Oil spill impacts on mangroves: Recommendations for operational planning and action based on a global review

Study Number

104

Author

Norman C. Duke

Abstract

Mangrove tidal wetland habitats are recognized as highly vulnerable to large and chronic oil spills. This review of current literature and public databases covers the last 6 decades, summarizing global data on oil spill incidents affecting, or likely to have affected, mangrove habitat. Over this period, there have been at least 238 notable oil spills along mangrove shorelines worldwide. In total, at least 5.5 million tonnes of oil has been released into mangrove-lined, coastal waters, oiling possibly up to around 1.94 million ha of mangrove habitat, and killing at least 126,000 ha of mangrove vegetation since 1958. However, there were assessment limitations with incomplete and unavailable data, as well as unequal coverage across world regions. To redress the gaps described here in reporting on oil spill impacts on mangroves and their recovery worldwide, a number of recommendations and suggestions are made for refreshing and updating standard operational procedures for responders, managers and researchers alike.

Main Results and Conclusions

• Oil spills are extremely detrimental to mangroves.
  • “Mangroves are highly vulnerable to oil spills because oil deposits on sensitive plant surfaces, affecting soils and dependant marine life causing death and sublethal impacts” (701)
  • “This disruption affects ecosystem services of mangroves, like fisheries production and shoreline protection worldwide.” (701)
  • “…oil coats breathing surfaces of mangrove roots, stems, seedlings, and surrounding sediments, as well as fauna present in burrows and root hollows. When smothered with oil, shorter plants and animals, die mostly within days. By contrast, taller mature trees and shrubbery, oiled only on their exposed roots and sediments, might persist for six or more months before dying. Plants are accordingly smothered, poisoned and starved by oil spills; and the lighter the oils are, the most damaging they are.” (701)
• Impacts differ across mangrove species.
  • “…studies, mangrove species were tested in planthouse trials where species showed a range of sensitivities from highest in Aegiceras corniculatum and Avicennia marina, to lesser levels in Rhizophora stylosa and Ceriops tagal.” (702)
  • “The more sensitive species were those with greater salt excretion strategies as their likely co-related physiological character.” (702)
  • “…further concluded that sediment type was also important where those plants in more porous sediments were more vulnerable than those growing in fine clay mud.” (702)
• Some oil types affect mangroves more severely than others.
  • “…oil types were ranked by increasing impact on plants, from least sensitive: Bunker C fuel oil; Arabian light crude; Gippsland light crude; Thevenard crude; to Woodside condensate, the most harmful.” (702)
  • “In general, impact level rankings corresponded to oil density, where light oils were more harmful than dense heavy oils.” (702)
• Mangroves and oil spills regularly co-occur.
  • “The distribution of reported incident locations (Fig. 2) generally match the global distribution of mangrove habitat…demonstrating that oil spills occur very widely affecting mangroves where ever this habitat occurs.” (703)
  • “Four types of oil spills are represented including: pipeline ruptures; vessel incidents; shore tank facility disruptions; and, well head damage.” (703)
  • “…there was a notable concentration of incidents in areas of both high vessel traffic and extraction sources, like the Arabian Sea, the Gulf of Mexico, South eastern Brazil, and the Niger Delta. However, the wider distributions dominate, and oil-affected mangroves could be found in all 6 global sub-regions.” (703)
• Mangroves are likely to retain oil in their sediments after an oil spill.
  • “Oil can at times be retained within sediments of the intertidal zone for decades following an oil spill.” (707)
• Oil-damaged mangroves take long periods of time to recover.
  • “Post spill assessments suggest that structural recovery of oil-damaged mangrove forests takes place over a period of at least 3 decades.” (708)
  • “The history of oil spills affecting mangroves suggests that recovery and rehabilitation generally takes at least 3 decades…depending on climate, tidal range and geographic circumstances” (711)
• Recovery times are dependent on several factors.
  • “…the primary factors most likely to influence recovery time are oil type (heavy versus light), quantity, and oil condition (fresh and concentrated versus weathered and dispersed).” (709)
  • “High initial impacts are most likely where fresh, light oils are involved, but also possible also with concentrated, heavy oils where they smother exposed surfaces.  Broadly applied dense oiling is also more likely to result in delayed recovery than small patches of oiling over several hectares or less. Recovery clearly depends on biotic factors also, like the supply of propagules for colonization, and variations in species tolerances.” (709)
  • “…recovery may proceed more rapidly in areas of high tidal ranges where the effects of more flushing appears to break down deposited oil faster.” (709)
• There is a need for an operational response guide for oil-damaged mangroves. (709-711)
• This article provides 9 recommendations for monitoring oil-damaged mangroves. (Table 3 and pp 711-714)