Application of Porous Media Theories in Marine Biological Modeling - Porous Media: Applications in Biological Systems and Biotechnology - page 387

when ∆ ρ p ≥ 0 the sphere decelerates as it enters the heavier layer, due to the

decreased specific gravity. The sphere settles in response to the diffusional
exchange of fluid from the pore space. In this case, the retention time at
the pycnocline scales with the diffusional relaxation time, a 2 /D . Second, if
∆ ρ p < 0, particles can sink into the lower layer even in the absence of diffu-
sional exchange of interstitial fluid. For large negative ∆ ρ p , also implying large
Re 0 , the buoyancy of the interstitial fluid becomes negligible with respect to
the effect of entrainment and the particle motion resembles that of a solid
sphere in the lower layer.
In conclusion, Kindler et al. (2010) identified and verified a mechanism
that can account for porous particle accumulation. However, the coupling of
entrainment and diffusive effects for intermediate excess densities has to be
clarified to consider more widely occurring, weaker stratifications. A better
understanding of marine particle transport and retention within the water
column will provide the basis for carbon transport modeling at the basin
scale.

10.3.5 Enhanced Nutrient Exchange by Burrowing
Macrozoobenthos Species
Chironomid larvae, known also as bloodworms, live on the river bed or lakes
in u-tube-like burrows made from detritus (Figure 10.17a). The pupae of
midges drift to the surface, where they rest before the adult fly emerges (Fig-
ure 10.17b). What makes these species interesting is that they enhance the
exchange of dissolved substances between pore water and the overlying water
body by their body motion while being in their burrows, and cause the so-
called bioirrigation activity.
Microbial consequences and biogeochemical impacts of bioirrigation in ben-
thic sediments have been long recognized and described in studies such as
those related to filter feeding (Walshe 1947; Osovitz and Julian 2002), sed-
iment biogeochemistry (Aller 1994; Stief and de Beer 2002; Lewandowski
and Hupfer 2005) metabolic demand for oxygen (Polerecky et al. 2006;
Timmermann et al. 2006), and the solute exchange between sediment and
water (Meysman et al. 2006, 2007).
However, despite their high abundances (

4 , 000 / m 2 ) and their significant
ecological role for processes both within and above the sediment, Chirono-
mus plumosus provide challenging unsolved questions. Specifically, it was not
clear until recently, how to quantify the flow rate pumped into the burrow.
Using particle image velocimetry, Morad et al. (2010) and Roskosch et al.
(2010) studied three different experimental setups to mimic the natural flow
generated by the larvae. For this purpose, a setup was made allowing larvae
to burrow their natural tubes in the sediment. A schematics of the burrow
has been shown in Figure 10.18a. On the basis of velocity measurements,
the volumetric flow rates could be calculated by integrating the velocities
obtained by PIV (Figure 10.18b). Rigorous experiments performed showed

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