Thick accumulations of flocculent organic sediment, or floc, are nearly ubiquitous in shallow freshwater ecosystems lacking strong physical disturbance regimes. Despite the prevalence of these sediments in a diversity of shallow water bodies, there is little information on their biogeochemical and ecological importance. Importantly, floc forms a transitional zone that is more connected to overlying water than relatively consolidated sediments, and is subject to dynamic variation in reduction-oxidation (redox) status. In Fall 2014, we monitored heat-exchange processes across the floc-water interface using vertical fiber-optic high-resolution distributed temperature sensors (HRTS) to reveal the timing and modes of hydrologic exchange (i.e., conduction vs. advection dominated) in thick floc layers (e.g. up to 1 m) of a through-flow wetland. The profiles revealed discrete zones of floc influenced by groundwater upwelling that limited vertical heat and solute exchange from surface waters, while other profiles showed deeper exchange patterns. The vertical porewater chemical profiles collected adjacent to each HRTS showed diel changes in redox sensitive solutes along the shallow profiles. This poster explores how dominant hydrologic exchange processes influence biogeochemical gradients in floc.