Sediment Dynamics by Bioturbating Organisms

Judging from the few pages dealing with bioturbation in books on coastal sediment dynamics (e. g. Hails and Carr 1975; Dyer 1986), one might conclude bioturbation to be unimportant in sediment dynamics: waves and (tidal) currents have a predominant role in sediment transport and erosion. Is this conclusion wrong? Probably not: For the Lister Basin near Sylt, Bayerl et al. (1998) conclude that reworking of sediments is mainly physical, certainly in exposed parts hardly influenced by bioturbation. With an increase in current strength and/or wave exposure (energy in the environment), sediment transport will increase. This is not the case with bioturbation. In very high-energy environments, sediment transport is too high for benthic organisms to live, so bioturbation is also absent. Under absence of water movement, benthic organisms also will not thrive. If oxygen deficiency also occurs they may be even completely absent. In between these extremes of environmental energy, benthic organisms and, therefore, bioturbation play a role. Sediment transport by bioturbation is mainly vertical, within the sediment column, whereas currents and waves transport sediment along the bottom. However, both are linked, e. g. sediment brought to the surface by organisms may be taken up and dispersed by currents. We can visualise sediment transport in a very simple model (Fig. 6.1) in which energy is on the X-axis, sediment dynamics on the Y-axis.