Floodplain hydrogeomorphic connectivity of abandoned channels regulates the content and diversity of dissolved organic matter within sediment

Abstract:

Retention and transformation of organic matter within floodplains has strong implications for carbon budgets, food webs, and the fate and transport of contaminants. Carbon content does not appear to vary greatly across the floodplain on the East River, Colorado, USA, nor does it contain significant trends with distance from the channel. However, carbon content decreases with depth, as expected, and does appear to vary in relation to the degree of hydrologic connectivity with the river channel. Using the relative elevation and distance from the channel as a proxy for connectivity, the quality and diversity of organic matter appears to be associated with hydrologic connectivity of geomorphic surfaces. Results from FT ICR MS indicate that drier geomorphic surfaces like elevated cutbanks retain more OM compounds with increasing depth, compared to more frequently wetted point bars that exhibit a greater loss of OM compounds with increasing depth. Additionally, cutbanks have a lower ratio of protein to lignin than more frequently wetted features like gravel bars, point bars, and abandoned channels, suggesting transformation and mineralization of OM. The composition of OM compounds in abandoned channels and oxbow lakes, however, vary in their (1) age since abandonment, (2) rate of abandoned channel sedimentation, and the (3) relative degree of hydrologic connectivity. Abandoned channels that are partially connected to the floodplain and much more likely to experience frequent wetting-and-drying periods exhibit a significant loss in OM compound abundance and diversity. Less hydrologically connected abandoned channels that may experience anoxic conditions periodically throughout each year exhibit substantially high diversity and abundance of OM compounds. This abundance in OM compounds decreases only minimally with increasing depth in older disconnected abandoned channels.

Retention and transformation of organic matter within floodplains has strong implications for carbon budgets, food webs, and the fate and transport of contaminants. Carbon content does not appear to vary greatly across the floodplain on the East River, Colorado, USA, nor does it contain significant trends with distance from the channel. However, carbon content decreases with depth, as expected, and does appear to vary in relation to the degree of hydrologic connectivity with the river channel. Using the relative elevation and distance from the channel as a proxy for connectivity, the quality and diversity of organic matter appears to be associated with hydrologic connectivity of geomorphic surfaces. Results from FT ICR MS indicate that drier geomorphic surfaces like elevated cutbanks retain more OM compounds with increasing depth, compared to more frequently wetted point bars that exhibit a greater loss of OM compounds with increasing depth. Additionally, cutbanks have a lower ratio of protein to lignin than more frequently wetted features like gravel bars, point bars, and abandoned channels, suggesting transformation and mineralization of OM. The composition of OM compounds in abandoned channels and oxbow lakes, however, vary in their (1) age since abandonment, (2) rate of abandoned channel sedimentation, and the (3) relative degree of hydrologic connectivity. Abandoned channels that are partially connected to the floodplain and much more likely to experience frequent wetting-and-drying periods exhibit a significant loss in OM compound abundance and diversity. Less hydrologically connected abandoned channels that may experience anoxic conditions periodically throughout each year exhibit substantially high diversity and abundance of OM compounds. This abundance in OM compounds decreases only minimally with increasing depth in older disconnected abandoned channels.