Bioturbation and irrigation in natural sediments, described by animal-community parameters

The key objective of this study was to compare tracer-based methods that measure bioturbation and irrigation rates with predictive models based on benthic community parameters such as animal size and abundance. The individual parameters of each model were investigated separately to assess their relative influence on model output. An important aspect of this study was its focus on benthic processses in natural marine sediments, where the well-defined model assumptions are not necessarily accomodated. Faunal parameters were incorporated into a biodiffusion model proposed to describe bioturbation, and direct comparisons were made to tracer-based depth distributions of an inert particle tracer, Cr-51. The results showed that this biodiffusion model under-estimated bioturbation rates (D-b) compared to Cr-51 tracer measurements. It was shown that the model predictions were highly sensitive to estimates of faunal weight and sediment reworking depth. The model suggests that large, deposit-feeding animals dominate biogenous mixing and that mixing rates are more influenced by animal size than density. In the box-cosm system, the large urchins Echinocardium cordatum did in fact dominate sediment-mixing processes. Irrigation was modelled by a combination of the non-local transport model and the radial-diffusion model, using animal size and density, and was compared to down-core distributions of a dissolved, conservative tracer bromide (Br-). Modelled irrigation rates increased as a function of burrow radius and animal density, with the strongest dependency on animal density. Given the simplicity of the assumptions for the idealized radial-diffusion model, agreement between measured and predicted rates was relatively good in 50% of the box-cosms. Over-estimation in the remainder may be attributable to unrealistic assumptions that all individuals inhabit vertical burrows and irrigate them continuously.