Oil spills in nearshore environments may eventually move into sensitive coastal habitats such as coastal marshes and impact marsh organisms. Application of dispersants to spilled oil in nearshore environments before the oil drifts into marshes was simulated, and the toxicity, impact and effectiveness of dispersants were investigated. The tolerance of the marsh plant Sagittaria lancifolia to the recently marketed dispersant JD-2000 was about 20 to 80 times higher than that of the standard test-organisms Menidia beryllina and Mysidopsis bahia, respectively. The LC50 of the dispersant JD-2000 for Sagittaria lancifolia was greater than 8000 ppm. Furthermore, the application of the dispersant JD-2000 significantly relieved the adverse effects of crude, diesel and No. 2 fuel oil on marsh vegetation. Upon contact with plant shoots on the rising tide, the un-dispersed oils detrimentally impacted the marsh plants Spartina alterniflora and Sagittaria lancifolia. Mortality rates significantly increased even at a 150-ppm oil dosage. The 750-ppm No. 2 fuel oil without the dispersant application resulted in more than 90 % mortality for Spartina alterniflora in 3 weeks. In contrast, the oils chemically dispersed with JD-2000, regardless of oil type and oil concentration, did not significantly affect the marsh plants compared to the no-oil control. Therefore, the dispersant application greatly reduced oil impact on marsh vegetation, indicating the potential for using dispersants as alternative countermeasures to protect sensitive coastal habitats during nearshore oil spills.

The use of dispersants in oil spill cleanup has attracted great attention since the Exxon Valdez oil spill in 1989. However, dispersants have been a controversial oil spill response technique because of disagreement about their effectiveness and concerns of their toxicity since their introduction during the Torrey Canyon oil spill in 1967 (Cunningham et al. 1991; Venosa et al. 1999). Dispersant use has been recommended for oil slicks in the sea before coastal habitats are reached, though minimal guidelines have been outlined (Page et al. 2000). Therefore, the effects of dispersants, including Corexit 9527 and Corexit 9500 listed in the National Contingency Plan (EPA, 2001), have mainly been focused on marine organisms, such as fishes, shrimps, and the larvae of fishes, crabs, and corals (Singer et al. 1994; Rhoton et al. 1999; Gulec and Holdway 2000; Epstein et al. 2000; Wolfe et al. 2001). Most of these studies on marine organisms were acute toxicity tests. However, decisions to use oil spill response chemicals should not be based solely on aquatic toxicity (George and Clark 2000).

A handful of studies have been conducted on the effects of dispersants on plants from salt to freshwater marshes. Some studies indicated that dispersants, such as BP 1100WD (Baker et al. 1984), Corexit 9527 (Lane et al. 1987) and BP Enersperse 1037

(Little and Scales 1987), were ineffective in cleaning the oiled salt marshes, and had greater detrimental impact on salt marsh plants, such as Spartina anglica, Salicornia spp, Spartina alterniflora and Aster spp than oils without applying dispersants. In contrast, other studies (Smith et al. 1984, DeLaune et al. 1984) demonstrated that dispersants applied to Louisiana crude oil contaminated Spartina alterniflora had short term benefits to plant photosynthesis although it did not have long-term effects on plant biomass.

Dispersants used today are more effective and less toxic (NCR 1989). For example, the dispersant JD-2000 recently marketed by the GlobeMark Resources Inc. in 2001 is especially effective for south Louisiana crude oil (EPA 2001) for both salt and fresh water environments. It is a high performance, biodegradable oil spill dispersants listed in the NCP (EPA 2001). However, little information is available on the toxicity and effects of dispersants on marsh habitats and the strategy of dispersant-use in the nearshore to protect sensitive coastal habitats.

The overall goal of the study was to determine the potential use of dispersants as oil spill countermeasures in nearshore environments in which spilled oil may eventually move into coastal marshes with tide and wind, and impact sensitive wetland habitats. The specific objectives of the study were (1) to evaluate the toxicity of dispersants on coastal marsh plants by determining the doseresponse of plants to dispersants, and (2) to evaluate effects and effectiveness of dispersants on protecting coastal marsh habitats from impact of different oils (crude, diesel, and fuel oil).

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