Agriculture and Water Quality in the Canadian Great Lakes Basin: III. Phosphorus

Abstract A major objective of the International Joint Commission Pollution from Land Use Activities Reference Group (PLUARG) was to determine the P contribution to the Great Lakes from the various agricultural activities in the basin. In Canada, two approaches were used. The total agricultural contribution was estimated by relating measured P loads at the outlets of 10 representative agricultural watersheds to land characteristics and agricultural activities in the watersheds. In the second approach the P contribution from four sources—runoff from cropland, livestock operations and unimproved land, and erosion of farm streambanks—was estimated from detailed studies of each source. Percent clay in the surface soil and proportion of the area in rowcrop accounted for 85% of the variability in total P unit‐area loads from the agricultural watersheds. Dissolved reactive P unit‐area loads were related to clay content of surface soil and the P added to soils in the watershed ( R 2 = 0.90). The total P load to the Great Lakes from the more than 300 subbasins of the agricultural portion of the Canadian Great Lakes Basin was estimated to be 3,000 t year −1 . The source studies indicated that about 70% of this load was attributable to cropland runoff, 20% to livestock operations, and 10% to a combination of runoff from unimproved land and streambank erosion.