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

PeerJ. 2015 Mar 19:3:e841. doi: 10.7717/peerj.841. eCollection 2015.

Abstract

We documented significantly increasing trends in atmospheric loading of ammonium (NH4) and nitrate/nitrite (NO2/3) and decreasing trends in total phosphorus (P) and sulfate (SO4) to Flathead Lake, Montana, from 1985 to 2004. Atmospheric loading of NO2/3 and NH4 increased by 48 and 198% and total P and SO4 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 NO3 was the primary N form in tributary inputs. Atmospheric loading of NH4 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.

Keywords: Aerosol index; Aerosols; Air inversions; Air quality; Atmospheric deposition; Atmospheric loading; Flathead Lake; Limiting nutrient; Nitrogen; Phosphorus; Sulfate.

Grants and funding

Funding for sample collection and chemical analyses was provided by Montana Department of Environmental Quality Contract #201041, 201042, 202051, and 204041. Support from U.S.E.P.A. #EP-R8-12, PRR812-00509, private donations and the Jessie M. Bierman Professorship, Flathead Lake Biological Station made data analysis and manuscript preparation possible. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.