Influences of beaver (Castor canadensis) on the storage of sediment and nitrogen removal in low order streams on the Des Moines Lobe, Iowa, USA.
Date
2024-08
Authors
Rupiper, Andrew Wayne
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Beck, William
Isenhart, Thomas
Moore, Peter
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Abstract
Beaver (Castor canadensis) were effectively extirpated from the Iowa landscape around the turn of the 19th century. Known widely as premier ecosystem engineers, these fastidious rodents have been shown to impart significant impacts on stream hydrology, sediment dynamics, and nutrient processing, but there is a significant knowledge gap in their efforts in the agricultural Midwest. The Mississippi River Basin (MRB), particularly the Western Corn Belt (WCB) in Iowa, has undergone significant land use change and hydrologic alteration due to historic agricultural intensification. These changes have led to “flashy” stream hydrology and accelerated nutrient loading within WCB watersheds. This study, a multi-year field and laboratory investigation, was conducted primarily in two agriculturally dominated watersheds, Caton Branch (CAT) and Prairie Creek (SME) on the Des Moines Lobe landform in central Iowa. As beaver populations recover across this region, we examined the ability of their damming efforts to preferentially remove excess nitrogen in the form of nitrate (NO3-N) and to store fine mobile sediment (FMS). Field sampling for water quality, probing of FMS volume, and cores extracted from the stream bed material were processed with an ex-situ tube-based assay and later analyzed for physiochemical properties. The sediment core assay findings indicate that beaver dam sediments have higher median N removal rates (0.59 ± 0.04, 0.46 ± 0.02 g/m²/day) than corresponding undammed reference reaches (0.24 ± 0.02, 0.26 ± 0.03 g/m²/day). When applied to dam site surface areas and temperature corrected for the study period, the total N-removal results aligned with grab sample reduction modeling. The tube assay estimated a NO3-N load reduction of 6.23 Mg at CAT and 2.31 Mg at SME during the study period, or 11.1% and 4.1% of total growing season loads retained in the beaver dam system, respectively. Field grab sample modeling estimated load reductions of 6.4 Mg at CAT and 3.4 Mg at SME, or 11.4% and 6.1% reduction of growing season loads, respectively. FMS was significantly greater in dammed stream segments in total mass (p < .0001), with as much as 372,608 kg in the CAT study dam, as well as a normalized per unit area storage (p < .001). Total phosphorous concentrations of FMS in dam pools were 259.03 ± 18.26 ppm at CAT and 366.65 ± 16.10 ppm at SME and when normalized per unit area, dammed segments stored between 17 and 73 g/m2. This study provides insights into recognizing beaver dams as a cost-effective method for improving water quality in agricultural landscapes heavily impacted by accelerated nutrient loading, especially when paired with other in-stream restoration and edge-of-field practices.
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