Marsh-ing Through Time: Resolving the temporal and spatial variability of tidal marsh sediment dissolved organic carbon sorption

Abstract

Tidal marsh ecosystems are among the most economically and ecologically valuable environments in the world, providing critical ecosystem services and a continuous exchange of carbon between these systems and their surrounding environments. Tidal marshes are an important overall net carbon sink, while simultaneously being a substantial source of dissolved organic carbon (DOC) to estuaries and the coastal ocean. The temporal and spatial variability in these carbon fluxes is large, difficult to measure, and currently considered to be one of the most daunting challenges to carbon exchange quantification. Sorption, despite being known as a dominant DOC exchange process at the sediment-water interface, is still understudied in tidal marsh ecosystems, with exchange kinetics largely unquantified. This research combined observational data with sediment flux modeling to answer a suite of questions addressing sorption speed, its variability, and its impacts to DOC fluxes between sediments and adjacent waters. Sediment flux models must incorporate sorption processes to more accurately simulate DOC fluxes between tidal marsh sediments and adjacent waters. Kinetics of these processes were quantified for the first time through a set of 24 hour sorption laboratory experiments, from which results showed that the majority of sorption processes occur rapidly, within 15 minutes of sediment exposure to water. Sorption rate parameters were determined through a numerical modeling study that simulated the laboratory experiments. These rates were used to parameterize a sediment flux model that included sorption processes formulated with varying degrees of complexity. The sorption kinetics of individual pools of DOC (colored and non-colored) were also measured, revealing that these separate pools sorb quickly but independently of one another, with preferential adsorption of humic colored DOC over time, and preferential desorption of native non-colored DOC over time. Sorption kinetics were also shown to be spatially variable within a marsh site, with adsorption decreasing with sediment depth and distance from the creek edge. This research provided important new information on sorption in tidal marsh sediments that allows these processes to be incorporated into models, which will, ultimately, facilitate efforts to simulate and quantify coastal carbon fluxes.