Bioirrigation

Since many organisms live below the oxic layer in surface sediments, they need to irrigate their tubes with oxygenated water from the water column. In addition to advecting oxygen into anoxic sediments, the renewal of water in the sediment structure fulfills other needs of the benthic organisms. These include the transport of food, metabolic wastes, gametes, and environmental stimuli.

Bioirrigation of the tubes induces an increase of the fluxes of compounds in solution at the sediment-water interface, a phenomenon that has been the subject of various modeling attempts. This process can be viewed as a nonlocal transport of solutes, with an exchange rate characterized by the irrigation coefficient a (s-1), that can be estimated from the size and density of the tubes as well as the solute gradients around a tube. In general, a decreases to zero with depth, in order to reflect the decreasing density of active fauna.

Although bioirrigation is a 3D process, a 1D equation has been used successfully in various models. This equation considers that the speed of intake or input of a solute by bioirrigation at a depth x can be described as

with Ctube the solute concentration in the tube (moll-1) and t the time (s). In the calculation, Ctube is usually considered to be similar to the concentration measured in water overlying the sediment.

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