Atmospheric Loading of Phosphorus and Other Chemicals to Lake Michigan

Abstract

Bulk precipitation was collected on a monthly basis in the Lake Michigan basin for 18 months during 1975-1976 to determine atmospheric total phosphorus (TP) loadings to Lake Michigan. The sampling network consisted of bulk collectors at 23 land stations and 2 in-lake buoys located off urban and rural areas. Annual TP loading to Lake Michigan for 1976 based on loading rates of 0.184 and 0.303 μg/cm2 /mo for the north and south basins, respectively, was 1.69 × 106 Kg/year, representing ∼ 16% of the total P budget. North and south basins showed different TP loading properties with ∼ 62% depositing in the south basin. Seasonal variations in loading rates showed maxima in late spring and early summer, and minima in winter. In-lake buoy samplers yielded enhanced loading of TP compared to land stations. Wet-only precipitation concentrations for TP average 15% of bulk deposition values. Weighted-average TP concentrations in bulk precipitation were 0.050 and 0.064 mg/l in the north and south basins, respectively. Atmospheric TP loading was closely correlated to Ca and Mg loading, independent of precipitation amount, and exhibited a large dry fall component. Wind-blown soil and re-entrained dust are believed to be the major source of atmospheric phosphorus addition to Lake Michigan.