Asterionella formosa

Asterionella formosa is common in mesotrophic and eutrophic lakes globally and is one of the most common planktonic diatoms in these lakes in the northern hemisphere. Studies of the seasonality of A. formosa date to the classic works of Lund (1950) in Windemere.

In North America, populations of A. formosa, along with Fragilaria crotonensis Kitton, have been shown to increase greatly with increases of reactive nitrogen (Nr) in oligotrophic alpine lakes of the Rocky Mountains. This increase has been interpreted as a response to atmospheric nutrient enrichment (Saros et al. 2005, Saros et al. 2010). Many diatoms in low nutrient lakes respond with population increases even with relatively low loads of reactive nitrogen. In particular, A. formosa has been used as a marker of “critical loads”, defined as the threshold of the nitrogen deposition rate above which there is a discernible ecological effect (Porter and Johnson 2007).

Although A. formosa is common in mesotrophic and eutrophic lakes globally, it has been increasing in presence and abundance in oligotrophic alpine lakes. This increase is interpreted as a response to atmospheric nutrient enrichment. Diatoms respond, even to relatively low loads of anthropogenic reactive nitrogen, in low nutrient lakes. In particular, A. formosa has been a marker of "critical loads", defined as the threshold of nitrogen deposition rate below which there is no discernible ecological effect (Porter and Johnson 2007). For example, relative abundance of A. formosa in Rocky Mountain National Park lake sediments approaches 60% in some lakes (Wolfe et al. 2003), 20-25% in lakes of the Beartooth Mountains (Saros et al. 2010) and to a lesser extent in shallow lakes of the Tetons (Spaulding et al. 2015). Hoh Lake, in western Washington, increased to more than 20% A. formosa in response to an estimated summer bulk deposition load of 1.0± 0.3 kg N ha-1 yr-1 (Sheibley et al. 2014).