Impact of bioroughness on interfacia solute exchange in permeable sediments

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 ident~fy small-scale horizontal pressure gradients of < 1 Pa cm-' 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-') compared to smooth-bed fluxes. Advective porewater flows were associated with structures as small as 700 pm at water flows as low as 3 cm S-' 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 rest~ng crabs, produced pressure patterns similar to those recorded for small mounds, and at an abundance of 17 m-' 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.

Enhanced benthic activity in sandy sublittoral sediments: Evidence from13C tracer experiments

“…Uptake down to 6 cm sediment depth was found after 132 h. One possible explanation for this transport is advective porewater flow, although at our station with a permeability of 3.029/1.66 )/ 10 (12 m 2 , advection should only cause transport down to 2 cm into the sediment (Huettel & Gust 1992). Recent results from Janssen et al (2005) using the in situ chamber system Sandy at the same station suggest that it is even less likely that advection is the predominant transport process at our station.…”

“…The amount of carbon added to each chamber corresponded to 0.31 g C m (2 (HE 145) and 0.36 g C m (2 (HE 148). The experiments were performed in the dark in acrylic cylindrical chambers (Huettel & Gust 1992), 20 cm in diameter and 31 cm in height. A horizontal disk stirred a water column of approximately 10 cm height at 20 rpm.…”

Calcification acidifies the microenvironment of a benthic foraminifer (Ammonia sp.)

“…The Petri dish was filled with natural seawater of reduced salinity (to about 26) and contained >30 individuals. The friction velocity at the bottom of the Petri dish was adjusted to 0.2 cm s − 1 by measuring particle movement over the bottom and directing an air jet onto the water surface, to emulate natural flow conditions of tidal flat sediment surfaces (Huettel and Gust, 1992;Shimeta et al, 2001). Temperature and pH (total scale) in the bulk seawater were measured using a micro-thermometer and a handheld pH meter (WTW pH 330i), respectively.…”

Quantifying biologically and physically induced flow and tracer dynamics in permeable sediments

“…Physically driven pore water flow may result from wave action leading to oscillatory pressure fields and currents (Riedl et al, 1972;Precht and Huettel, 2003) or from unidirectional bottom currents interacting with the topography of the sediment-water interface (Thibodeaux and Boyle, 1987). This bottom topography may have a pure physical origin, such as sand ripples induced by currents, or can be created by biology, such as the pits and sediment mounds of various benthic organisms, like mud shrimp, lugworms and rays (Huettel and Gust, 1992b;Ziebis et al, 1996). Yet, pore water flow may also be directly induced by burrowing organisms, a process typically referred to as bio-irrigation (Aller, 2001;Meysman et al, 2006a).…”