Ammonium uptake and growth models in marine diatoms: Monod and Droop revisited

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 approachesMonod 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.

Modeling ecosystem disruptive algal blooms: positive feedback mechanisms

“…and T.p. were estimated previously by Sunda et al (2009), and those for A.l. were estimated for this analysis using similar procedures (Supplement 2).…”

“…com/ articles/ suppl/m447p031_supp.pdf). For all 3 phytoplankton species, specific growth rates (μ i ) were modeled using a modified Droop equation (Supplement 1; Sunda et al 2009), in which growth is a function of the cellular N:C ratio. Parameter values for T.w.…”

“…and T.p. were modeled using a modified Michaelis-Menten equation (Supplement 1; Sunda et al 2009). Uptake rates for both species increased approximately linearly with nutrient concentration until saturating at a maximum rate.…”

“…and T.w. (Sunda & Hardison 2007, Sunda et al 2009). A similar approach was then used here to model data for ammonium limitation of Aureoumbra lagunensis published previously (Sunda & Hardison 2007.…”

Dissolved organic matter (DOM) release by phytoplankton in the contemporary and future ocean

“…Most models of phytoplankton growth use ratios, as nutrient quotas are normalized to carbon (e.g. Geider et al, 1998;Sunda et al, 2009). Determining the degree to which C : N : P stoichiometry of phytoplankton deviates from the canonical Redfield ratio of 106 : 16 : 1 is seen as an important step in understanding of the role of phytoplankton in biogeochemical cycling (Falkowski, 2000;Geider & La Roche, 2002).…”