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 of organic matter were predominantly detritivores. This classification was then compared to a conceptual stream 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 of organisms collected).
The potential for recolonization of stream benthos appears to be relatively high because of the creation of new habitats near backwaters and pools from springs that were exposed by the scouring action of the mudflows. These environments are very stable and provide ideal conditions for algal growth and for chironomid larvae to thrive. It is believed that these "seed habitats," in conjunction with downstream drift from unaffected tributaries and aerial oviposition by winged adults of aquatic insects, will be the principal sources of stream benthos for the affected streams in the coming years. However, successful recolonization will be dependent mainly on substratum and channel stability, with some improvement of the chemical quality of the stream water.
Several lakes and Coldwater Creek, north of the volcano, as well as some lakes formed on top of the mud flow that went down the North Fork Toutle River valley, were visited in July to document survival of stream and lake benthos and to determine whether the new lakes had indications of colonization by stream benthos. Of the lakes visited, Spirit Lake, Lake St. Helens, and Tradedollar Lake, the latter had the greatest variety of lake benthos. Mosquito larvae (Culex sp.) were the only organisms found in Spirit Lake. Coldwater Creek contained rat-tailed maggots (Tubifera sp. (?)) in areas of decaying organic matter. Numerous chironomids (midges) and blackfly larvae were found on some of the lakes on the mudflow area.
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