Monday, 3 October 2011

Can rigs-to-reefs benefit deep-sea communities?

There are conflicting opinions as to whether human activities have much of an impact on the deep sea; while some feel that anthropogenic disturbance is likely already effecting deep sea communities, others argue that the deep sea is, and will remain, relatively untouched by our activities far above.

However, in a paper out last week, collaborators from the University of Technology in Sydney, Australia wonder maybe humans should have an impact on the deep sea environment--by deliberately installing artificial reefs as part of the rigs-to-reefs (RTR) program. This initiative was developed over 30 years ago, and the first rig-to-reef conversion occurred in 1979. Theoretically, the RTR technique is win-win, as it appears to have beneficial environmental affects while also acting as a cost-saving device for those in the oil/gas industry. RTR programs are already in place in the United States, SE Asia, and Mexico, and talks are underway to instigate them in North Sea countries, as well.

Traditionally, RTR sites are found in shallower waters; as a result, much of the research on the efficacy of RTR has been conducted relatively close to the ocean surface--the deepest study taking place at 235 m, about half the depth of where the proposed "deep sea" rigs should be deployed. This means that not much is known about how RTR efforts might impact species and their habitats, but observations of the deep-water coral Lophelia pertusa on decommissioned oil rigs in the North Sea suggest that these human structures may provide habitat suitable for development of deep-sea coral reefs. Thus, the authors of the current study have reviewed the shallow-water RTR literature in an attempt to make extrapolations and best guesses so as to predict whether rigs might be beneficial at greater depths.

One of the biggest worries about rig reefs is whether they might attract species--fish, in particular--away from their natural habitats, thus relocating part of the existing population rather than offering additional habitat space for more individuals to live. However, at least some species--octopi, for example--have been confirmed to utilize the rig habitat as a means of population expansion. Additionally, in locations that are far from natural reefs, immobile species like corals may literally find rigs to be lifesavers: The larvae can only settle where they happen to bump in to appropriate substrate, so the rigs may prevent the larvae from being doomed to drift aimlessly until they die. Although not all species use artificial reefs to the same extent, some, such as the Boccaccio rockfish (Sebastes paucispinis), benefit greatly; approximately 20% of their current population is thought to reside in rig reefs, and rockfish population numbers have soared not just within the artificial habitats but also in surrounding natural areas--indicating that RTR efforts can have widespread beneficial effects on some species. In fact, experts have estimated that, in this species at least, removing 1 rig structure would be comparable to removing 29 hectares of average natural habitat.

(Controlled sinking of an oil rig)

Rigs can also act as physical barriers to enforce marine parks and other protected areas. This is because their large internal spaces offer shelter to a variety of species, while their external spaces pose a hazard to vessels--particularly to trawlers, which cannot effectively deploy their nets in RTR areas. Unfortunately, the physical layout of the rigs probably make them suitable habitat for only a subset of species. The large open areas in the middle of the metal framework are appropriate for large species, but the lack of small hiding areas make them unattractive for small fish or invertebrates--at least until the structure is colonized by marine epifauna.

There are several concerns related to the transport and installation of rigs. When rigs are moved from their original location to their final resting places, they may inadvertently distribute invasive species that are attached to their surfaces; it is not clear how much of a danger this is and whether these potentially invasive organisms might somehow be cleaned off prior to rig movement. Rig installation could be associated with physical damage to the sea floor, such as redistributing sediment or smothering bottom-dwelling organisms. It is also possible that the mostly metal structures may begin to corrode over time, releasing toxins into the environment. Their removal from their original locations is certainly associated with release of potentially toxic buildup around the site of oil/gas drilling, but this is a problem that occurs regardless of how the rigs are treated once they are decommissioned.

(Artificial coral reef)

Although rigs probably only represent a tiny amount of all potential deep sea habitat--the largest ones offering up to a football field's worth of area--they provide a type of dwelling relatively rare in the deep sea; thus, their presence could be particularly useful. Indeed, population persistence in the deep sea is influenced by connectivity between communities in isolated areas. Over large distances, even small, isolated reefs could act as important stepping stones. Then again, this might not always be a good thing, if this leads to a disruption of natural dynamics--for instance, if a particularly good colonizer, or even an invasive species, uses the stepping stones to reach habitats where it eventually excludes other individuals or species. By impacting the diversity and abundance of native communities, reefs could also alter food-web dynamics and community structure.

Clearly, though RTR areas have the potential to offer some valuable ecosystem services, they may also do real damage. Thus, additional research is necessary before any rigs are installed at deep sea sites. Most obviously, there is a need for long-term investigations of what happens after a rig is deployed in the deep sea. It will also be important to investigate connectivity of deep sea communities and to understand how the presence of rigs alters natural dynamics. This will require profiles of species abundance and composition at various depths. Marine wildlife near RTR installations will need to be sampled for contaminants in order to see whether the rigs are poisoning the habitat; if they are, it will be necessary to research potential methods of cleaning up the rigs prior to deployment.

Only through these research efforts--and the continuance of those in shallow-water habitats--can we answer the question of whether RTR is a scheme that is too good to be true. A 2006 survey estimated that there were at least 7500 offshore rigs worldwide, many of which are soon due for decommissioning. In other words, it appears that there is no time like the present for determining whether these structures have any real conservation/management value.

Macreadie, P.I., Fowler, A.M., Booth, D.J. 2011. Rigs-to-reefs: will the deep sea benefit from artificial habitat? Frontiers in Ecology and the Environment 9(8):455-461.

Thanks to the following websites for providing the images used in this post:

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