Application of Porous Media Theories in Marine Biological Modeling - Porous Media: Applications in Biological Systems and Biotechnology - page 367

(Emery 1968; Vance-Harris and Ingall 2005). However, it is believed that N 2
production in high-permeability coastal sediments may play an important
role in the global nitrogen cycle (Rao et al. 2007). This is another important
evidence for the significant role of porous media theories in understanding
global cycles.
When seabed sediments are permeable, the advective flux predominates
the diffusive one (Huettel and Gust 1992a) drastically. In the context of per-
meable sediments, a variety of interesting phenomena exists in the field of
marine microbiology, which can benefit from the knowledge available in porous
media. Examples include, but are not limited to, topography effects in nutri-
ent transport into deeper sediment layers (Huettel and Gust 1992b), enhanced
bottom transport by gravity waves (Shum 1992a), reactive solute transport
below rippled beds (Shum 1992b), and tide-driven deep pore-water flow in
intertidal sands flats (Røy and Lee 2008).
Also the classification of different sediment types providing a habitat for
marine species depend on how well the physical properties of the sediment have
been described. It has been found that, beside permeability and porosity, the
knowledge over tortuosity plays a significant role, for example, in exchange
processes in the porewater (Iversen and Jørgensen 1993).
Furthermore, a variety of microorganisms inhabit the seabed, which have
the ability of altering or influencing the ongoing interfacial exchange. Prominent
examples of this group are burrowing animals, which construct U-, V-, or L-
shaped tubes into the seabed, and ventilate the overlying seawater and generate
an enhanced mixing. Using peristaltic or oscillatory motions, larvae are able to
transfer oxygen into deeper sediment layers, and perform an ecologically signif-
icant interfacial nutrient exchange (Riisg ard and Larsen 2005). Theoretically it
seems obvious that the seawater ventilated by the larvae might also penetrate
laterally into the ambient sediment, and generate, in addition to the currently
accepted diffusive transport, yet another new mode of transport, namely the
advective one. Modeling studies considering flow through a composite region
made of saturated sediment and pure-fluid layers can provide useful hints bring-
ing more light into this complex and important phenomena of bioirrigation.
Also in the water column of world oceans, a great deal of situations arise
where porous media theories can be applied. An interesting example is that
of marine aggregates. It has been found that particle settling has a significant
effect on the biogeochemistry and ecology of the oceans due to the fact that
particles are the key factor for carbon sequestration, and indirectly responsible
for the amount of CO 2 that is released into the atmosphere from the seabed
(Chisholm 2000; Azam and Long 2001).
When marine particles coagulate, bigger aggregates are formed that sink
from the ocean surface down to the seabed within several hours or days
depending on their sinking velocity and the ocean depth. The release of nutri-
ents from sinking aggregates into the ambient seawater or vice versa plays
an important role for the marine life. Although some simple models exist
in which aggregates were considered as solid bodies (Kiørboe et al. 2001),