Assessing the long-term effects of a catastrophic oil spill on subtidal coral reef communities off the Caribbean coast of Panama (1985–2017)

Accidental oil discharges pose acute and chronic risks on coral communities, but knowledge on the ecological long-term implications is fragmentary. Here, we examine the potential short-, mid-, and long-term effects of a major oil spill on subtidal reef communities over a 30-year period using a multicontrol before-after-control-impact (BACI) approach. In April 1986, 8000 t (~ 9.3 106 L) of crude oil were released from a refinery in Bahia Las Minas (Caribbean Panama) contaminating an area of about 40 km2 consisting of intertidal and subtidal mangrove, seagrass, sandy, and coral reef habitats. Surveys of oiled and unpolluted control sites have been conducted at different times between 1985 and 2017 and changes in community metrics (i.e., percent live cover, diversity, community composition, and recruitment) were compared with pre-spill data. The main focus was on scleractinian corals, but impacts on other major benthic taxa were also considered. Short-term oil effects on scleractinian corals included substantial declines in live cover, and diversity as well as changes in community structure being detectable up to 4 years after the spill, while other benthic taxa were hardly affected. Branching corals, such as Acropora palmata, seemed to suffer more, but strong incident-related declines could also be seen in two massive species (i.e., Pseudodiploria clivosa and Porites astreoides). Recruitment rates were not significantly different relative to oil exposure, but number of recruits showed strong temporal variation both at the oiled and control sites. While short-term effects (1 year post-spill) could be unequivocally linked to the spill, assessment of mid-term impacts was complicated by cumulative, albeit different stressors (diseases, bleaching, warming, additional accidental oil discharges) that have been driving changes at oiled and control sites respectively and thus ultimately concealing any effects of the spill. Our data did not provide evidence of a long-term (> 10 years) chronic impact of the oil spill, but instead showed that a variety of factors have contributed to reef degradation both at oiled and control sites over the survey period.