TROPICS: 30-year Follow-up and Analysis of Mangroves, Invertebrates, and Hydrocarbons

Abstract

ABSTRACT TROPICS (TRopical Oil Pollution Investigations in Coastal Systems) has been the seminal study on trade-offs for Net Environmental Benefit Analysis (NEBA) for dispersant use in tropical ecosystems. The study began in 1983/84 with the identification of suitable tropical island sites in Bahia Almirante, Bocas del Toro, Panama that contained mangrove, seagrass and coral habitats in close enough proximity to establish three 30m X 30m test sites. Controlled releases of Prudhoe Bay crude oil (dosed at 1L/m2) and Prudhoe Bay crude oil pre-dispersed with Corexit 9527 (to maintain 50 ppm water soluble fraction), were introduced into the Non- dispersed oil (Site O) and the Dispersed oil (Site D) sites, respectively, for 48 hours. A nearby Reference site (Site R) was not treated with oil/dispersed oil. Treatments were designed to simulate a realistic oil spill in adjoining mangrove, seagrass, and coral habitats. Following exposure and removal of oil, sites were studied periodically over 30 years for relative effects of dispersed and non-dispersed oil in coral, seagrass, mangrove, and invertebrate populations, as well as hydrocarbon presence. Early research focused on short- and mid-term effects compared to the Reference site (R), while later work focused on long-term effects and ecosystem recovery. In general, researchers found that Site O exhibited more overall long-term ecosystem disruption than Site D, and that Site D had recovered quickly to Site R and baseline levels. In November 2013 (29 years after oil and dispersed oil exposure), the TROPICS sites were re-visited under a grant provided by Clean Caribbean & Americas. Researchers collected data on mangroves, mangrove invertebrates, and hydrocarbons. The density of mangrove trees at Site D had remained at Site R and baseline levels. Site O, which had experienced early die off of trees, followed by peak production at 10 years (far in excess of Site R and baseline levels), exhibited a decline dominated by small trees. Mangrove snails and oysters increased sharply at Site O after the spill, but declined over 10-20 years. Sites D and R maintained gradual invertebrate increases during this time. This paper focuses on research from the November 2013 visit and draws on previous observations and TROPICS papers on overall ecosystem disruption and recovery as it pertains to the NEBA for nearshore dispersant use in tropical marine ecosystems.