Shift-up and the nitrate kinetics of phytoplankton in upwelling systems

The Michaelis-Menten model of enzyme kinetics has been applied widely to nutrient uptake in phytoplankton. Practically, the heavy nitrogen isotope (15N) is used to measure nitrogen incorporation, and the uptake velocity of the Michaelis-Menten expression has been obtained by normalizing the rate to particulate N. NO3‒ uptake velocity normalized to total particulate organic N (PON) in newly upwelled phytoplankton increases over several generation times, a process that has been described by the term “shift-up” borrowed from bacterial physiology. Michaelis-Menten kinetics do not adequately describe the process and more complicated descriptions have evolved in the literature. In upwelling systems the proportion of phytoplankton biomass in the PON increases with time, affecting calculated NO3‒ uptake velocities. A simple model demonstrates that shift-up and related phenomena may be the result of normalizing to particulate N with a time-dependent phytoplankton N content.