Oregon Sea Grant

Streams that dry during part of the year are common throughout the world, yet studies of themacroinverte​brate assemblages in these types of streams are rare compared to those in permanentstream​s; and studies that assess the effects of agriculture on temporary stream invertebrates are evenrarer. We studied macroinvertebra​te assemblages in lowland temporary streams of a region with highagricultura​l land use, the southern Willamette Valley, Oregon, USA. Overall assemblages were dominatedby non-insects, and invertebrates tolerant of organic pollution. Nonetheless, these invertebratesdi​splayed adaptations to life in temporary habitats, and as such they may be unique to temporary streamsand seasonal wetlands, providing an important addition to regional biodiversity. Stream invertebratesar​e also important as a prey base for native fish and amphibians using these channels. Benthicinverteb​rate densities were higher at sites with slower water and more in-stream vegetation; to a lesserdegree greater agricultural land use was associated with lower densities. Taxon richness was alsonegatively affected by agriculture, but this was most evident when least disturbed and highly agriculturalsit​es were compared. Sites in watersheds with a lower proportion of their area under agriculture (mostlywest of the Willamette River) had a variety of taxa in disturbance-​sensitive insect orders Ephemeroptera,P​lecoptera, and Trichoptera (EPT), plus flies in the family Simuliidae present. In addition, they had greaterrelative abundances of 2 types of flies in the family Chironomidae. In contrast, sites in watersheds withhigh agricultural land use (mainly east of the Willamette River) had greater relative abundances of noninsects, including ostracods, nematodes, and oligochaete worms. In highly agricultural watersheds, when stream-bottom vegetation was abundant, it was associated with greater benthic invertebrate density, butnot with higher taxon richness. Our results suggest that increasing stream-bottom vegetation could beuseful when food is limiting for native vertebrates. On the other hand, reduced agricultural land useallows for the development of more diverse benthic invertebrate assemblages.

Authors: Gerth, William J.; Li, Judith; Giannico, Guillermo R.