In describing the propagation and attenuation of high-frequency acoustic waves in porous media, several models emphasize the importance of bulk fluid properties and/or processes at grain contacts. In surficial marine sediments, bacteria and their secretions may have a significant impact on acoustic penetration. First, average bacterial abundances are theoretically dense enough to raise the viscosity of the fluid phase. Second, grain contacts likely harbor dense assemblages of bacteria and their secretions, as they may provide spatial refuge from predation. Our goal is to investigate the potential for bacterial effects on acoustic penetration.
The total number of bacteria in most surficial sediments is constant (Schmidt et al. 1998), and thus bulk measurements of abundance are not sufficient to predict acoustic effects. Our goal in SAX99 is to visualize the spatial distribution of bacteria on and among sediment grains with the native geometry intact. The methods involve a combination of histological and petrographic techniques for sample preparation, and epifluorescence microscopy and digital image processing for analysis. Sediment cores will be collected using SCUBA and processed shipboard as promptly as possible.
We predict that bacterial distributions, and their potential to impact acoustic processes, will vary considerably within the study site on a scale of centimeters, depending on macrofaunal activity, predation pressure, and hydrodynamic interactions with sediment topography. Thus, we will collect samples from diverse areas within the study site and conduct manipulations with experimental cores placed in situ. The experimental cores will be collected over a time course, providing additional information about the colonization of abraded sediments after vigorous storm activity.
While the total number of bacteria may be constant, the number of actively respiring bacteria may vary considerably on a centimeter scale. We will conduct bulk analyses of actively respiring bacteria in conjunction with DJ Tang's conductivity probe used for the in-situ measurement of porosity. Using high porosity as a proxy for processes stimulating microbial activity, e.g., macrofuanal burrowing or increased advective transport, we predict that high-porosity samples will contain higher proportions of actively respiring bacteria. The combination of micrometer-scale analysis of grain contacts and centimeter-scale analysis of bulk variation will allow better predictions of microbial impacts on acoustic penetration.
Schmidt, J. L., J. W. Deming, P. A. Jumars and R. G. Keil (1998). Constancy of bacterial abundance in surficial marine sediments. Limnology and Oceanography, 43:976–982.
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