Data from: Phytoplankton responses to nitrogen enrichment in Pacific Northwest, USA mountain lakes

Limited information is available about threshold lake nitrogen concentrations necessary to stimulate phytoplankton species and biomass responses in remote nitrogen-limited mountain lakes. We conducted in situ enrichment bioassays in mountain lakes within Mount Rainier, North Cascades, and Olympic National Parks in Washington State, USA to characterize phytoplankton species and biomass responses to nitrogen enrichment, and associated dissolved inorganic nitrogen (DIN) concentration thresholds. Based on biomass and growth measurements, phytoplankton were nitrogen-limited or co-limited by nitrogen and phosphorus in the nine bioassay lakes. We identified 20 taxa that responded to nitrogen enrichment, and estimated response thresholds using nitrogen Monod half-saturation constants (K s) for 18 of these taxa. DIN thresholds in nitrogen-limited lakes were 13 μg N l−1 for any increase in chlorophyll a, and 25 μg N l−1, for an increase beyond typical inter-annual chlorophyll a variation. K s values ranged from 0.02 to 77 μg N l−1 across most N-responsive taxa, and diatom K s values were higher than those previously quantified in U.S. Rocky Mountain lakes. Approximately, 75% of sampled mountain lakes in the parks have summer dissolved inorganic nitrogen concentrations below biomass response thresholds. This finding suggests that phytoplankton in park mountain lakes are likely sensitive to future deposition-induced lake nitrogen enrichment.experiments_phyto_12_26_15cell densities of phytoplankton taxa in initial lake water and in each experimental replicate. column definitions are provided in ReadMe.txt.park_lakes_db675 dissolved inorganic nitrogen (NO3-N and NH3-N) observations from lakes above 1,200 m sampled between 1988 and 2014 across Mount Rainier, North Cascades, and Olympic National Parks. The database was compiled by Jason Williams. Column definitions and information about data origins and compilation are provided in ReadMe.txtChlachlorophyll a concentrations in initial lake water and in experimental replicates. column definitions are provided in ReadMe.tx