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Abstract:
We demonstrate the contribution of biogenic sediment microtopography, termed bioroughness, comprising all surface relief created by benthic organisms, on interfacial solute fluxes in permeable beds. Rhodamine-WT dye injected in sandy intertidal sediments revealed strong advective porewater replacement down to 5 cm sediment depth caused by small aggregates of mussel shells exposed to boundary layer flows. In a set of laboratory flume experiments we quantify the impact of various types of bioroughness elements on interfacial solute exchange and identify small-scale horizontal pressure gradients of < 1 Pa cm-1 as the driving force for the advective processes. Roughness size, sediment permeability, and characteristics of the boundary layer flow determined magnitude and depth range of the resulting advective porewater flows, which provided a fast, conveyor-belt-type link between sediment layers as deep as 15 cm and the water column. Simulated bioroughness elements in natural sizes and abundances increased porewater fluxes in sandy sediment up to 7-fold at subcritical flows (friction velocity u* < 0.8 cm s-1) compared to smooth-bed fluxes. Advective porewater flows were associated with structures as small as 700 mum at water flows as low as 3 cm s-1 and sediments with permeabilities of K > 3 Darcy (D). Interfacial fluxes associated with protruding bioroughness exceeded those with recessed structures of comparable size. Elevated biogenic structures just above an undisturbed smooth surface, such as resting crabs, produced pressure patterns similar to those recorded for small mounds, and at an abundance of 17 m-2 increased dye flux up to 5-fold relative to smooth control runs. Moving on 3-dimensional streamlines, bioroughness-driven advection is an important structuring process in permeable sediments, increasing the surface area of geochemical reaction zones and creating a variety of microenvironments for interstitial life. We conclude that roughness-related advective porewater processes play a key role in permeable sediments.
