Competition and nutrient kinetics along a temperature gradient: An experimental test of a mechanistic approach to niche theory

A resource-based model of competition correctly predicted the equilibrium outcome and approximated the population and nutrient dynamics of many nutrient-limited competition experiments with two algal species at temperatures from 8° to 24°C. Silicate growth kinetics of temperature-acclimated Asterionella formosa and Synedra ulna predicted that Asterionella should competitively displace Synedra below 20°C and Synudra should displace Asterionella above 20°C, as was observed. Although the growth kinetics showed no significant difference in silicate requirements at 20°C, Synedra displaced Asterionella for all starting conditions at 20°C. These experiments support a major prediction of an equilibrium theory of competition for a single resource: independent of initial population densities, the species with the lowest resource requirement should displace all competitors. Thus resource requirement, as defined by a mechanistic model, may be appropriate for defining niche boundaries. In single species experiments, fit to the Monod model with a nutrient threshold, the nutrient-saturated growth rate of Synedra increased from 0.16 d^-1 at 8°C to 0.75 at 13°C and above; its half-saturation constant for growth was ca. 5.0 μM SiO₂, independent of temperature from 8° to 24°C. The nutrient-saturated growth rate of Asterionella increased from 0.35 d¹ at 4°C to 0.8 at 13°C, and decreased to 0.66 at 24°C; its half-saturation constant for growth had a temperature-independent value of 2.1 μM SiO₂, from 4° to 13°C, but increased to 10 μM SiO₂, at 24°C. Half-saturation constants for silicate-limited growth may be temperature-independent for temperatures within or below the optimal temperature range, but increase sharply for temperatures above it.