Stream invertebrate communities, water quality, and land-use patterns in an agricultural drainage basin of northeastern Nebraska, USA

We used invertebrate bioassessment, habitat analysis, geographic informatio n system analysis of land use, and water chemistry monitoring to evaluate t ributaries of a degraded northeast Nebraska, USA, reservoir. Bimonthly inve rtebrate collections and monthly water chemistry samples were collected for two years on six stream reaches to identify sources contributing to reserv oir degradation and test suitability of standard rapid bioassessment method s in this region. A composite biotic index composed of seven commonly used metrics was effective for distinguishing between differentially impacted si tes and responded to a variety of disturbances. individual metrics var led greatly in precision and ability to discriminate between relatively impacte d and unimpacted stream reaches. A modified Hilsenhoff index showed the hig hest precision (reference site CV = 0.08) but was least effective at discri minating among sites. Percent dominance and the EPT (number of Ephemeropter a, Plecoptera, and Trichoptera taxa) metrics were most effective at discrim inating between sites and exhibited intermediate precision. A trend of high er bi otic integrity during summer was evident, indicating seasonal correct ions should differ from other regions. Poor correlations were evident betwe en water chemistry variables and bioassessment results. However, land-use f actors, particularly within 18-m riparian zones, were correlated with bioas sessment scores. For example, there was a strong negative correlation betwe en percentage of rangeland in 18-m riparian zones and percentage of dominan ce in streams (r(2) = 0.90, P < 0.01). Results demonstrate that standard ra pid bioassessment methods, with some modifications, are effective for use i n this agricultural region of the Great Plains and that riparian land use m ay be the best predictor of stream biotic integrity.