Modelling Biological Interactions in Aquatic Sediments as Coupled Reactive Transport

Biogeochemical processes in surface sediments are characterized by a reciprocal coupling between macrofauna, microbiology, and geochemistry. Up to present, reactivetransport models have been mainly implemented from a geochemical perspective, so-called early diagenetic models. In this chapter, we evaluate the possibilities and limitations of such diagenetic models as ecological tools, i.e. to assess the interactions between microbial and macrofaunal components. Despite the strong biological abstraction, diagenetic models can be considered as rudimentary ecosystems models, because the metabolism of bacteria and the activity of macrofauna is implicitly modelled. Effectively, present models incorporate microbial competition for metabolic resources (organic matter [OM], terminal electron acceptors) and the effect that macrofauna exerts on this competition via the redis-tribution of solute and solid reactants. To illustrate these interactions, a highly idealized sediment ecosystem model is constructed, incorporating five functional groups, i.e. three microorganisms (oxic respirers, sulphate reducers, sulphide oxidizers) and two macroorganisms (small bioturbators and large bioirrigators). The model predicts the steady-state values for a number of biogeochemical variables (so-called ecosystem functions) for a load of OM that varies from deep-sea to near-shore conditions. We performed a sensitivity analysis for the biological parameters in the model and compared numerical results with theoretical first-order approximations. Despite the high level of abstraction, we show that the model captures some important features of the benthic ecosystem. We conclude that the present reactive-transport formalism could serve as a basic platform for developing more advanced benthic ecosystem models. Future extensions could include (1) the feedback of microbial metabolism on macrofaunal activity via the environment, (2) the incorporation of biomass dynamics of bacteria and macrofauna, and (3) the inclusion of direct interactions between biological components.