The Droop model: producer growth limited by internal nutrient quota Q (Liebig's law of the minimum). dP/dt = μ(Q)P − dP, dQ/dt = ρ(S) − μ(Q)Q, dS/dt = −ρ(S)P + recycling. Carbon:Nitrogen:Phosphorus ratios (Redfield ratio C:N:P = 106:16:1) shape ecosystem dynamics.