MEPS 386:29-41 (2009)  -  DOI: https://doi.org/10.3354/meps08077

ABSTRACT: Mathematical models are a useful tool for predicting the responses of marine phytoplankton to changes in nutrient inputs and other environmental factors. Two modeling approaches—Monod and Droop—have been traditionally used. These 2 model types were fitted to empirical data for specific growth rate, cellular N:C ratio, cellular ammonium uptake rate, and ammonium concentration measured in N-limited cyclostats at different dilution rates and in nutrient-saturated batch cultures. The modeled data were for a small, fast-growing coastal diatom Thalassiosira pseudonana (~4.5 µm diameter) and for a larger, slower growing diatom T. weissflogii (~11 µm diameter) cultured in seawater medium at 20°C and 14 h d^–1 of light. The observed data did not conform well to the classic Monod equation, but could be fit to a modification of this equation in which the maximum growth rate was assigned a value higher than the observed maximum rate. Likewise, data for cellular N uptake rate versus ammonium concentration did not conform well to the standard saturation equation, but could be fit to a modification of the equation in which the maximum uptake rate was set above the empirically measured value and the x-intercept was shifted from the origin to a finite positive value. Both modified models accurately fit the observed steady-state relationships between ammonium concentrations and specific growth rates of the 2 species. However, the 2 models showed different transient dynamics in response to a change in the concentration of inflowing nutrients in time-course simulations. Such differences suggest that the choice between Droop and Monod approaches, when used as part of larger food web models, could lead to widely divergent predictions of algal blooms and other nonequilibrium dynamics of ecosystems.

KEY WORDS: Ammonium · Model · Nutrient uptake · Growth rate · Phytoplankton · Diatom · Monod · Droop