Sensitivity of stoichiometric ratios to temporal variability in streamflow

Increasing nutrient fluxes through river systems can contribute to the eutrophication and degradation of downstream aquatic ecosystems. The severity of these environmental impacts partially depends on the relative abundance of nutrients, which limit primary productivity and species composition. Stoichiometric ratios of nitrogen (N), phosphorus (P) and silicon (Si) in river systems are regulated by river discharge both directly, through the movement of dissolved constituents, and indirectly, through the transport of particulate nutrients adsorbed to sediment. As a result, hydrologic flow patterns can potentially inform predictions on the biogeochemical state of river systems. This study examines 18 independent sub-basins in the Mississippi River Basin to assess the relationship between nutrient ratios and temporal variability in river discharge. Monthly-averaged data show that, in general, N:P ratios increase with increasing river discharge for agriculturally dominated basins, and decrease with increasing discharge for basins with low agricultural activity. This suggests that nitrogen supply is virtually unlimited in rivers that drain agricultural basins and that nitrogen losses may accelerate more rapidly in these watersheds. Seasonal patterns reveal a complex relationship between stoichiometric ratios and streamflow that depends on the relative timing of peak flow with agricultural activity and fertilizer/manure inputs, as well as the dynamics of in-stream nutrient removal and sediment transport.