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

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

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海上油气平台退役通常涉及拆除部分或全部相关基础设施，并由此破坏历经数十年形成的关联海洋生态系统。现有越来越多的证据表明，海上平台发挥着重要的生态功能。诸如新生生态系统（novel ecosystems）这类概念，能够帮助利益相关方在平台退役流程中，考量每一座平台所承担的生态角色。本研究以澳大利亚西北部的旺杜（Wandoo）油田为案例，探究新生生态系统概念在海上油气平台退役工作中的应用。研究人员采用诱饵式立体远程水下视频系统，对旺杜油田的生境组成与鱼类群落展开评估，同时设置两处对照样地：一处为沙地样地，模拟旺杜油田平台安装前的原生生境；另一处为天然礁盘样地，作为天然硬质基底与垂直地形起伏的对照。研究发现，旺杜油田样地的大型底栖生物生境密度高于两处对照样地，我们将这一现象归因于旺杜油田基础设施周边海域禁止海底拖网作业。同时，研究发现旺杜油田的底栖与浮游生物类群组成，相较于平台安装前的原生群落，更接近于天然礁盘样地的类群组成，但在区域尺度下，这些类群仍具有独特性。底栖生物类群以礁盘关联物种为主，其多样性高于沙地对照样地与天然礁盘对照样地；浮游生物群落则以其他海域已被报道与石油平台相关的物种为特征。上述研究结果表明，旺杜油田已形成一处新生生态系统。若采用部分或完全拆除的退役方案，该生态系统的诸多独特属性大概率会随之消失。本研究为将海上油气平台归类为新生生态系统提供了实践范例。
本数据集通过诱饵式立体远程水下视频系统（Stereo-BRUVS）采集得到。海底型Stereo-BRUVS由两台相距80厘米的GoPro相机组成，安装于水平基座杆上，每台相机以4度夹角汇聚至同一焦点。系统末端的1.5米长诱饵臂上挂载镀锌钢网诱饵笼，内装800克粉碎后的沙丁鱼。海底型Stereo-BRUVS的布设间距不小于200米，单次部署时长不少于60分钟。中水型Stereo-BRUVS采用与海底型相同的水平基座杆，安装于1.45米长的钢制立杆以保证稳定性，悬挂于水下10米处。其诱饵系统为：1.5米长诱饵臂上挂载带孔诱饵罐，内装1千克粉碎后的沙丁鱼，该诱饵臂可兼作舵体，确保部署全程相机朝向顺流方向。中水型Stereo-BRUVS单次部署时长不少于120分钟，本研究中对其进行锚定，以避免与海底基础设施缠绕。视频分析工作始于海底型Stereo-BRUVS稳定放置于海底后的60分钟窗口期，或中水型Stereo-BRUVS在水下10米稳定后的120分钟窗口期。所有进入视野范围内的动物均被鉴定至尽可能低的分类阶元，种群丰度采用保守丰度指标MaxN进行估算——即单帧画面中某一分类群的最大个体数（Cappo等人，2006年）。针对视野清晰、未被其他个体遮挡的个体，采用立体测量法记录其适宜体长指标，例如叉长（FL）、盘宽（DW）或甲壳长（CL）。针对海底型Stereo-BRUVS，视野范围内的可见生境被大致划分为三类：沙地（无可见大型底栖生物或其他海洋附着生物的裸露基底）、稀疏大型底栖生物覆盖区（以裸露基底为主，生物覆盖度不足50%）以及密集大型底栖生物覆盖区（可见基底以生物覆盖为主，覆盖度超过50%）。
使用说明详见名为"Stereo-BRUVS data usage"的文件。
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