This World Oceans Day, we’re diving into the deep sea and coral recovery. Fanny Girard discusses the recent article, Projecting the recovery of a long‐lived deep‐sea coral species after the Deepwater Horizon oil spill using state‐structured models.

Although the deep sea is the largest ecosystem on earth, it is also the least understood due to its inaccessibility. Advances in technology in the last few decades have allowed scientists to significantly increase our knowledge on how deep-sea ecosystems function. Unfortunately, the advances that have allowed exploration into increasing depths also spurred the economic activities that threaten deep-sea ecosystems. Among these threatened ecosystems are deep-sea corals.

Deep-sea corals, also named cold-water corals to differentiate them from their tropical counterparts, are generally considered hotspots of diversity in the deep sea as they provide habitat for several other organisms including commercially valuable fishes or invertebrates. If we are now starting to really understand the importance of deep-sea corals as an ecosystem, their response and ability to recover after impact from human activities is still poorly known. The general consensus is that cold-water corals, like most deep-sea organisms, grow and reproduce slowly, and thus are expected to recover slowly, if at all. In other words, we know that recovery will take a long time but we do not know exactly how long. The main reason why this information is lacking is because the best way to study recovery is through long-term monitoring, which is technically challenging and extremely expensive to do in the deep sea.

The April 2010 Deepwater Horizon oil spill was the largest accidental oil spill in recorded history. Various ecosystems from the coast to the deep Gulf of Mexico were severely impacted due to exposure to oil and, potentially, to the chemical dispersant used in response to the spill. The discovery of three impacted deep-sea coral communities in the few months that followed the spill provided us with a unique opportunity to study how corals respond to impact from human activities.

As part of the research efforts based on substantial funding in the aftermath the spill, we started an image-based long-term monitoring programme in November 2010 to follow changes in the health of corals that were impacted by the spill. Because of its planar morphology, the dominant coral species at the impacted sites, a gorgonian (also known as sea fan corals), was an ideal candidate for image analysis. Indeed, all coral branches were always visible in the same photograph, and the proportion of the coral covered with injuries could easily be quantified. We took high resolution photographs of the same individual corals every year between 2010 (or 2011 for the sites discovered later) and 2017. We then used these photographs to quantify impact and assess recovery. In our paper, we use this unique data set to estimate how long it will take for impacted corals to recover, and then give recommendations to optimise monitoring strategies (cost and efficiency) and better manage vulnerable deep-sea coral ecosystems.

To do so, we used data collected between 2011 and 2017 to parameterise and validate a state-structured matrix model, a type of model commonly used in ecological studies. For every impacted coral, we used the model to estimate the time necessary for the coral to recover from its injuries. We found that most corals will have recovered within ten years, but that some of the most impacted corals could take up to three decades to recover. In order to assess the recovery of corals affected by the Deepwater Horizon oil spill, we recommend monitoring once every two years for a decade (annual monitoring is not necessary since recovery is so slow) and then less frequent monitoring for another two decades in order to follow the potential recovery of the most impacted corals.

We also used our model to estimate the number of branches that broke on each coral and found that, depending on their initial level of impact, corals will lose a large number of branches by the time they recover. Because the growth of deep-sea corals is generally extremely slow, it will likely take hundreds of years for some of these corals to grow back to their original size. These findings, as well as results from other studies, indicate that the best management strategy to conserve deep-sea corals is to prevent impact in the first place rather than rely on restoration after the fact. Therefore, we suggest the establishment of coral monitoring sites implemented as part of Marine Protected Areas to both prevent and detect impact on corals.

In this study, we estimated the time necessary for the recovery of corals impacted by an oil spill, but our modelling approach could be used as a tool to optimise the duration and frequency of a monitoring programme following any impact on gorgonians.

Read the full open access article, Projecting the recovery of a long‐lived deep‐sea coral species after the Deepwater Horizon oil spill using state‐structured models in Journal of Applied Ecology.