Channel were distinct due to the high cover of a non-native bryozoan, Watersipora subatra (Ortmann, 1890). The existence of geographical patterns in the composition of platform invertebrate assemblages and the colonization of one platform by a native bryozoan with southern affinities during elevated SST of 2014–2015 suggest that these assemblages may be useful over broad spatial scales as barometers of short- and longer-term changes in ocean climate. However, over smaller spatial scales, the idiosyncratic differences in invertebrate assemblages among platforms within regions indicates that these assemblages would have to be considered on a platform-by-platform basis under various decommissioning scenarios.