『Abstract
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.
Keywords: Catchment hydrology; Connectivity; N:P ratios; Stoichiometry;
Stream ecology』
Introduction
Methods
Nutrient and discharge data
Data analysis
Results
Total variability
Seasonal patterns
Seasonal relationship between TN:TP ratios and discharge
Isolating the relationship between TN:TP ratios and discharge
Discussion
Hydrologic connections across the landscape and TN:TP ratios
TN:TP ratios at high and low flows
Implications
Acknowledgments
References