Functional approach to sediment reworking by gallery-forming macrobenthic organisms:: modeling and application with the polychaete Nereis diversicolor

A mechanistic model has been developed to characterize and quantify sediment-mixing due to macrobenthic organisms that construct gallery systems. The mixing model is time- and space-dependent and employs ordinary differential equations. It uses (1) biological parameters-the size of the bioturbated zone, rate of biodiffusion and rate of biotransport; (2) physical parametes-output to the water-column coefficient and rate of physical mixing due to local water currents; and (3) biogeochemical parameters-decay rate of the tracer. This gallery-diffusor model is based on a combination of 2 processes: biodiffusion in the sediment layer containing very dense gallery systems, and biotransport in the region of tube bottoms. The performance of this gallery-diffusor model is compared with that of the biodiffusor model classically used to describe mixing of such organisms, Both models are applied to conservative tracer profiles measured in laboratory experiments with the polychaete Nereis diversicolor. Our new model provides mechanisms to describe and explain the tracer-profile shapes observed in sediments. It includes rapid particle transport from the upper layer of the sediment to the tube bottom zone, which is not taken into account with the biodiffusor model but which is of great importance in understanding the processes of organic matter degradation in the sedimentary column, It also makes possible the accurate quantification of the different components of the mixing process of an organism (in this study, the polychaete N. diversicolor). The gallery-diffusor model constitutes 1 of 5 elementary components in a global bioturbation model that allows the study, quantification and prediction of sediment reworking by macrobenthic communities according to their functional group and composition and/or to the specific characteristics of the individual organisms.