Changes in Near-Stream Soil Phosphorus Resulting from Extreme-Event Driven Erosion

Movement of sediment, and associated phosphorus, from streambanks to freshwater lakes is predicted to increase with increasing extreme precipitation events. This higher phosphorus load has the potential to accelerate harmful algal blooms in affected water bodies, such as Lake Champlain in Vermont, New York and Quebec. Although it is generally assumed that ‘legacy’ phosphorus (primarily from agricultural activities) will be high in streambank soils in this region, samples taken along many of Vermont’s tributaries have been found relatively low in phosphorus (< 650 mg/kg)—close to the mean for parent material. It appears that the streambanks are largely comprised of reworked sediment that could actually act as a sink for soluble phosphorus. However, intensive agricultural land use adjacent to the streams can result in elevated phosphorus concentrations (> 1400 mg/kg) in soils within the stream corridor. Extreme events have the capacity to move and expand the stream channel, bringing the high-phosphorus soils closer to the actual streambank and increasing the likelihood of erosion. Tropical storm Irene in 2011 was such an event that primarily affected the southeastern Lake Champlain Basin. In the Mad River, a subwatershed of Vermont’s Winooski River, extreme flooding damaged infrastructure and changed the stream channel (see example in figure). We sampled soils at nine stream erosion sites along the Mad River, using a design to measure changes both in depth (to 90 cm) and distance from stream. In addition to total phosphorus, we determined available (soil test) phosphorus and the degree of phosphorus saturation. Where the extent of erosion was high, the present-day near-stream soils were elevated in some or all of the soil phosphorus measurements, reflecting the legacy phosphorus from agricultural land use. Imagery before and after the extreme event also shows a clear loss of well-established riparian buffer. Where erosion was low and the riparian buffer largely intact, near-stream phosphorus values were low. Measurement of stream channel change, coupled with the soils data, will allow an estimate of phosphorus losses due to Irene. An increase in these extreme events will not only increase sediment loads but also likely increase the concentration of phosphorus in the eroded sediment.