Interactive effects of nitrogen and phosphorus on growth and stoichiometry of lake phytoplankton

Phytoplankton responses to nutrient enrichment are often assumed to be universal, but in practice they can significantly vary because the effect size to an increased supply of one nutrient may depend on the availability of the other. Here, we used two complementary two-way factorial experiments to determine how responses of lake phytoplankton to increased N and P supply vary with increasing concentrations of the other nutrient. We manipulated dissolved N and P concentrations in a 4-d bioassay conducted with lake phytoplankton and measured/determined chlorophyll a and carbon (C)-specific growth rates, phytoplankton size-classes, chlorophyll-specific N and P removal, and seston C : N : P ratios. These data were used to assess the presence and type of interactive effects between N and P on phytoplankton. For most response variables, the effects of increased N supply depended on the background concentration of P and vice versa. The specific nature of effects differed among response variables and depended on the N and P concentrations present in the bioassay. Generally, increases of N or P alone elicited few and weaker effects than increasing the supply of one element in combination with the other. Overall, our results show that phytoplankton responses to increased N or P supplies are context dependent and likely reflect multiple processes involved in nutrient uptake and use. These results also show that phytoplankton responses to N and P supply ratios can depend on the supply rate of the other nutrient, which complicates understanding how single nutrient enrichment affects lake phytoplankton producers.