Resource requirements of Asterionella formosa and Fragilaria crotonensis in oligotrophic alpine lakes:: implications for recent phytoplankton community reorganizations

A widespread increase in the relative abundances of Asterionella formosa and Fragilaria crotonensis has occurred in oligotrophic alpine lakes across the western United States. Previous investigations have suggested that enhanced atmospheric nitrogen (N) deposition is driving these shifts in diatom community structure; however, little information is available on N requirements of these taxa. We examined the distributions of these two taxa in relation to a variety of physicochemical parameters in a suite of lakes situated in the Beartooth Mountain Range (Montana-Wyoming, USA). We also conducted a series of nutrient enrichment experiments to assess the response of these taxa to changes in N, phosphorus (P), and silica (Si) supply. The distributions of both taxa were positively correlated with C:P, N:P, and Si:P seston ratios, revealing that these taxa are abundant when P availability is very low and the supply of N and Si are moderate to high. In the enrichment experiments, both taxa responded strongly to N additions, whereas P or Si enrichment alone had no effect. While these two taxa are indicative of P enrichment in temperate lakes, our results indicate that in these oligotrophic alpine lakes, N enrichment is driving their recent increase.