Monday, August 6, 2007

PS 1-8: Response of riparian invertebrate predators to allocthonous prey availability in two rivers impacted by surface water diversions

Sandra J. DeBano, David E. Wooster, and Philip Brown. Oregon State University

Most rivers in the western United States are managed, to some degree, to provide surface water for human uses. In many areas, irrigated agriculture is highly dependent upon surface water withdrawals, yet its effect on many aspects of riparian and aquatic food webs is poorly understood. We hypothesized that changes in stream condition associated with surface water withdrawals would impact terrestrial riparian predators through their effect on the abundance of emerging aquatic invertebrates. We tested this hypothesis in two fifth order rivers in eastern Oregon (the Umatilla and Walla Walla) that are highly impacted by water withdrawal. Terrestrial invertebrate predators were sampled with pitfall traps and emerging aquatic invertebrates were sampled with emergence traps in mid and late summer of 2006. In both river systems and both time periods, the most common riparian invertebrate predators were ants, carabid and staphylinid beetles, spiders, and harvestmen. In the Walla Walla River, chironomids formed a major component (85%) of emerging invertebrates during both sampling periods. In the Umatilla, chironomids were also important, but other groups, including aquatic pyralid moths, baetid mayflies, and hydropsychid and hydroptilid caddisflies were also abundant. Certain predator groups appeared to track availability of some components of emerging aquatic invertebrates, although these patterns varied by sampling period and river. For example, carabid beetle abundance was significantly positively correlated with pyralid moth emergence on the Umatilla River in late summer, when pyralid emergence peaked. However, the abundance of most emerging invertebrate groups did not vary predictably with water withdrawal, suggesting that riparian invertebrate predators may not be strongly affected by water withdrawal impacts on allocthonous prey availability

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