Five segments of a mesohaline marsh located off the York River in Virginia were physically isolated from the surrounding area, except for allowing subtidal flow, and dosed with fresh and artificially weathered South Louisiana crude oil. The experimental design and field site utilized in this study are described. The mini-ecosystems each contained about 695 m2 of marsh, 100 m2 of open water and 15 m2 of intertidal mud flat. In September 1975, three barrels (5701) of each of the experimental oils were spilled into replicate systems.

Overall, the artificially weathered oil was shown to have as great an ecological impact on the communities as the fresh crude. Phytoplankton and fish populations all showed greater declines following the spills in the weathered oil systems. Phytoplankton production declined immediately after both oil spills but had recovered to control values within seven days. Species composition was not affected by the oils, while periphyton biomass, as measured by adenosine triphosphate (ATP), increased after both treatments. Marsh grass production was reduced in both spill units. Benthic animals, showing population declines after both oil spills, included nereid polychaetes, insect larvae and amphipods. Oligochaete populations decreased shortly after the fresh crude spill, returned to normal within 30 days, and then declined again relative to the control in both treatments 11 weeks after the spill. Mortalities offish, Fundulus heteroclitus, held in live boxes were noted only in the weathered treatment systems.

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