Rethinking Aerobic Respiration in the Hyporheic Zone under Variation in Carbon and Nitrogen Stoichiometry

Hyporheic zones (HZs)zones of groundwater–surface
water mixingare hotspots for dissolved organic matter (DOM)
and nutrient cycling that can disproportionately impact aquatic ecosystem
functions. However, the mechanisms affecting DOM metabolism through
space and time in HZs remain poorly understood. To resolve this gap,
we investigate a recently proposed theory describing trade-offs between
carbon (C) and nitrogen (N) limitations as a key regulator of HZ metabolism.
We propose that throughout the extent of the HZ, a single process
like aerobic respiration (AR) can be limited by both DOM thermodynamics
and N content due to highly variable C/N ratios over short distances
(centimeter scale). To investigate this theory, we used a large flume,
continuous optode measurements of dissolved oxygen (DO), and spatially
and temporally resolved molecular analysis of DOM. Carbon and N limitations
were inferred from changes in the elemental stoichiometric ratio.
We show sequential, depth-stratified relationships of DO with DOM
thermodynamics and organic N that change across centimeter scales.
In the shallow HZ with low C/N, DO was associated with the thermodynamics
of DOM, while deeper in the HZ with higher C/N, DO was associated
with inferred biochemical reactions involving organic N. Collectively,
our results suggest that there are multiple competing processes that
limit AR in the HZ. Resolving this spatiotemporal variation could
improve predictions from mechanistic models, either via more highly
resolved grid cells or by representing AR colimitation by DOM thermodynamics
and organic N.