Irrigation patterns in permeable sediments induced by burrow ventilation: a case study of Arenicola marina

In sandy sediments, a strong connection exists between the physics of flow and the ecology of burrow-ventilating macrofauna. We developed a general modelling procedure that quantifies this link involving 3 steps. (1) Burrow-ventilating organisms can be described as mechanical pumps. (2) The pumping of burrow water into blind-ending tubes induces advective flow in the sediment. (3) The resistance to pore water flow is governed by the friction between solid and fluid, i.e. Darcy's law. This analysis allows the determination of the operation point of an 'organism pump' under in situ conditions, and we applied it in a detailed modelling study of the lugworm Arenicola marina. A 3-dimensional finite element model encompasses the lugworm's J-shaped burrow and represents a typical lugworm territory at in situ density. We simulated the associated flow patterns in the sediment and analysed the factors that influence the lugworm's ventilation rate. Since the lugworm's oxygen supply critically depends on the burrow ventilation rate, we advance the following 2 ecological hypotheses: (1) decreasing the permeability of the burrow lining greatly increases the efficiency of oxygen supply, as it prevents the re-entry of anoxic pore water; and (2) the permeability of the bulk sediment constrains the lugworm's habitat. When permeability falls below a critical threshold, the sediment's resistance becomes too high, thus resulting in an insufficient oxygen supply. Overall, we show that permeability exerts an important control on ventilation activity, and hence resource availability, in sandy sediment ecosystems. From an evolutionary point of view, we anticipate biological feedbacks on this physical control-in particular, behavioural adaptations that increase the permeability in the bulk sediment but decrease the permeability near the burrow wall.