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Lindsay Chipman
7/12/10

My current research involves investigating benthic oxygen dynamics in sandy sediments using the eddy correlation method, as well as development of the technique.  Oxygen flux measurements at the seafloor are fundamental for our understanding of the cycles of matter (Glud 2008), such as aerobic decomposition of organic matter, animal respiration, and oxidation processes.  Processes that transport water and its dissolved substances through the sediment-water interface include molecular diffusion, bioturbation, bioirrigation, and current- or wave-driven advection.  These processes have a large effect on oxygen availability to sediment communities and thus on oxygen consumption and fluxes.  It is therefore important to measure benthic fluxes in a way that minimally influences boundary layer flows and light.  The eddy-correlation method is a new technique that permits the non-invasive measurement of oxygen flux at the seafloor with high spatial and temporal resolution, and thus, is expected to produce unbiased flux data.  This method was adapted for aquatic environments by Berg et al. (2003). The assumption behind the method is that oxygen travelling vertically towards or away from the sediment is transported by turbulent motion.  By measuring the fluctuating vertical velocity and the corresponding oxygen concentration at the same time and point above the sediment surface, it is possible to derive the vertical oxygen flux.  

The aim of my project is to improve the technology and accessibility of the method.  I am experimenting with optodes as sensors to measure oxygen concentrations in place of the traditionally used electrodes (Fig. 1).  Optodes are optical sensors that measure oxygen based on the interaction of oxygen molecules with a luminescent dye on the tip, which causes a quenching of the luminescence. They have the advantage over electrodes of being more durable in the field conditions to which the system is typically exposed and may therefore be used in a broad range of aquatic environments and turbulent conditions. 

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Fig. 1. Optodes and AVD measuring above sand ripples.

 

The response times of our optodes are comparable to those of microelectrodes.  We are working on gathering in situ data that correlates oxygen fluxes measured with optodes to those measured with electrodes and benthic chambers.  We hope to develop a relatively simple non-invasive flux measuring instrument that allows measurements in environments which precluded realistic measurements when using traditional techniques (chambers, electrodes, etc.). 

For my masters project, I researched the fate and degradation of dissolved organic carbon (DOC) in permeable coastal sediments.  Through a series of laboratory flow-through column experiments, flume experiments, and in situ benthic chamber experiments, I investigated what happens to DOC when it is exposed to the abundant sediment microbial communities.  Results showed that large amounts of DOC are removed by microbial degradation when flushed though permeable sediments, as compared to that removed in only seawater (Fig.2).

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Fig. 2. Percent of DO13C mineralized in sediment columns as compared to that in seawater-only columns, at two different flushing speeds.

 

Ripples and surface layers of coastal marine sand may act as efficient decomposers for highly degradable DOC pumped by bottom currents, tidal flows and waves.  Exposure of DOC to a relatively large surface area covered with biofilm and to different biogeochemical zones in the sediment accelerates its degradation.  The DOC consumption rates in column reactors corresponded to fluxes up to 379 mmol DOC m-2 d-1, suggesting that the permeable sands can act as effective sinks for degradable DOC.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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