Constraints on primary producer N:P stoichiometry along N:P supply ratio gradients

A current principle of ecological stoichiometry states that the nitrogen to phosphorus ratio (N:P) of primary producers should closely match that from environmental nutrient supplies. This hypothesis was tested using data from ponds in Michigan, USA, a freshwater mesocosm experiment, a synthesis of studies from diverse systems (cultures, lakes, streams, and marine and terrestrial environments), and simple dynamic models of producer growth and nutrient content. Unlike prior laboratory studies, the N:P stoichiometry of phytoplankton in Michigan ponds clustered around and below the Redfield ratio (7.2:1 by mass), despite wide variation in N:P supply ratios (2:1-63:1 by mass) and the presence of grazers. In a mesocosm experiment, the N:P stoichiometry of phytoplankton cells again deviated from a nearly 1:1 relationship with N:P supply. Phytoplankton seston exhibited lower N:P content than expected at high N:P supply ratios, and often higher N:P content than anticipated at low N:P supply ratios, regardless of herbivore presence. Similar deviations consistently occur in the N:P stoichiometry of algae and plants in the other diverse systems. The models predicted that both high loss rates (sinking, grazing) and physiological limits to nutrient storage capacity could attenuate producer stoichiometry. In the future, research should evaluate how limits to elemental plasticity of producers can influence the role of stoichiometry in structuring communities and ecosystem processes. Key words: aquatic ecosystems; food webs; grazers; nitrogen:phosphorus ratio; N:P; primary producers; quota saturation; Redfield ratio; stoichiometry.