Bioirrigation in permeable sediments: Advective pore-water transport induced by burrow ventilation

The physical mechanism that drives bioirrigation is strongly dependent on the permeability of the sediment. We advance two mechanisms, each described by a corresponding microenvironment model. In muds, burrow water cannot penetrate the sediment, so bioirrigation is intrinsically driven by diffusional transfer across the burrow wall. This ‘‘diffusive’’ mode of bioirrigation is accurately described by the classical tube irrigation model. In sands, ventilation flows can penetrate the surrounding sediment via dead end burrows. To quantify this ‘‘advective’’ mode of bioirrigation, we propose a novel two-dimensional pocket injection model. This model’s principal features are that (1) organisms indent the sediment–water interface with burrow structures, (2) the specific structure of the burrow can be neglected except for the location of a feeding pocket, and (3) burrow water is injected from this feeding pocket into the surrounding sediment. We tested the adequacy of the pocket injection model in a detailed case study of the lugworm Arenicola marina, comparing model simulations and experimental data from core incubations. Simulation of two different sets of inert tracer experiments shows good agreement between model and data, indicating that our model captures the relevant aspects of lugworm bioirrigation in permeable sediments. Diverse macrobenthic communities inhabit the surface layer of marine and estuarine sediments, supported by fluxes of organic matter and oxygen across the sediment–water interface. A major fraction of these bottom dwellers create burrows or burrow networks that penetrate deeply into the anoxic zone of the sediment (Anderson and Meadows 1978). The metabolic demand for oxygen is satisfied through passive or active flushing of the burrows with oxygen-rich water from the overlying water column (Gust and Harrison 1981; Webb and Eyre 2004). Besides oxygen supply, burrow flushing has also been linked to metabolite removal and filter feeding (Aller 2001). This process of burrow flushing and its geochemical consequences is typically referred to as bioirrigation (Rhoads 1974; Aller 2001). Previous studies have shown that bioirrigation exerts a major control on sediment biogeochemistry (Davis 1974; Aller and Aller 1998; Wenzhöfer and Glud 2004), microbial ecology (Hylleberg 1975; Reichardt 1988; Marinelli et al. 2002), and solute exchange across the sediment–water interface (Christensen et al. 1984; Archer and Devol 1992; Meile and Van Cappellen 2003). Therefore, the development of reactive transport models for aquatic sediments crucially depends on a—preferably mechanistic—mathematical description of bioirrigation. In the past, a suite of bioirrigation models have been proposed that can be classified into two general approaches 1 Corresponding author ([email protected]).