Phytoplankton growth and stoichiometry under multiple nutrient limitation

Phytoplankton growth and stoichiometry depend on the availability of multiple nutrients. We use a mathematical model of phytoplankton with flexible stoichiometry to explain patterns of phytoplankton composition in chemostat experiments and nutrient drawdown dynamics that are found in the field. Exponential growth and equilibrium represent two distinct phases, each amenable to mathematical analysis. In a chemostat at a fixed dilution (growth) rate, phytoplankton stoichiometry matches the nutrient supply stoichiometry over a wide range at low growth rates and over a narrow range at high growth rates. In a chemostat with a fixed nutrient supply stoichiometry, phytoplankton stoichiometry varies with dilution rate nonlinearly, between the supply stoichiometry at low dilution rates and a species-specific optimal ratio at high dilution rates. The flexible-stoichiometry model we study predicts low equilibrium concentrations of two nutrients over a wide range of supply ratios, contrary to the predictions of a traditional fixed-stoichiometry model. The model is in quantitative agreement with experimental data, except at extreme nutrient supply ratios, which require a negative feedback from quota to uptake to fit the data. Our analysis points to the importance of better understanding the regulation of uptake rates in determining phytoplankton stoichiometry and incorporating this knowledge into phytoplankton models. Phytoplankton require multiple nutrients for growth. Knowledge of how multiple nutrients interact to limit growth is essential to understanding the causes of variation in phytoplankton stoichiometry (Rhee 1978; Goldman et al. 1979), the identity of the nutrient(s) limiting biomass and primary production (Smith 1982), and the effect of resource competition on community structure (Tilman 1982). Of particular interest are nitrogen (N) and phosphorus (P), two macronutrients that are commonly thought to limit phytoplankton (Smith 1982; Downing 1997). Classic chemostat experiments under multiple-nutrient– limited growth conditions were performed in the 1970s and 1980s (Sterner and Elser 2002). Rhee (1978) grew Scenedesmus sp. at a fixed dilution rate with the N : P ratio in the input medium varying from 5 to 80 (by atoms, as throughout this paper). He found that phytoplankton N : P stoichiometry matched the input ratio and that residual nutrients were undetectable. Sterner and Elser (2002) interpreted this as a complete absence of homeostasis over the range of input ratio studies. Other researchers (Goldman et al. 1979; Healey and Hendzel 1979; Ahlgren 1985) fixed the N : P input ratio but controlled the equilibrium growth rate by varying the dilution rate of the chemostat. These studies show that phytoplankton stoichiometry is most variable at low growth rates, with N : P varying from 5 to 100 and carbon (C) : P 1 Corresponding author ([email protected].