Long-term atmospheric deposition of nitrogen, phosphorus and sulfate in a large oligotrophic lake

We documented significantly increasing trends in atmospheric loading of ammonium (NH(4)) and nitrate/nitrite (NO(2/3)) and decreasing trends in total phosphorus (P) and sulfate (SO(4)) to Flathead Lake, Montana, from 1985 to 2004. Atmospheric loading of NO(2/3) and NH(4) increased by 48 and 198% and total P and SO(4) decreased by 135 and 39%. The molar ratio of TN:TP also increased significantly. Severe air inversions occurred periodically year-round and increased the potential for substantial nutrient loading from even small local sources. Correlations between our loading data and various measures of air quality in the basin (e.g., particulate matter <10 µm in size, aerosol fine soil mass, aerosol nutrient species, aerosol index, hectares burned) suggest that dust and smoke are important sources. Ammonium was the primary form of N in atmospheric deposition, whereas NO(3) was the primary N form in tributary inputs. Atmospheric loading of NH(4) to Flathead Lake averaged 44% of the total load and on some years exceeded tributary loading. Primary productivity in the lake is colimited by both N and P most of the year; and in years of high atmospheric loading of inorganic N, deposition may account for up to 6.9% of carbon converted to biomass.