Intercalibration of benthic flux chambers

Abstract

We have compared 14 different sediment incubation chambers, most of them were used on bottom landers. Measurements of mixing time, pressure gradients at the bottom and Diffusive Boundary Layer thickness (DBL) were used to describe the hydrodynamic properties of the chambers and sediment-water solute fluxes of silicate (34 replicates) and oxygen (23 replicates) during three subsequently repeated incubation experiments on a homogenized, macrofauna-free sediment. The silicate fluxes ranged from 0.24 to 1.01 mmol m−2 day−1 and the oxygen fluxes from 9.3 to 22.6 mmol m−2 day−1. There was no statistically significant correlation between measured fluxes and the chamber design or between measured fluxes and hydrodynamic settings suggesting that type of chamber was not important in these flux measurements. For verification of sediment homogeneity, 61 samples of meiofauna were taken and identified to major taxa. In addition, 13 sediment cores were collected, sectioned into 5-10-mm slices and separated into pore water and solid phase. The pore water profiles of dissolved silicate were used to calculate diffusive fluxes of silicate. These fluxes ranged from 0.63 to 0.87 mmol m−2 day−1. All of the collected sediment parameters indicated that the sediment homogenization process had been satisfactorily accomplished. Hydrodynamic variations inside and between chambers are a reflection of the chamber design and the stirring device. In general, pump stirrers with diffusers give a more even distribution of bottom currents and DBL thicknesses than paddle wheel-type stirrers. Most chambers display no or low static differential pressures when the water is mixed at rates of normal use. Consequently, there is a low risk of creating stirrer induced pressure effects on the measured fluxes. Centrally placed stirrers are preferable to off-center placed stirrers which are more difficult to map and do not seem to give any hydrodynamic advantages. A vertically rotating stirrer gives about five times lower static differential pressures at the same stirring speed as the same stirrer mounted horizontally. If the aim is to simulate or mimic resuspension at high flow velocities, it cannot be satisfactorily done in a chamber using a horizontal (standing) rotating impeller (as is the case for most chambers in use) due to the creation of unnatural conditions, i.e. large static differential pressures and premature resuspension at certain locations in the chamber.

The impact of sedimentary alkalinity release on the water column CO<sub>2</sub> system in the North Sea

“…Flux chamber incubations are potentially susceptible to various methodological artifacts resulting from sediment enclosure, which relate to the sensitivity of benthic fluxes to changes in ambient hydrodynamics and altered benthic faunal activity (Santschi et al, 1991;Tengberg et al, 2005;Lehrter et al, 2011). In permeable sediments, fluxes are particularly susceptible to the imposed stirring regime and the associated local pressure gradients that are generated in the flux chamber, which drive the advective pore-water exchange between sediment and overlying water (Huettel and Gust, 1992;Janssen et al, 2005).…”

Eddy correlation measurements of oxygen fluxes in permeable sediments exposed to varying current flow and light

“…The most common way to measure benthic oxygen fluxes is by using in situ chambers. Traditional chamber incubations determine oxygen fluxes for a small, enclosed sediment area (typically , 0.25 m 2 ) from concentration changes measured in the enclosed water through relatively long time periods (hours to days; Wild et al 2004;Tengberg et al 2005). This is an intrusive method that excludes natural flow and exchange of water-bound constituents and can bias flux measurements significantly for permeable sands Berg and Huettel 2008;Glud 2008).…”

Recycling and Burial of Biogenic Silica in an Open Margin Oxygen Minimum Zone

“…They provide the net recycling rate of BSi (BSi rec ): where d [H 4 SiO 4 ]/ dt is the concentration‐time gradient and H is the height of water in the chamber. The uncertainties associated with lander fluxes have been discussed in detail (Dale et al., 2015; Hammond et al., 2004; Tengberg et al., 2005). The mean standard error of each flux was equivalent to ±13% (range 2%–61%).…”