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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.

To examine seasonal and spatial factors affecting prey consumption by Oncorhynchus trout, we examined trout diet from mainstem and tributary sites at Hinkle Creek, Oregon. Benthic invertebrate densities were similar across seasons and did not differ between tributaries and the mainstem. Fluctuations in diet followed seasonal changes in invertebrate sizes and abundances. Average prey biomass consumed was positively correlated with fish size. Consumption rates were high in spring and summer but fell significantly in fall when fewer and smaller prey were eaten. A switch in consumption from 36% terrestrial prey biomass in spring to 85% in summer coincided with an increase in terrestrial prey size and a decrease in benthic prey size. Location within the watershed also affected prey consumption. Despite similarities in diet composition, tributary trout consumed somewhat more biomass than trout in the mainstem but grew relatively slower. Because stream fishes such as Oncorhynchus trout feed opportunistically on varied prey, studies incorporating multiple seasons and stream types are important to understanding energy exchanges between terrestrial and aquatic ecosystems.

The numerous ponds formed in the weeks following the 1980 eruption of Mount St. Helens provided a unique opportunity to investigate macroinvertebrate communities in young (23- to 25-year-old) fishless ponds following a large natural disturbance. Over a 3-year period, Large-bodied macroinvertebrates were sampled from 97 ponds with a range of hydroperiods and biophysical characteristics. Macroinvertebrate communities from these ponds were spatially and temporally variable in spite of their close geographical proximity, shared regional species pool, and similar creation history. We identified 110 taxa, many of which were observed at only one pond or in only one year. Community differences appeared to be influenced more by pond size and riparian tree development rather than by hydroperiod alone, and stochastic colonization events were likely an important factor determining community composition.

Wetland loss throughout the United States has contributed substantially to landscape fragmentation and loss in biodiversity. In Oregon’s Willamette Valley, only 1 % of native-wet prairie habitat still exists, though seasonal wetlands are still common. These hydrogeomorphic Flats wetlands predominately occur on privately owned and actively farmed lands. We studied these wetlands in spring of 2009 and 2010 in order to quantify and compare aquatic invertebrate communities in two agricultural land-use groups (annual and perennial-grass-seed fields) with native-wet prairie habitat. Community composition in native-prairie, including higher taxa richness and greater diversity, differed from farmed wetlands. Invertebrate densities did not differ among land-uses. However, biomass in perennial-grass wetlands was greater than in annual-grass wetlands both years, and during 2009, it was more than in native-prairie. Lower turbidity, lower conductivity levels, and greater availability of rooted vegetation in native-prairie habitat were conditions associated with differences in invertebrate composition among land-uses. Though invertebrate communities in farmed wetlands differed from native-prairie, the importance of these seasonal wetlands in an altered and fragmented Willamette Valley landscape speaks to their potential contribution for the region’s biodiversity and to their inclusion for management of agricultural lands.

Historically, the upper Willamette River valley in western Oregon was characterized by seasonal floods and large expansions of its stream network. During the past century, human activities have altered or eliminated many intermittent stream and floodplain habitats in the valley. As a result, the remaining intermittent streams and ditches, referred to as watercourses, may still provide habitat that is critical for native fish. Our objectives were to determine (1) fish presence, (2) the spatial gradients of fish distribution (including species identity, native versus nonnative status, and numbers), (3) fish use of the intermittent streams as spawning and nursery habitats, and (4) the main factors that influence the numbers of fish and fish species. In the winter and spring of 2002-2003, we examined the distributions of fish species in five subbasins within the Willamette River valley. The sampling sites were in intermittent watercourses that drained grass seed- producing fields. We collected water samples and sampled fish from December to May with minnow traps and an electrofishing unit and collected data on the standard fish habitat variables at all sites in spring. Thirteen fish species were found and only three of them were exotic. The presence of recently hatched and juvenile fish shows that intermittent watercourses offer conditions suitable for spawning and juvenile rearing. The two watershed-scale variables with the most influence on fish species richness were the percentage of the watershed covered by forest and the distance to perennial water, the first of which had a direct relationship to species diversity and the second an inverse relationship. In turn, fish abundance had a negative, albeit modest, relationship with the distance to perennial water. Among local-scale variables, water velocity and conductivity were inversely related to species richness and fish numbers. Our results highlight the relevance of intermittent agricultural watercourses for native fish species in the Willamette River valley and call for the promotion of agricultural conservation practices that benefit farmers while maintaining aquatic biodiversity in floodplain habitats.

Summary 1. To examine the effects of forest harvest practices on headwater stream macroinvertebrates, we compiled a 167 site database with macroinvertebrate, fish, physical habitat and catchment land cover data from the three forested ecoregions in western Oregon. For our analysis, headwater streams were defined by catchment areas