Impact of biogenic sediment topography on oxygen fluxes in permeable seabeds

Abstract

Boundary layer flows, interacting with roughness elements at the sediment surface, alter the small-scale flow regime. Consequently, pressure differences are generated that are the driving forces for advective pore- water flow. We investigated topography-induced transport of oxygen in a permeable coastal sediment from the Mediterranean Sea (Isola del Giglio, Italy). The sediment surface was characterized by a high abundance (120 m-2) of sediment mounds (average height: 4 cm) built by the mud shrimp Callianassa truncata (Decapoda, Thalassinidea). Boundary layer flow velocities recorded in situ ranged between 2 and 16 cm s-1. Detailed experiments were performed in a recirculating laboratory flow channel. A natural sediment core, 20 cm deep with a surface area of 0.3 m2, was exposed to a unidirectional flow of varying current velocity (3, 6, 10 cm s-1). The alteration of the small-scale flow regime at a sediment mound was documented by vertical velocity profiles measured in 1 mm resolution with temperature- compensated thermistor probes. Oxygen distribution in the sediment was investigated with Clark-type microelectrodes. At a smooth surface, oxygen penetration depth in the permeable sediment did not exceed 4 mm, independent of flow velocity. In contrast, the topography-induced advective oxygen transport increased significantly with current speed. Oxygen reached down to almost 40 mm at the upstream foot of a 1 cm high sediment mound at a flow velocity of 10 cm s-1. Thus, the oxic sediment volume increased locally by a factor of 4.8 compared to the oxic zone underneath a smooth surface. At a natural abundance of 120 mounds m-2 the oxic sediment volume per m2 seabed was calculated to be 3.3-fold higher than in a seabed with a smooth surface. In a parallel experiment, advective solute transport was also demonstrated in a less permeable sediment (k = 5 x 10-12 m2) from the North Sea intertidal flat. Due to the lower permeability the effect on O2 transport was less than in the Mediterranean sand, but oxygen penetration depth increased locally 2-fold at a sediment mound under a flow velocity of 10 cm s-1. The experiments showed the high spatial and temporal variability of oxygen distribution in a coastal seabed depending on sediment surface topography, boundary layer flow velocities and sediment permeability.