Effect of benthic-pelagic coupling on dissolved organic carbon concentrations in permeable sediments and water column in the northeastern Gulf of Mexico

Large areas of the continental shelf are covered by permeable sand beds that filter substantial volumes of coastal water. This study investigated the temporal changes in and coupling between dissolved organic carbon (DOC) concentrations in the water column and pore water of nearshore permeable sediments in the northeastern Gulf of Mexico. Time series samples, collected in the nearshore environments at an exposed and a nearby protected site, showed very similar patterns of sedimentary DOC concentrations at both sites indicating that large-scale, seasonal, and weather-related processes controlled these distributions. A summer situation, with the sediment surface layer depleted in DOC and upward diffusion of recalcitrant DOC from deeper layers, was separated by a fall transition phase, where increasing winds and waves caused a mixing of the sedimentary DOC, resulting in relatively even concentrations over the investigated sediment depth (0–12 cm). The subsequent winter situation was characterized by increased DOC in the surface layer caused by filtration of degradable organic material into the sands and subsurface removal of degradable DOC. A second transition phase in early spring marked the end of the winter situation, with strong winds and waves and thorough pore water mixing, leading again to even DOC distribution in the investigated sediment layer. This transition phase initiated the next summer situation with depletion of DOC in the sediment surface layers. Our results indicate that DOC in the upper layer (≤12 cm) of the shallow sands is controlled by benthic-pelagic coupling facilitated by relatively rapid solute and particle exchange in the highly permeable sediments at our study sites. The prompt responses of the surface layer DOC concentrations to the changes in the overlying water underline the dynamic character of substrate supply in the permeable sediments, setting them apart from fine-grained cohesive beds characterized by relatively stable DOC distributions.