Characterizing organic matter inputs to sediments of small, intermittent, prairie streams: a molecular marker and stable isotope approach

Small rivers and streams are ecologically important because they contribute to the export of organic carbon to coastal environments, likely influencing the global carbon cycle. While organic matter (OM) dynamics in large rivers has been studied in quite some detail, less is known about small streams. Sources of OM in streams ultimately determine its availability to the food web and downstream transport. In this study, sediment samples were collected from the King’s Creek watershed in Konza Prairie (Kansas, USA) and analyzed using molecular biomarkers and bulk 13C stable isotopes with the objective to comparatively assess OM inputs between riparian forest vegetation and watershed grassland to small, intermittent streams. We are interested in the potential influence of woody riparian expansion that has been ongoing at the site. Biomarkers typical of the local C4 grasses (branched n-alkanes, phytadienes) were more abundant in some of the sediments of the upper reaches. The sediments of the lower reaches contained biomarkers of algae (short-chain aliphatic compounds, C25:5 highly branched isoprenoid, brassicasterol) and vascular plant-derived material (triterpenols). Degraded OM (triterpene/triterpenol ratio) was found throughout the watershed with no pattern between the upper and lower reaches. Bulk 13C isotope analysis showed that the upper reaches of the watershed receive significant OM inputs from the C4 grasses (74–99 %) while the lower reaches are more strongly influenced by riparian trees (26–27 %) and algae (21–22 %). These results suggest that the environmental dynamics of bulk OM and the biomarker composition of small prairie streams are highly complex and likely a function of several factors such as light availability, riparian vegetative composition and density, and varying degrees of OM storage, retention and transport along the river continuum