Survey of invertebrate and algal communities on offshore oil and gas platforms in southern California: Final report

Abstract

From the Summary: “In summary, the major findings pertinent to invertebrate and algal communities inhabiting six of California’s platforms and two natural rock outcrops include the following: (1) Results of videotape, quantitative slide, and point contact analyses, in general, produced similar biotic zonation patterns at the platforms studied, although an alternate, three-zone approach to platform characterization may be viable. (2) A total of 173 species (or higher taxa) were identified during quantitative slide analysis of 821 photographs from six platforms and two rock outcrops, while a total of 55 species or higher taxa were identified during point count analysis of 883 photographs from six platforms; invertebrate fauna dominated, with only a few algal species/species groups evident (filamentous and foliose red algae; other red and green algae). (3) In general, the presence and relative ranking of percent cover dominants from the point count analysis were very consistent with the results of whole slide analysis. Random point count analysis provides a straightforward means of identifying percent cover dominants, whereas quantitative slide analysis (i.e., individual counts or percent cover) provides a more comprehensive characterization of all taxa present, within the limitations of photographic interpretation and identification. (4) A total of 130 species or higher taxa (primarily cryptic, interstitial forms typically not visible via photographic techniques) were identified during analysis of scraping samples from the mussel and barnacle zones of Platform Harvest; a total of 90 species or higher taxa were identified from scraping samples from the intertidal, barnacle, and mussel zones of Platform Gail. Findings highlight the complexity and three-dimensional structure of these zones. In total, 181 separate taxonomic entities were noted from scrapings at both platforms, of which 123 (68%) were monospecific taxa. Only 41 taxa occurred at both platforms. (5) A mussel zone was present at each platform, although the vertical extent and maximum depth of each mussel zone varied between platforms; considerable variability in mussel density and percent cover was evident. (6) Highest species- or taxa-specific density levels were typically encountered in the upper portions of each platform, normally within the upper 30 m (100 ft); within this depth range, the intertidal and mussel zones also contained several other numerically dominant species (e.g., anemones, ophiuroids, barnacles, algae). (7) Middle and lower portions of each platform showed considerable variability, but typically contained encrusters (e.g., sponges) and various cnidarian species (e.g., Metridium, Corynactis); some prominent species at depth were limited in their geographic distribution (e.g., Desmophyllum dianthus only at Platform Gail; red encruster only at platforms Harvest, Hidalgo, and Irene). On the deeper water platforms (i.e., Gail and Harvest), deepest zones at the bottom of each platform exhibited a strong affinity to basin fauna. (8) Upper portions of the platforms exhibited similar dominant taxa; taxa unique to each platform tended to occur at depth. Overall, comparisons of density- and percent cover-dominants from adjacent platform legs were very similar, although species-specific enumerations were variable. (9) Clustering analyses comparing density- and percent cover-dominants at the six platforms identified strong similarities between adjacent platform legs and relatively strong similarity between all platforms, with minor exceptions (i.e., few trends suggesting existence of a north-south gradient in species composition). (10) Correspondence analysis of the scraping data revealed that Mytilus and barnacle zones were distinct in terms of their respective taxonomic compositions; taxonomic composition of the Mytilus zone was more variable than the barnacle zone; and (11) Species diversity at the platforms was relatively high, although the number of species present in individual photographs was relatively low and species diversity at natural outcrops was higher (within comparable, shallow depth zones) than that observed at the platforms.

There are few substrates comparable to those afforded by existing platform structures. Naturally-occurring rock substrates that mimic the entire depth range (i.e., continuous substrate) found on platforms do not exist, while intermittent or discontinuous outcrop features exhibiting limited vertical extent are limited (e.g., shallow nearshore outcrops; shelf and slope hard bottom features with vertical relief of 1 to 3 m). Submerged platform surfaces, including platform legs, are optimal settling plates which provide habitat for a variety of attached and motile biota, particularly in the shallower portions of a platform where Mytilus, Metridium senile, barnacles, and associated fauna and flora predominate. Mid and lower platform zones exhibit decreasing species diversity with depth, yet frequently contain unique taxa which suggest subtle differences in platform community structure affected by platform location.