Interactions between nitrogen form, loading rate, and light intensity on Microcystis and Planktothrix growth and microcystin production

•Nitrate, ammonium, and urea enrichment increased biomass and microcystins.•Different N loading rates resulted in similar biomass and microcystins.•Phosphorus only enrichment did not affect biomass and microcystins.•mcyD was up-regulated within 4 h of ammonium and urea enrichment.•Highest light level (300 PAR) with N resulted in greatest biomass and microcystins. The toxin-producing, bloom-forming cyanobacterial genera Microcystis and Planktothrix require fixed nitrogen (N), such as nitrate, ammonium, or organic N (e.g., urea) for growth and production of microcystins (MC). Bioavailable N can enter lakes in pulses via tributary discharge and through in-lake recycling, which can maintain low N concentrations. Additionally, light intensity has been suggested to play a role in MC production. This study examined how three forms of N (nitrate, ammonium, and urea) interacted with N loading rate (one large pulse vs. many small pulses) and light intensity to stimulate Microcystis and Planktothrix growth and MC production using nutrient enrichment experiments. Enrichments of nitrate, ammonium, and urea resulted in greater cyanobacterial biovolumes and MC concentrations than phosphorus-only enrichments, and there was no difference between pulse (100 μmol/L) and press treatments (8.3 μmol/L every 4 h). Analysis of mcyD transcripts showed significant up-regulation within 4 h of ammonium and urea enrichment. High light intensities (300 μmol photons/m2/s) with N enrichment resulted in greater cyanobacterial biovolumes and MC concentrations than lower light intensities (30 and 3 μmol photons/m2/s). Overall, the results suggest Microcystis and Planktothrix can use many forms of N and that high light intensities enhance MC production during elevated N concentrations. Moreover, the results here further demonstrate the importance of considering N, as well as P, in management strategies aimed at mitigating cyanobacterial blooms.