Unintended stoichiometric consequences of Michaelis-Menten nutrient uptake

Background/Question/Methods

Inorganic nitrogen (N) and phosphorus (P) often limit primary production in the euphotic zone and much effort has been directed toward understanding the influence of inorganic N:P stoichiometry on phytoplankton growth. Difficulties associated with studying the interaction between inorganic N and P availability and phytoplankton N and P uptake in situ have lead to widespread laboratory experimentation and development of mathematical models. We derive a simple prediction about euphotic zone N:P stoichiometry from models employing the Michaelis-Menten function to characterize N and P uptake kinetics: the ratio of phytoplankton N:P to inorganic N:P in the euphotic zone equals the ratio of phytoplankton maximum uptake rates of N and P. Next, we quantify this prediction using nutrient uptake parameters estimated from laboratory growth experiments and compare the relationship predicted from models and laboratory data to empirical observations. Results/Conclusions The model predictions for the ratio of phytoplankton N:P to inorganic N:P are at odds with the ma jority of data from extensive long-term oceanic sampling in the Atlantic and the Pacific. This discrepancy potentially calls into question predictions of ecosystem models that explicitly describe phytoplankton growth as a function of N and P availability, and needs to be addressed by performing more comprehensive N and P uptake experiments and by re-examining how we model nutrient uptake.