Megabenthic invertebrates on shell mounds under oil and gas platforms off California

Abstract

From Conclusions: "In contrast to natural reefs, shell mounds in both shallow and deepwater are dominated by predatory and scavenging taxa, especially asteroid echinoderms, and except for the anemone Metridium farcimen, large structure-forming invertebrates are scarce. Asteroids are at least an order of magnitude more abundant on shell mounds than on reefs at similar depths and are largely dependent on the food subsidy of mussels and other fouling organisms growing on the upper reaches of the platforms. Therefore, only those decommissioning options retaining platform structure in shallow water will result in the continued dominance of this taxon. As freespawning organisms with limited migratory ability as adults, the high densities of sea stars on the deepwater shell mounds may be important as spawning aggregations, especially if shallow water populations continue to be impacted by disease outbreaks associated with long-term ocean warming. However, the magnitude of the reproductive contribution by shell mound asteroids is unknown, and their unusually high densities could also render them more susceptible to disease outbreaks, should infectious agents reach them. Limited evidence obtained in this study suggests that a few shell mound fishes may be partitioning habitats created by those few structure-forming invertebrates found on or intermingled with the shell mounds. However, the overall paucity of structure-forming invertebrate taxa on the shell mounds compared to natural reefs likely suppresses their habitat value to many other fishes and invertebrates. If decisions are ultimately made to retain part or all of the platforms as artificial reefs, we recommend adding large, non-calcareous, hard substrata to the shell mounds in order to promote the settlement and growth of more structure-forming invertebrates. This would enhance biodiversity on the shell mounds regardless of the decisions made about the shallow water portions of the platforms, and indeed, could be accomplished on a large scale by toppling entire platforms and then using the legs and crossbeams for theattachment of extensive amounts of artificial secondary structure.

In California the non-native bryozoan Watersipora subtorquata had previously been known only from shallow waters, largely in bays and harbors. This species commonly reaches basketball size on hard substrata in calm water, and its occurrence both on and under platform Gilda presents new risks for spread to previously unavailable deepwater habitats, including ecologically valuable natural reefs. If the colonies of this species on the shell mound are determined to be viable and reproducing every effort should be made to eradicate this species from both the jacket and shell mound. Based on this study, deeper shell mounds have more megabenthic invertebrate taxa than mounds in shallow water. Therefore, deeper shell mounds may be the most valuable in terms of their conservation potential as artificial reefs, especially since they stand in waters as yetrelatively unaffected by long-term ocean warming.