Patterns of hydrologic control over stream water total nitrogen to total phosphorus ratios

Many ecologists and biogeochemists explore the interaction of the nitrogen (N) and phosphorus (P) cycles by addressing N:P ratios. While N:P ratios are recognized as broadly important to the composition and functioning of lotic ecosystems, the fundamental controls on stream water N:P ratio variation remains poorly understood. Low N:P ratio (less than 16) streams appear more likely in arid climates than in mesic climates, suggesting possible hydrologic or landscape controls. We explored the importance of watershed hydrology to the variation of total N to total P (TN:TP) ratios in stream water, and whether such variation is characteristically different across watershed classes based on mean annual precipitation and median observed TN:TP ratio. Nonparametric scatter plot analysis was applied to normalized TN:TP ratios and associated discharge (Q) measurements from 57 minimally-impacted watersheds from the contiguous United States. At the seasonal scale, TN:TP ratios showed a negative relationship with Q in semiarid climates and a positive relationship with Q in humid climates. Over storm event scales, TN:TP ratios decline with increasing Q across all watershed classes. The results broadly indicate hydrology is an important driver of TN:TP ratio variation over multiple time scales. We hypothesize that the broad differences across watershed classes are driven by variation in the nature of connectivity (frequency and magnitude of connections) of the landscape to streams. A strong physical control of N:P ratios in stream water is in stark contrast to the biological control of N:P ratios in the oceans, suggesting that application of stoichiometric theory—developed using marine systems—to lotic systems requires a broader consideration of controlling factors.