A four year analysis of vegetation following an oil spill in a freshwater marsh.
After an accidental spillage, the total plant cover, total number of species, mean number of species per quadrat and the Shannon-Wiener function progressively decreased in both high and mid-marsh zones for two years after the spillage. Eighteen of the species found before the spill were not found the following season. Perennial species generally were less affected than annual species immediately after the spill. Some species, e.g. Dulichium arundinaceum and Eleocharis palustris, became less abundant in the second season after the spill. Marked changes in the relative abundance of the dominant species of high and mid marsh zones occurred from year to year, with Onoclea sensibilis more prominent in high marsh at the end of the study period than prior to the spill and Pontederia cordata and Nuphar variegatum more prominent in different segments of mid-marsh. The vegetation of the high marsh and mid-marsh zones had substantially recovered by the third and fourth years. The low marsh vegetation was apparently unaffected immediately following the spillage but in succeeding years the species diversity declined and luxuriant growth of Elodea nuttallii, Potamogeton crispus and P. ephihydrus occurred.