Is the hyporheic zone a source or a sink of DOM?

The role of streams in dissolved organic matter (DOM) fluxes is widely acknowledged, yet the contribution of the hyporheic zone (Hz) to these dynamics remains unclear. For inorganic species of nitrogen, the Hz has been reported to be a sink, but what could be expected for DOM? We propose that the contribution of the Hz to stream DOM dynamics is conditioned by the water connectivity between surface and Hz (i.e., hyporheic flow). As hyporheic flow increases, DOM will tend to be removed by the microbial activity associated to the sediments in the Hz. While as hyporheic flow decreases, the microbial community will release DOM, acting as a source. We tested this hypothesis in two reaches, one with high hyporheic flow (connected reach) and another without hyporheic flow (disconnected reach), combining measurements at reach- and patch-scale of relative hyporheic flow with pore water sampling to determine DOM quantity and properties. We observed that at the reach scale, the connected reach tended to be a sink while the connected one was rather a source. Within the hyporheic zone, at the patch-scale, the areas with low hyporheic flow tended to have higher production of DOM than those more connected. Our results suggest that the contribution of the HZ to reach-scale DOM dynamics may be driven by hyporheic flow, and whether it is a source or a sink will result from the interplay among hyporheic areas with different degree of hyporheic flow.