Sampling pore water at a centimeter resolution in sandy permeable sediments of lakes, streams, and coastal zones

Concentration gradients in the solid fraction and pore water of modern sediments allow to identify benthic biogeochemical processes and the associated fluxes. Gradients are often obtained from layers sampled below the sediment-water interface from sediment cores. Numerous examples in the literature show results from cores collected in impermeable porous sediments, but very few from permeable sediment cores. Indeed, the acquisition of vertical profiles in sandy permeable sediments with a spatiotemporal resolution equivalent to that of muddy sediments is a challenge. We present here a simple protocol that allows sampling of sandy sediments and their interstitial waters with a vertical resolution of 1 cm. This method is suitable for shallow environments, such as intertidal zones, lakes, lagoons, and stream beds. The method is based on rapid conditioning of hand-collected cores. The cores are pre-cut lengthwise. Immediately after recovery, they are laid horizontally, opened, and sliced. Interstitial water is collected by centrifugation on site shortly after cutting the core with tubes equipped with a 0.2 mu m membrane. Optimally, 30 min is sufficient between core collection and conditioning of 12 sections of a 20-cm long core. The examples shown indicate that the method is reproducible. Reduced dissolved compounds such as Fe(II), Mn(II), and NH4+ show profiles without significant oxidation. At the time of core cutting, small volumes of pore water are seeping out of the core, but for sediments with a permeability of up to 10(-10) m(2), this seepage does not disturb the shape of the pore-water profiles.

Automated summary

Concentration gradients in modern sediments can help identify benthic biogeochemical processes and fluxes, but there is limited research on gradients in permeable sediment cores. A simple protocol is presented here to sample sandy sediments and their interstitial waters with a 1 cm vertical resolution, suitable for shallow environments. This method provides reproducible results for studying dissolved compounds and pore-water profiles.