Graduate Thesis Or Dissertation
 

Invertebrate composition and distribution in desert springs of Oregon

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  • In the summers of 1998 and 1999, aquatic invertebrate and plant communities were sampled from nineteen springs in the Warner Basin of southeastern Oregon. Across the landscape, these springs exhibited a broad range in water temperature (5-24°C), pH, conductivity, elevation, and gradient. Within a particular spring, water temperature and chemistry fluctuated little diurnally or annually providing a constant environment for aquatic organisms. Benthic hand net samples, emergence traps, and hand-picking methods were employed to determine the invertebrate composition of each spring. Non-metric Multidimensional Scaling (NMS) invertebrate ordination showed a strong temperature and chemical gradient. For example, invertebrate communities on Abert Rim and Hart Mountain were similar because water temperature, chemistries, and elevation were similar. On the second NMS ordination axis, communities responded to topographic gradients. Differences in the presence of specific taxa in Abert Rim and Hart Mountain springs were related to topographic separation of these sub-basins. For example, only Abert Rim springs contained nemourid stoneflies, Malenka sp., and limniphilid caddisflies, Pseudostenophylax edwarsii. Hart Mountain springs were distinctive in the presence of certain dytiscid beetle and chironomid taxa. TWINSPAN analysis confirmed differences in invertebrate composition in Abert Rim and Hart Mountain springs and identified variation in invertebrate communities within sub-basins. When riparian and emergent plant taxa and plant-type variables were overlaid on the NMS invertebrate ordination, neither were related to invertebrate composition. However, there was a significant correlation between invertebrate taxa and percent open area and percent vegetative cover. Open water may be an important habitat attribute for more active invertebrates such as Labiobaetis sp., a mayfly, and Rhyacophila sp., a free-swimming caddisfly that were correlated with open water and faster-flowing springs in this study. Dixa sp., a midge, was prevalent in marshy systems. Longitudinal patterns of invertebrate taxa richness showed an increase as distance from the spring source increased, and may be related to increased temperature fluctuations as groundwater influences decrease. These springs make a significant contribution to the invertebrate diversity of the Warner Basin; forty-three taxa were collected in this study that have not been found in Warner Basin streams.
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