Land Use, Stream Habitat and Benthic Invertebrate Assemblages in a Highly Altered Iowa Watershed
Herringshaw, Cassie J., Stewart, Timothy W., Thompson, Janette R., Anderson, Paul F., The American Midland Naturalist
Knowledge of relationships between land cover (i.e., land use) and abiotic and biotic features of headwater streams enhances our ability to predict and effectively assess conditions in a variety of aquatic ecosystems. We evaluated land use effects on stream condition in an Iowa watershed dominated by intensive row crop agriculture and low-intensity urban development by quantifying relationships among land cover, stream invertebrate assemblages and other stream biophysical characteristics (i.e., invertebrate habitat) at 29 sites. On average, 81% of subbasin land cover was agricultural and 6% of land cover was urban across study sites. High nitrate concentrations (range = 5.6-29.0 mg/L) and high relative abundance of oligochaetes and chironomid midges reflected degraded conditions at all sites. However, agriculture and urban land use appeared to have different effects on stream features. Nitrate concentrations were positively related to agricultural land cover, and turbidity and nitrate concentrations were negatively related to urban land cover (P ≤ 0.05). Invertebrate densities and taxonomic diversity (i.e., total taxa richness, % EPT) were also positively related to agricultural land cover and negatively related to urban land cover. Regardless of land use, highest invertebrate abundance and taxonomic diversity occurred at sites with abundant coarse particulate organic matter, plants and coarse inorganic substrate. Relationships between land cover and invertebrate variables were strong at both local and subbasin measurement scales. Based on invertebrate assemblages, which integrate multiple instream features, we conclude that urban land use had greater adverse effect on stream condition than agriculture in our study watershed. Although impacts of urbanization on stream invertebrates frequently exceed effects of agriculture, this has not previously been demonstrated in Iowa or other Midwestern landscapes so heavily dominated by agriculture.
Biophysical features of headwater streams are tightly coupled with the surrounding terrestrial landscape, and are therefore useful tools for evaluating human land use impacts on aquatic ecosystem condition (i.e., health, integrity; Allan, 2004; Clarke et al, 2008). Landscape change and other activities associated with agricLilture and urban development can adversely affect headwater stream condition in multiple ways. Loss of natural vegetation, construction of artificial drainage systems, increased impervious surface area, application of fertilizers and biocides and discharge of human and animal waste contribute to altered stream hydrology and contaminant inputs to streams (Allan, 2004; Walsh et al, 2005). Increased fluctuations in water flow velocity and discharge (i.e., increased flashiness) can alter stream depth and channel width, and sediment input results in higher turbidity and reduced benthic substrate complexity (Allan, 2004; Walsh et al, 2005). A variety of contaminants cause pH shifts, and high nutrient concentrations promote increased biological production and oxygen demand, and ultimately, low dissolved oxygen concentrations (Walsh et al, 2005; Weijters et al, 2009).
Whereas it is generally agreed that land cover (i.e., land use) is an excellent predictor of human impacts on stream condition, the apparent magnitude of effect on stream condition can depend on spatial scale of measurement (Walsh et al, 2007; Weijters et al, 2009). Furthermore, the appropriate scale of land cover measurement must be identified and used for ecological condition assessment and restoration practices to be successful (Walsh, 2004). Although some investigators have found significant relationships between local (e.g., riparian) land cover and stream condition indicators, results from other studies suggest that land cover measured at the watershed or subbasin scale is the better predictor of stream condition (Walsh, 2004; Walsh et al, 2007). Riparian vegetation can mitigate effects of agricultural or urban land use occurring elsewhere in the watershed by reducing sediment and nutrient inputs to streams, reducing solar irradiance and maintaining lower water temperatures, and contributing organic matter that provides invertebrates and fish with habitat and food (Allan, 2004; Walsh et al, 2007). …