Projecting the recovery of a long-lived deep-sea coral species after the Deepwater Horizon oil spill using state-structured models

1. Deepwater coral communities are hotspots of diversity and biomass. Most deep-sea coral species are long-lived and slow-growing and are, thus, expected to recover slowly after disturbance. A better understanding of the recovery potential of these organisms is necessary to make appropriate management decisions. 2. We used data from high-resolution monitoring of individual coral colonies that were impacted by the Deepwater Horizon oil spill (April 2010) to parameterize and validate an annual, impact-dependent, state-structured matrix model to estimate the time to recovery for each coral colony. We projected the dynamics of three branch states: visibly healthy, unhealthy and hydroid-colonized. Although we implicitly included branch loss in the model, we focused on the short-term return of extant, damaged branches to a visibly healthy state and did not consider the far longer term regrowth of lost branches. 3. Our model estimates that, depending on the initial level of impact, corals impacted by the spill will take up to three decades to recover to a state where all remaining branches appear healthy, though the majority of corals are projected to reach that state within a decade. By that time, some of these colonies will have lost a significant number of branches, leading to approximately 10% reduction in total biomass at all impacted sites. 4. Overall, our model overestimates recovery, but branch loss estimates were reliable. Thus, the available growth rate data suggest that hundreds of years may be necessary for impacted communities to grow back to their initial biomass. 5. Policy implications. Our study quantifies the very slow recovery rate of deep-sea corals impacted by the Deepwater Horizon oil spill and demonstrates the imperative of prioritizing a precautionary approach for deep-sea ecosystems over restoration after the fact. As anthropogenic pressure on the deep sea is likely to increase, we suggest the establishment of coral monitoring sites implemented as part of Marine Protected Areas to limit and detect impact to deep-sea corals. Furthermore, our model may be used to plan shorter-and longer-term monitoring programmes after impact and to provide a timeline for policy.