Determinants of headwater stream condition in central Iowa: An empirical study

Headwater streams constitute the majority of stream length of river systems, and have important roles in terms of landscape processes and as habitat for organisms. These bodies of water are also tightly coupled with the local terrestrial landscape, making them ideal for studying linkages between land use and stream condition. Despite the ubiquity and importance of these streams, they are often overlooked and damaged. Previous research has shown that both urban and agricultural land use negatively impact stream condition, and that the magnitude of anthropogenic effects is generally greater in urban landscapes. Understanding of agricultural and urban land use impacts on streams depends on identifying cause and effect linkages between human system (demographic) features, terrestrial landscape features, and biophysical characteristics of streams. Using correlation analysis, path analysis, and non-metric multidimensional scaling, I simultaneously investigated these relationships in 29 central Iowa watersheds that collectively represented a strong urban-rural gradient. These analyses enabled me to quantify the impact of human land use on headwater streams, and identify direct and indirect mechanisms for human impacts on benthic macroinvertebrate communities. Aquatic macroinvertebrates were a primary focus of this study because they have well-established utility as indicators of stream condition due to their diverse environmental requirements, and importance in food webs. Results suggested that urban land use had a greater negative impact than agricultural land use on stream condition, as indicated by spatial variation in total invertebrate taxon richness and the percentage of the total number of invertebrates that were mayflies (Ephemeroptera), stoneflies (Plecoptera), and caddisflies (Trichoptera). Urban impacts appeared to be caused by pollutants delivered to streams by impervious surfaces, while agricultural effects were the result of nitrogen enrichment from croplands. Regardless of land