Abstract
Simple general equations are used to show that when riverine water mixes with seawater, the N : P ratio is a nonlinear function of salinity. Thus, a shift in the limiting‐nutrient status at a particular salinity along a continuum is a mathematical certainty, albeit complicated by biological consumption and remineralization. The salinity where the shift occurs is a function of biological nutrient uptake ratio. Four general modes, each with a corresponding formula, are deduced here to explain the factors that govern changes in nutrient ratios vs. salinity. These formulas help to explain why changes in the seasonal nutrient limitation status in some estuaries are in some cases triggered by variations in nutrient loading, and in other cases by mixing.