Atmospheric Ammonia Deposition: A Significant Source of Nitrogen to an Oligotrophic Lake and Its Watershed

Abstract The increasing frequency of toxic cyanobacterial blooms (TCBs) in freshwater lakes in recent years has been problematic, both in terms of our understanding of basic drivers, and attempts to manage them. While phosphorus has been determined to limit the growth of blooms, some cyanophytes manufacture toxins with a high nitrogen (N) demand, so toxin production may be N‐limited. Skaneateles Lake, an oligotrophic lake in New York, has experienced TCBs in recent years, despite its status as one of the cleanest lakes in the United States, and strict watershed regulations regarding agricultural and other nutrient sources. This study investigated whether gaseous ammonia (NH 3 ) deposition contributes to the nitrogen (N) load in the lake, potentially exacerbating the toxicity of these blooms. Using a network of NH 3 air samplers, we estimated both direct and indirect NH 3 deposition to the lake, and compared these fluxes to other sources of N to the lake and its watershed. Follow up sampling campaigns were conducted to extend the deposition estimates to Skaneateles into a second and third year, and to compare estimates of NH 3 deposition to nearby Owasco Lake. Our findings indicate that, not including potential contributions from reduced N deposition to the watershed, dry gaseous NH 3 deposition directly to Skaneateles Lake alone represents 11%–12% of the watershed N load to the lake with ammonium (NH 4 + ) deposition contributing another 7%–10% suggesting that strategies to manage atmospheric reduced N emissions from nearby agricultural sources could reduce the N loading to Skaneateles.