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

The Michaelis-Menten model ofenzyme 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. NO,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. MichaelisMenten 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 NO,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. Acknowledgments I am grateful to many colleagues for discussions that led to the ideas in this note, Janet Campbell, John Cuilen, Hugh Ducklow, John Marra, James McCarthy, Pat Wheeler, and two anonymous reviewers for comments on it, and my wife Jean who assisted with the program. This work was supported by NSF grants OCE 8713166. OCE 87-16908. and OCE 90-00263. This is Bigelow Laboratory Contribution 9 1006. Marine phytoplankton nutrient kinetics for single species and populations are usually modeled with the Michaelis-Menten expression describing enzyme kinetics: