The potential contribution of streambanks to phosphorus loads in the Lake Champlain Basin, with a focus on the Missisquoi River

Erosion of streambank soils has been identified as a major source of sediment to Lake Champlain, especially in the Missisquoi River watershed. The phosphorus (P) in this sediment contributes to P loads and may contribute to continued harmful algal blooms. We sampled streambank soils and adjacent land-uses in numerous tributaries of the Lake and measured total P, bioavailable P and the degree of P saturation. Land-use, land-cover included active dairy farms, abandoned agriculture, forests, wetlands and suburban development. In the Missisquoi River watershed, soils in actively managed silage corn and hay crops, and wetlands, were elevated in total P relative to the regional average (600 mg/kg). The total P of the corresponding streambanks of each land-use was statistically significantly lower. Forest soils and nearby streambanks were relatively low in total P. Bioavailable P (estimated with soil test P) was very low in forests, wetlands, and all the streambanks of the different land uses, but elevated in the active agricultural fields. The degree of P saturation averaged 36% in the corn fields, but was less than 21% in all of the streambanks. Little legacy effect from abandoned agriculture was observed, likely because P additions were not historically high. The combination of relatively low soil P and low saturation suggests that the streambank soils have little potential for short-term release of P if eroded into the adjacent streams. However, depending on buffer widths and stream channel behavior, continued erosion could easily lead to deposition of high-P sediment from the active agricultural land-use.