Bioturbation

Bioturbation is the transport of solutes and solids by the activities (e.g., feeding and movement) of macrobenthos.

Figure 2 Conceptual diagram of transport and exchange processes near the sediment-water interface.

Figure 3 Bioturbation of subsurface sediments by benthic organisms (e.g., mollusks, arthropods, and annelids). The structures and activities of those organisms ventilate and extend the depth of oxic conditions in adjacent sediments, illustrated by the light area highlighted by the dashed line, above anoxic sediments, indicated by their darker coloration.

Figure 3 Bioturbation of subsurface sediments by benthic organisms (e.g., mollusks, arthropods, and annelids). The structures and activities of those organisms ventilate and extend the depth of oxic conditions in adjacent sediments, illustrated by the light area highlighted by the dashed line, above anoxic sediments, indicated by their darker coloration.

These include arthropods, annelids, and mollusks, which live in biogenic structures buried in the sediment. For example, chironomids dig and ventilate semipermanent U-shaped tubes; tubificid oligochetes live upside down, feeding on bottom sediment; and bivalves burrow into sediments with only their siphon linked to the surface -as illustrated in Figure 3.

This biologically mediated flux of molecules and ions Fb (molcm~2s-1) is generally considered as a random phenomenon similar to diffusion. For a solute, the flux is defined as

Fb - - ^ Tx with Db the biodiffusion coefficient (cm2s_1). For a solid, the flux, Fbs, is defined as

with Fbs in gcm_2s_1, DBs the bioturbation coefficient of the solid, and Cs the concentration of the solid compound in molg-1. In addition to the slight difference in formulations, it has been found that the effect of bioturbation on solutes can be 20 times more important than on solids.

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