The stoichiometry of spiraling: Concurrently assessing nitrogen and phosphorus uptake in streams

With few exceptions, spiraling research has focused on individual elements since its conception.  As climate and landuse change have become pronounced, scientist have become increasingly interested in the coupling of elemental cycles.  In this presentation, we combine theory, modeling, and field measurements to explore the coupling of nitrogen and phosphorus spiraling.  Information on the stoichiometric plasticity of bacteria and algae were combined with existing knowledge of stream function to develop models of coupled N and P spiraling in purely autotrophic and heterotrophic ecosystems.  We assess these predictions using NH₄⁺, NO₃^-, and PO₄^-2 enrichment experiments in several N-limited streams to estimate element-specific uptake lengths and calculate uptake rate ratios (U_N:U_P).  Our field results suggest that uptake ratios largely reflect the relative availability of water column nutrients, but considerable variation exists.  As stream size increased and autotrophy becomes more pronounced, P uptake lengths became insignificant (approached infinity) even under elevated nitrogen conditions.  This observation is consistent with stoichiometric theory if algal assimilation emerged as an important sink for water column nutrients.  We argue that ecological stoichiometry and spiraling provide complimentary frameworks for examining the simultaneous cycling of multiple elements in streams.