Temporal progression of oil spill impact on a cold-water coral community

Open Access
Author:
Hsing, Pen-yuan
Graduate Program:
Biology
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
January 10, 2013
Committee Members:
  • Charles Raymond Fisher Jr., Thesis Advisor
Keywords:
  • cold-water corals
  • Paramuricea
  • multiple imputation
  • oil spill impact
  • time series
  • Deepwater Horizon
  • Gulf of Mexico
Abstract:
Usually found in deep and cold marine environments around the world, azooxanthellae cold-water corals (CWCs) are not as well studied as their tropical reef counterparts. However, the diversity of biological communities on hard scleractinian cold-water coral reefs have been compared to that of tropical reefs (Cordes et al. 2008), and dense assemblages of soft gorgonian corals near the Aleutian Islands have been described as coral “gardens” (Stone 2006). They are slow growing and long lived, sometimes up to thousands of years old (Roark et al. 2009), suggesting they and the habitats they support might take a long time to recover from major disturbances, if at all. Historically, those disturbances include fishing and trawling (Hall–Spencer et al. 2002; Davies et al. 2007). Also, cold-water corals are being affected by thermal stress (Cerrano et al. 2000), changes in surface primary productivity (Roberts et al. 2009), and ocean acidification (Guinotte et al. 2006) as a result of anthropogenic induced climate change. In recent years, mining and energy production activities have expanded into increasingly deeper waters, including cold-water coral habitats (Roberts et al. 2009). A major accident that occurred as a result of those activities was the Deepwater Horizon oil spill in April 2010 in the deep Gulf of Mexico. On 3 November 2010, a coral community 11 km southwest of the site of the Deepwater Horizon blowout was discovered at 1,370 m with gorgonian corals partially covered by a brown flocculent material (“floc”) and commensal brittle stars with abnormal coloration and behavior (White et al. 2012). This thesis will describe the work that quantified the visible changes in impact to the corals, where images of individual branches of each colony were digitized and categorized based on their condition after four repeat visits. Most of the floc was absent from the corals by the first 2011 visit, and there was a decrease in the median proportions of the colonies showing obvious signs of impact between the November 2010 and later visits. During the second visit in 2010, early onset of hydroid colonization on impacted coral branches was observed and it increased over the course of the study. The probability of impacted portions of the coral dying and being colonized by hydroids by March 2012, was positively correlated with the proportion of the colony covered by floc in late 2010. Similarly, the probability of impacted portions of the coral showing signs of recovery and persisting through March 2012 was negatively correlated with the proportion of the coral covered with floc in late 2010. A notable feature of the impact at all time points was its patchy nature, both with and among colonies, suggesting the impacting agent was not homogenously dispersed during initial contact with the corals. While the median level of visible impact decreased over time, the conservative method used to categorize branches as obviously visibly impacted only when they were covered in floc, dead, or dripping tissue ignored more subtle indications of impact such as polyp condition and coloration. As a result, it is likely that future visits will reveal additional deterioration in the condition of these normally very long-lived corals.