Abstract
Previous research suggests that fertilization of surface waters with chemically reduced nitrogen (N), including ammonium (NH 4 + ), may either enhance or suppress phytoplankton growth. To identify the factors influencing the net effect of NH 4 + , we fertilized natural phytoplankton assemblages from two eutrophic hardwater lakes with growth‐saturating concentrations of NH 4 Cl in 241 incubation experiments conducted biweekly May–August during 1996–2011. Phytoplankton biomass (as chlorophyll a ) was significantly ( p 4 + increased with abundance of diatoms, cryptophytes, and unicellular cyanobacteria, particularly when water temperatures and soluble reactive phosphorus (SRP) concentrations were low. In contrast, phytoplankton was often stimulated by NH 4 + when chlorophytes and non‐N 2 ‐fixing cyanobacteria were abundant, and temperatures and SRP concentrations were high. Progressive intensification of NH 4 + effects over 16 yr reflects changes in both spring (cooler water, increased diatoms and cryptophytes) and summer lake conditions (more chlorophytes, earlier cyanobacteria blooms), suggesting that the seasonal effects of NH 4 + will vary with future climate change and modes of N enrichment.