Agricultural land use and macroinvertebrate assemblages in lowland temporary streams of the Willamette Valley, Oregon, USA

Streams that dry during part of the year are common throughout the world, yet studies of the macroinvertebrate assemblages in these types of streams are rare compared to those in permanent streams; and studies that assess the effects of agriculture on temporary stream invertebrates are even rarer. We studied macroinvertebrate assemblages in lowland temporary streams of a region with high agricultural land use, the southern Willamette Valley, Oregon, USA. Overall assemblages were dominated by non-insects, and invertebrates tolerant of organic pollution. Nonetheless, these invertebrates displayed adaptations to life in temporary habitats, and as such they may be unique to temporary streams and seasonal wetlands, providing an important addition to regional biodiversity. Stream invertebrates are also important as a prey base for native fish and amphibians using these channels. Benthic invertebrate densities were higher at sites with slower water and more in-stream vegetation; to a lesser degree greater agricultural land use was associated with lower densities. Taxon richness was also negatively affected by agriculture, but this was most evident when least disturbed and highly agricultural sites were compared. Sites in watersheds with a lower proportion of their area under agriculture (mostly west of the Willamette River) had a variety of taxa in disturbance-sensitive insect orders Ephemeroptera, Plecoptera, and Trichoptera (EPT), plus flies in the family Simuliidae present. In addition, they had greater relative abundances of 2 types of flies in the family Chironomidae. In contrast, sites in watersheds with high agricultural land use (mainly east of the Willamette River) had greater relative abundances of non-insects, including ostracods, nematodes, and oligochaete worms. In highly agricultural watersheds, when stream-bottom vegetation was abundant, it was associated with greater benthic invertebrate density, but not with higher taxon richness. Our results suggest that increasing stream-bottom vegetation could be useful when food is limiting for native vertebrates. On the other hand, reduced agricultural land use allows for the development of more diverse benthic invertebrate assemblages.