The linear relationship between a conservative element and salinity during mixing of water masses is widely used to study biogeochemistry in estuaries and the oceans. Even though nutrient ratios are widely used to determine the limiting nutrient in aquatic environments, the rules of nutrient ratios change through the mixing of freshwater and seawater are still unstudied. This study provides general rules for nutrient ratios change via mixing. A simple mixing model is developed with the aims to illustrate that nutrient ratio is a nonlinear function of salinity, thus, shift in limiting nutrient over the salinity gradient can be simply a result of river water and seawater mixing, albeit complicated by biological consumption or remineralization. This model explains a natural phenomenon that rivers contain relatively high dissolved inorganic nitrogen (DIN) to soluble reactive phosphorus (SRP) ratios start to decrease the ratios as salinity increases when seawater contains higher SRP:DIN ratios. Although additional sources of P have been implicated as the cause for such change, this change can be a result of riverine water and seawater mixing. Four mixing rules are presented here to explain the factors governing the change in nutrient ratios vs. salinity; thus, answering why in some cases variations in nutrient loading and in other cases mixing triggers changes to seasonal limitation status in some estuaries. Shift in nutrient ratios can be explained by the change in nutrient inventories via mixing. After the P-limited riverine water shifts in N limitation by mixing with N-limited seawater, new production of the estuary in general becomes limited by the amount of N inputs from the riverine water and the seawater. The result may help to explain a current consensus that N and not P riverine loadings lead to eutrophication in estuaries which are influenced by P-limited riverine waters. Further, new production which is generated by N-limited riverine input and N-limited seawater input mainly depends on the amount of N inputs from the riverine water and the seawater.

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