LAUSR

Human release of reactive nitrogen (N) to the environment has increased 10-fold since 1860 and is expected to rise by a further ~75% by 2050. Much of this N enters phosphorus (P)-rich, eutrophic lakes in agricultural and urbanized watersheds. While N pollution of eutrophic lakes can promote toxic cyanobacterial growth, some cases of extreme N loading have led to the dominance of chlorophytes (green algae). As N-loads required to shift communities from cyanobacterial to chlorophyte dominance are unclear, we experimentally tested phytoplankton responses to a gradient of N loading in a P-rich lake. Low-to-moderate doses (1-3 mg N L-1 week-1) promoted toxic cyanobacterial dominance and elevated concentrations of the hepatotoxin microcystin. Conversely, loads characteristic of pure urban or agricultural effluents (up to 18 mg N L-1 week-1) led to dominance of chlorophytes over cyanobacteria, and lower microcystin content. This indicates that N loads needed to sustain chlorophyte dominance are uncommon, likely restricted to select shallow lakes directly exposed to urban or agricultural effluents. As most N-pollution regimes in P-rich lakes will favour toxic cyanobacterial dominance, restricting future N pollution will help curb further cyanobacterial dominance in lakes both directly, and by constraining the capacity for future P-loading and climate warming to drive cyanobacterial growth.