| Benthic invertebrate samples were collected in the Toutle River, Muddy River, and Pine Creek drainage basins on March 28-30, 1980, to document pre-eruption conditions in watersheds surrounding Mount St. Helens, in the event of a major eruption. A highly diverse fauna was present at all sampling sites. Mayfly and midge (chironomid) larvae were the most numerous organisms. The mayflies were most common in the third-order streams, and the chironomid larvae were most common in first- and second-order streams. An attempt was made to classify the streams studied according to the functional role of organisms in the processing of organic matter. It was hypothesized that many of the organisms exhibiting multiple functional roles in processing or organic matter were predominantly detritivores. This classification was then compared to a conceptual steam ecosystem model. The volcanic eruption of May 18 destroyed most, if not all, of the benthic fauna present at the sites sampled. The cataclysmic events that took place during the explosion modified the topography to the point of eliminating some of the watersheds, while in others the riparian vegetation was scoured out, along with the natural substrate of the streams. Of the remaining sites, a very simplified community was found in July, composed primarily of midges and blackfly larvae (98-100 percent of the total number or organisms collected). -- Fueste, 1981 |
