A report on the macroinvertebrates of the Columbia River estuary found in deposits of volcanic ash from the May 18, 1980 eruption of mount St. Helens

Following the May 18, 1980 eruption of Mount St. Helens, up to 11.5 cm of volcanic ash was deposited in sections of the upper Columbia River estuary. A survey of the benthic infauna of this area indicates that most taxa were able to inhabit the ash, suggesting that the material is nontoxic to most groups. However, the abundance of the taxa examined was dependent on the distribution of the ash within the sediment column. Except for the oligochaetes, animal densites were reduced in areas where volcanic ash lay atop the sediment surface as compared to areas where the ash layer had been buried beneath, or mixed with sands and/or muds. The ash apparently affects the infauna through some physical means, possibly related to its fine grain size.