The effects of geology and land use on the export of phosphorus from watersheds

Abstract The export of total phosphorus from 34 watersheds in Southern Ontario was measured over a 20-month period. The annual average export for igneous watersheds (i.e. those of the Canadian Shield) that were forested was 4.8 mg m−2 yr−1, significantly different from the average (11.0 mg m−2 yr−1) for watersheds that included pasture as well as forest. Similarly, on sedimentary rock, the mean export from forested watersheds (10.7 mg m−2 yr−1) differed significantly from those with forest and pasture (28.8 mg m−2 yr−1). The differences between watersheds of different geology but similar land use were also highly significant. Additional data from the literature supported our conclusions. Other forested igneous watersheds of plutonic origin averaged 4.2 mg m−2 yr−1 of total phosphorus exported: forested igneous watersheds of volcanic origin, however, averaged 72 mg m−2 yr−1. The overall average export from each type of watershed as classified by geology and land use was very similar to that for the same classification found in our study. The effects of agriculture and urbanization were to greatly increase the total phosphorus exported. Wide ranges of values probably reflect the intensity of land use.

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