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Presented at the NABS Annual meeting, Pittsburgh, Pennsylvania, 2002
in Biogeochemistry I
INFLUENCE OF A NEWLY CONSTRUCTED BEAVER DAM ON NUTRIENT AND POC TRANSPORT/RETENTION IN A SAND BED STREAM, THE SHINGOBEE RIVER, MN, USA.
F.J. Triska1, A.P. Jackman2, J.H. Duff1, and R.J. Avanzino1. 1U.S. Geological Survey, Menlo Park, CA 94025, 2University of California at Davis, Davis, CA 95616
Wherever beavers colonize lowland streams the overall impact is profound including: flooding riparian vegetation, killing bank-side trees, creating distinct vegetation patches and producing "beaver meadows" as abandoned dams are breached. Previous studies have typically considered overall habitat impacts since dam age is typically unknown and channel characteristics prior to colonization are unavailable. Here we examine the impact of an ~9 month-old beaver dam on DIN and POC transport/retention using upstream-downstream comparisons, ammonium/tracer injections and comparisons with pre-impoundment data. The new beaver dam drastically altered channel properties increasing mean width from 4.4±1.2 m to 17.6± 3.9 m. Mean channel depth increased from 20±6 cm near the face of the dam to 110±15 cm tapering to normal 180-200 m upstream. The dam effectively reduced SPOM load from 3.1± 0.05 mg L-1 upstream to 1.1±0.05 mg L-1 at the dam under base flow conditions. In a former scour pool ~160 m upstream from the dam, SPOM accumulated to a depth of ~25 cm. Pore water ammonium at 10 cm depth increased from 25-95 µg-N L-1 (prior to inundation), to 11240 µg-N L-1, presumably from ammonification of deposited POM. A bromide tracer- ammonium injection study indicated retention of ammonium in both the beaver pond and an equivalent length of free flowing channel downstream. However, comparing tracer dilution relative to ammonium disappearance indicates that most ammonium retention occurred behind the beaver dam. New beaver impoundments actively store and transform dissolved and particulate nutrient phases resulting in long-term retention of nutrients from the adjacent terrestrial landscape.
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