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Offshore platforms as novel ecosystems: a case study from Australia’s Northwest Shelf

Citation

van Elden, Sean; Meeuwig, Jessica; Hobbs, Richard (2022), Offshore platforms as novel ecosystems: a case study from Australia’s Northwest Shelf, Dryad, Dataset, https://doi.org/10.5061/dryad.mpg4f4r1s

Abstract

The decommissioning of offshore oil and gas platforms typically involves removing some or all of the associated infrastructure and the consequent destruction of the associated marine ecosystem that has developed over decades. There is increasing evidence of the important ecological role played by offshore platforms. Concepts such as novel ecosystems allow stakeholders to consider the ecological role played by each platform in the decommissioning process. This study focused on the Wandoo field in Northwest Australia as a case study for the application of the novel ecosystem concept to the decommissioning of offshore platforms. Stereo-baited remote underwater video systems were used to assess the habitat composition and fish communities at Wandoo, as well as two control sites: a sandy one that resembled the Wandoo site pre-installation, and one characterised by a natural reef as a control for natural hard substrate and vertical relief. We found denser macrobenthos habitat at the Wandoo site than at either of the control sites, which we attributed to the exclusion of seabed trawling around the Wandoo infrastructure. We also found that the demersal and pelagic taxonomic assemblages at Wandoo more closely resemble those at a natural reef than those which would likely have been present pre-installation, but these assemblages are still unique in a regional context. The demersal assemblage is characterised by reef-associated species with higher diversity than those at the sand control and natural reef control sites, with the pelagic community characterised by species associated with oil platforms in other regions. These findings suggest that a novel ecosystem has emerged in the Wandoo field. It is likely that many of the novel qualities of this ecosystem would be lost under decommissioning scenarios that involve partial or complete removal. This study provides an example for classifying offshore platforms as novel ecosystems.

Methods

This dataset was collected using stereo-baited remote underwater video systems (Stereo-BRUVS). Seabed stereo-BRUVS consist of two GoPro cameras mounted 80 cm apart on a horizontal base bar, each converging at an angle of four degrees to a common focal point. A galvanised steel mesh bait cage containing 800 g of crushed pilchards is attached to the end of a 1.5 m long bait arm. Seabed stereo-BRUVS are deployed at least 200 m apart for a minimum of 60 minutes. Mid-water stereo-BRUVS consist of the same horizontal base bar as seabed stereo-BRUVS, mounted on a 1.45 m long steel upright to provide stability, and suspended 10 m below the surface. They are baited with 1 kg of crushed pilchards in a perforated bait canister on a 1.5 m long bait arm, which acts as a rudder to keep the cameras facing down-current for the duration of the deployment. Mid-water stereo-BRUVS are deployed for a minimum of 120 minutes, and in this study, are anchored to prevent entanglement with subsea infrastructure.

Video processing commenced either once seabed stereo-BRUVS had settled on the seabed, for a period of 60 minutes, or when the mid-water stereo-BRUVS had stabilised at 10 m depth following deployment, for a period of 120 minutes. All animals entering the field of view were identified to the lowest possible taxonomic level, and abundance was estimated using the conservative abundance metric MaxN, which is the maximum number of individuals of a given taxon in a single frame (Cappo et al. 2006). The appropriate length metric (e.g. fork length FL, disc width DW, or carapace length CL) was measured in stereo with individuals measured where they were well positioned relative to the camera and not occluded by other individuals. For seabed stereo-BRUVS, the habitat visible in the field of view was broadly categorised into three groups: sand (bare substrate with no visible macrobenthos or other marine growth); sparse macrobenthos (predominantly bare substrate with less than 50% biotic cover); and dense macrobenthos (the visible substrate was dominated by more than 50% biotic cover).

Usage Notes

Usage notes can be found in the file titled "Stereo-BRUVS data usage".