Point-scale organic-matter decomposition in streambeds is weakly associated with reach-scale respiration

Abstract. Stream and river ecosystems play a central role in the movement and decomposition of particulate organic matter, serving as a conduit between terrestrial hillslopes and coastal environments. Microbe-catalyzed decomposition generates simpler organic molecules that fuel respiration, often in the sediments of these ecosystems. However, the degree of connection between sediment-associated respiration (ERsed) and organic-matter-decomposition potential remains poorly understood. It is also unclear whether organic-matter-decomposition potential is more closely associated with ERsed, whole-ecosystem respiration (ERtot), or water-column respiration (ERwc). We examined the link between particulate organic-matter-decomposition potential – using cellulose-based cotton strips as a standardized substrate – and all three components of respiration across 48 sites in the environmentally diverse Yakima River Basin (Washington State, USA). We hypothesized that decomposition within sediments would be most strongly related to ERsed, but decomposition rates were more closely associated with ERtot, less so with ERsed and not at all with ERwc. This suggests that point-scale particulate organic-matter-decomposition potential within stream/river sediments is more closely associated with integrated system respiration rather than with processes confined to sediments or the water column alone though these relationships were weak overall. Further, across the basin, decomposition rates nearly spanned the previously reported global range for streams and rivers and were best explained by total dissolved nitrogen (TDN), sediment grain size, and aridity of the upstream drainage area. These results highlight the strong influence of land cover and basin-scale biophysical variation on sediment-associated decomposition processes and indicate that mechanistic models of organic-matter decomposition in streams and rivers should account for coupled sediment–water–land interactions.