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The results of a long-term program to determine the effects of oil and dispersant on red mangroves and black mangroves are presented. Laboratory experiments were conducted to determine the effects of three oils and dispersant on juvenile red mangroves and black mangroves. A field experiment was conducted to determine the effects of a crude oil and dispersant on a mature mangrove forest in Panama. Our studies indicate that exposure of mangrove seedlings to oil and dispersant in the laboratory resulted in changes of growth, respiration, and transpiration, and led to uptake of petroleum hydrocarbons. Exposure of a mature red mangrove forest to oil and dispersant resulted in many of the same effects observed in the laboratory and at other oil spill sites. These effects were greatly reduced at the site treated with oil and dispersant when compared to the site treated with whole oil.

Field experiments were conducted to determine the effects of oil and dispersant on red mangroves (Rhizophora mangle). Two experimental sites were chosen on the basis of their biological and physical similarity and treated with either undispersed Prudhoe Bay crude oil or with Prudhoe Bay crude and dispersant Corexit 9527. Short- and long-term effects on biota were monitored to determine the effects of using dispersants on spilled oil in mangrove forests. Significant differences were noted in effects on seedling and adult mangroves, most notably in survival and growth of adults. Undispersed oil caused significantly lower survival and growth of seedlings, and significantly lower growth and foliage cover in adults. Addition of dispersant to crude oil greatly increased its mobility in the water column, and decreased its residence time in the intertidal zone. Retention of oil within the mangrove forest was the primary factor in controlling impacts to mangroves, and dispersants appeared to greatly reduce the amount of oil retained in the intertidal zone of the mangrove forest. Additional research is underway to investigate the impact of dispersed oil on the subtidal communities associated with mangrove forests.

Relatively few data exist on the effects of oil and dispersing agents on tropical wetlands. This is true despite extensive near-shore and onshore drilling, onshore refining, and transportation of oil through the past decade in tropical areas of the world. This paper reports on preliminary results from a laboratory experiment on the toxicity of oil and dispersants to mangroves (tropical marine forests). A controlled-growth chamber with regulated light, temperature, and humidity was used to monitor growth and stress responses. Seedlings of the mangrove genera Rhizophora and Avicennia were selected for study since they represent species found growing along most oil-sensitive, tropical shorelines. Mangrove seedlings were treated with three oil types (bunker C, No. 2 fuel oil, and light Arabian crude), both with and without a dispersant concentrate added. The plants were dosed with oil at 25, 50, 500, 5,000, and 50,000 parts per million; dispersant concentrates were added as a well-mixed, unweathered dilution with oil in a 1:22 ratio. From these results, preliminary conclusions are drawn regarding the differential response of mangrove species, different oil and dispersant combinations, and the difference in response to oil and dispersants by stocks of mangroves from chronically oiled environments.