Sheetflow Effects and Canal Backfilling on Sediment Source and Transport in Everglades Freshwater Marshes: Analysis of Molecular Organic Biomarkers

Abstract:

Historic freshwater sheetflow in the Florida Everglades distributed sediment to form a ridge-and-slough landscape. However, drainage along with reduction and obstruction of flow has resulted in degradation of this ridged topography. The DECOMP Physical Model is a landscape-scale project aiming to reestablish natural sheetflow to the central and southern Everglades by redesigning barriers to flow. To validate proof of concept that increased flow will rebuild ridge-slough microtopography, biomarker proxies were established for ridge and slough organic matter sources. In addition, partial and complete canal backfill options were assessed via sediment trap accumulation in each backfill treatment area. Flocculent matter (floc) and sediment samples were collected, solvent extracted, chromatographically separated, and analyzed on a GC/MS using internal standard for quantification. Four molecular organic biomarkers were evaluated: the aquatic proxy (P_aq), highly-branched isoprenoids (C₂₀ HBI), kaurenes and botyrococcenes. P_aq, an aquatic proxy of mid to long-chain n-alkanes, was shown to clearly differentiate between ridge-derived and slough-derived organic matter with P_aq values increasing along ridge-to-slough transects. Kaurenes indicated presence of ridge-derived organic matter while C₂₀ HBI and botyrococcenes were indicative of periphyton-derived organic matter which is commonly more abundant in sloughs. Biomarker distributions during both low (present day) and high (managed) water flow through the DECOMP experimental parcel were determined and discussed comparatively.