Insights from Long-Term Monitoring of Deep-Sea Coral Communities Impacted by the Deepwater Horizon Oil Spil

Deep-water corals form one of the most complex biological habitats in the deep sea, and house a high diversity of associated fauna. Yet, they are very vulnerable to anthropogenic impact due to their lack of mobility, exposed tissue, and generally low growth rates. In April 2010, the blowout of the Deepwater Horizon drilling platform in the northern Gulf of Mexico led to the largest oil spill in US history. The first impacted coral community was discovered three months after the well was capped. Corals there, were covered in a brown flocculent material (floc) that contained traces of oil, directly linking the observed damages to the spill. Eleven months later, two additional affected communities were discovered and, although corals were no longer covered in floc, the characteristic patchy impact distribution on the colonies, previously observed at the first site discovered, indicated that these corals had also been impacted by the spill. I quantified the impact and assessed the recovery of deep-sea corals using high-definition photographs of individual colonies. Paramuricea spp. colonies, well suited for visual monitoring due to their planar morphology, were imaged every year between 2011 and 2017 at five sites (three impacted and two reference sites). Images were then digitized to quantify impact and track recovery patterns. Overall recovery was slow. Although the health of lightly impacted corals improved, heavily impacted colonies showed little or no sign of recovery by 2017. The initial level of visible impact on corals had a significant effect on the improvements in the condition of individual branches between consecutive years. Furthermore, branch loss at two of the impacted sites was still significantly higher between 2016 and 2017 than at the reference sites. Even after seven years, the fate of the corals that were impacted by the Deepwater Horizon oil spill is still uncertain and the effects of the oil spill appear to be ongoing.;The high-resolution images collected between 2011 and 2014 were also used to investigate the relationship between Paramuricea biscaya and its ophiuroid associate Asteroschema clavigerum, based on the hypothesis that both species benefit from this association. Coral colonies associated with ophiuroids were on average less impacted than coral colonies that had no associates. After defining the area clearly under the influence of ophiuroids for each coral, I found that the level of visible impact to coral branches was lower in the area influenced by ophiuroids than outside that area, and that impacted branches within this area were more likely to recover than branches outside the area of influence. These results suggest a mutualistic symbiosis between P. biscaya and A. clavigerum; Ophiuroids use corals to gain access to food particles brought by currents, and corals likely benefit through the physical action of ophiuroids removing particles deposited on polyps and perhaps inhibiting the settlement of hydroids. The beneficial role of ophiuroids was demonstrated on corals impacted by an oil spill, but these benefits could also extend to corals in environments exposed to natural sedimentation events, perhaps allowing corals to live in environments where heavy sedimentation would otherwise limit their survival.;In order to assess recovery over the long term and to plan for future monitoring, I developed an impact-dependent, state-structured matrix model. The model, parameterized using data collected as part of the long-term monitoring project, projected the dynamics of three-branch states: visibly healthy, unhealthy and hydroid-colonized. Although branch loss was implicitly included in the model, I focused on the return of extant damaged branches to a healthy state rather than on the slower re-growth of lost branches. The model estimated that, whereas most corals will recover to a visibly healthy state within a decade, the most impacted coral colonies will take up to three decades to visibly recover. Impact-related branch loss will lead to a 10% reduction in total biomass at the impacted sites by the time all coral colonies are projected to appear healthy. Given the very slow growth rates estimated for these corals, hundreds of years may be necessary for coral communities to re-grow to their original biomass.;Overall, even with the help of associated ophiuroids, the recovery of corals impacted by the oil spill is extremely slow, demonstrating the necessity to prevent impact to deep-sea corals rather than relying on restoration after the fact. Deep-sea corals are reliable indicators of anthropogenic impact in the deep sea because they are sessile, their skeleton is almost entirely covered with living tissue, making potential damage easily detectable, and natural mortality is an extremely rare event. The methods I employed allow the detection of small changes in the health of coral colonies that would not be visible with monitoring based on transects.